Substitution and Elimination Reactions
Substitution involves replacing a leaving group with a nucleophile and elimination removes H and a leaving group to form an alkene. Both types of reactions require a good leaving group. Good leaving groups are very weak bases (pKa conjugate acid < 0). What are some examples of good leaving groups?
In class, we have seen examples of alcohols undergoing Sn1 and E1 reactions. The names of these reactions come from experimental evidence that supports the mechanism. Substitution (S), elimination (E), and the 1 because it is unimolecular, meaning that one molecule is involved in the rate-determining step.
What we know from experiments:
Kinetics experiments show that the rate law depends only on alcohol concentration
Sn1 and E1 reactions proceed fastest at tertiary centers (supports carbocation formation).
Sn1 products are a mixture of stereoisomers when the alcohol is at a chiral center (also supports carbocation formation; the geometry is trigonal planar and attack can occur from the front or back).
E1 products are regioselective for the more-substituted alkene and stereoselective for the E alkene (connect to PLA 21).
Based on the above data, it is proposed that the mechanism goes through a carbocation intermediate and formation of the cation is the the rate-determining step. Review your class notes for these mechanisms or Section 9.2 (Sn1) and 9.4 (E1) of the text. Be sure to consider the reactivity trends with respect to alcohol substitution and how this makes sense with the proposed mechanism.
Now, we will consider how the above reactions (Sn1 and E1) as well as Sn2 and E2 reactions look with respect to alkyl halides as reactants.
The same trends as above are seen with alkyl halides under Sn1/E1 conditions. See sections 8.3-4 and pages 282-283.
However, a two other reaction pathways are possible depending on conditions: Sn2 and E2. The "2" comes from the fact that the reaction is bimolecular, meaning that two molecules are involved in the rate determining step.
What we know from experiments:
Kinetics experiments show that the rate law depends only on both concentration of alkyl halide and nucleophile/base.
Sn2 reactions proceed fastest at primary centers and do not go at all at tertiary centers (page 267).
When the halide is at a chiral center, only one stereoisomer is produced (p.269).
E2 products are regioselective for the more-substituted alkene and stereoselective for the E alkene (connect to PLA 21).
It is proposed that the Sn2 and E2 mechanisms are each one step where substitution or elimination occur at the same time as loss of a leaving group (pages 266 and 282).