SN1: Introduction and Characteristics

Nucleophilic Substitution: Focus on Aliphatic Reactions

Nucleophilic substitution reactions fall into two broad categories depending on the type of carbon atom involved:

• Acyl substitution involves a nucleophile attacking a carbonyl carbon (C=O). These reactions are typical of acid derivatives such as esters, amides, and acid chlorides. They proceed through a tetrahedral intermediate and will be discussed later in the course.
• Aliphatic substitution involves a nucleophile replacing a leaving group on an sp³-hybridized carbon (a saturated carbon). These are the classic SN1 and SN2 reactions and are the focus of this section.  Recall that aliphatic means alkane

I. Definition: Unimolecular Nucleophilic Substitution (SN1)

The SN1 reaction is a type of aliphatic substitution in which the rate-determining step involves only the substrate. The nucleophile attacks after a carbocation has formed.

II. Key Characteristics

• Two-step mechanism – The reaction begins with the departure of the leaving group to form a carbocation intermediate. The nucleophile then attacks this carbocation.
• Carbocation intermediate – A key feature of the SN1 mechanism. The stability of this intermediate strongly influences the reaction rate.
• Rate law: Rate = k[substrate] – The reaction rate depends only on the concentration of the substrate.
• Racemization (or partial racemization) – If the reaction center is chiral, the planar nature of the carbocation leads to loss of stereochemical information.
• Favored by:
  • Substrates that form stable carbocations (e.g., tertiary, allylic, benzylic)
  • Polar protic solvents (e.g., water, alcohols)
  • Good leaving groups (e.g., I⁻, TsO⁻)