NEET Chemistry Notes Hydrocarbons – Alkenes

NEET Chemistry Notes Hydrocarbons – Alkenes

Alkenes

Alkenes

Alkenes are unsaturated hydrocarbons containing at least one double bond. Their general formula is C nH2jn  Alkenes show structural as well as geometrical isomerism. For exhibiting geometrical isomerism restricted rotation of double bond should be present.

• Polarity of alkene
  cis > trans
•  Boiling point of alkene
  cis > trans
•  Melting poing of alkene
  trans > cis

Preparation of Alkenes

Alkynes when treated with Lindlar’s catalyst
[Pd/C/H₂ /g] give cis-alkenes while on reduction with Na/liquid NH₃ give trans-alkenes.


In this reaction intermediate carbocation is formed and rearrange, therefore double bond is formed at maximum degree of carbon. Dehydration and dehydrohalogenation both are governed by Saytzeffs rule, i.e. the greater is the number of alkyl groups attached to double bonded ‘C’ atoms, the more stable is the alkene.
If dehydrohalogenation of alkyl halide takes place in the presence of potassium f-butoxide, less substituted alkene is obtained due to steric hindrance.
4. Vicinal dihalides when treated with zinc metal give alkenes. This reaction is known as dehalogenation.

Physical Properties of Alkenes

The physical properties of alkenes are listed below:

• Melting Point
  The melting point of alkenes do not show regular gradation. Alkenes have higher melting point than corresponding alkanes.
•  Boiling Point
  The boiling point of alkenes vary in the same manner as in alkanes. Their boiling points are higher than that of the corresponding alkanes.
• Solubility
  Alkenes are quite soluble in non-polar solvents such as benzene,CCl₄, etc.

Chemical Properties of Alkenes
Alkenes show different types of reactions such as:

•  Electrophilic Addition Reactions

Addition takes place according to Markownikoffs rule which states that negative part of the adding molecule gets attached to that carbon atom which possesses lesser number of hydrogen atoms, e.g.


In this reaction intermediate carbocation is formed, which rearrange to form an alcohol.

Free Radical Addition Reactions

Alkenes with HBr only in the presence of benzoyl peroxide show free radical addition reaction according to anti-Markownikoffs rule. This is also known as Kharasch effect.

Free Radical Substitution Reactions

Alkenes in the presence of sunlight or high temperature show free radical substitution reactions.

Oxidation

Cold dilute KMnO ₄ (Baeyer’s reagent) when treated with alkene gives glycol (vicinal dihydric alcohol).

• It is a test for unsaturation as purple colour of KMnO ₄ disappears during the reaction and a brown precipitate of Mn02 is obtained.
• Acidic KMnO ₄ or acidic K₂Cr₂0₇  oxidises alkenes
to ketones and or acids depending upon the nature of the alkene.

Ozonolysis is used to locate the position of double bond in an alkene.

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