Question

Why do elimination reactions complete with substitute reactions when alkyl halides react with a nucleophile?

Answer 1

All that you must know is that the concept of elimination reaction is complete and the substitute reaction.
You must understand that they vary from one to another reaction. These two reactions are influenced by several other factors and compete with each other.
In this we can see about this and why do the elimination reaction is complete with a substitute reaction when alkyl halides react with a nucleophile? As per the given question.

Complete answer:
Both reaction mechanisms favour the same conditions due to this Elimination reactions compete with substitution reactions. It undergoes the same condition for both Alkyl halide and a nucleophile.
The elimination reactions cannot occur and vice versa so, they cannot compete with substitution reactions.
Example,
When there is no Elimination reaction:
The acetate $C{H_3}COO$− has a weak base but it is a strong nucleophile, therefore, the substitution $SN1$ will occur at a very fast rate and, before any elimination product is formed it consume all the alkyl halide.
Example,
When there is no substitution reaction:
The tert-butoxide$\left( {C{H_3}} \right)3CO$− has a weak nucleophile but it is a strong bulky base. By this, we can understand that the elimination will occur at a very fast rate and consume the alkyl halide before any substitution product is formed.
Mechanism:
Elimination Reaction:
Elimination reactions are basically divided into two categories one is $E1$ reactions and another is $E2$ reactions.
$E1$ reactions will have two steps in reaction.
$E2$ reactions will have only a single-step mechanism.
Substitution Reaction:
Based on their reaction mechanism, substitution reactions are divided into two categories.
${S_N}1$ reactions and ${S_N}2$ reactions.

Note:
In organic chemistry, there are two types of chemical reactions. They are elimination and substitution reactions. The difference between elimination and substitution reaction by using their mechanism can be explained clearly. The rearrangement of previous bonds occurs after the reaction in the elimination reaction.
But in the substitution reaction,it replaces a leaving group with a nucleophile. They vary from one to another reaction. These two reactions compete with each other and are influenced by several other factors.