MECHANISM FOR DEHYDROHALOGENATION REACTION

Dehydrohalogenation reaction involves the removal of hydrogen and halogen, this reaction basically yields alkene. foe example, dehydrohalogenation of t-buthyl bromide to give but-2-ene as shown by the equation below;

                                                           dehydrogenation Reaction

It is also important to know that this reaction only occurs in the presence of a strong base like alcoholic KOH.

NOTE: That dehydrohalogenation reaction of alkyl halide follows E2 mechanism (i.e elimination bimolecular).

It is paramount to know that the knowledge of nucleophilic substitution reaction is needed to understand elimination reaction because the same species are involved (i.e alkyl halid and base) in both mechanism.

How Can We Now Know That a Reaction Will Follow Elimination Or Nucleophilic Substitution Mechanism?
                               ↓                    ↓                   ↓                      ↓

For example, in nucleophilic substitution reaction of an alkyl halide, say t-buthyl bromide, a weak base or strong nucleophile is needed for the reaction as shown below;

                                                            Intermediate (carbocation)

NOTE: this reaction has an element effect (i.e removal of bromine)

In this reaction the rate determining step involves the loss of of bromine (Br) (that is there is element effect). After the loss of Br, the nucleophile then attacks the carbon in the second step.
while elimination reaction occurs when the nucleophile attacks hydrogen instead of carbon as shown below:

                                                              Formation of Alkene

This reaction is taking place because the nucleophile is a strong base. we should also note that the base (hydroxyl ion) is involved in the rate determining step of elimination reaction.

                          Why Is a Strong Base Needed For Elimination Reaction
                                       ↓                         ↓                            ↓ 

According to bronsted lowry definition of a base, "a base is a substance that accepts a proton" (i.e a base is a proton acceptor). Since the rate determining step of elimination reaction involves the attack of hydrogen by the base (strong nucleophile), therefore a strong base (good nucleophile) is needed for the removal of hydrogen.

in addition, only a strong base will be able to remove hydrogen at the rate determining step of elimination reaction. A nucleophile will only attack the carbon atom supporting nucleophilic substitution mechanism.

NOTE: That all nucleophiles are base, but not all bases are nucleophile.