Preparation and Properties of Alkenes

Theory

Methods of preparation of alkenes:

1. From Alcohols : 

Alkenes can be prepared from monohydric alcohols or alkanols by the loss of H₂O and the reaction is known as dehydration reaction. 

(i) Dehydration with Al₂O₃:

(ii) Dehydration with mineral acid upon heating:

The order of acidic dehydration in different alcohols is Tertiary > Secondary > Primary

Mechanism of dehydration:

Regioselectivity of β-elimination is governed by Saytzeff's rule. The rule states that when two possible alkenes are obtained by the elimination reaction than that alkene containing maximum number of alkyl group on double bonded C–atoms will be in good yield.

Example:

Mechanism:

2. From alkyl halides: Removal of HX from a substrate by alcoholic KOH or NaNH₂

The ease of dehydrohalogenation shows the order:
For alkyl group tertiary > secondary > primary
For halogen in halide Iodide > Bromide > Chloride > flouride

It is a single step and synchronous process. Removal of proton, the formation of multiple bond between C_α and C_β and the release of the leaving group X take place simultaneously. (E₂ mechanism)

E₁ mechanism:  These are shown by tertiary alkyl halides which are capable of producing stable tert-carbonium ion.

E₂ mechanism : Primary and secondary alkyl halides undergo elimination reaction by E₂ mechanism. These alkyl halides do not give stable carbonium ion on ionization.

3. By Kolbe's method: Electrolysis of potassium or sodium salt of saturated dicarboxylic acid gives alkene.

4. By hydrogenation of alkyne: 

(i) By catalytic Hydrogenation of Alkynes in presence of poisoned catalyst gives rise to cis-alkenes : This is performed by Lindlar's catalyst : Metallic palladium deposited on calcium carbonate conditioned with lead acetate and quinoline. 

(ii) Birch reduction gives rise to trans-alkenes by using Na/Li and liquid NH₃.

5. Wittig Reaction:  The aldehydes and ketones are converted into alkenes by using a special class of compounds called phosphorus ylides, also called Wittig reagents.

Physical properties:

1. State: C₁ to C₃: Gas,
   C₄ to C₂₀: Liquids and above C₂₀ are solids.
2. Dipole moment: Cis > trans
3. Melting point: increases with molecular weight. Trans > cis.
4. Boiling point: increases with molecular weight. Cis > Trans.
5. Solubility : Alkenes are insoluble in water but soluble in non-polar solvents. Example : C₆H₆, CCl₄, ether etc. cis > trans

Chemical Properties : Alkenes are more reactive than alkanes because -

(i) The Ï€ electrons of double bond are located much far from the carbon nuclei and are thus less firmly bound to them.

(ii) Ï€ bond is weaker than Ïƒ bond and more easily broken.

The reactivity order for alkenes -

CH₂=CH₂ > R—CH=CH₂  > R₂C=CH₂ ≈ R₂CH=CHR > R₂C=CHR > R₂C=CR₂
(Trans < Cis)

(1) Electrophilic addition reactions : 

Mechanism: The addition in alkenes is generally electrophilic in nature which means that attacking reagent which carries the initial attack is an electrophile (E+).  This is quite expected also as there is high electron density in the double bond. 

For asymmetrical alkenes, Markownikoff's rule decides the addition, where the halogen atom goes to the unsaturated carbon atom bearing lesser number of hydrogen atoms.

Mechanism: 

However, In the presence of oxygen of peroxides, the addition of HBr on unsaturated unsymmetrical compound takes place contrary to Markownikoff's rule. This is called peroxide effect/anti-markownikoff's and is due to the difference in the mechanism of the addition and the mechanism involves free radicals. 

Mechanism: 

2. Oxidation reaction:  Alkene on combustion gives CO₂ and H₂O.  One mole of alkene requires (3n/2) moles of O₂ for complete combustion.

Ozonolysis: The addition of ozone on the double bonds and subsequent a reductive hydrolysis of the ozonide formed is termed as ozonolysis.

Oxidation by strong oxidising agent (Oxidative cleavage) : The alkenes themselves are readily oxidised to acid or ketone by means of acid permagnate or acid dichromate. If HCOOH is formed, it further oxidized to CO₂ and H₂O.