Synthetic Uses of Reagents – Chemistry Optional PYQs (2000–2026)

Here are all 126 Synthetic Uses of Reagents questions asked in UPSC Chemistry Optional Paper II from 2000 to 2026, arranged under the 3 topics of the official syllabus. Within each topic the latest questions come first.

Each question ends with a code such as [2024/15m/3b] – the year, the marks, the word limit (where UPSC set one) and the question number in the paper.

5.1] Oxidising reagents

54 questions

1. Write the structure of the major product indicating the stereochemistry and mechanism of the following reactions:

[Figure: Substrate: cis-4-tert-butyl-3-methylcyclopentene (cyclopentene; tBu on a wedge at C4 and CH3 on a wedge at C3, C3 adjacent to the C=C). Left: –m-CPBA→ ?; right: –(i) NBS; (ii) 30% NaOH→ ?] [2026/10m/5b]

2. Write the structure of the major product formed in the following reactions and provide the mechanism:

[Figure: (i) methyl 3-formylbenzoate (COOCH3 and CHO meta) –NaBH4, EtOH→ ?; (ii) 4-tert-butylcyclohexanone drawn as a chair with tBu equatorial and the ring H on C4 axial (down) –(i) LiAlH4; (ii) H2O→ ?; (iii) anisole (C6H5OMe) –Na, NH3 (l), ether, ethanol→ ?; (iv) RHC=CMe2 (H and R on one carbon, two Me on the other) –SeO2→ ?; (v) 2,5-dihydroxybenzonitrile (OH at C1 and C4 of the ring, CN ortho to the C1–OH) –Pb(OAc)4→ ?] [2026/20m/7c]

3. Describe the synthesis of ketone (A) when only isobutanol is available as the starting material:

[Figure: Ketone A: 2,5-dimethylhexan-3-one, (CH3)2CH–CH2–CO–CH(CH3)2] [2025/5m/3a(ii)]

4. Provide the structure of missing reagents and intermediates in the following synthesis:

[Figure: Scheme: 3-methylbutan-1-ol ((CH3)2CHCH2CH2OH) –M→ 3-methylbutanal ((CH3)2CHCH2CHO) –N→ O –P→ 1-amino-4-methylpentan-2-ol ((CH3)2CHCH2CH(OH)CH2NH2)] [2025/10m/3c(i)]

5. Describe the role of NMO during the dihydroxylation of an alkene using catalytic amount of $Os{O}_4$ in the presence of N-methylmorpholine N-oxide (NMO). [2025/5m/6a(ii)]

6. Predict the structure of P, Q and R in the following sequence of reactions:

[Figure: Scheme: $H_{3}C-C\left(C{H}_3\right)=CH-C{H}_2-C\equiv C-C{H}_3$ –Na, NH3(l)→ P –1) OsO4; 2) NaHSO3, H2O→ Q –HIO4 (downward arrow)→ R] [2024/10m/5b]

7. Predict the structure of X, Y and Z in the following sequence of reactions:

[Figure: Scheme: tert-butyl 4-(2-oxoethyl)-3-methylidenepiperidine-1-carboxylate (N-Boc piperidine, Boc = tBuO–C(=O)–; CH2CHO on C4 and exocyclic =CH2 on C3) –LiAlH4, THF, 0 °C→ X –1) BH3/THF; 2) HO${}^{-}$, H2O2, H2O→ Y –CrO3, H2SO4→ Z] [2024/15m/7b]

8. Write the structure of the compounds (A), (B), (C), (D) and (E).

[Figure: (1) 2-methylpentan-3-ol, ${\left(C{H}_3\right)}_{2}CH\mathrm{\text{–}}CH\left(OH\right)\mathrm{\text{–}}C{H}_{2}C{H}_3$ –i. CS2, NaOH; ii. MeI→ (A) –SeO2→ (B); (2) a 6–7–6 fused tricyclic compound –mcpba, CH2Cl2, 0 °C→ (C): left ring is a cyclohexene whose C=C is the fusion bond to the central seven-membered ring and which carries a gem-dimethyl group on the ring carbon next to the lower fusion carbon; the central seven-membered ring contains, besides the two fusion bonds, a CH2 (between the upper left fusion carbon and the upper benzene fusion carbon) and a cis CH=CH (between the lower benzene fusion carbon and the lower left fusion carbon); the right ring is a benzene ring bearing OMe on the carbon next to the CH2-side fusion carbon, a second OMe on the next carbon, an isopropyl group on the next carbon, and H on the remaining carbon; (3) 2,2,3,3-tetramethylbutanedioic acid, $H{O}_{2}C\mathrm{\text{–}}C{\left(C{H}_3\right)}_2\mathrm{\text{–}}C{\left(C{H}_3\right)}_2\mathrm{\text{–}}C{O}_{2}H$ –Pb(OAc)4, O2→ (D); (4) 3-methylcyclohexene with the CH3 on a wedged bond at the allylic sp3 carbon –CrO3-Pyridine→ (E)] [2023/10m/7c(i)]

9. From the reaction sequence below, answer the following questions: (I) Write structure of (A) and (B) (II) Write the oxidation state of Osmium in the intermediate (A) and its geometry.

[Figure: Reaction: cyclohexene –OsO4, Pyridine→ (A) –(B)→ cis-cyclohexane-1,2-diol (both OH on wedged bonds)] [2023/5m/7c(ii)]

10. Write the structure of (A) and (B) in the above reactions.

[Figure: Reaction: hexane-1,2,3,4,5,6-hexol drawn as a zigzag HOCH2–CH(OH)–CH(OH)–CH(OH)–CH(OH)–CH2OH with C-2 OH on a wedge (up), C-3 OH on a hash (down), C-4 OH on a hash (up), C-5 OH on a wedge (down) (D-mannitol) –cyclohexanone, BF3·Et2O, (MeO)3CH→ (A) –HIO4→ (B)] [2023/5m/7c(iii)]

11. Complete the following reaction by showing stepwise reaction mechanism for the formation of products:

[Figure: Reaction: (E)-but-2-ene (Me and H on each carbon, the two Me groups trans) + PhCOOOH –$O{H}^{\ominus }$→] [2022/10m/4a(i)]

12. Write the structures of the products $X$, $Y$ and $Z$ in the following reactions and indicate the mechanism for the formation of $X$:

[Figure: Reaction: a tricyclic lactone – a γ-butyrolactone (C=O and ring O) cis-fused to a cyclopentane, which is fused at its other edge to a cyclopropane; the two cyclopropane ring-fusion H atoms on wedges (one up, one down as drawn); the exocyclic cyclopropane carbon bears H (wedge) and a side chain –CH=CH–CH2–CH2–CH2–Ph (double bond drawn cis) –(1) OsO4, Pyridine; (2) $H^{+}$, H2O→ $X$ –$HI{O}_4$→ $Y+Z$] [2022/10m/6c(i)]

13. Predict the product(s) and suggest mechanism in each of the above chemical transformations.

[Figure: I. acetophenone ($C_6{H}_{5}COC{H}_3$) –$Se{O}_2$→ ?; II. benzoic acid ($C_6{H}_{5}COOH$) –Na, $N{H}_3$, $C_2{H}_{5}OH$→ ?] [2021/10m/5c]

14. Write the products X/Y in the above chemical transformations.

[Figure: I. 1-phenyl-1,2-dihydronaphthalene (C-1 bearing Ph and H, C3=C4 double bond conjugated with the benzene ring) –MCPBA→ X; II. 1,4-dihydronaphthalene (C2=C3 double bond between two CH2 groups) –$Os{O}_4$→ X –$NaI{O}_4$→ Y; III. 3-hydroxybutanoic acid ($H_{3}C\mathrm{\text{–}}CH\left(OH\right)\mathrm{\text{–}}C{H}_2\mathrm{\text{–}}COOH$) –$B{H}_3/THF$→ X; IV. cyclohexene –$Cr{O}_3$-Pyridine, $C{H}_{2}C{l}_2$→ X] [2021/15m/8a(i)]

