Organic Chemistry: Sn1E1 Reactions: SN1 and E1 Reactions | SparkNotes (2024)

S_N2 and E2 reactions require a good nucleophile or a strong base. S_N1 and E1 reactions occur with strong bases with molecules whose α-carbon is secondary or tertiary and in the absence of good nucleophiles.

S_N1 and E1 Rate Law and Mechanism

This reaction yields S_N1 and E1 products:

If we disregard ethanol's poor nucleophilicity and weak basicity, this reaction looks very much like an S_N2 or E2. The fact that it is not a bimolecular reaction becomes apparent in the S_N1/E1 rate law.

Remember that the rate law shows which molecules are present in the transitionstate of the rate-limiting step. Since S_N1 and E1 share the same rate law (including k), it is reasonable to assume that both reactions go through the same rate-limiting transition state. That transition state involves a forming carbocation.

Once the carbocation intermediate forms, the two reactions follow divergent pathways. In the S_N1 pathway, ethanol acts as a nucleophile. In the E1 pathway, ethanol is a base.

A base/nucleophile as weak as ethanol can substitute or eliminate because the carbocation is an incredibly reactive species. Without the carbocation or a very good leaving group, S_N1 and E1 would be impossible.

S_N1 vs. E1

S_N1 and E1 reactions are not synthetically useful because they almost always give a mixture of substitution and elimination products. The proportion of these products does vary with the α-carbon branching, however. Generally speaking, greater branching gives more elimination products. These elimination products generally follow Saytzeff's rule. Keep in mind that the α-carbon must be secondary or tertiary for S_N1 or E1 to occur at all. Thus the branching effect is much less pronounced than in the S_N2 and E2 reactions.

In the broader context of all substitution and elimination reactions, remember that S_N1 and E1 will not occur in the presence of a strong base or a good nucleophile. In these cases, E2 and S_N2 dominate their unimolecular cousins.