Dehydrohalogenation of Alkyl Halides

Dehydrohalogenation is a chemical reaction of alkyl halides in which halide atom and hydrogen atom eliminate to form alkenes products in organic chemistry. Dehydrohalogenation is an example of elimination reaction.

Key words: Dehydrohalogenation, Elimination reaction, Unimolecular reaction, bimolecular reaction, Zaitsev rule, Hoffmann product, 

In this reaction, hydrogen and a halogen which is bonded to adjacent carbon gets eliminate to produce carbon-carbon double bond. The reaction takes place by E1 and E2 mechanisms. The abbreviation E1 represents Elimination, unimolecular reaction whereas E2 represents Elimination, bimolecular reaction.

E1 Reaction

The E1 reaction is called as unimolecular reaction because rate of the reaction depends upon the concentration of one molecule (i.e. alkyl halide). This reaction consists of two steps; first step is formation of carbocation which is slow and rate determining step. In the second step base abstract hydrogen from carbon which is adjacent to the carbocation to form carbon-carbon double bond.

E1 reaction is first order reaction that is rate of the reaction is depending upon concentration of the alkyl halide. The energy diagram shows that ionization step is endothermic and gives rate limiting Transition State 1. The second step is deprotonation by base. The base is not involved in rate limiting step so rate of the reaction depends only on the concentration of alkyl halide.

Rate of E1 reaction = k[Alkyl halide]

k is rate constant

Figure 1:  Energy diagram of E1 reaction

Structure of alkyl halide plays important role in elimination reaction. In general secondary and tertiary alkyl halides are better for the formation of carbocation. In the second step weak base is required abstract a proton. Polar protic solvent are necessary for ionisation of bond. Hence for E1 reaction polar protic solvents are preferred.

Example: Reaction of tert-butyl bromide with methanol to form alkene. In this reaction ionisation step gives carbocation and bromide ion. In the second step methanol acts as base to abstract a proton to give alkene product.

E1 reaction is a slow process and carbocation formed in this reaction can be attacked by nucleophile and gives substitution product. For example in reaction of bromocyclohexane with ethanol under heating condition gives Elimination as well as Substitution (S_N1) product.

The carbocation intermediate can also undergo rearrangement to from more stable carbocation. Hence E1 reaction often gives mixture of products. For example; reaction of (1-bromoethyl)cyclopentane with boiling ethanol gives mixture of alkene products due to rearrangement of intermediate carbocation.

Zaitsev rule

In elimination reactions the product with more substituted double bond is predominates. This is known as Zaitsev rule. Usually unhindered weak bases like ethanol or methanol gives more substituted alkene product.

E2 Reaction  

The E2 reaction is bimolecular reaction that means rate of the reaction is depend upon two molecules (alkyl halide and base). It is a one step reaction and usually strong base is required to abstract a proton and leaving of halide ion to give an alkene product.

The energy diagram E2 elimination shows that it is a single-step bimolecular reaction.

Figure 2: Energy diagram of E2 reaction

Example: Reaction of tert-butyl bromide with sodium methoxide to form an alkene.

The E2 elimination reaction needs strong base to abstract a proton. In general alkoxide (RO-) bases are better for dehydrohalogenation reaction. To enhance nucleophilicity of the nucleophile; polar aprotic solvents are used for E2 reactions. Unhindered bases like ethoxide / methoxide gives highly substituted alkene product. It is known as Zaitsev product. The bulky base like tert-butoxide gives less substituted alkene product. It is called as Hoffmann product.

In organic synthesis for the preparation of alkene compounds second order elimination (E2) reactions are preferred because E1 reactions are known to give multiple products due to rearrangements.

Finally to summarize this topic, dehydrohalogenation of alkyl halides give alkene compounds via E1 or E2 reactions. Comparison of E1 / E2 elimination reactions is shown below. 

	E1 Reaction	E2 Reaction
Molecularity	Unimolecular reaction	Bimolecular reaction
Rate equation	Rate = k[Alkyl halide]	Rate = k[Alkyl halide] [Base]
Rate limiting step	Carbocation	Bimolecular transition state
Transition states	Two	One
Substrate	Secondary/ Tertiary alkyl halide	Secondary/ Tertiary alkyl halide
Base	Weak base required (MeOH, EtOH)	Strong base required (NaOMe, NaOEt)
Solvent	Polar protic (MeOH, EtOH)	Polar aprotic (DMF, DMSO)
Product	Multiple products	One major product
Rearrangements	Possible	Not possible

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