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Synthesis Of Alkenes

We can synthesize alkenes by processes such as dehydrogenation of alkanes, dehydrohalogenation of alkyl halides, and dehydration of alcohols.

From Alkanes

An alkene can be synthesized by the process of dehydrogenation (removal of hydrogen atoms), by heating an alkane up to about 700-7500.

Sample reaction

From Alcohols

When alcohols undergo dehydration reactions, alkenes are generated. There is a chance of rearrangement of the carbocation intermediates to form more stable carbocation intermediates, whenever possible, resulting in the formation of more than one type of alkenes. The mechanisms of such rearrangements are discussed in Chapter 21.

Sample reaction


The only product in the above reaction is ethylene, since there is no rearrangement and more importantly, there is no need for rearrangement. This is because the intermediate formed in this reaction is a primary carbocation, and rearrangement cannot produce a more stable carbocation.

From Alkyl Halides by Dehydrohalogenation

Alkenes can be synthesized by reacting the corresponding alkyl halides with a suitable strong base that can abstract a proton from one of the carbon atoms, while the leaving group attached to the adjacent carbon leaves. The general mechanism is shown below.

image\Ch 16sec E, g3.png

According to Zaitsev's rule, the major alkene product is the one that is the most highly substituted. Consider the following example involving 2-bromobutane with potassium hydroxide.

Sample reaction

image\Ch 16 sam 16-3.png

If bulky bases are used, the least substituted alkene may predominate as the product. In the next example, the base used is tert-butoxide ion. Since it is a bulky base it will preferentially abstract the less hindered hydrogen, leading to the formation of the least substituted alkene–the Hofmann product.

Sample reaction

image\Ch 16 sam 16-4.png

From Vicinal Dibromides by Dehalogenation

Vicinal dibromides have two bromines on adjacent carbon atoms. Vicinal dibromides can be converted to alkenes by reduction with zinc or iodide ion. A sample reaction is given below.

Sample reaction

image\Ch 16 sam 16-5.png

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