Hydrocarbons
, fundamental to organic chemistry, are compounds composed solely of hydrogen and carbon atoms. Classified into alkanes, alkenes, and alkynes based on carbon-carbon bonding, they serve as the building blocks for countless organic substances. Alkanes exhibit single bonds, alkenes have double bonds, and alkynes possess triple bonds. With diverse applications, from fuels like gasoline to plastics and pharmaceuticals, hydrocarbons play a pivotal role in various industrial and scientific fields.
Alkanes (Paraffins)
General Formula:
C
n
H
2n+2
Hybridization:
sp
3
Physical Properties:
Non-polar, insoluble in water, less dense than water, melting and boiling points increase with increasing molecular weight.
Preparation:
Reduction of alkyl halides:
R−X + H
2
→ R−H + HX
From unsaturated hydrocarbons:
RCH=CH
2
+ H
2
→ RCH
2
CH
3
Chemical Properties:
Combustion:
CH
4
+ 2O
2
→ CO
2
+ 2H
2
O
Halogenation:
CH
4
+ Cl
2
→ CH
3
Cl + HCl
Isomerization: Changing straight-chain alkanes to branched isomers.
n−C
4
H
10
→ iso−C
4
H
10
Pyrolysis (Cracking): Breaking down larger alkanes to smaller ones by heating.
C
8
H
18
→ C
6
H
14
+ C
2
H
4
Oxidation: In the presence of oxygen and heat or light, alkanes slowly oxidize to form alcohols, ketones, and carboxylic acids.
Download PDF Hydrocarbons Formula
Alkenes (Olefins)
General Formula
C
n
H
2n
Hybridization:
sp
2
Physical Properties:
Similar to alkanes but with slightly higher reactivity due to the double bond.
Preparation:
Alcohol dehydration:
CH
3
CH
2
OH → CH
2
=CH
2
+ H
2
O
Halide dehydrohalogenation:
CH
3
CH
2
Br + KOH → CH
2
=CH
2
+ KBr + H
2
O
Chemical Properties:
Addition of halogens:
CH
2
=CH
2
+ Br
2
→ CH
2
BrCH
2
Br
Hydrogenation:
CH
2
=CH
2
+H
2
→CH
3
CH
3
Hydration: Addition of water in the presence of an acid.
CH
2
=CH
2
+H
2
O→CH
3
CH
2
OH
Polymerization: Forming polymers.
n CH
2
=CH
2
→ [−CH
2
−CH
2
−] n
Oxidation: With cold dilute KMnO
4
, alkenes are converted to glycols.
CH
2
=CH
2
+ KMnO
4
→CH
2
(OH)−CH
2
(OH)
Also Check –
Discovery of Proton Formula
Alkynes (Acetylenes)
General Formula:
C
n
H
2n−2
Hybridization:
sp
Physical Properties:
Similar to alkanes and alkenes, but even more reactive due to the triple bond.
Preparation:
From calcium carbide:
CaC
2
+2H
2
O →C
2
H
2
+ Ca(OH)
2
From vicinal dihalides:
CH
2
Br−CH
2
Br + 2NaNH
2
→ CH≡CH + 2NaBr + 2NH
3
Chemical Properties:
Addition of halogens:
CH≡CH+2Br
2
→CHBr
2
−CHBr
2
Hydrogenation:
CH≡CH+2H
2
→CH
3
CH
3
Acidic Hydrogen: Terminal alkynes have acidic hydrogen which reacts with strong bases.
CH ≡ CH + NaNH
2
→ CH≡C−Na + NH
3
Addition of Water: Produces ketones.
CH
3
−C≡CH + H
2
O → CH
3
−CO−CH
3
Oxidation: With KMnO
4
, alkynes are cleaved to form carboxylic acids or carbon dioxide.
CH
3
−C≡C−CH
3
+ KMnO
4
→2CH
3
−COOH
Also Check –
Nucleophile Formula
Benzene
General Formula:
C
6
H
6
Hybridization:
sp
2
Physical Properties:
Aromatic odor, non-polar, insoluble in water, more stable than expected due to delocalization.
Preparation:
From acetylene:
3C
2
H
2
→C
6
H
6
Decarboxylation:
C
6
H
5
COOH→C
6
H
6
+CO
2
Chemical Properties:
Halogenation (in presence of halogen carrier):
C
6
H
6
+ Cl
2
→ C
6
H
5
Cl + HCl
Nitration:
C
6
H
6
+HNO
3
→ C
6
H
5
NO
2
+H
2
O
Alkylation (Friedel-Crafts): Introduction of an alkyl group using a Lewis acid.
C
6
H
6
+ RCl + AlCl
3
→C
6
H
5
R + HCl
Acylation (Friedel-Crafts): Introduction of an acyl group.
C
6
H
6
+RCOCl+ AlCl
3
→C
6
H
5
COR + HCl
Sulfonation: Adding a sulfonyl group to benzene.
C
6
H
6
+ H
2
SO
4
→C
6
H
5
SO
3
H + H
2
O
Desulfonation: Removal of sulfonyl group under hydrolytic conditions.
C
6
H
5
SO
3
H + H
2
O→ C
6
H
6
+ H
2
SO
4
Oxidation: With strong oxidizing agents like
KMnO
4
, side chains on benzene are oxidized to carboxylic acid groups. Benzene ring remains intact.
Also Check –
Modern Periodic Table Formula
Aromaticity
Aromatic molecules are highly stable, cyclic compounds with delocalized π electrons. This delocalization provides additional stability beyond what would be expected based on simple resonance structures. The classic example of an aromatic compound is benzene.
Criteria for Aromaticity:
Cyclic Structure:
The molecule must be cyclic.
Planarity:
All atoms in the ring must be in the same plane. Complete
Delocalization:
The π electrons must be completely delocalized over the entire ring. This is typically accomplished through conjugated double bonds.
Hückel Rule:
The molecule must have a total of 4n+2 π electrons (where n is a non-negative integer including zero).
Examples:
Benzene (C
₆
H
₆
): 6 π electrons
(from three double bonds), which fits the
4n+2
rule when n = 1. Thus, benzene is aromatic.
Cyclobutadiene:
4 π electrons,
which doesn't fit the
4n+2 rule
. Thus, cyclobutadiene is not aromatic. In fact, it's anti-aromatic.
Cyclooctatetraene
(C
₈
H
₈
): 8 π electrons
, which also doesn't fit the 4n+2 rule. However, it is neither aromatic nor anti-aromatic. Instead, it adopts a non-planar conformation to avoid anti-aromaticity.
