Hydrocarbons

Compounds of carbon and hydrogen.
Classification of Hydrocarbons:

Alkane

Open chain saturated hydrocarbon with general formula (CnH2n+2).
All the C atoms are single bonded i.e. sp³ hybridised.

Conformations of Alkane

Conformations are the different arrangement of atoms that can be converted into one another by rotation about single bonds.
Eclipsed Conformation: H atoms on two adjacent carbon atoms are closest to each other i.e. dehydral angle is 0.
Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dehydral angle is 60.

Preparation of Alkanes:

Reduction of Alkyl Halides:

RX + Zn: + H⁺ → RH + Zn²⁺ + X^-

4RX + LiAlH4 → 4RH + LiX + AlX3 (X≠ F)

RX + (n - C4H9)3 SnH → R-H + (n - C4H9)3 SnX

Grignard Reagent:

Hydrogenation of Alkenes:

Wurtz Reaction:

2RX + 2Na → R-R + 2NaX

2Na + 2CH3CH2CH2Cl → CH3CH2CH2CH2-CH2CH3 + 2NaCl

Corey House Reaction:

Decarboxylation of a mixture of the sodium salt of a carboxylic acid:

RCOONa +NaOH(CaO) → RH + Na2CO3

Kolbe's electrolytic method:

2 RCOOK + 2H2O → R-R + 2CO2 + H2+ 2KOH

Chemical Properties of Alkane

Direct Halogenation

RH + X2→ RX + HX

Order of Reactivity of X2: F2 > Cl2 > Br2; I2 does not react

?a. Initiation Step

Cl-Cl $\overset{uv}{\rightarrow}$ 2Cl^.

b. Propagation Step

H3C-H +Cl^. → H3C^. + H-Cl

H3C^. + Cl-Cl → H3C-Cl +Cl^.

c. Termination Step

Cl^. + Cl^. →Cl-Cl

H3C^. + H3C^.→ H3C-CH3

Cl^. + H3C^. → Cl-CH3

Nitration

Nitration of alkane is made by heating vapours of alkanes and HNO3 at about 400oC to give nitroalkanes.

¨This is also known as vapour phase nitration.

Combustion:

?Alkanes burn readily with non luminous flame in presence of air or oxygen to give CO2 & water along with evolution of heat.

C2H6 + 7O2 → CO2 +6H2O + heat

Aromatization

?¨Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst.

Oxidization of 3⁰alkane:?

Tertiary alkanes are oxidized to tertiary alcoholsby KMnO4

R3CH + KMnO4 → R3COH

Alkene (olefins)

Open chain, Unsaturated hydrocarbons with general formula (CnH2n).
At least one >c=c< (double bond) group i.e. sp2 hybridisation, is present throughout the chain.
Allene: alkene molecule in which at least one C has double bonds with each of the adjacent carbon i.e. -c=c=c- group.
Isomeric with saturated cycloalkanes.

Geometric Isomers:

Z is used if the higher - priority substituents on each C are on the same side of the double bond.letter E is used if they are on opposite sides

Heats of Hydrogenation: Heat of hydrogenation increases with increase in stability of alkene.

Order of heat of hydrogenation: 1-Butene> cis-2-Butene > trans-2-Butene

Order of stability: 1-Butene> cis-2-Butene > trans-2-Butene

Preparation of Alkenes:

1. Cracking of petroleum:

2. Dehydrohalogenation of alkyl halides: RCH2CH2X + alc.KOH → RCH = CH2

3. Dehydration of Alcohols :

Saytzeff Rule: In dehydration and dehydrohalogenation the preferential order for removal ofan H is 3° > 2° > 1°

4. Reduction of alkynes:

Chemical Properties:

1. Electrophilic Polar Addition Reactions

Reagent		Product
Name	Structure	Name	Structure
Halogens (Cl2, Br2 only)	X:X	Ethylene dihalide	CH2XCH2X
Hydrohalic acids	H:X	Ethyl halide	CH3CH2X
Hypohalous acids	X:OH	Ethylene halohydrin	CH2XCH2OH
Sulfuric acid (cold)	H:OSO2OH	Ethyl bisulfate	CH3CH2OSO3H
Water (dil. H3O⁺)	H:OH	Ethyl alcohol	CH3CH2OH
Borane	H2B:H	Ethyl borane	(CH3CH2BH2) → (CH3CH2)3B
Peroxyformic acid	H:O-OCH=O (HCO3H)	Ethylene glycol	CH2OHCH2OH

2. Addition of Hydrogen Halides to Alkenes: Markovnikov’s Addition:

R - CH = CH2 + HBr → R – CHBr – CH3

Mechanism:

R - CH = CH2 + HBr → R – CH⁺ - CH3 +Br^-
R – CH⁺ - CH3 + Br^-→ R – CHBr - CH3

Anit- Markovnikov’s Addition (Peroxide Effect):

R - CH = CH2 + HBr + (C6H5CO)2O2 → R – CHBr – CH3

Mechanism

Initiation:

R - O - O - R → 2RO^.

RO^. + HBr → Br^. + ROH

Propagation

CH3CH = CH2 + Br^. → CH3·CH - CH2Br

CH3·CHCH2Br + HBr→ CH3CH2CH2Br + Br^.

Termination:

2RO^. → R - O - O - R

Br^.+ Br^.→Br2

3. Addition of Water to Alkenes: Acid Catalyzed Hydration:

Reagent		Product
Name	Structure	Name	Structure
Halogens (Cl2, Br2 only)	X:X	Ethylene dihalide	CH2XCH2X
Hydrohalic acids	H:X	Ethyl halide	CH3CH2X
Hypohalous acids	X:OH	Ethylene halohydrin	CH2XCH2OH
Sulfuric acid (cold)	H:OSO2OH	Ethyl bisulfate	CH3CH2OSO3H
Water (dil. H3O⁺)	H:OH	Ethyl alcohol	CH3CH2OH
Borane	H2B:H	Ethyl borane	(CH3CH2BH2)®(CH3CH2)3B
Peroxyformic acid	H:O - OCH = O (HCO3H)	Ethylene glycol	CH2OHCH2OH

4. Oxymercuration-Demercuration:

Examples:

5. Hydroboration-Oxidation:

Examples:

6. Halogen Addition in Non-polar Solvent:

7. Halogen Addition in Aqueous Medium:

8. Syn – Hydroxylation: Formation of di-oles.

9. Ozonolysis of Alkenes:

Alkyne

Saturated open chain hydrocarbon with general formula (CnH2n-2).
At least one -c≡c- (triple bond) group i.e. sphybridisation, is present throughout the chain.
Physical properties of alkynes are similar to those of the corresponding alkenes