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.
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.
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.