15. Identify the compounds A, B and C in the following conversions: (i) cyclohexene + NBS $\to$ [A] $\stackrel{1)O{H}^{-};2)KMn{O}_4,0{}^{\circ }C}{\to }$ [B] $\stackrel{HI{O}_4}{\to }$ [C] (ii) $C{H}_{3}C{H}_{2}CH=C{H}_2\stackrel{\left[A\right]}{\to }C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}OH\stackrel{\left[B\right]}{\to }C{H}_3-CH=CH-C{H}_3\stackrel{1)Os{O}_4;2)Pb{\left(OAc\right)}_4}{\to }$ [C]. [2020/20m/6c]

16. Identify X and Y in the following reactions and outline the mechanism for the formation of X in both the reactions:

[Figure: (i) fused bicyclic substrate drawn as two six-membered rings: right ring = cyclohexane carrying a CH3 on the carbon next to the left-ring fusion carbon; left ring (as drawn) = C(SiMe3)=CH–CH2–CH(ring-fusion)–C(ring-fusion)=O–(back to C(SiMe3)), i.e. the ring O is drawn bonded to the $M{e}_{3}Si$-bearing vinylic carbon and double-bonded (=) to the ring-fusion carbon that is adjacent to the CH3-bearing carbon –mCPBA→ [X] –$H_3{O}^{+}$→ [Y]; (ii) 2-ethyl-3-methylcyclopentanone tosylhydrazone (C=N–NHTs; Et on C2, Me on C3) –1) n-BuLi, 2 equiv., THF; 2) $H_3{O}^{+}$→ [X]] [2020/15m/8b]

17. Identify ($X$) and ($Y$) in the following: (i) ($X$) $\underset{\varphi .OOH}{\stackrel{NBS}{\leftarrow }}$ cyclohexene $\underset{{\left(C{H}_3\right)}_4\stackrel{+}{N}B{r}^{-}}{\stackrel{NBS}{\to }}$ ($Y$) (ii) methylenecyclobutane $\stackrel{\mathrm{\text{Dil.}}KMn{O}_4}{\to }$ ($X$) $\stackrel{NaI{O}_4}{\to }$ ($Y$) + HCHO (iii) $C_6{H}_6\underset{Os{O}_{4}in𝑡-C_4{H}_{7}OH}{\stackrel{H_2{O}_2}{\to }}$ ($X$) [2019/15m/6b]

18. Write the products ($X$) and ($Y$) of the following reactions: (i) (ii) (iii) (iv)

[Figure: (i) $HOOC-CH=CH-COOH$ (drawn as CH–COOH ‖ CH–COOH, no geometry shown) –$H_2{O}_2$, $Se{O}_2$→ ($X$); (ii) o-phenylenediamine (benzene-1,2-diamine) –$Pb{\left(OAc\right)}_4$→ ($X$); (iii) cyclopentene –$Os{O}_4$, Aq. $N{a}_{2}S{O}_3$→ ($X$); (iv) 3-methyl-3-phenylcyclopropene –(1) $Os{O}_4$; (2) $NaS{O}_{3}H$→ ($X$) –$HI{O}_3$→ ($Y$)]

Comment on the stereochemistry of product ($X$) in each case. [2019/20m/7c]

19. When trans-4-tert-butylcyclohex-2-enol is reacted separately with mCPBA and V(acac)${}_2$/tert-butylhydroperoxide, in the second reaction only one product is formed, whereas in the first case a mixture of two isomeric products is obtained. Write the structure of the product in each case with proper stereochemical outcome and justification. [2018/15m/6a]

20. Write the structures of the products expected from hydroxylation of maleic acid and fumaric acid using $Os{O}_4/Py$, with the stereochemistry. What is the role of pyridine in the above reaction? [2018/10m/7b]

21. Predict the final product obtained after the following sequence of reactions and also suggest the corresponding plausible mechanistic pathway.

[Figure: Reaction: (E)-2-ethylhept-5-enenitrile (Me–CH=CH–CH2–CH2–CH(CN)–CH2–Me, trans double bond) –(i) mCPBA; (ii) LiN(iPr)2, −78°C, THF; (iii) H3O+→ ?] [2017/10m/4a]

22. Write the reagent(s) for the following conversions and write the mechanism for reaction (i).

(i)

[Figure: 2-methylhept-2-ene (Me2C=CH–CH2CH2CH2CH3) → (E)-2-methylhept-2-en-1-ol (HOCH2–C(Me)=CH–CH2CH2CH2CH3, the CH2OH group trans to the butyl chain); reagent to be supplied] [2017/6m/4c(i)]

(ii)

[Figure: 1-methylcyclohexene → cis-1-methylcyclohexane-1,2-diol (labelled “cis – diol”) → 2-hydroxy-2-methylcyclohexan-1-one; reagents to be supplied for both steps] [2017/4m/4c(ii)]

23. Identify the major product ($Z$) formed in the following reactions.

[Figure: (i) 1,2-bis(benzyloxy)hept-3-ene with the double bond drawn cis: C6H5–CH2–O–CH2–CH(O–CH2–C6H5)–CH=CH–CH2–CH2–CH3, the OCH2C6H5 at C2 on a bold wedge, cis C3=C4 double bond –OsO4, pyridine (above arrow), THF (below arrow)→ (Z); (ii) ethyl acetoacetate (CH3–CO–CH2–CO–OC2H5) with two routes: LiAlH4, C2H5–O–C2H5 → (Z); and (1) HO–CH2CH2CH2–OH (propane-1,3-diol), H+; (2) LiAlH4; (3) H3O+ → (Z); (iii) a 4-methylcyclohex-3-en-1-yl ring attached to C(=CH2)–CH2–CH2–CH=C(CH3)–CH2OH (the CH2 next to the =CH drawn with both H atoms shown; the CH2OH cis to the chain on the trisubstituted double bond) –Na, NH3, C2H5OH→ (Z)] [2017/15m/7b]

24. Identify the major product $\left(X\right)$ and discuss its stereochemistry:

[Figure: Reaction: norbornene (bicyclo[2.2.1]hept-2-ene) –OsO4 (above arrow), NaHSO3 (below arrow)→ (X)] [2016/10m/1b]

25. Discuss the stereoselectivity of products in the reaction of MCPBA with cis- and trans-butene. [2016/10m/3b]

26. Glucose on periodic acid oxidation gives 5 molecules of formic acid, whereas fructose gives only 3 molecules of formic acid. Explain with reactions. [2016/10m/4c]

27. Identify the major product $\left(X\right)$ formed in the following reactions:

[Figure: (i) 2-acetylnaphthalene (naphthalene with –CO–CH3 at C2) –SeO2→ (X); (ii) cyclopentene –1) NBS; 2) hν/heat→ (X); (iii) trans-decalin-4a,8a-diol (decalin with OH on both ring-fusion carbons, one on a bold wedge and one on a hashed bond) –Pb(OAc)4→ (X)] [2016/15m/8d]

28. Give the mechanisms involved in the epoxidation of an alkene and Baeyer-Villiger oxidation of a ketone using MCPBA. [2015/10m/7a]

29. Complete the following reactions and suggest a suitable reaction mechanism for each:

[Figure: (i) 1-(hydroxymethyl)cyclopentan-1-ol (cyclopentane C1 bearing OH and CH2OH) –$HI{O}_4$→ ?; (ii) 3-nitrobenzaldehyde (O2N and CHO meta on a benzene ring) –$NaB{H}_4$, methanol→ ?; (iii) methyl phenyl sulfoxide ($C_6{H}_5-S(=O)-C{H}_3$) –$LiAl{H}_4$→ ?; (iv) cyclohexanone –$Cr{O}_3$→ ?] [2015/20m/7d]

30. Suggest appropriate reagents/conditions, with justification, for the desired chemical transformations shown below:

[Figure: 1-methylcyclohexene at the centre with four arrows, each labelled “?”, to: (1, left) cis-1-methylcyclohexane-1,2-diol (C-2 bears H and a hashed OH; C-1 bears CH3 and a hashed OH); (2, right) trans-2-methylcyclohexan-1-ol (C-1 bears H and a hashed OH; C-2 bears a wedged CH3 and a hashed H); (3, lower left) trans-1-methylcyclohexane-1,2-diol (C-2 bears a wedged OH and a hashed H; C-1 bears CH3 and a hashed OH); (4, lower right) 1-methylcyclohexan-1-ol (C-1 bears a wedged CH3 and a hashed OH)] [2014/15m/6c]

