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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.
     

                                          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 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:

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 (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

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).                         :

          Aromatic Hydrocarbons

Benzene (C6H6)

1. Structure:

Kekual Structure of Benzene

 2. Chemical Reactions of Benzene:

  

 Anti-aromatic Hydrocarbons:

Highly unstable compounds.

Number of π electrons in ring = 4n. 

Example: 

 Anti-aromatic Hydrocarbons

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