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. dihedral angle is 0.
• Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dihedral 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:
• 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
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.
• 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 30 alkane:?
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:
Chemical Properties:
1. Electrophilic Polar Addition Reactions
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:
