Introduction:
In organic chemistry, alkyl halides can undergo two types of reactions: substitution and elimination reactions. Substitution reactions involve the replacement of a halogen atom with another atom or group, while elimination reactions result in the removal of a halogen atom and the formation of a double bond or pi bond. The choice between elimination and substitution reactions depends on several factors.

Factors favoring elimination over substitution:
1. Low polarity of solvent: Elimination reactions are favored in nonpolar solvents because they do not stabilize the charged intermediates formed during the reaction. Substitution reactions, on the other hand, are favored in polar solvents that can solvate and stabilize the charged intermediates. Therefore, low polarity of the solvent favors elimination over substitution.

2. Strong base: Elimination reactions require the presence of a strong base that can abstract a proton from the alkyl halide and form a double bond. Strong bases such as hydroxide ion (OH-) or alkoxide ion (RO-) are commonly used in elimination reactions. Substitution reactions, on the other hand, do not require a strong base. Instead, a nucleophile attacks the alkyl halide and replaces the halogen atom. Therefore, the presence of a strong base favors elimination over substitution.

3. High concentration of base: The concentration of the base affects the rate of both elimination and substitution reactions. However, elimination reactions are more sensitive to the concentration of the base. Higher concentrations of base increase the likelihood of proton abstraction and double bond formation, favoring elimination over substitution.

Factor not favoring elimination over substitution:
4. Sterically unhindered haloalkene: Steric hindrance refers to the obstruction of a reaction by bulky substituents around the reacting atoms. In the case of elimination reactions, the formation of a double bond requires the alignment of the leaving group and the hydrogen being abstracted. If the alkyl halide has bulky substituents, it becomes difficult for the leaving group and the hydrogen to align properly, leading to steric hindrance. This hinders the elimination reaction and favors substitution instead.

Therefore, sterically unhindered haloalkenes do not favor elimination reactions because they are less hindered and allow for proper alignment of the reacting atoms, resulting in efficient substitution reactions.

Low polarity of solvent, strong base and high concentration of base favour elimination reaction.