Nucleophilic Reactions

Nucleophile

Region of high electron density

• This can be a lone pair / negative charge

Electrophile

Region of low electron density

• This can be next to an electronegative atom or cation

What does a curly arrow represent?

The movement of a pair of electrons

What does SN mean?

Substitution nucleophilic

What is S_N2?

• Substitution Nucleophilic Bimolecular

• One step reaction (all at once)

• The nucleophile attacks the carbon at the same time the leaving group leaves

• The rate depends on both the concentration of the substrate (the molecule being attacked), and the concentration of the nucleophile

Example of an S_N2 reaction

CH₃Br + OH^- → CH₃OH + Br^-

• OH^- is the nucleophile and it attacks

• Br^- is the leaving group and leaves

Key points of an S_N2 reaction:

• Happens in 1 step

• Strong nucleophile needed

• Works best with primary carbons

• Inversion of configuration (the molecule flips like an umbrella)

What is S_N1?

• Substitution Nucleophilic Unimolecular

• Two step reaction (one at a time)

• Leaving group leaves first, forming a carbocation (C⁺)

• Nucleophile attacks the carbocation

• The formation of the carbocation is the slow step, so it is the rate determining step

• Therefore the rate depends only on substrate concentration

Example of S_N1:

(CH₃)₃CBr  -->  (CH₃)₃C⁺   +   Br^-

(CH₃)3C⁺   +  H₂O  -->  (CH₃)₃COH  +  H⁺

Key points of S_N1:

• Happens in 2 steps

• Carbocation intermediate

• Works best with tertiary carbons

• Weak nucleophile is okay

• Causes racemization (the mix of both enantiomers, no inversion)

What is steric hindrance?

The crowding around a molecule’s reactive site makes it hard for another molecule (like a nucleophile) to get close enough to react.

Imagine a nucleophile (like OH⁻) trying to attack the carbon where Br is attached:

• In methyl or primary cases, it’s easy — not much in the way.

• In tertiary cases, all those CH₃ groups are bulky and block access to the carbon.

That’s steric hindrance — the bulky groups literally get in the way.

Steric hindrance in S_N1?

The leaving group leaves first, forming a carbocation, and the nucleophile attacks after when there’s more space

What is E1 and E2?

• The elimination versions of S_N1 and S_N2.

• E = Elimination

• Instead of a nucleophile replacing the leaving group (like in S_N1 and S_N2), the base removes a hydrogen atom and the leaving group leaves, forming a double bond

E2 reaction

So three things happen simultaneously:

1. The base takes a hydrogen.

2. The C–H bond breaks.

3. The C–X (leaving group) bond breaks and a C=C double bond forms.

E1 reaction

• 2 step reaction like S_N1

• Leaving group leaves first, forming a carbocation.

• A base removes a proton (H⁺) from a β-carbon → double bond forms.

Key features of E1:

• Two step reaction

• Carbocation intermediate

• Weak base

• Tertiary carbocations

In E1, what does R =

Aryl alkyl

In E2, what does R =

H