Revisal Problems (Past 13 Year) JEE Advanced (Alkyl Halides) - JEE MCQ

H and Br from anti position eliminated.

Drawing Sawhorse projection of the given compound

E2 elimination first gives cyclopentene. In the next step, addition of bromine gives anti vicinal dibromide.

S_N2 reaction gives inversion of configuration at α-carbon.

S_N2 reaction gives inversion of configuration at α-carbon. If the compound also has chiral carbon other than α-carbon, configuration at those chiral carbons remains unchanged after tthe reaction.

S_N1 reaction involves planar carbocation, giving .racemic products predominantly. However same carbocation is not completely deteched from leaving group, exists as intimate ion pair. Such ion pairs prevent attacks from front side favouring backside attacks.

Due to above reason, in S_N1 reaction, partial racemisation and net inversion is usually observed.

S_N1 reaction proceeds via carbocation intermediate, hence reactivity follow the order of stability of carbocation. In the present case, besides resonance stability of benzylic carbocations, hyperconjugation by alkyl group at para position is also affecting stability. Methyl group from para position will stabilise, the most by hyperconjugation effect due to three α - H followed by, ethyl group with two a-H and least by isopropyl group with only one α-H.

Hydrolysis will proceed by S_N1 mechanism. Ill will be least reactive as it will involves the least stable, anti aromatic carbocation. IV will be most reactive as its carbocation will involve ring expansion of highly strained cyclopropyl ring.

(III) is most reactive as it forms most stable, aromatic carbocation:

(II) is least reactive as it involves least stable, anti aromatic carbocation,

It involves least stable, anti aromatic carbocation.

(III) is most reactive as it pro cee ds via m ost stable aromatic carbocation (II) is least reactive as it proceeds ! via least stable anti aromatic carbocation.

(IV) is most reactive as it involves the most stable carbocation.

(III) is least reactive as it is vinylic halide. (II) is more reactive than (I) because (II) proceeds via benzylic carbocation.

The reaction may proceed either by E2 or S_N2 mechanism. The only elimination product possible here is 1-pentene. I 2-pentene is not possible because its formation will involve I rearrangement which is not possible in E2 reaction.
Also due to following equilibrium : 

Both tertiary butoxide and an ethoxide undergo S_N2 reaction forming (b) and (d).

E1cb is a modification of E2 mechanism that involve formation of a carbanion intermediate in the first, fast step followed by unimolecular elimination of halide ion in the second slow rate determining step. Overall order is two. Electron withdrawing group stabilises carbanion, favours the mechanism. Since first step is reversible, deuterium incorporation may take place as :

Cis and trans 2-butenes are pair of diastereomers. C—H bond is broken in the rate determ ining slow step, stronger C—D bond will slow down the reaction. C—X bond is broken in slow, ate determining step, C—Cl bond will break slowly than C—Br bond.
Bulky base like tertiary butoxide results in less substituted Hofmann’s elimination product.

Reaction-I has involved rearrangement, must have proceeded by S_N1 mechanism. Reaction-ll involves strong nucleophile and devoids of rearrangement, proceeded by S_N2 mechanism.

In option (a) and option (b) achiral products are formed via tertiary carbocations.

In S_N2 reaction, order of reactivity is 1° > 2° > 3°. In option (c), although both are primary bromides, tertiary butyl group at a-carbon gives large steric hindrance, decreases reactivity in S_N2 reaction.

Quarternary ammonium chloride brings CN^- in organic phase by the following reaction:

If AgCN is used in place of KCN, isocyanide (linking from nitrogen) is preferred due to covalent character of Ag—C bond.

In free radical chlorination reaction, the major product is determined by combining the stability of free radicals and probability of reaction at a particular carbon both. Hence, only the basis of stability of tertiary free radical alone, tertiary chloride can't be predicted as major product.

B being 1° halide, undergoes faster S_N2 reaction than 2° halideC. Both B and C are chiral while A is achiral.
Grignard's reagent (R^- Mg X⁺) with most stable carbanion (1°) is formed with B.

B being 1° halide, undergoes faster S_N2 reaction than 2° halideC. Both B and C are chiral while A is achiral.
Grignard's reagent (R^- Mg X⁺) with most stable carbanion (1°) is formed with B.

B being 1° halide, undergoes faster S_N2 reaction than 2° halideC. Both B and C are chiral while A is achiral.
Grignard's reagent (R^- Mg X⁺) with most stable carbanion (1°) is formed with B.

Only the following chlorination products, upon E2 reaction can give 1-methyl cyclopentene as one of the product.

(i) With SOCI₂, an intramolecular nucleophilic substitution (S_N1) reaction takes place. In absence of base like pyridine, complete retention of configuration takes place.

(ii) With SOCI₂, in the presence of pyridine, S_N2 reaction takes place with inversion of configuration at chiral α-carbon

(iii) S_N1 reaction takes place giving racemic mixture of product.