31. Suggest suitable reagents and provide mechanism for the following chemical conversions:

[Figure: (i) octahydro-2H-chromene-4a,8a-diol (two fused six-membered rings, one containing a ring O next to the lower ring-fusion carbon; an OH on each ring-fusion carbon) –?→ the same bicyclic outline (the bond between the two former ring-fusion carbons is still drawn) with a C=O in place of each OH (the lower one next to the ring O, i.e. a lactone C=O), intended as the ten-membered keto-lactone from cleavage of the fusion bond; (ii) 7-(methoxymethyl)bicyclo[2.2.1]heptan-2-one (H3COH2C– on the one-carbon bridge) –?→ the corresponding bicyclic lactone with O inserted between the C=O and the bridgehead (2-oxabicyclo[3.2.1]octan-3-one carrying H3COH2C– on the one-carbon bridge); (iii) bicyclo[2.2.1]heptan-2-one (norcamphor) –?→ bicyclo[2.2.1]heptane-2,3-dione; (iv) norbornene –?→ ? → cis-exo-bicyclo[2.2.1]heptane-2,3-diol (both OH exo, drawn up; both H endo, drawn down)] [2014/20m/7c]

32. Write the products (X) and (Y) in the following reactions:

[Figure: Reaction scheme: CH≡CH branching into two arrows: upper arrow labelled H2CrO4 → (X); lower arrow labelled alk. KMnO4 → (Y)] [2013/10m/1e]

33. What is the major product of each of the following reactions? (i) ${\left(C{H}_3\right)}_{3}COO\mathrm{\text{–}}OH/H{O}^{\ominus }$ and (ii) $Os{O}_4/t\mathrm{\text{–}}BuOH$ Give the mechanisms of the reactions.

[Figure: Printed before “(i) …”: cyclohexene drawn with the two vinylic H atoms shown on the C=C carbons –(i) and (ii)→ (no product shown)] [2013/10m/5b]

34. Give structures of compounds (A), (B), (C) and (D) in the following reactions: (i) n-Octaldehyde + Ethyl bromo acetate + Zn and then $H_{2}O\to \left(A\right)\left(C_{12}{H}_{24}{O}_3\right)$ (ii) $\left(A\right)+Cr{O}_3$ in glacial acetic acid $\to \left(B\right)\left(C_{12}{H}_{22}{O}_3\right)$ (iii) $\left(B\right)+C_2{H}_5{O}^{-}N{a}^{+}$, then benzyl chloride $\to \left(C\right)\left(C_{19}{H}_{28}{O}_3\right)$ (iv) $\left(C\right)+O{H}^{-}$, heat, then $H^{+}$ warm $\to \left(D\right)\left(C_{16}{H}_{24}O\right)$ [2012/20m/2b]

35. What are the reagents (A, B and C) used for the following conversion: (i) $R_{2}C\left(OH\right)-C\left(OH\right)R_2\stackrel{A}{\to }{R}_{2}C=O$ (ii) $C_6{H}_5-CO-C{H}_3\underset{\mathrm{\text{in acetic acid}}}{\stackrel{B}{\to }}{C}_6{H}_5-CO-CHO$ (iii) [structure] $\underset{\mathrm{\text{Heat}}}{\stackrel{C}{\to }}$ [structure].

[Figure: (i) drawn as R–C(R)(OH)–C(R)(OH)–R (a pinacol, each carbon bearing two R and one OH) –A→ R(R)C=O; (ii) acetophenone C6H5–C(=O)–CH3 –B, in acetic acid→ phenylglyoxal C6H5–C(=O)–C(=O)–H; (iii) decalin (two fused saturated six-membered rings) –C, Heat→ naphthalene] [2012/15m/7d]

36. How Malaprade oxidation is used to determine cis-glycolic units in sugars? Illustrate with one example showing mechanism. [2011/10m/5b]

37. Predict the structure of the products in the following reactions. Give the mechanism. (i) $\stackrel{mCPBA}{\to }$ ? (ii) $\stackrel{Se{O}_2}{\to }$ ? (iii) $C_3{H}_7-C\equiv C-Et\underset{liq.N{H}_3}{\stackrel{Li}{\to }}?$ (iv) $C{H}_2=C\left(C{H}_3\right)\left(Et\right)\underset{H_2{O}_2,NaOH}{\stackrel{B{H}_3}{\to }}?$

[Figure: (i) substrate: partial structure: cyclopentanone ring whose α-carbon is a quaternary ring-fusion carbon carrying an angular methyl group, the next ring carbon a ring-fusion CH; the bonds to the rest of the fused ring system are shown as wavy lines (as in a steroid D-ring 17-ketone); (ii) substrate: 2-methylcyclohexanone] [2011/20m/7c]

38. Write the product formed in the oxidation of quinoline by $Cr{O}_3$. [2010/10m/6c]

39. Which among the following (A or B) is formed when Pent-2-ene is oxidised by $Os{O}_4$ and $NaHS{O}_3$? Explain its formation with the mechanism.

[Figure: two Fischer projections of pentane-2,3-diol with $C{H}_{2}C{H}_3$ at top and $C{H}_3$ at bottom. (A) upper carbon H left, OH right; lower carbon H left, OH right (both OH on the same side: erythro). (B) upper carbon H left, OH right; lower carbon HO left, H right (OH on opposite sides: threo)] [2010/10m/7c]

40. Identify the compound susceptible for $HI{O}_4$ oxidation and justify (i) $C{H}_3-CHN{H}_2-C{H}_{2}OH$ (ii) $C{H}_{2}N{H}_2-C{H}_2-C{H}_{2}OH$ [2010/10m/7d]

41. Write the reagents required for the following conversions.

[Figure: (i) indane-1,3-dione (benzene ring fused to a five-membered ring with C=O at C1 and C3, C2 a $C{H}_2$) → five-membered ring with three C=O groups (C1, C2, C3) fused to a six-membered ring drawn without any double bonds or aromatic circle (as printed); (ii) p-xylene (1,4-dimethylbenzene) → terephthalaldehyde (benzene-1,4-dicarbaldehyde)] [2010/10m/7e]

42. $\stackrel{Se{O}_2}{\to }E\stackrel{Cr{O}_3}{\to }F\underset{inalkalinemedium}{\stackrel{heat}{\to }}G\underset{Catalyst}{\stackrel{C{N}^{-}}{\to }}H$

[Figure: starting compound acetophenone ($C_6{H}_5-CO-C{H}_3$, drawn as benzene ring with $C(=O)C{H}_3$)] [2009/16m/4b(iii)]

43. Write suitable reagents for converting cyclohexene to $OHC{\left(C{H}_2\right)}_{4}CHO$. [2009/10m/7e]

44. Write reagent(s) for the following conversion:

[Figure: cyclopentene → cyclopentane-1,2-diol (both OH drawn on plain bonds on adjacent carbons)] [2008/20m/5d(ii)]

45. Identify A-D in the following How adipic acid can be produced from phenol (only two steps)? [2008/10m/6c]

46. How is azelaic acid obtained from oleic acid? Azelaic acid $HOOC{\left(C{H}_2\right)}_{7}COOH$ Oleic acid = $C{H}_3{\left(C{H}_2\right)}_7-CHCH{\left(C{H}_2\right)}_{7}COOH$. [2008/10m/8b]

47. Explain the following transformations:

[Figure: (i) cyclohexene –$Os{O}_4$→ cis-cyclohexane-1,2-diol (both OH on wedged bonds); (ii) indoline (2,3-dihydro-1H-indole, N–H) –$Pb{\left(OAc\right)}_4$→ indole (N–H); (iii) 2-methylbut-2-ene –$B_2{H}_6$ and then $H{O}^{\ominus }$→ 3-methylbutan-2-ol (${\left(C{H}_3\right)}_{2}CH-CH\left(OH\right)-C{H}_3$); (iv) cyclohexanone –NBS and then $H{O}^{\ominus }$→ cyclohex-2-en-1-one] [2007/20m/6c]

