Nucleophiles and Electrophiles in Substitution Reactions

Nucleophile

A 'friend' of positive charges; usually a highly electronegative atom with a lone pair of electrons that attacks atoms with positive charge density.

Examples of nucleophiles

H-S, H2-N, H-O

Electrophile

A 'friend' of negative charges; usually a carbon atom with positive charge density and a good leaving group.

Good leaving groups

Cl, I, Br

Nucleophilic substitution at saturated carbons

If leaving group departs first then nucleophile arrives: Sn1. If nucleophile attacks while leaving group departs: Sn2.

Pentavalent carbon

An impossible intermediate with 5 bonds and 10 shared electrons.

Sn1 mechanism

Two-step: 1) Leaving group departs (slow), 2) Nucleophile arrives (fast)

Sn1 rate law

Rate depends only on concentration of electrophile

Sn1 rate determining step

Step 1 is slow and UNImolecular (involves only the electrophile)

Sn2 mechanism

One-step: nucleophile attacks as leaving group departs

Sn2 transition state

State where bonds are partially formed and broken simultaneously

Sn2 rate law

Rate depends on concentration of both nucleophile and electrophile

Sn2 rate determining step

BImolecular (involves both nucleophile and electrophile)

Electrophile structure vs mechanism

Methyl: Sn1 no, Sn2 yes | Primary: Sn1 no, Sn2 yes | Secondary: Sn1 ?, Sn2 ? | Tertiary: Sn1 yes, Sn2 no