Alkenes

Topics: Alcohol, Alkane, Alkene Pages: 36 (5090 words) Published: July 22, 2013
Preparation
u 

Alcohols when heated in presence of H2SO4, H3PO4, P2O5, Al2O3 or BF3 undergo loss of water molecule with the formation of alkene. Conc. H2SO4 180oC H3PO4/P2O5 200oC Al2O3/BF3 350oC

CH3-CH=CH2 + H2O

CH3-CH2-CH2-OH

CH3-CH=CH2 + H2O

CH3-CH=CH2 + H2O

u  u 

Mechanism: In the first step OH group of the alcohol is protonated in a fast reversible reaction. Unlike OH group, protonated OH group is a good leaving group. Step 1: CH3 CH3-C-CH3 + H+ HO CH3 CH3-C-CH3 H2O+ Fast

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In the second step, water molecule is lost with the formation of a carbonium ion. This is the rate determining step. CH3 CH3 u  Step 2: Slow CH3-C-CH3 CH3-C-CH3 + H2O + H2O+ In the final step carbonium ion loses proton from its adjacent carbon atom which results in more stable alkene. The anions of the acid or another alcohol molecule will function as a base and facilitate loss of proton. CH3 CH3 u  Step 3: CH3-C-CH2 + HSO4CH3-C=CH2 +H2SO4 + H

u 

u  u 

The ease of dehydration follows the order tertiary > secondary > primary alcohols. This Mechanism is called E1. It implies that it is Elimination unimolecular as the rate determining step has only one molecule of the reactant. Primary alcohols require the most stringent conditions to undergo dehydration i.e. use of conc. H2SO4 and high temperature (180 - 200°C). Secondary alcohols can be dehydrated under relatively milder conditions by the use of 85% H3PO4 and a temperature of 160°C. Tertiary alcohols can be easily dehydrated by using 25% H2SO4 at 85°C. To decide which proton is to be abstracted of various available, that alkene is formed in major amounts that has maximum number of alkyl groups (preferably methyl) attached to both the carbons. This is because of maximum hyperconjugation. This is known as Zaitsev Rule (after Alexander Zaitsev in 1875 and Aleksandr Nikolaevich Popov in 1872 Though it was Popov who proposed the rule first, it is still credited to Zaitsev. Interestingly the prime motive for Zaitsev to work on elimination reaction was to counter the view of his bitter rival Vladimir Vasilyevich Markovnikov who believed that the less substituted alkene is formed. Since Markovnikov had already published his work on addition on alkenes in 1870, Zaitsev worked on its reverse reaction, i.e, elimination to disprove his rival)

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u  u 

CH3 CH3-CH-CH2-OH + H+ CH3 CH3-C-CH2-OH + H+ CH3

CH3 CH3-C=CH2 CH3 CH3-C=CH CH3 CH3 HC=C-CH3

OH + H+

OH + H+ OH + H+

CH3 CH3-C-CH2-OH + H+ Ph

OH + H+ OH + H+ OH

OH + H+

OH + H+

+ H+ OH CH3 HC≡C-CH2-CH-CH-CH3 + H+ + H+ OH CH3 HC≡C-CH=CH-CH-CH3

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Alkyl halides when treated with a strong base like hot alcoholic solution of KOH undergo elimination of hydrogen halide leading to the formation of alkene. The yield of alkene depends on the nature of alkyl halide used. It is fair with primary and secondary and very good with tertiary alkyl halides. CH3-CH2-CH2-X Alc. KOH

u 

good with

CH3-CH=CH2

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In this type of elimination reaction the two leaving groups are lost from the adjacent carbon atoms. One of the leaving groups is the halogen atom and the C-atom from which halogen is lost is usually designated as 1 or (α)-carbon atom. Hydrogen the other leaving group is lost from the neighbouring carbon which is designated as 2 or (β)-carbon atom and the reactions are referred to 1, 2-elimination or βelimination (α- is commonly omitted). If the alkyl halide contains only one β-carbon atom, only one alkene is formed as in the above sited example. However, if the alkyl halide has two or more β-carbon atoms, two or more alkenes are possible. Major Alkene is formed based on the Zaitsev Rule.

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Mechanism: u  Consider the C2-C3 elimination of meso-2,3-Dichlorobutane u 

CH3 H Cl

H C

Cl

H C

Cl

H CH3
u  u 

Cl

Me

Me

First convert the Fischer projection to Sawhorse projection. It is observed that the...
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