Question

Primary and secondary alcohols can be converted to alkyl chlorides via treatment with thionyl chloride. The reaction proceeds via an SN2 mechanism with inversion of configuration at the chiral carbon.

Draw curved arrows to show the movement of electrons in this step of the mechanism.

Answer 1

1. Electron Movement with Curved Arrows:

Here's the mechanism for the conversion of a secondary alcohol (R-CHOH-R') to an alkyl chloride (R-CHCl-R') using thionyl chloride, with curved arrows showing the movement of electrons:

(check image)

How to explain

Step 1: The lone pair of electrons on the oxygen atom of the alcohol attacks the sulfur atom of thionyl chloride, which is a good electrophile due to its positive partial charge. This forms a tetrahedral intermediate with a positive charge on the oxygen atom.

Step 2: The chloride ion (Cl-) from thionyl chloride leaves, forming a double bond between the carbon atom and the chlorine atom, resulting in the alkyl chloride product.

2. SN2 Mechanism and Configuration Inversion:

The reaction proceeds via an SN2 mechanism because:

The nucleophile (the alcohol's lone pair) attacks the backside of the electrophile (the sulfur atom of thionyl chloride) simultaneously with the leaving group (the chloride ion) departing.

This results in a concerted transition state with partial bonding to both the nucleophile and the leaving group.

The inversion of configuration at the chiral carbon occurs because the attack by the nucleophile occurs from the backside of the carbon atom, leading to a new stereochemical arrangement of the substituents.

3. Role of Pyridine:

Pyridine is often added to this reaction as a base. It captures the proton released from the alcohol in step 1, preventing it from reacting with the chloride ion and forming unwanted byproducts. This improves the yield of the desired alkyl chloride product.

The Complete Question

Mechanism for Alcohol Conversion to Alkyl Chloride with Thionyl Chloride

Scenario: Primary and secondary alcohols can be converted to alkyl chlorides via treatment with thionyl chloride (SOCl₂). The reaction proceeds via an SN2 mechanism with inversion of configuration at the chiral carbon.

Task:

Draw curved arrows to show the movement of electrons in the rate-determining step of the mechanism for the conversion of a secondary alcohol (R-CHOH-R') to an alkyl chloride (R-CHCl-R') using thionyl chloride.

Explain why the reaction proceeds via an SN2 mechanism and why there is inversion of configuration at the chiral carbon.

Briefly discuss the role of pyridine in this reaction.