Important notes of Hydrocarbons for CBSE & JEE

Go through these Important notes of Hydrocarbons to score the most in you CBSE examinations and In IIT – JEE  main and advance.


Compounds of carbon and hydrogen

Classification of Hydrocarbons:


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


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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:


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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       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


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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.


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


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.


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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°


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4. Reduction of alkynes:

Chemical Properties

1. Electrophilic Polar Addition Reactions

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

R – CH= CH2 + HBr → R – CHBr – CH3


R – CH= CH2 + HBr → R – CH+ – CH3 +BrR

CH+ – CH3 + B → R – CHBr- CH3

Anit- Markovnikov’s Addition (Peroxide Effect):

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



R – O – O – R → 2RO

RO. + HBr → Br. + ROH


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CH3CH = CH2 + Br. → CH3·CH – CH2Br

CH3·CHCH2Br + HBr→ CH3CH2CH2Br + Br


2RO. → R – O – O – R

Br. + Br.→Br2

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

4. Oxymercuration-Demercuration:


5. Hydroboration-Oxidation:



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6. Halogen Addition in Non-polar Solvent

7. Halogen Addition in Aqueous Medium:

8. Syn – Hydroxylation: Formation of di-oles

9. Ozonolysis of Alkenes


Saturated open chain hydrocarbon with general formula (CnH2n-2). At least one -c≡c- (triple bond) group i.e. sp hybridisation, is present throughout the chain.

Physical properties of alkynes are similar to those of the corresponding alkenes.


1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides

2. Dehalogenation of vic-Tetrahalogen Compounds


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3. Alkyl Substitution in Acetylene; Acidity of º C-H

4. From Calcium Carbide:

CaC2 +2H2O → Ca(OH)2+ C2H2

5. Kolbe’s Electrolysis:

Chemical Properties

1. Hydrogenation:

RC ≡ CCH2CH3 + 2H2 → CH3CH2CH2CH2CH3

2. Hydro-halogenation:

Markovnikov addition: RC≡CH +HBr → RCBr=CH2 +HBr→ RCBr2-CH3

Anti-markovnikov addition: RC≡CH +HBr +peroxide → RCH=CHBr


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Aromatic Hydrocarbons:

For being aromatic a hydrocarbon should

  • be a cyclic compounds.
  • have planarity in geometry.
  • have complete delocalization of electrons over ring.
  • follow Huckel Rule i.e. number of ?? electrons in ring = (4n+2).

Benzene (C6H6)

1. Structure:

2. Chemical Reactions of Benzene:


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Anti-aromatic Hydrocarbons:

Highly unstable compounds.

Number of π electrons in ring = 4n.


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