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:

Alkane

Open chain saturated hydrocarbon with general formula (C_nH_2n+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

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 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

Mechanism

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

Mechanism

Initiation:

R – O – O – R → 2RO

RO. + HBr → Br. + ROH

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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:

4. Oxymercuration-Demercuration:

Examples:

5. Hydroboration-Oxidation:

Examples:

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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

Alkyne

Saturated open chain hydrocarbon with general formula (C_nH_2n-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.

Preparation

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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