Q63E - Organic Chemistry (Mcmurry)

Question

Would you expect the Friedel–Crafts reaction of benzene with (R)-2- chlorobutane to yield optically active or racemic product? Explain.

Step-by-Step Solution

Answer

Reaction of (R)-2-chlorobutane with AlCl₃ produces an ion pair $\left[{{\mathrm{CH}}_3}^{+}{\mathrm{CHCH}}_2{{\mathrm{CH}}_3}_{-}{\mathrm{AlCl}}_4\right]$. The planar, ${\mathrm{sp}}^2$- hybridized carbocation is achiral, and its reaction with benzene can occur on either side of the carbocation to yield racemic product. 

Step 1: Friedel-Crafts alkylation

Friedel craft Alkylation is an organic reaction refers to the replacement of an aromatic proton with an alkyl group. This is done through an electrophilic attack on the aromatic ring with the help of a carbocation. The Friedel-Craft alkylation reaction is also known as a method of generating alkylbenzenes by using alkyl halides as reactants.

The Friedel craft alkylation of benzene is shown below:

Step 2: Racemic mixtures

Racemic mixture are equimolar mixtures of enantiomers that are optically inactive (not capable of rotating the plane polarized light). They can be made through adding together equal portions of enantiomers. The solution containing equal amounts of (R)-2-butanol and (S)-2-butanol is an example of racemic mixture.

Step 2: Identify the type of product

Reaction of (R)-2-chlorobutane with AlCl₃ produces an ion pair $\left[{{\mathrm{CH}}_3}^{+}{\mathrm{CHCH}}_2{{\mathrm{CH}}_3}_{-}{\mathrm{AlCl}}_4\right]$. The planar ${\mathrm{sp}}^2$, hybridized carbocation is achiral, and its reaction with benzene can occur on either side of the carbocation to yield racemic product.

 

Racemic product