Q.63. The major product of which of the following reaction is not obtained by rearrangement reaction

Q: What is the correct answer for JEE Main Chemistry Q63?

The correct answer is B. Sol. Let's analyze each reaction: (1) Benzene + $CH_3CH_2Cl$ with Anhyd. AlCl$_3$ (Friedel-Crafts alkylation): Reactant: $CH_3CH_2Cl$. This forms a primary carbocation $CH_3CH_2^+$. This carbocation can rearrange via a hydride shift to form a more stable secondary carbocation $CH_3CH_2CH_2^+ \rightarrow CH_3CH^+CH_3$. Alkylation of benzene with $CH_3CH_2Cl$ typically gives ethylbenzene as the majo

Q: Q.63. The major product of which of the following reaction is not obtained by rearrangement reaction? [Figure: Four reaction schemes with reactants and products]

The correct answer is B. Sol. Let's analyze each reaction: (1) Benzene + $CH_3CH_2Cl$ with Anhyd. AlCl$_3$ (Friedel-Crafts alkylation): Reactant: $CH_3CH_2Cl$. This forms a primary carbocation $CH_3CH_2^+$. This carbocation can rearrange via a hydride shift to form a more stable secondary carbocation $CH_3CH_2CH_2^+ \rightarrow CH_3CH^+CH_3$. Alkylation of benzene with $CH_3CH_2Cl$ typically gives ethylbenzene as the majo

Q: What are the options for this JEE Main Chemistry question?

A. [Figure: Reaction 1] · B. [Figure: Reaction 2] · C. [Figure: Reaction 3] · D. [Figure: Reaction 4]

Question

Q.63. The major product of which of the following reaction is not obtained by rearrangement reaction?
[Figure: Four reaction schemes with reactants and products]

FirstInTest · Accepted Answer

Sol. Let's analyze each reaction:
(1) Benzene + $CH_3CH_2Cl$ with Anhyd. AlCl$_3$ (Friedel-Crafts alkylation):
Reactant: $CH_3CH_2Cl$. This forms a primary carbocation $CH_3CH_2^+$. This carbocation can rearrange via a hydride shift to form a more stable secondary carbocation $CH_3CH_2CH_2^+ \rightarrow CH_3CH^+CH_3$. Alkylation of benzene with $CH_3CH_2Cl$ typically gives ethylbenzene as the major product, which implies no rearrangement. However, if the carbocation is formed, rearrangement is possible. But for ethyl chloride, the primary carbocation is not stable enough to rearrange significantly before reacting, or the reaction proceeds via a complex with the Lewis acid. If rearrangement occurs, isopropylbenzene would be formed. The product shown is ethylbenzene, which is the direct product without rearrangement.
(2) Phenol + $CHCl_3$ + NaOH (Reimer-Tiemann reaction):
Reactant: Phenol. Reagents: $CHCl_3$ and NaOH. This is the Reimer-Tiemann reaction, which introduces a formyl group (-CHO) ortho to the -OH group of phenol, forming salicylaldehyde. This reaction involves the formation of a dichlorocarbene intermediate ($:CCl_2$), which is an electrophile. The reaction mechanism does not involve carbocation rearrangement. The product shown is salicylaldehyde.
(3) $CH_3(CH_2)_4CH_3$ (n-hexane) with Anhyd. AlCl$_3$/HCl (Isomerization):
Reactant: n-hexane. Reagents: Anhyd. AlCl$_3$/HCl. This is an isomerization reaction of alkanes, which proceeds via carbocation intermediates. Primary carbocations rearrange to more stable secondary or tertiary carbocations through hydride or alkyl shifts. For n-hexane, this leads to branched isomers like 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane. The product shown is 2-methylpentane, which is a rearranged product.
(4) Cyclohexanol with acid (Dehydration):
Reactant: Cyclohexanol. Reagent: Acid (e.g., H$_2$SO$_4$). This is an acid-catalyzed dehydration of an alcohol to form an alkene. The mechanism involves the formation of a carbocation intermediate. If the carbocation can rearrange to a more stable carbocation (e.g., from secondary to tertiary, or by ring expansion/contraction), it will. For cyclohexanol, a secondary carbocation is formed, which can undergo rearrangement if a more stable carbocation is possible. However, in this specific case, direct elimination from the secondary carbocation leads to cyclohexene, which is the product shown. If there were a possibility of a more stable carbocation, rearrangement would occur. For simple cyclohexanol, direct elimination is the primary pathway, but if the question implies *any* rearrangement, it's about the mechanism. The formation of cyclohexene from cyclohexanol does not typically involve a rearrangement of the carbon skeleton, but rather a direct elimination from the carbocation.
Revisiting (1): Friedel-Crafts alkylation with primary alkyl halides can involve rearrangement if a more stable carbocation can be formed. For ethyl chloride, rearrangement to a more stable carbocation is not possible (primary to primary). So, ethylbenzene is formed without rearrangement.
Revisiting (4): Dehydration of cyclohexanol forms cyclohexene. This involves a carbocation intermediate, but no carbon skeleton rearrangement is needed to form cyclohexene. The carbocation is formed, and then a proton is removed from an adjacent carbon.
Comparing (1), (2), (3), (4):
(1) Friedel-Crafts alkylation with ethyl chloride: No rearrangement of carbon skeleton.
(2) Reimer-Tiemann reaction: Involves dichlorocarbene, no carbocation rearrangement.
(3) Isomerization of n-hexane: Involves carbocation rearrangement (hydride/alkyl shifts).
(4) Dehydration of cyclohexanol: Involves carbocation, but no carbon skeleton rearrangement for cyclohexene formation.
The question asks which product is *not* obtained by rearrangement. Both (1) and (2) fit this description. However, the options are single choices. Let's re-evaluate the common understanding.
Friedel-Crafts alkylation with primary halides *can* lead to rearrangement if a more stable carbocation can be formed (e.g., n-propyl chloride gives isopropylbenzene). For ethyl chloride, rearrangement is not observed because the ethyl carbocation cannot rearrange to a more stable carbocation. So, product (1) is formed without rearrangement.
Reimer-Tiemann reaction (2) does not involve carbocation rearrangement.
Isomerization of alkanes (3) *is* a rearrangement reaction.
Dehydration of alcohols (4) *can* involve carbocation rearrangement (e.g., pinacol rearrangement, or 1,2-hydride/alkyl shifts if a more stable carbocation can be formed). For cyclohexanol, the secondary carbocation directly loses a proton to form cyclohexene, without a skeletal rearrangement. So, this product is also formed without rearrangement of the carbon skeleton.
If the question means 'rearrangement of the carbon skeleton', then (1), (2), and (4) would be products not obtained by rearrangement. This implies the question expects a single answer.
Let's consider the most definitive case of 'no rearrangement'. The Reimer-Tiemann reaction (2) proceeds via a carbene intermediate, not a carbocation, and thus no carbocation rearrangement is possible. This is the most unambiguous answer for 'not obtained by rearrangement reaction'.
For (1), while ethylbenzene is the direct product, Friedel-Crafts alkylation is known for rearrangements with longer chain alkyl halides. For (4), while cyclohexene is the direct product, dehydration of other alcohols frequently involves rearrangements.
Therefore, the Reimer-Tiemann reaction (2) is the most clear-cut case where rearrangement (specifically carbocation rearrangement) is not involved in forming the major product.

Answer

[Figure: Reaction 1]

Answer

[Figure: Reaction 2]

Answer

[Figure: Reaction 3]

Answer

[Figure: Reaction 4]

Related Questionsin Chemistry

More from JEE Main

Browse Engineering Questions by Subject

Practice More JEE Main Questions