Question

1-Chloro-2-butene undergoes hydrolysis in warm water to give a mixture of these allylic alcohols. Propose a mechanism for their formation.

Short answer

Answer: The mechanism for the formation of allylic alcohols from 1-chloro-2-butene involves the following steps: 1. Formation of a carbocation intermediate through ionization. 2. Carbocation intermediate rearrangement via hydride shift, resulting in a more stable allylic carbocation. 3. Nucleophilic attack by water, adding a water molecule to the molecule and forming a carbon-oxygen bond. 4. Deprotonation of the water molecule, forming the final allylic alcohol product.

Step-by-step solution

Identify the reactants and products

We are given 1-chloro-2-butene as the reactant. The products will be a mixture of allylic alcohols. Allylic alcohols have a hydroxyl group connected to a carbon adjacent to the C=C bond.

Formation of carbocation intermediate

In the first step of the mechanism, the chlorine atom in 1-chloro-2-butene is leaving, resulting in the formation of a carbocation intermediate. This step is called the ionization step. The chlorine atom takes the bonding pair of electrons with it, leaving behind a positive charge on carbon 2, adjacent to the carbon-carbon double bond. The carbocation intermediate is \(CH_2=CH-C^+\!H-CH_3\).

Carbocation Intermediate Rearrangement

The carbocation intermediate formed can undergo a rearrangement called a hydride shift. A hydride shift occurs when a hydrogen atom along with its two electrons migrate from a neighboring carbon atom to the positively charged carbon. This results in a more stable carbocation intermediate. In this case, a hydride will shift from carbon 3 to carbon 2, and the carbocation will be formed at carbon 3, which is an allylic carbon having the positive charge adjacent to the double bond.

Nucleophilic attack by water

Water, as a nucleophile, will now attack the positively charged allylic carbon (carbon 3). This results in the addition of a water molecule to the molecule, forming the carbon-oxygen bond and removing the positive charge on carbon 3.

Deprotonation to form the allylic alcohol

The final step involves deprotonation of the water molecule attached to the molecule. A water molecule acts as a base, abstracting the hydrogen atom from the carbon 3 and creating another water molecule in the process. This step forms the C-O-H bond in the allylic alcohol and leaves a mixture of allylic alcohols as the products. The proposed mechanism for the formation of allylic alcohols from 1-chloro-2-butene is as follows: 1. Formation of carbocation intermediate 2. Carbocation intermediate rearrangement via hydride shift 3. Nucleophilic attack by water 4. Deprotonation to form the allylic alcohol product.