48. How will you carry out the following transformations in good yields? Indicate the reagents used, conditions employed & reasons thereof:

[Figure: (i) cyclohex-2-en-1-ol → cyclohexane-1,2,3-triol (C-1 OH plain, C-2 OH hashed, C-3 OH on a wedged/bold bond); (ii) $C{H}_{3}C{H}_{2}COC{H}_3$ → $PhCH\left(OH\right)-CH\left(C{H}_3\right)-CO-C{H}_3$ (aldol: OH on hashed bond, $C{H}_3$ wedged; 4-hydroxy-3-methyl-4-phenylbutan-2-one); (iii) 1-chloro-3-phenylpropan-2-one ($PhC{H}_{2}COC{H}_{2}Cl$) → phenyl cyclopropanone [as drawn: benzene ring attached to a cyclopropanone ring carbon]; (iv) phenyl 3-methylbut-2-enyl ether ($PhOC{H}_{2}CH=C{\left(C{H}_3\right)}_2$) → 2-(1,1-dimethylprop-2-enyl)phenol (OH at C-1, $C{\left(C{H}_3\right)}_{2}CH=C{H}_2$ at C-2)] [2006/48m/4a]

49. Indicate the steps by means of which you will effect the following conversions in good yields:

[Figure: Conversions: (i) 2-acetylfuran (furan-2-yl–$COC{H}_3$) → furan-2-yl–$COCH=CHC{H}_{2}C{H}_3$; (ii) acetophenone ($C_6{H}_{5}COC{H}_3$) → phenol ($C_6{H}_{5}OH$); (iii) cyclohexanone → 4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (bicyclic octalone: C=O at C-2 conjugated with the C-1=C-8a ring-fusion double bond); (iv) cis-alkene (R and R on the same side, H and H on the same side, RHC=CHR) → trans-alkene (R and R on opposite sides)]

The only reagents available are m-chloroperbenzonic acid and triphenyl phosphine. [2005/20m/2c]

50. Predict the product(s) in the reactions given below with mechanism wherever possible.

[Figure: (i) tetralin (1,2,3,4-tetrahydronaphthalene) –$Cr{O}_3$, ACOH→ ; (ii) 3-methylthiophene –NBS, Pyridine / Peroxide→ ; (iii) cyclohex-2-en-1-ol –$Os{O}_4$ [?], Pyridine / THF [?] (printed “CHF”)→ ; (iv) methyl 3-oxohex-4-enoate, $C{H}_3-CH=CH-CO-C{H}_2-COOC{H}_3$, with two arrows: –$NaB{H}_4$, $C{H}_{3}OH$→ and –$LiAl{H}_4$, ether→] [2003/20m/5b]

51. Suggest suitable reagents for the synthesis of the following.

[Figure: (i) cyclohexane-1,2-dione from cyclohexanone; (ii) 3-(but-3-enyl)cyclohexan-1-ol (cyclohexanol with $C{H}_{2}C{H}_{2}CH=C{H}_2$ at C-3) from 3-(2-bromoethyl)cyclohexan-1-ol (cyclohexanol with $C{H}_2-C{H}_{2}Br$ at C-3); (iii) 1-methylcyclopentan-1-ol from 1-methylcyclopentene; (iv) 2,4-dimethylpyrrole ($C{H}_3$ on C-2 and C-4 of 1H-pyrrole) from $C{H}_3-CO-C{H}_2-N{H}_2$] [2003/20m/7b]

52. In cyclohexane series it is observed that with chromic acid it is the axial hydroxyl group that is oxidised more rapidly than equatorial hydroxyl group by a factor of about 3, but the saponification of the esters of related compounds however shows a reverse order of reactivities i.e., axial ester group hydrolyses very slowly than the equatorial ester group. Explain. [2002/13m/3b(i)]

53. Which types of compounds are oxidised by the periodate? Give an example in each case. Mention the product formed by the periodate oxidation of the following compound.

[Figure: Two fused six-membered rings (decalin-type) with OH on each ring-fusion carbon (one drawn up, one drawn down); the left ring has a ring O attached to the lower ring-fusion carbon (octahydro-2H-chromene-4a,8a-diol)] [2000/15m/7a]

54. How is $Se{O}_2$ used in organic chemistry? Give two examples and discuss their mechanisms. [2000/15m/7c]

5.2] Hydride, borane and dissolving-metal reagents

51 questions

1. Write the structures of the products formed in the following reactions:

[Figure: (i) cyclohex-2-en-1-one –NaBH4/CeCl3, MeOH→ ?; (ii) cyclohex-2-en-1-one –Na/NH3 (l)→ ?; (iii) EtO2C–CH2–CH(Me)–CH2–CO2H –(i) B2H6; (ii) H${}^{+}$→ ?; (iv) EtO2C–CH2–CH(Me)–CH2–CO2H –(i) LiAlH4; (ii) H${}^{+}$→ ?; (v) methyl 1-benzyl-5-oxopyrrolidine-3-carboxylate (N-CH2Ph lactam, CO2Me on C4 of the ring, β to N) –B2H6, THF→ ?] [2026/10m/6a]

2. Write the structure of the major product formed in the following reactions and provide the mechanism:

[Figure: (i) methyl 3-formylbenzoate (COOCH3 and CHO meta) –NaBH4, EtOH→ ?; (ii) 4-tert-butylcyclohexanone drawn as a chair with tBu equatorial and the ring H on C4 axial (down) –(i) LiAlH4; (ii) H2O→ ?; (iii) anisole (C6H5OMe) –Na, NH3 (l), ether, ethanol→ ?; (iv) RHC=CMe2 (H and R on one carbon, two Me on the other) –SeO2→ ?; (v) 2,5-dihydroxybenzonitrile (OH at C1 and C4 of the ring, CN ortho to the C1–OH) –Pb(OAc)4→ ?] [2026/20m/7c]

3. Identify the missing reagent and intermediates in the following chemical conversion:

[Figure: Reaction scheme: cyclohexyl methyl ketone (1-cyclohexylethan-1-one, cyclohexyl–CO–CH3) –PCl5, 0 °C (–POCl3)→ M –1) 3 equiv. NaNH2, Heat; 2) HA→ N –NaNH2, C2H5Br→ O –P (reagent to be identified, vertical arrow)→ (E)-1-cyclohexylbut-1-ene (cyclohexyl–CH=CH–CH2CH3, drawn with the two vinylic H trans to each other)] [2025/10m/1b]

4. Provide the structure of missing reagents and intermediates in the following synthesis:

[Figure: Scheme: 3-methylbutan-1-ol ((CH3)2CHCH2CH2OH) –M→ 3-methylbutanal ((CH3)2CHCH2CHO) –N→ O –P→ 1-amino-4-methylpentan-2-ol ((CH3)2CHCH2CH(OH)CH2NH2)] [2025/10m/3c(i)]

5. Elucidate the structure of the product and the intermediate (if any) in the following reactions:

[Figure: (A) 3-methylpentanedioic acid monoethyl ester, EtO2C–CH2–C(H)(Me)–CH2–CO2H, with H on a hashed bond and Me on a wedge at the central carbon –1) BH3; 2) H${}^{+}$→ ?]

[Figure: (B) (E)-3-cyclohexylprop-2-enal (cyclohexyl–CH=CH–CHO) –NaBH4→ ?] [2025/10m/6a(i)]

6. Predict the structure of P, Q and R in the following sequence of reactions:

[Figure: Scheme: $H_{3}C-C\left(C{H}_3\right)=CH-C{H}_2-C\equiv C-C{H}_3$ –Na, NH3(l)→ P –1) OsO4; 2) NaHSO3, H2O→ Q –HIO4 (downward arrow)→ R] [2024/10m/5b]

7. Identify A, B, C and D in the following reaction sequence: Write the mechanism of the first step of the above reaction sequence.

[Figure: Scheme: A –NaBH4, MeOH→ cyclohexanol –H${}^{+}$→ B –C→ 3-bromocyclohexene –mCPBA (downward arrow)→ D] [2024/20m/6c]

8. Predict the structure of X, Y and Z in the following sequence of reactions:

[Figure: Scheme: tert-butyl 4-(2-oxoethyl)-3-methylidenepiperidine-1-carboxylate (N-Boc piperidine, Boc = tBuO–C(=O)–; CH2CHO on C4 and exocyclic =CH2 on C3) –LiAlH4, THF, 0 °C→ X –1) BH3/THF; 2) HO${}^{-}$, H2O2, H2O→ Y –CrO3, H2SO4→ Z] [2024/15m/7b]

9. Complete the above reactions and give suitable mechanism.

[Figure: Reaction: $P{h}_{2}C=C{\left(C{H}_{2}C{H}_3\right)}_2$ (2-ethyl-1,1-diphenylbut-1-ene) –$B{H}_3\cdot THF$→ (A) –$H_2{O}_2$, $\stackrel{\ominus }{O}H$→ (B)] [2023/10m/4a(iii)]

10. Reduction of camphor with $LiAl{H}_4$ leads to 90% of the isomer in which the OH group is cis to the bridge. Give a suitable explanation of this observation.

[Figure: Reaction: Camphor (1,7,7-trimethylbicyclo[2.2.1]heptan-2-one: bridgehead C-1 bearing H3C, C-7 bridge bearing two CH3 (H3C, CH3), C=O at C-2) –$LiAl{H}_4$→ the corresponding bicyclic alcohol with C-2 bearing OH (drawn pointing right) and H (drawn pointing down)] [2023/10m/5d]

11. Write the structure of product(s) in the above reactions:

[Figure: (1) methyl 4-oxocyclohexane-1-carboxylate (O= on ring C-4, C(=O)OCH3 on C-1) –(i) NaBH4, H2O, $O{H}^{-}$; (ii) H2O, $H^{+}$→ A; (2) estrone (3-hydroxyestra-1,3,5(10)-trien-17-one: aromatic A ring with HO at C-3, C-13 Me on a wedge, C-17 ketone) –(i) HC≡CH, BuLi; (ii) $H^{+}$, H2O→ B; (3) anisole (methoxybenzene, OMe) –Na, NH3(l); Et2O, EtOH→ C] [2023/10m/6b(i)]

12. Complete the following reactions and give suitable mechanisms for the formation of products: (i) $C{H}_3\mathrm{\text{–}}C{H}_2\mathrm{\text{–}}C{H}_2\mathrm{\text{–}}COCl\underset{\left(2\right)H_3{O}^{\oplus }}{\stackrel{\left(1\right)2NaB{H}_4}{\to }}?$ (ii) $C{H}_3\mathrm{\text{–}}C\equiv C\mathrm{\text{–}}C{H}_3\stackrel{\mathrm{\text{Na in liq.}}N{H}_3}{\to }?$ [2022/15m/7b]

13. Predict the product(s) and suggest mechanism in each of the above chemical transformations.

[Figure: I. acetophenone ($C_6{H}_{5}COC{H}_3$) –$Se{O}_2$→ ?; II. benzoic acid ($C_6{H}_{5}COOH$) –Na, $N{H}_3$, $C_2{H}_{5}OH$→ ?] [2021/10m/5c]

14. Discuss the solvent compatibility for $LiAl{H}_4$ and $NaB{H}_4$ reagents and the factors responsible for differential reactivity. Also suggest preferred reagent between the two for the above transformations.

[Figure: (1) 4-acetylbenzamide ($H_{2}NOC\mathrm{\text{–}}{C}_6{H}_4\mathrm{\text{–}}COC{H}_3$, para) –?→ 4-(1-hydroxyethyl)benzamide ($H_{2}NOC\mathrm{\text{–}}{C}_6{H}_4\mathrm{\text{–}}CH\left(OH\right)C{H}_3$); (2) $C{H}_3\mathrm{\text{–}}CH\left(OH\right)\mathrm{\text{–}}C\equiv C\mathrm{\text{–}}C{H}_3$ –?→ (E)-pent-3-en-2-ol, $C{H}_3\mathrm{\text{–}}CH\left(OH\right)\mathrm{\text{–}}CH=CH\mathrm{\text{–}}C{H}_3$ with the two alkene H atoms trans] [2021/15m/7b]

15. Write the products X/Y in the above chemical transformations.

[Figure: I. 1-phenyl-1,2-dihydronaphthalene (C-1 bearing Ph and H, C3=C4 double bond conjugated with the benzene ring) –MCPBA→ X; II. 1,4-dihydronaphthalene (C2=C3 double bond between two CH2 groups) –$Os{O}_4$→ X –$NaI{O}_4$→ Y; III. 3-hydroxybutanoic acid ($H_{3}C\mathrm{\text{–}}CH\left(OH\right)\mathrm{\text{–}}C{H}_2\mathrm{\text{–}}COOH$) –$B{H}_3/THF$→ X; IV. cyclohexene –$Cr{O}_3$-Pyridine, $C{H}_{2}C{l}_2$→ X] [2021/15m/8a(i)]

16. Predict the product in the reaction of 2-methyl-1-butene with diborane. Account for the regioselectivity observed in the reaction. [2021/5m/8a(ii)]

17. Identify the compounds A and B in the following reaction sequence:

[Figure: Reaction scheme: methyl (4-oxocyclohexyl)acetate (cyclohexanone ring with $C{H}_2-C(=O)-OC{H}_3$ at C4) –$NaB{H}_4$, MeOH→ [A] –1) $LiAl{H}_4$; 2) $H_3{O}^{+}$→ [B]] [2020/10m/5b]

18. Name the reagent for the following regioselective conversion. Propose the mechanism:

[Figure: Conversion: 3-methylpentanedioic acid monoethyl ester ($Et{O}_{2}C-C{H}_2-CH\left(C{H}_3\right)-C{H}_2-C{O}_{2}H$, CH3 on a wedged bond) –?→ 4-methyltetrahydro-2H-pyran-2-one (δ-valerolactone ring with CH3 on a wedged bond at C4)] [2019/15m/7b]

19. By taking 1-methylcyclopentene as an example, explain hydroboration-oxidation reaction. Justify that it is stereoselective and regioselective. [2018/10m/2a]

20. Outline the mechanism for the following reaction: In what way is $NaB{H}_4$ different from $LiAl{H}_4$ in ‘chemoselectivity’?

[Figure: Reaction: carboxylic acid R–C(=O)–OH –i) $LiAl{H}_4$; ii) $H_{2}O$→ $R-C{H}_2-OH$] [2018/15m/2c]

21. Predict the product(s) with mechanism for the following:

[Figure: (i) chiral 1-phenylethyl chlorosulfite: stereocentre bearing Ph (up), CH3 (wedged, down), H (hashed) and $-OSOCl$ –1,4-dioxane, Δ→; (ii) 4a-(tosyloxymethyl)-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (octalone with angular $C{H}_{2}OTs$, enone C=O at C2 conjugated with C1=C8a) –1) Li/liq. $N{H}_3$, Tetrahydrofuran; 2) $H_3{O}^{+}$→; (iii) naphthalene –Na, 6-bromohex-1-ene ($C{H}_2=CH-C{H}_{2}C{H}_{2}C{H}_{2}C{H}_{2}Br$, drawn below the arrow)→] [2018/15m/3d]

22. What is Birch reduction? Suggest its mechanism by taking a suitable example. Why does Birch reduction on benzene derivatives with an electron-donating group happen on ortho and meta positions? [2018/15m/7c]

23. Write specific reagents necessary for each of the following transformations. Out of ‘stereospecific’ and ‘stereoselective’, which one suits for each reaction?

