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.
   

                                                

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 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 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:

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:

 

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. 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

 

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

 

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:

  

 Anti-aromatic Hydrocarbons:

Highly unstable compounds.

Number of π electrons in ring = 4n. 

Example: 

 

 

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