CHE 230

University of Kentucky

Substitution or elimination under basic conditions

A nucleophile or base is classified as:

• a good nucleophile and a poor base if it is in the 2nd row of the main group or below (P, S, Cl, Br, I);
• a good nucleophile and a good base if it is in the 1st row of the main group and fairly unhindered;
• a poor nucleophile and a good base if it is in the 1st row of the main group and it is very hindered (t-BuOK, i-Pr₂NEt, LiN(i-Pr)₂).

	good nuc, poor base	good nuc, good base	poor nuc, good base
1° elec	SN2	SN2	E2, rarely SN2*
2° elec	SN2	SN2 < E2**	E2
3° elec	No reaction	E2	E2

Notes:

If elimination is to occur, there must be at least one H-bearing C adjacent to the electrophilic C.

*Some very unhindered electrophiles, especially those that lack H-bearing C atoms adjacent to the electrophilic C (e.g., CH₃X and PhCH₂X), may undergo S_N2 reactions even with poor nucleophiles.

**In the middle square, the balance of S_N2 and E2 can easily be tilted in one direction or the other depending on various factors.

• Polar aprotic solvents such as DMSO and DMF promote substitution.
• The less basic the nucleophile, the larger the proportion of substitution. So AcO^– gives more substitution product than HO^–, HC≡C^– more than H₂C=CH^–, and (EtO₂C)₂CH^– more than EtO₂CCH₂^–. The less basic forms of the nucleophile can often be converted into the more basic forms after the substitution reaction has been executed.
• Compounds in which the electrophilic C atom is adjacent to a π bond, such as PhCH(CH₃)X or H₂C=CHCH(CH₃)X, tend to give more substitution.
• Cycloalkyl halides tend to give more elimination.

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