Synthesis of Alkenes from Alkyl Halides

Alkene compounds are the hydrocarbons which consist of at least one carbon-carbon double bond. Various synthesis methods for the synthesis of alkene are described herein.

Hi Friends, in this article we will discuss about few basic reactions in organic chemistry which are used for the synthesis of alkenes starting from alkyl halides. 

Key words: Alkene, Alkyl halide, Dehydrohalogenation, Dehalogenation, Carbocation, Zaitsev product, Hoffmann product.

Alkene compounds are the hydrocarbons which consist of at least one carbon-carbon double bond. They are important intermediates in organic synthesis because they can be converted in to alkyl halides or alcohols by addition reactions and carbonyl compounds by oxidation reactions. There are two important and basic reactions that are useful for synthesis of alkene compounds. They are;

1. Dehydrohalogenation of alkyl halides

2. Dehalogenation of vicinal dihalides

Let’s see each reaction in detail;

1.    Dehydrohalogenation of alkyl halides

Dehydrohalogenation is a elimination reaction. In this reaction, hydrogen and a halogen gets eliminate to produce carbon-carbon double bond. The reaction takes place by E1 and E2 mechanisms. 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. Nature of the alkyl halide is also play important role here, usually secondary and tertiary alkyl halides gives excellent yield of the elimination product.  

E1 Mechanism

E1 reaction is unimolecular reaction it means that rate of the reaction is completely depend upon alkyl halide. In this reaction tertiary alkyl halide reacts faster than secondary alkyl halide. E1 reaction consists of two steps;

1. First step is formation of carbocation intermediate.

2. Second step is deprotonation by weak base or solvent. 

The reaction follows Zaitsev rule and provides highly substituted alkene product.

Example 1:

1-iodo-1-methylcyclohexane undergoes dehydrohalogenation when heated with methanol. The reaction follows E1 mechanism. In this case after formation of carbocation intermediate, methanol acts as base and causes deprotonation to form alkene product.

Since the reaction proceeds through carbocation intermediate; it can form multiple products. Also carbocation can undergo rearrangements to form thermodynamically stable products.

Example 2:

3-bromo-2-methylpentane undergoes dehydrohalogenation reaction when heated with water.

E2 Mechanism

E2-reaction is bimolecular elimination reaction that means rate of the reaction is depend upon concentration of alkyl halide and base. In this reaction strong base is required. The mechanism consists of single step. In this mechanism H and halide should be anti periplaner to each other. The strong base abstract H from the carbon which is next to halide bonded carbon. In this reaction also Zaitsev product is favored.  

Example 3:

Reaction of ter-butyl bromide with sodium methoxide gives alkene product. Here methoxide anion abstract H and elimination of bromide takes place to form alkene compound. 

Choice of Base for Elimination Reaction

As we know that for E2 elimination reaction there is need of strong base. Outcome of the reaction is highly substituted alkene. In general alkoxide (RO-) bases are better for dehydrohalogenation reaction. Unhindered bases like ethoxide / methoxide gives highly substituted alkene product. It is known as Zaitsev product. The bulky base like tert-butoxide gives least substituted alkene product. It is called as Hoffmann product.

Dehydrohalogenation of germinal dihalides can give alkene product. The reaction follows E2 mechanism and strong base is required for elimination. Bases like potassium hydroxide (KOH) or sodium amide (NaNH₂) are generally used in this reaction.

Example 4: 

1,1-dichloro-2-methylbutane reacts with alcoholic potassium hydroxide to give alkene product.

If there are two H on carbon which is next to halide bonded carbon; then reaction doesn’t stop at alkene stage and second dehydrohalogenation gives alkyne product.

Example 5:

2.    Dehalogenation of vicinal dihalides

Vicinal dihalides react with zinc (Zn) in acetic acid to produce alkene compound. Here the halogen atoms should be “anti” to each other.

Mechanism

The reaction mechanism consists of two steps;

1. First step is incersion of metal in to carbon-halogen bond.

2. Second step is elimination of halide to form alkene prouct.

Example 6:

1,2-dibromobutane reacts with zinc metal in acetic acid to produce but-1-ene as elimination product.

Write the major product of the following reactions;

That's all for this topic. If you have any questions please feel free to ask me in the comment box. Thank you..!