Overview of SN1 & SN2 Substitution Reactions

The most basic substitution reaction:

So what happened?

• C-O bond is made.
• C-Br bond is broken.
• Lewis Base (OH, in this case) donates electrons and forms a new bond to the central carbon.
• Lewis Acid (Br), accepts electrons.

There are two different kinds of substitution:

• The C-Br bond is broken first, then later the C-O is made.
• This is a 2-step reaction, which we call SN1.

• The C-Br and C-O bonds are made and broken at the same time.
• This is a 1-step concerted reaction called SN2.

SN1 (2-Step Reaction):

A few things to notice:

• The Lewis Base, OH, is not involved in the first step at all. The Br falls off on its own!
• A carbocation (carbon with a positive charge) is formed after the Br falls off.
• In this case the OH attaches to the carbocation from below, but it can attach from any angle. This causes SN1 reactions to have products with mixed stereochemistry.

What conditions cause the Br to just fall off?

• A stable carbocation is formed.
• Polar solvent (helps stabilize the carbocation).
• A good leaving group. Br is a good leaving group, as are most halides (I, Cl). These are all stable after they fall off, even with a negative charge.
• Bad leaving groups, unlikely to fall off, include Fluorine (F) and Alcohol (OH).
• Note: these are the exact same conditions as E1 Elimination!

• In the top two reactions, unstable carbocations would be formed. This wouldn’t happen!
• In the 3rd reaction, the carbocation is stabilized by resonance from the double bond, distributing the positive charge between two carbons.
• In the 4th reaction, a stable tertiary (3°) carbocation is formed.

So in order for the 2-Step, SN1 reaction to occur a stable carbocation must be formed!

SN2 (1-Step Reaction):

A few things to notice:

• The OH “pushes” the Br off, in one single step.
• In SN2, the Lewis Base (OH) will always attach opposite to the Lewis Acid (Br), resulting in 100% inversion of stereochemistry.

What conditions cause the OH to attack the carbon, pushing off the Br?

• Negative charge on the OH is attracted to the partial positive charge on the central carbon.
• There are not very many large groups to “get in the way” of the OH.

• In the above picture, if A, B and C are large, they’ll get in the way of the OH. The step above won’t happen, preventing SN2! This is called steric hindrance.
• So, 3° halides (3 carbons attached to central carbon) have high steric hindrance, and won’t typically participate in SN2. But, remember that 3° halides will form a stable carbocation. Perfect for SN1!
• 1° halides (1 carbon, 2 hydrogens attached to central carbon) have low steric hindrance, and are perfect for SN2.