Question

Reaction of unknown A with HCl forms chlorohydrin B as the major product. A shows no absorptions in its IR spectrum at \({\bf{1700}}{\rm{ }}{\bf{c}}{{\bf{m}}^{ - 1}}\) or \({\bf{3600--3200 c}}{{\bf{m}}^{{\bf{ - 1}}}}\), and gives the following 1H NMR data: 1.4 (doublet, 3 H), 3.0 (quartet of doublets, 1 H), 3.5 (doublet, 1 H), 3.8 (singlet, 3 H), 6.9 (doublet, 2 H), and 7.2 (doublet, 2 H) ppm.

(a) Propose a structure for A, including stereochemistry.

(b) Explain why B is the major product in this reaction.

Short answer

(A) Structure of A:

(B)

Chlorine attacks at the more substituted carbon center since the carbocation that is formed after the opening of the bond is resonance stabilized.

Step-by-step solution

Determination of A

The structure of chlorohydrin B is given as:

It is given that; the reaction of A yields the given chlorohydrin.

Since it is mentioned that the compound A shows no absorption in the IR spectrum at\({\rm{1700}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 1}}}}{\rm{,3600}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 1}}}}{\rm{,3200}}\;{\rm{c}}{{\rm{m}}^{{\rm{ - 1}}}}\), it has no carbonyl group present, and the absence of any absorption on these bands suggests that the compound might be an epoxide since epoxides have no easily distinguishable IR bands.

Chlorohydrin B can form from an epoxide with the possible structure:

A gives the following NMR data: 1.4 (doublet, 3 H), 3.0 (quartet of doublets, 1 H), 3.5 (doublet, 1 H), 3.8 (singlet, 3 H), 6.9 (doublet, 2 H), and 7.2 (doublet, 2 H) ppm. This data is similar to the HNMR data of the proposed structure, as shown below:

Mechanism of formation of B from A

The reaction that leads to the product B is known as epoxide ring opening in acidic medium, since HCl is present.

The first part of the reaction is the protonation of the epoxide oxygen, as given below:

Since the attack of chlorine can occur in two ways, the two carbocations that are formed upon the opening of the bond is compared below:

Since the first carbocation is more stable, the attack of chlorine at that position yields the major product

Since the attack occurs below the plane, the product can also be written as: