CHM 25500 Lecture Notes - Lecture 31: Electrophile, Nucleophilic Substitution, Nucleophile

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CHM 255 - Lecture 31 - Substitution Vs. Elimination Reactions
Substitution and Elimination Reactions
Substitution and elimination reactions often occur at the same time and compete
with one another
The ratio of substitution versus elimination products therefore depends on the
relative rates of the reactions
Determining Which Reactions Will Occur
There are several factors that determine which reactions (substitution or
elimination or both) will occur as well as the relative rates of the reactions
Structure of the substrate
Methyl halides only go through substitution because they do not have
beta carbons or beta hydrogens
Primary alkyl halides only go through Sn2 reactions and E2 reactions, E2
reactions with primary alkyl halides only taking place in the presence of a
strong base and heat
For secondary and tertiary alkyl halides, the type of reaction is
determined by other factors
Qualities of the nucleophile
If the nucleophile is also a strong base, elimination reactions are favored
If the nucleophile is not a strong base, then mostly substitution reactions
will occur, however there may be some elimination reactions occurring
Some bases cannot be nucleophiles because they are heavily substituted and
therefore sterically hindered, making it difficult to impossible for them to attack
the electrophile
If species such as this are present, the elimination reactions may take
place but not nucleophilic substitution reactions
Solvents also contribute to determining the predominant reaction taking place
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