A solution of (–) – 1–chloro –1– phenylethane on toluene racemises slowly in the presence of a small amount of SbCl5 due to the formation of :
This question involves understanding why a chiral alkyl halide undergoes racemization (loss of optical activity) in the presence of a Lewis acid like SbCl5. Let's break it down step by step.
The compound is (–)-1-chloro-1-phenylethane. Its structure is:
This is a chiral molecule because the carbon bonded to Cl and the phenyl group is a stereocenter (it has four different substituents). SbCl5 is a strong Lewis acid. Its role is to accept a pair of electrons, making it an excellent catalyst for reactions involving the removal of leaving groups.
SbCl5 interacts with the chlorine atom (the leaving group) on the chiral carbon. Chlorine has lone pairs of electrons. The Lewis acid (SbCl5) coordinates with these lone pairs, weakening the C-Cl bond and facilitating its heterolytic cleavage (breaking where both electrons go to one atom).
The reaction is: R-Cl + SbCl5 → R+ + SbCl6–
This generates a carbocation (R+) intermediate. In this case, R+ is the 1-phenylethyl carbocation:
The phenyl ring stabilizes this carbocation through resonance, making it relatively easy to form.
The key to racemization is the nature of the carbocation intermediate. A carbocation is sp2 hybridized and has a trigonal planar geometry. This means it is flat and achiral.
The nucleophile (in this case, the SbCl6– ion or another chloride ion from solution) can attack this planar carbocation from either the top face or the bottom face with equal probability.
This results in the reformation of the alkyl chloride, but now a 50:50 mixture of the original (–) enantiomer and its mirror image (+) enantiomer is produced. This 1:1 mixture is called a racemic mixture and is optically inactive.
The racemization occurs due to the formation of a planar, achiral carbocation intermediate, which allows for nucleophilic attack from both sides with equal probability.
SN1 Reaction: This is a classic example of a unimolecular nucleophilic substitution (SN1) reaction. The mechanism involves two steps: 1) slow ionization to form a carbocation, and 2) fast nucleophilic attack. The rate law is first order: Rate = k[Alkyl Halide]. Racemization is a hallmark of SN1 reactions at chiral centers.
Carbocation Stability: The stability of the carbocation intermediate follows the order: tertiary > secondary > primary > methyl. Carbocations are also stabilized by adjacent phenyl rings (benzyllic position) through resonance delocalization of the positive charge, which is why this reaction proceeds readily.
The order of stability of the following carbocations:
is :
In the scheme given below, the total number of intramolecular aldol condensation products formed from ‘Y’ is
Amongst the following, the total number of compounds soluble in aqueous NaOH is