[Figure: But-2-yne (Me–C≡C–Me) with two branches: (i) → cis-but-2-ene ((Z)-Me–CH=CH–Me); (ii) → trans-but-2-ene ((E)-Me–CH=CH–Me)] [2017/10m/4e]

24. Identify the major product ($Z$) formed in the following reactions.

[Figure: (i) 1,2-bis(benzyloxy)hept-3-ene with the double bond drawn cis: C6H5–CH2–O–CH2–CH(O–CH2–C6H5)–CH=CH–CH2–CH2–CH3, the OCH2C6H5 at C2 on a bold wedge, cis C3=C4 double bond –OsO4, pyridine (above arrow), THF (below arrow)→ (Z); (ii) ethyl acetoacetate (CH3–CO–CH2–CO–OC2H5) with two routes: LiAlH4, C2H5–O–C2H5 → (Z); and (1) HO–CH2CH2CH2–OH (propane-1,3-diol), H+; (2) LiAlH4; (3) H3O+ → (Z); (iii) a 4-methylcyclohex-3-en-1-yl ring attached to C(=CH2)–CH2–CH2–CH=C(CH3)–CH2OH (the CH2 next to the =CH drawn with both H atoms shown; the CH2OH cis to the chain on the trisubstituted double bond) –Na, NH3, C2H5OH→ (Z)] [2017/15m/7b]

25. Identify the product in the following reaction and provide the mechanism for the formation of the product.

[Figure: Reaction: α-cyperone-type bicyclic enone: 4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one with CH3 at C1 (C1=C8a double bond conjugated with the C2=O), angular CH3 at C4a on a bold wedge, and an isopropenyl group (C(CH3)=CH2) at C7 on a bold wedge –(i) Na, NH3, ether; (ii) NH4Cl→ ?] [2017/10m/8c]

26. Give the structures of $\left(X\right)$ and $\left(Y\right)$:

[Figure: 4-nitrobenzaldehyde (benzene ring with NO2 at the top and CHO para at the bottom) with two branches: LiAlH4 → (X); NaBH4 → (Y)] [2016/10m/1e]

27. Discuss the stereospecificity of reduction of $C{H}_3{\left(C{H}_2\right)}_2-C\equiv C-{\left(C{H}_2\right)}_{2}C{H}_3$ to alkene with sodium and liquid $N{H}_3$. [2016/10m/5d]

28. Predict the major product and explain the origin of the regioselectivity and stereoselectivity observed in the following hydroboration-oxidation reaction:

[Figure: Reaction: 1-methylcyclopentene –(i) $B_2{H}_6$; (ii) $H_2{O}_2$, ${}^{-}OH$→ ?] [2015/10m/5b]

29. Give the mechanism for reduction of a ketone by $LiAl{H}_4$ and predict the products in the following reactions:

[Figure: (i) 1,2-epoxydecane ($C{H}_2-CH{\left(C{H}_2\right)}_{7}C{H}_3$ with an O bridging the two carbons) –$LiAl{H}_4$→ ?; (ii) hexa-2,4-dienoic acid (sorbic acid, $C{H}_{3}CH=CH-CH=CH-COOH$, drawn as a zigzag) –$LiAl{H}_4$→ ?; (iii) $H_{3}C-C_6{H}_4-C{H}_{2}CN$ (p-tolylacetonitrile) –$LiAl{H}_4$→ ?] [2015/15m/6d]

30. Complete the following reactions and suggest a suitable reaction mechanism for each:

[Figure: (i) 1-(hydroxymethyl)cyclopentan-1-ol (cyclopentane C1 bearing OH and CH2OH) –$HI{O}_4$→ ?; (ii) 3-nitrobenzaldehyde (O2N and CHO meta on a benzene ring) –$NaB{H}_4$, methanol→ ?; (iii) methyl phenyl sulfoxide ($C_6{H}_5-S(=O)-C{H}_3$) –$LiAl{H}_4$→ ?; (iv) cyclohexanone –$Cr{O}_3$→ ?] [2015/20m/7d]

31. Propose the mechanism for the following chemical conversion:

[Figure: Reaction: benzoic acid (C6H5–COOH) –Na, liq. NH3→ cyclohexa-2,5-diene-1-carboxylic acid (ring carbon bearing H and COOH, the para CH2 drawn with two H, two C=C between them)] [2015/10m/8c]

32. Explain why hydroboration-oxidation protocol gives anti-Markonikoff’s alcohol from unsymmetrical alkene. Complete the following reaction giving predominant stereoisomer and regioisomer.

[Figure: Reaction scheme: 2-methylbicyclo[2.2.1]hept-2-ene (norbornene with CH3 on one alkene carbon) –(i) B2H6; (ii) H2O2, NaOH→ ?] [2014/10m/5c]

33. Suggest appropriate reagents/conditions, with justification, for the desired chemical transformations shown below:

[Figure: 1-methylcyclohexene at the centre with four arrows, each labelled “?”, to: (1, left) cis-1-methylcyclohexane-1,2-diol (C-2 bears H and a hashed OH; C-1 bears CH3 and a hashed OH); (2, right) trans-2-methylcyclohexan-1-ol (C-1 bears H and a hashed OH; C-2 bears a wedged CH3 and a hashed H); (3, lower left) trans-1-methylcyclohexane-1,2-diol (C-2 bears a wedged OH and a hashed H; C-1 bears CH3 and a hashed OH); (4, lower right) 1-methylcyclohexan-1-ol (C-1 bears a wedged CH3 and a hashed OH)] [2014/15m/6c]

34. Provide mechanism for the following conversion:

[Figure: 4-tert-butylcyclohexanone drawn as a chair with the t-butyl group equatorial and the ring H on that carbon axial –Na/liq. NH3→ ? followed by the product drawn as a chair: trans-4-tert-butylcyclohexanol with t-butyl equatorial, OH equatorial and the H on the carbinol carbon axial (drawn up)] [2014/5m/8d]

35. Describe using appropriate reagents and conditions a method to achieve the following transformations:

[Figure: Multistep conversion (three stacked arrows): methyl 6-oxobicyclo[2.2.1]heptane-2-carboxylate (norbornane skeleton with a ring C=O on one two-carbon bridge and a –C(=O)–OCH3 group on the other two-carbon bridge, both on the carbons attached to the same bridgehead) ⇒ 6-(hydroxymethyl)bicyclo[2.2.1]heptan-2-one (same skeleton, ketone retained, the ester replaced by CH2OH)] [2013/20m/3b]

36. Identify [X] and [Y] in the following:

[Figure: 2-Pentyne with two branching arrows: upper arrow labelled Na/NH3 → [X]; lower arrow labelled Lindlar’s Catalyst → [Y]] [2013/10m/5c]

37. Identify (X) and (Y) in the following:

[Figure: Reaction scheme: cyclohexene –O3→ (X) –NaBH4→ (Y)] [2013/10m/8b]

38. Write down the products of the following reactions with chemical equation: (i) $R-CH=C{H}_2\underset{\mathrm{\text{(ii)}}{H}_2{O}_2}{\stackrel{\mathrm{\text{(i)}}{B}_2{H}_6}{\to }}Product$ (ii) $R-CO-H+C_2{H}_5-O-CO-CH{N}_2\underset{C{H}_{2}C{l}_2}{\stackrel{H^{\oplus }}{\to }}Product$ (iii) $C_6{H}_5-\stackrel{\oplus }{N_2}C{l}^{\ominus }\underset{\mathrm{\text{(ii)}}{H}^{\oplus },O{H}^{\ominus }}{\stackrel{\mathrm{\text{(i)}}NaHS{O}_3}{\to }}Product$ [2012/15m/8a]

39. Predict the structure of the products in the following reactions. Give the mechanism. (i) $\stackrel{mCPBA}{\to }$ ? (ii) $\stackrel{Se{O}_2}{\to }$ ? (iii) $C_3{H}_7-C\equiv C-Et\underset{liq.N{H}_3}{\stackrel{Li}{\to }}?$ (iv) $C{H}_2=C\left(C{H}_3\right)\left(Et\right)\underset{H_2{O}_2,NaOH}{\stackrel{B{H}_3}{\to }}?$

[Figure: (i) substrate: partial structure: cyclopentanone ring whose α-carbon is a quaternary ring-fusion carbon carrying an angular methyl group, the next ring carbon a ring-fusion CH; the bonds to the rest of the fused ring system are shown as wavy lines (as in a steroid D-ring 17-ketone); (ii) substrate: 2-methylcyclohexanone] [2011/20m/7c]

40. Benzoic acid on reduction with Na in liq $N{H}_3$ gives 1,2-dihydroproduct, whereas, anisole gives 2,5-dihydroproduct. Explain. [2010/10m/5b]

