Alkenes are homologues series having general formula CnH2n. the first member of alkene is ethene (C2H4). They are unsaturated hydrocarbons having at least one double bond between any two carbons. There is one sigma bond and a pi bond between the two doubly bonded carbons. Due to the presence of weak pi bond alkene are less stable and more reactive than alkanes. The first three member of alkene are gases. They are also called olefin which mean oil like.
Alkene being unsaturated are prepared by elimination reaction of saturated compounds. General methods for their preparation are the following.
From alkyl halides
Alkenes can be prepared by elimination of hydrogen halide from alkyl halide. The reaction take place in basic medium therefore alcoholic KOH solution is used as a medium in this reaction. Elimination of β hydrogen take place from β carbon. The carbon next to the carbon having functional group is called β carbon.
Order of reactivity of alkyl halides
Reactivity of primary alkyl halides depend upon the halogen atom. Alkyl iodides are more reactive than other. The order of reactivity of alkyl halides is; alkyl iodide ˃ alkyl bromide ˃alkyl chloride.
Mechanism
A strong base give hydroxyl group which remove proton from the alkyl halides. As the β hydrogen of alkyl halides is slightly acidic which can for bond with the lone pair of OH group. The detachment of hydrogen leave electron to the carbon which are attracted by the positive carbon having halogen group. As a result of this a double bond is formed between the two carbon atoms and breaking of halide ion take place.
From vicinal dihalides
Those alkyl halides which have two halogen attached to the two adjacent carbon atoms are called vicinal dihalides. When vicinal dihalides are gently heated with methanolic zinc solution, they form alkene due to the loss of two halogens.
From alcohol
Alkenes can be prepared by the elimination of a water from an alcohol molecule. When alcohol are treated with concentrated sulphuric acid at 180oC they undergo elimination to form double bond.
Heat of dehydration
Heat of dehydration of alcohol depend upon the number of substituents attached to the alcohol. Greater the number of substituents attached to the alcohol, lesser amount of heat will be required. Therefore the heat of dehydration for primary alcohol is about 180 OC, while it is 140 OC for secondary alcohol and 80 OC for tertiary.
Order of reactivity of alcohols
Among the alcohol tertiary alcohols are more reactive than secondary and primary alcohol. Their order of reactivity is as follow
Tertiary ˃secondary ˃primary˃methanol.
Mechanism
Mechanism of the dehydration of primary alcohol is that the proto from sulphuric acid make bond with the lone pair of oxygen of alcohol forming oxonium ion. Then the HSO4– attack on the β hydrogen on the back side. A double bond is formed as a result of detachment of water molecule from α carbon and hydrogen from β carbon.
Alkene can also be prepared by catalytic dehydrogenation of alcohol where Al2O3 is used as catalyst.
Zaitsev rule.
According to the Zaitsev rule if there are more than one beta hydrogens, then elimination of that hydrogen will take place which results in the formation of highly substituted alkene. The clear example of this is elimination of water from 2-butanol can form 1-butene as well as 2-butene but the product will be 2-butene according to the Zaitsev rule.
Zaitsev rules is also applicable in case of elimination of alkyl halides for example elimination of halogen from 2-chloro butane.
From Alkynes
Alkenes can be prepared by the catalytic hydrogenation of alkynes. Here is the problem of limited hydrogenation of alkynes to stop the reaction at alkene otherwise alkane will be formed. For this purpose Lindlar’s catalyst is used instead of Ni catalyst. Lindlar’s catalyst is a mixture of Pd deposited over CaCO3 and poisoned with quinolone. As a result of this reaction cis-alkene can be produced.
If trans alkene are required then reduction of alkynes is done by uing sodium in liquid ammonia.
From alkane
In petrochemical industry alkenes can be brepared from alkanes by dehydrogenation. This process is often used for production of styrene and aromatic compounds. This process is highly exothermic and require temperature for the process is 500 OC or more.
Dr. Michael Pa
Dr. Michael Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a Ph.D. in organic chemistry from the University of Arizona. His research focus was on novel pain killers which were more potent than morphine but designed to have fewer side effects. There may even be a patent or two that came out of it. Prior to all of this, he was a chemist at Procter and Gamble. After all of that, he (briefly) worked as a post-doctoral assistant at Syracuse University, working on novel organic light-emitting diodes (OLEDs). In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. #fail During this entire time, he always loved helping students, especially if they were struggling with organic chemistry. In 2006, Dr. Pa founded AceOrganicChem.com in order to make learning organic chemistry fast and easy. 14 years and about 60,000 students later we are still helping students to learn organic chemistry one reaction at a time at https://www.aceorganicchem.com
