What is the SN1 mechanism?

Here’s the explanation of the SN1 mechanism in plain words, without chemical equations:

SN1 Mechanism Explained

1. Step 1 – Leaving group departs
   The reaction begins when the bond between the carbon atom and the leaving group breaks. This produces a positively charged carbon center, called a carbocation. This is the slowest and most important step, so the reaction speed depends only on the concentration of the starting molecule.
2. Step 2 – Nucleophile attacks
   A nucleophile (an electron-rich species) then attacks the carbocation. Because the carbocation is flat in shape, the nucleophile can approach from either side. This often produces a mixture of stereoisomers, leading to partial or full racemization if the carbon is chiral.
3. Step 3 – Proton removal (sometimes)
   If the nucleophile that attaches is neutral (such as water or alcohol), the product may first be positively charged. A proton is then removed to give the stable, neutral final product.

Key Characteristics

• Reaction rate depends only on the substrate (not the nucleophile).
• Works best with tertiary carbons, since they form the most stable carbocations.
• Favored in polar protic solvents, like water or alcohols, which stabilize the carbocation.
• Can compete with elimination reactions, since carbocations may also lose a proton to form alkenes.
• Leads to racemization at stereocenters because the carbocation intermediate is planar.