Hydrocarbons

Compounds of carbon and hydrogen.
Classification of Hydrocarbons:

Alkane

Open chain saturated hydrocarbon with general formula (C_nH_2n+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. dihedral angle is 0.
Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dihedral angle is 60.

Sawhorse Representation Newman Representation

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 (C_nH_2n-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