Introduction
Hydrocarbons are organic compounds that contain only carbon (C) and hydrogen (H) atoms. They are the simplest class of organic compounds and form the foundation of organic chemistry.
General representation:
CxHy
Hydrocarbons are mainly obtained from natural sources like petroleum and natural gas.
Classification of Hydrocarbons
Hydrocarbons are broadly classified into three types:
1. Alkanes (Saturated Hydrocarbons)
Contain only single bonds between carbon atoms.
General formula:
CnH2n+2
Examples:
CH4, C2H6, C3H8
2. Alkenes (Unsaturated Hydrocarbons)
Contain at least one double bond.
General formula:
CnH2n
Example:
C2H4
3. Alkynes (Unsaturated Hydrocarbons)
Contain at least one triple bond.
General formula:
CnH2n−2
Example:
C2H2
4. Aromatic Hydrocarbons
Contain benzene ring.
General formula (basic):
C6H6
Alkanes
Structure and Bonding
All carbon atoms are sp3 hybridized.
Bond angle:
109.5°
Preparation of Alkanes
1. From Alkyl Halides
R − X + H → R − H
2. Wurtz Reaction
2R − X + 2Na → R − R + 2NaX
3. Decarboxylation
R − COONa + NaOH → R − H + Na2CO3
Chemical Reactions of Alkanes
1. Combustion
CnH2n+2 + O2 → CO2 + H2O
2. Halogenation
CH4 + Cl2 → CH3Cl + HCl
3. Isomerization
Straight chain → branched chain
Alkenes
Structure
sp2 hybridization
Bond angle:
120°
Preparation
1. Dehydration of Alcohol
C2H5OH → C2H4 + H2O
2. Dehydrohalogenation
R − CH2 − CH2X → R − CH = CH2 + HX
Reactions of Alkenes
1. Hydrogenation
C2H4 + H2 → C2H6
2. Halogenation
C2H4 + Br2 → C2H4Br2
3. Hydrohalogenation
C2H4 + HCl → C2H5Cl
Markovnikov’s Rule
Addition occurs such that hydrogen attaches to carbon with more hydrogens.
Anti-Markovnikov Rule
Occurs in presence of peroxide.
Alkynes
Structure
sp hybridization
Bond angle:
180°
Preparation
1. From Vicinal Dihalides
CH2Br − CH2Br → HC ≡ CH + 2HBr
Reactions of Alkynes
1. Hydrogenation
C2H2 + H2 → C2H4 → C2H6
2. Addition Reactions
C2H2 + HCl → CH2 = CHCl
Aromatic Hydrocarbons
Benzene Structure
Formula:
C6H6
Delocalized π electrons
Electrophilic Substitution Reactions
1. Nitration
C6H6 + HNO3 → C6H5NO2 + H2O
2. Halogenation
C6H6 + Cl2 → C6H5Cl + HCl
3. Friedel-Crafts Alkylation
C6H6 + RCl → C6H5R + HCl
4. Sulfonation
C6H6 + H2SO4 → C6H5SO3H + H2O
Isomerism in Hydrocarbons
Chain Isomerism
Different carbon skeleton
Position Isomerism
Different position of double/triple bond
Geometrical Isomerism
Cis and Trans forms
Physical Properties
- Alkanes are non-polar
- Boiling point increases with molecular mass
- Insoluble in water
Chemical Properties Summary
Alkanes → substitution
Alkenes → addition
Alkynes → addition
Aromatics → substitution
Important Equations Summary
Alkane formula:
CnH2n+2
Alkene formula:
CnH2n
Alkyne formula:
CnH2n−2
Combustion:
Hydrocarbon + O2 → CO2 + H2O
Markovnikov rule:
H adds to carbon with more H
Common Mistakes
- Confusing alkene and alkyne formulas
- Ignoring Markovnikov rule
- Mixing substitution and addition reactions
Exam Tips
- Learn general formulas
- Practice reaction mechanisms
- Focus on important named reactions
- Revise aromatic reactions carefully
Conclusion
Hydrocarbons are the basic building blocks of organic chemistry. Understanding their structure, preparation, and reactions is essential for mastering organic chemistry.
FAQs
Q1. What are hydrocarbons?
Compounds containing only carbon and hydrogen
Q2. What is Markovnikov rule?
Hydrogen adds to carbon with more hydrogens
Q3. What is general formula of alkanes?
CnH2n+2