1. Alkanes preparation methods and chemical properties:

2. Physical properties of alkanes:

(i) Boiling Point $(B . P .)$ :

(a) $B . P .$ increases with the increase in the molecular mass.

(b) Among the isomers, straight chain alkanes have higher $B . P .$ than branched chain alkanes.

(ii) Melting Point $(M . P .)$ :

(a) The melting points do not show a regular variation with an increase in the molecular size.

(b) The even number members have higher $M . P .$ as compared to the next alkanes with an odd number of carbon atoms (ALTERATION EFFECT).

(iii) Solubility:

They are soluble in non-polar solvents, and hence, insoluble in polar solvents such as water.

3. Alkenes preparation methods and chemical properties:

4. Physical properties of alkenes:

(i) Solubility: They are insoluble in water but soluble in organic solvents.

(ii) Boiling point: The boiling point of cis alkenes is usually higher than the corresponding trans-alkenes.

(iii) Melting point: The melting point of trans-alkenes is usually greater than cis alkenes.

5. Alkynes preparation methods and chemical properties:

6. Aromatic hydrocarbons:

(i) Electrophilic aromatic substitution:

(a) Bromination of Benzene:

Bromination follows the general mechanism for electrophilic aromatic substitution. Bromine itself is not sufficiently electrophilic to react with benzene, but a strong Lewis acid such as $F e B r_{3}$ , catalyses the reaction.

Step $1$ : Formation of a stronger electrophile.

Step $2$ : Electrophilic attack and the formation of the sigma complex.

Step $3$ : Loss of a proton gives the products.

(b) Nitration:

The electrophilic reagent, ${S O}_{3}$ , attacks the benzene ring to form the intermediate carbocation.

(d) Friedel Craft reaction:

Alkylation mechanism

(i)

(ii)

(iii)

Acylation mechanism

Acylation of benzene may be brought about with acid chlorides or anhydrides in the presence of Lewis acids.

Step $1$ : Formation of an acylium ion.

Step $2$ : Electrophilic attack.

Step $3$ : Loss of a proton. Complexation of the product.

For example:

Note: Friedel - Crafts acylation is generally free from rearrangements and multiple substitutions. They do not go on strongly deactivated rings.

7. Chemical Reactions of Benzene:

8. Nucleophilic Aromatic Substitution:

The reaction is a second order in which nucleophilic substitution occurs on the benzene ring. It is generally accepted that the reaction proceeds via an intermediate $σ$ -complex.

A hydrocarbon AC4H8 on reaction with HCl gives a compound BC4H9Cl which on reaction with 1 mol of NH3 gives compound CC4H10 N. On reacting with NaNO2 and HCl followed by treatment with water, compound C yields an optically active compound D. The D is

Important Questions on Hydrocarbons

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