13. Hydrocarbons Classification: Saturated Single bond Unsaturated Multiple bond Aromatic Alkanes: General formula is CnH2n+2. Structural isomer Difference in structure Chain isomer Difference in chain
Preparation From unsaturated hydrocarbons (hydrogenation) CH3 CH=CH2 H2
Pt/Pd/Ni
CH3 CH2 CH3
Wurtz reaction (Preparation of higher alkanes containing even number of carbon atoms) CH3Br 2Na BrCH3
dry ether
CH3 CH3 2NaBr
De-carboxylation (Elimination of carbon dioxide from carboxylic acid) CH3COONa NaOH
CaO
CH4
Na 2CO3
Physical properties Boiling point increases with increase in molecular mass. Further, it decreases with increase in number of branched chains. Chemical properties KMnO4 can oxidise alkanes having tertiary H atoms to corresponding alcohols. CH3 3 CH
KMnO4 Oxidation
Aromatization
Pyrolysis or cracking
CH3 3 COH
Conformations Sawhorse projections
Newman projections
Alkenes: General formula is CnH2n. Position isomer 1
2
3
4
5
C H2 = C H C H 2 C H 2 C H3 Pent-1-ene 1
2
3
4
5
C H 3 C H= C H C H 2 C H 3 Pent-2-ene
Geometrical isomerism
Preparation From alkynes
Pd/C
Lindlar’s catalyst
From alkyl halides (Dehydrohalogenation)
Physical properties Boiling point decreases with increase in number of branched chains. Chemical properties Addition of HBr to unsymmetrical alkenes (Markovnikov rule) CH3 CH=CH 2
HBr
CH3 C H CH3 | Br
Anti-Markovnikov addition (Peroxide effect or Kharash effect) CH3 CH=CH2 HBr
Ozonolysis
Alkynes: General formula is CnH2n–2. Preparation
C6 H5CO 2 O2
CH3 CH2 CH2Br
From vicinal dihalide
Physical properties All alkynes are colourless. Melting point, boiling point, and density increase with increase in molar mass. Chemical properties Acidic character HC ≡ CH > H2C=CH2 > CH3–CH3 HC ≡ CH > CH3–C ≡ CH >> CH3–C ≡ C–CH3 Addition of hydrogen HC CH H 2
Pt/Pd/Ni
H 2C=CH 2
H2
CH3 CH3
Cyclic polymerization
Aromatic hydrocarbon: Aromaticity Planarity Complete delocalization of the electrons in the ring Huckel rule Presence of (4n + 2) electrons in the ring (n = 0, 1, 2…) Preparation of benzene Cyclic polymerization of ethyne Decarboxylation of aromatic acids
Physical properties Immiscible with water but readily miscible with organic solvents Chemical properties Electrophilic substitution Nitration
Halogenation
Friedel-Crafts alkylation
Friedel-Crafts acylation
On treatment with excess of chlorine
Addition reaction
Directive influence of functional group in benzene ring Ortho and para directing groups: –OH, –NH2, –NHR, –NHCOCH3, –OCH3, –CH3, –C2H5 Meta directing groups: –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H Carcinogenicity and toxicity Benzene and polynuclear hydrocarbons containing more than two benzene rings are toxic and can cause cancer. Contribute to this Revision Note: If you find anything of importance missing from this note, email it to us at
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