41. Identify A and B and discuss the selectivity of the reagents.

[Figure: methyl 4-oxocyclohexane-1-carboxylate (cyclohexanone with $COOC{H}_3$ at C4) –($NaB{H}_4$; $H_{2}O$)→ A; same compound –($LiAl{H}_4$; $H_{2}O$)→ B] [2010/20m/6d]

42. Which reagent can be used to convert $C{H}_3-CH=CH-C{H}_2-C{O}_{2}Me$ to $C{H}_3-CH=CH-C{H}_2-C{H}_{2}OH$. [2009/6m/6b(ii)]

43. Why does $B_2{H}_6$ follow anti-Markovnikovs addition to $R_{2}C=C{H}_2$? [2008/10m/7c(iii)]

44. (i) Write the structures of the product/s formed in the Birch reduction of anisole and benzaldehyde and explain their formation. (ii) Describe a method for the conversion of 1-methyl-cyclohexene to 2-methylcyclohexanone and explain the mechanism. [2007/20m/5b]

45. Explain the following transformations:

[Figure: (i) cyclohexene –$Os{O}_4$→ cis-cyclohexane-1,2-diol (both OH on wedged bonds); (ii) indoline (2,3-dihydro-1H-indole, N–H) –$Pb{\left(OAc\right)}_4$→ indole (N–H); (iii) 2-methylbut-2-ene –$B_2{H}_6$ and then $H{O}^{\ominus }$→ 3-methylbutan-2-ol (${\left(C{H}_3\right)}_{2}CH-CH\left(OH\right)-C{H}_3$); (iv) cyclohexanone –NBS and then $H{O}^{\ominus }$→ cyclohex-2-en-1-one] [2007/20m/6c]

46. Identify the major product in each of the following reactions. Give mechanisms of the reactions involved and stereostructure, if any:

[Figure: (i) 3,5-dimethoxybenzoic acid ($C{O}_{2}H$ at C-1, MeO at C-3 and C-5) –1) Na/$N{H}_3$, 2) $C_5{H}_{11}Br$, 3) $Pb{\left(OAc\right)}_4$→ ?; (ii) 1,1′-bi(cyclobutyl)-1,1′-diol (two cyclobutane rings joined at carbons each bearing OH) –$H^{+}$→ ?; (iii) cyclohexane bearing on adjacent carbons: HO (wedge, top), $C{H}_{2}OH$ (HO–CH2 on a hashed/wedged bond) and OH (at the next carbon, plain bond down) –PhCHO, $H_3{O}^{+}$→ ?; (iv) 2-methoxybuta-1,3-diene [?] (drawn as a diene with OMe on C-2) + $C{H}_2=CH-CN$ –Δ→ ?] [2006/48m/3a]

47. Give steps and reagents involved in the conversion of compounds written on the left hand side of the arrow to compounds written on the right hand side of the arrow:

[Figure: (i) cyclohexanone → 4-cyclohexylpentan-2-one ($C_6{H}_{11}CH\left(C{H}_3\right)C{H}_{2}COC{H}_3$); (ii) 2-methoxybenzoic acid (OMe and $C{O}_{2}H$ ortho) → 2-heptylcyclohexanone (C=O with $C_7{H}_{15}$ on the adjacent ring carbon); (iii) cyclohex-2-en-1-one → cyclohexane-1,2,3-triol drawn as two stereoisomers: the first with all three OH on plain bonds, “and” the second with the C-2 OH on a hashed bond (OH at C-1 and C-3 plain); (iv) ethyl acetoacetate ($C{H}_{3}COC{H}_{2}C{O}_{2}Et$) → 3-methylcyclohex-2-en-1-one] [2005/20m/3b]

48. Predict the product(s) in the reactions given below with mechanism wherever possible.

[Figure: (i) tetralin (1,2,3,4-tetrahydronaphthalene) –$Cr{O}_3$, ACOH→ ; (ii) 3-methylthiophene –NBS, Pyridine / Peroxide→ ; (iii) cyclohex-2-en-1-ol –$Os{O}_4$ [?], Pyridine / THF [?] (printed “CHF”)→ ; (iv) methyl 3-oxohex-4-enoate, $C{H}_3-CH=CH-CO-C{H}_2-COOC{H}_3$, with two arrows: –$NaB{H}_4$, $C{H}_{3}OH$→ and –$LiAl{H}_4$, ether→] [2003/20m/5b]

49. Identify the products of the following sequence of reactions and explain how these are formed:

[Figure: (i) cyclohexanone [ring size uncertain, ?] –(i) LiAlH4; (ii) H3O+→ A –(i) NaH, Ether; (ii) CS2; (iii) CH3I→ B –Heat→ C. (ii) α-pinene –BH3→ [(pinanyl)2BH] (diisopinocampheylborane) –cis-2-butene (CH3, CH3 on one side, H, H on the other)→ A –H2O2, OH−→ B. (iii) benzophenone (C6H5)2C=O + benzyl bromide (Br–CH2–C6H5) –(i) Ph3P; (ii) n-BuLi→ A –N-bromosuccinimide→ B. (iv) cyclohexanone –NaBH4→ A –H2SO4→ B –Pb(OAc)4→ C + D + E] [2002/30m/6a]

50. Give mechanism to account for the formation of the following product(s)

[Figure: (i) dimethyl hexanedioate drawn as a chain with two COOMe ends –2 Na, Ether→ 2-hydroxycyclohexanone (acyloin). (ii) 2-methylcyclohexanone drawn as a chair with Me axial at C2 and C1=O –$LiAl{H}_4$→ A (Less stable) 25% [?] + B (More stable) 25% [?]; same ketone –$R_{2}BH$ ($R_2$ = bulky group)→ A 100%] [2001/20m/5c]

51. Describe the reduction of acid chlorides with lithium aluminium hydride and give its mechanism. What happens when 3 equivalents of t-butyl alcohol and LAH are used in this reaction? [2000/15m/7b]

5.3] NBS and n-BuLi

21 questions

1. Write the major products formed in the following reactions:

[Figure: Reaction 1: indole (N–H) –(i) nBuLi; (ii) CO2; (iii) H${}^{+}$→ ?; Reaction 2: indole –(i) nBuMgBr; (ii) CO2; (iii) H${}^{+}$→ ?] [2026/5m/4c(iii)]

2. Write the structure of the major product indicating the stereochemistry and mechanism of the following reactions:

[Figure: Substrate: cis-4-tert-butyl-3-methylcyclopentene (cyclopentene; tBu on a wedge at C4 and CH3 on a wedge at C3, C3 adjacent to the C=C). Left: –m-CPBA→ ?; right: –(i) NBS; (ii) 30% NaOH→ ?] [2026/10m/5b]

3. Describe the synthesis of ketone (A) when only isobutanol is available as the starting material:

[Figure: Ketone A: 2,5-dimethylhexan-3-one, (CH3)2CH–CH2–CO–CH(CH3)2] [2025/5m/3a(ii)]

4. Deduce the structure of the starting material (A) and all the intermediates formed in each step that would lead to the formation of the following product through the defined reactions:

[Figure: Scheme: A –1) NaNH2, EtI; 2) Lindlar’s cat., H2; 3) NBS, ROOR, Δ; 4) NaOMe, MeOH→ (E)-2-methoxy-5,5-dimethylhex-3-ene (CH3–CH(OMe)–CH=CH–C(CH3)3, C=C drawn trans)] [2025/10m/5b]

5. Identify A, B, C and D in the following reaction sequence: Write the mechanism of the first step of the above reaction sequence.

