Preparation of Alkenes

Preparation of Alkenes

1. Dehydrohalogenation of alkyl halides:

Alkenes can be prepared by the elimination of hydrogen halide from an alkyl halide. Hydrogen and halogen are removed from adjacent carbon atoms, resulting in the introduction of a carbon-carbon double bond in the molecule.

RCH₂—CH₂X → RCH=CH₂ + HX

One way to accomplish the elimination of hydrogen halide is to simply heat the alkyl halide at high temperature (300–400°C). However, if a strong base is used, the reaction can be carried out at a lower temperature. The most common reagent used for this purpose is sodium ethoxide in ethanol.

RCH₂—CH₂X + C₂H₅ONa → RCH=CH₂ + C₂H₅OH + NaX
                                                                      55°C

Another complication arises from the fact that if β-hydrogen is available at more than one position, a mixture of isomeric alkenes is obtained. The relative stability of the alkenes usually reflects the relative stability of the alkenes (see p.133); the most stable alkene generally predominates.

CH₃—CH=CH₂ + CH₂=CH—CH₃
25% (cis) / 75% (trans)

However, the more basic and bulkier potassium t-butoxide in the less polar solvent t-butyl alcohol, tends to give more of the terminal alkene.

2. Dehydration of alcohols:

Alkenes can also be prepared by the dehydration of alcohols. This is another method involving the elimination reaction. In this reaction, a hydrogen atom and the hydroxyl group are removed from adjacent carbon atoms for the introduction of a carbon-carbon double bond.

RCH₂—CH₂OH → RCH=CH₂ + H₂O

Whereas dehydrohalogenation is promoted by a base, dehydration is brought about by an acid.

Dehydration of an alcohol is generally carried out either by heating the alcohol with a strong acid, such as sulfuric acid or phosphoric acid (a laboratory method), or by passing the vapors of the alcohol over a Lewis acid, such as commercial alumina (Al₂O₃) containing SiO₂ and other oxides) at about 400°C (an industrial method).

Different types of alcohols differ in the ease of dehydration, the general order being:

 tertiary > secondary > primary.
The experimental conditions like the temperature and the concentration of the acid therefore depend on the nature of the alcohol, as shown below:

Alkenes produced from secondary and particularly from tertiary alcohols tend to polymerize under the influence of a concentrated acid.

The most accepted mechanism of dehydration involves three steps. In the first step, the alcohol is protonated by the acid to form an alkyloxonium ion, which then undergoes the removal of a water molecule to form a carbocation in the second step. A proton is then removed from a β-carbon atom (the carbon atom next to the positively charged carbon atom) of the carbocation in the third step, resulting in the formation of an alkene. These three steps are illustrated as under:

The behavior of alcohols toward dehydration supports this mechanism because the ease of dehydration of different types of alcohols is related to the stability of the carbocation formed as an intermediate in each case.

Similarly, 1-butanol on dehydration gives a mixture of products:

CH₃CH₂CH₂CH₂OH → trans-2-Butene (Major) + cis-2-Butene (Minor) + 1-Butene (Minor)
                        (conc. H₂SO₄, 170°C)

In this reaction, a hydride ion migrates from one carbon to the next to convert a primary carbocation into a more stable secondary carbocation.

CH₃CH₂CH₂CH₂OH → CH₃CH⁺—CHCH₃ → CH₃CH=CHCH₃.

3. Dehalogenation of vic-dihalides:

Alkenes can be prepared by the elimination of halogen atoms from vic-dihalides (compounds in which the halogen atoms are situated on adjacent carbon atoms). Dehalogenation of a vic-dihalide with zinc in an anhydrous solvent such as methanol or acetic acid is carried out as follows:

RCH—CH₂ + Zn → RCH=CH₂ + ZnBr₂
                                          (In presence of Zn, alcohol or CH₃COOH solvent)