[Figure: Scheme: A –NaBH4, MeOH→ cyclohexanol –H${}^{+}$→ B –C→ 3-bromocyclohexene –mCPBA (downward arrow)→ D] [2024/20m/6c]

6. Write the structure of product(s) in the above reactions:

[Figure: (1) methyl 4-oxocyclohexane-1-carboxylate (O= on ring C-4, C(=O)OCH3 on C-1) –(i) NaBH4, H2O, $O{H}^{-}$; (ii) H2O, $H^{+}$→ A; (2) estrone (3-hydroxyestra-1,3,5(10)-trien-17-one: aromatic A ring with HO at C-3, C-13 Me on a wedge, C-17 ketone) –(i) HC≡CH, BuLi; (ii) $H^{+}$, H2O→ B; (3) anisole (methoxybenzene, OMe) –Na, NH3(l); Et2O, EtOH→ C] [2023/10m/6b(i)]

7. Predict $\left[A\right]$ and $\left[B\right]$ in the following reaction:

[Figure: Reaction: 1,2-dihydronaphthalene (benzene ring fused to a six-membered ring containing CH2–CH2 and one C=C conjugated with the benzene ring) –NBS, CCl4→ $\left[A\right]$ –NaOEt, EtOH/Δ→ $\left[B\right]$] [2022/5m/2b(ii)]

8. (i) By using appropriate reactants, reagents and conditions and using acetylene as starting material, how will you synthesize the following compound? (ii) Identify the products $W$, $X$, $Y$ and $Z$ in the following reactions:

[Figure: (i) 1-ethynylcyclopentan-1-ol (cyclopentane carbon bearing OH and C≡C–H); (ii) Scheme: toluene –hν, NBS, Dibenzoyl peroxide ($B{z}_2{O}_2$)→ $W$ (arrow to the left); toluene –$Cr{O}_3$, $A{c}_{2}O$, $H_3{O}^{+}$→ $X$ –$A{c}_{2}O$, NaOAc, Δ→ $Y$ –$LiAl{H}_4$ excess→ $Z$] [2022/10m/5c]

9. Although carbon and magnesium are both common elements in the human body, Grignard reactions do not occur in cells. Why is it so? [2020/10m/1d]

10. Identify the compounds A, B and C in the following conversions: (i) cyclohexene + NBS $\to$ [A] $\stackrel{1)O{H}^{-};2)KMn{O}_4,0{}^{\circ }C}{\to }$ [B] $\stackrel{HI{O}_4}{\to }$ [C] (ii) $C{H}_{3}C{H}_{2}CH=C{H}_2\stackrel{\left[A\right]}{\to }C{H}_{3}C{H}_{2}C{H}_{2}C{H}_{2}OH\stackrel{\left[B\right]}{\to }C{H}_3-CH=CH-C{H}_3\stackrel{1)Os{O}_4;2)Pb{\left(OAc\right)}_4}{\to }$ [C]. [2020/20m/6c]

11. Identify X and Y in the following reactions and outline the mechanism for the formation of X in both the reactions:

[Figure: (i) fused bicyclic substrate drawn as two six-membered rings: right ring = cyclohexane carrying a CH3 on the carbon next to the left-ring fusion carbon; left ring (as drawn) = C(SiMe3)=CH–CH2–CH(ring-fusion)–C(ring-fusion)=O–(back to C(SiMe3)), i.e. the ring O is drawn bonded to the $M{e}_{3}Si$-bearing vinylic carbon and double-bonded (=) to the ring-fusion carbon that is adjacent to the CH3-bearing carbon –mCPBA→ [X] –$H_3{O}^{+}$→ [Y]; (ii) 2-ethyl-3-methylcyclopentanone tosylhydrazone (C=N–NHTs; Et on C2, Me on C3) –1) n-BuLi, 2 equiv., THF; 2) $H_3{O}^{+}$→ [X]] [2020/15m/8b]

12. Predict the structure of ($X$) and ($Y$) in the following sequence:

[Figure: Reaction: ethyl 4-oxopentanoate ($C{H}_{3}COC{H}_{2}C{H}_{2}C{O}_{2}C{H}_{2}C{H}_3$) –(1) Excess MeMgBr; (2) $E{t}_{2}O$→ ($X$) → ($Y$) → 2,2,5,5-tetramethyltetrahydrofuran] [2019/10m/4c(i)]

13. Identify ($X$) and ($Y$) in the following: (i) ($X$) $\underset{\varphi .OOH}{\stackrel{NBS}{\leftarrow }}$ cyclohexene $\underset{{\left(C{H}_3\right)}_4\stackrel{+}{N}B{r}^{-}}{\stackrel{NBS}{\to }}$ ($Y$) (ii) methylenecyclobutane $\stackrel{\mathrm{\text{Dil.}}KMn{O}_4}{\to }$ ($X$) $\stackrel{NaI{O}_4}{\to }$ ($Y$) + HCHO (iii) $C_6{H}_6\underset{Os{O}_{4}in𝑡-C_4{H}_{7}OH}{\stackrel{H_2{O}_2}{\to }}$ ($X$) [2019/15m/6b]

14. How is NBS obtained from succinimide? Write its reaction with propene in presence of sunlight. Outline the mechanism. [2018/10m/1b]

15. Carry out the following transformations:

[Figure: (i) indole → 2-(1H-indol-2-yl)ethan-1-ol (indole bearing $-C{H}_{2}C{H}_{2}OH$ at C2, N–H retained); (ii) 1-vinylcyclohex-1-ene (1-ethenylcyclohexene) → hexahydro-1-indanone (octahydro-1H-inden-1-one: cyclohexane ring fused to cyclopentanone, C=O on the five-membered-ring carbon adjacent to the ring fusion)] [2018/10m/3a]

16. Write the mechanism of the following:

[Figure: (i) 3-benzylidenephthalide (3-benzylidene-2-benzofuran-1(3H)-one: benzene ring fused to a five-membered lactone, exocyclic =CHPh on C3, C1=O) –PhMgBr→ –$H_{2}S{O}_4$, Δ→ 2,3-diphenyl-1H-inden-1-one (2,3-diphenylindenone); (ii) pyrrole-2-carbaldehyde (1H-pyrrole with CHO at C2, N–H) –$C{H}_2=CH-CH=PP{h}_3$, Δ→ (product not shown); (iii) 3-acetoxy-4a-methyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (octalone: OAc on the carbon α to C=O, C=O conjugated with the ring C=C at the ring fusion, angular CH3) –1) $H_{2}S{O}_4$–MeOH; 2) $H_3{O}^{+}$→ 4a-methyloctahydronaphthalene-1,7(2H,8H)-dione (bicyclic decalin-dione: angular CH3 at the upper ring-fusion carbon; in the right ring the C=O is on the carbon adjacent to the lower ring-fusion carbon; in the left ring the C=O is on the lower-left carbon, two carbons from the CH3-bearing ring-fusion carbon)] [2018/15m/3c]

17. What happens when 2,4-pentadione is treated with one equivalent of MeMgI? How can you convert the mentioned dione to 2,4-hexadione? [2017/10m/3a]

18. Write a method of preparation of a tertiary alcohol containing two identical alkyl groups. Write one method for distinguishing primary, secondary and tertiary alcohols. [2017/10m/3b]

19. Identify the major product $\left(X\right)$ formed in the following reactions:

[Figure: (i) 2-acetylnaphthalene (naphthalene with –CO–CH3 at C2) –SeO2→ (X); (ii) cyclopentene –1) NBS; 2) hν/heat→ (X); (iii) trans-decalin-4a,8a-diol (decalin with OH on both ring-fusion carbons, one on a bold wedge and one on a hashed bond) –Pb(OAc)4→ (X)] [2016/15m/8d]

20. The N-benzoyl methyl ester derivatives of the amino acids glycine. alanine and valine react with N-bromosuccinimide to give monobromination products containing bromine at the α-carbon atom of amino acids. The order of reactivity is glycine > alanine > valine (23 : 8 : 1) Account for this trend of reactivity. [2002/20m/3b(ii)]

21. Suggest the reaction product(s) and explain its mechanism:

[Figure: (i) methylenecyclohexane (cyclohexane ring with exocyclic =CH2) –(NBS over arrow; CCl4, Δ under arrow)→ ? (ii) trans-alkenyl iodide n-C7H15–CH=CH–I (n-C7H15 and H on one carbon, H and I on the other, n-C7H15 and I trans; the I label is faint) –(n-Bu Li over arrow; CuI, ether under arrow)→ ? (iii) C6H5–CH(OCH3)–CH2–Hg–O2CCF3 (benzene ring bearing a CH(OCH3) group whose other carbon is CH2–Hg–O2CCF3) –(1. NaBH4; 2. H2O)→ ?] [2000/20m/5b]

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Last updated: 30 October 2026