Hydrocarbons

Hydrocarbons contain only carbon and hydrogen. They form the backbone of organic chemistry.

Overview

graph TD
    A[Hydrocarbons] --> B[Aliphatic]
    A --> C[Aromatic]
    B --> B1[Alkanes - CnH2n+2]
    B --> B2[Alkenes - CnH2n]
    B --> B3[Alkynes - CnH2n-2]
    C --> C1[Benzene]
    C --> C2[Substituted Benzene]

Alkanes

General Formula

$$C_nH_{2n+2}$$

Nomenclature

• Root word based on number of carbons + “-ane”
• Branch: position-name (e.g., 2-methylbutane)

Conformations of Ethane

graph LR
    A[Staggered] -->|"Most stable
0 kJ/mol"| B[Energy]
    C[Eclipsed] -->|"Least stable
12.5 kJ/mol"| B

Newman Projections:

• Staggered: dihedral angle 60°
• Eclipsed: dihedral angle 0°

Preparation

1. From unsaturated hydrocarbons:

   $$\text{Alkene/Alkyne} + H_2 \xrightarrow{Ni/Pd/Pt} \text{Alkane}$$

2. Wurtz Reaction:

   $$2RX + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$$

3. Kolbe’s Electrolysis:

   $$2RCOO^- \xrightarrow{\text{electrolysis}} R-R + 2CO_2 + 2e^-$$

4. Decarboxylation:

   $$RCOONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3$$

Halogenation Mechanism (Free Radical)

$$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$

Steps:

1. Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl\cdot$
2. Propagation: $CH_4 + Cl\cdot \rightarrow CH_3\cdot + HCl$, then $CH_3\cdot + Cl_2 \rightarrow CH_3Cl + Cl\cdot$
3. Termination: Radical recombination

Reactivity: 3° H > 2° H > 1° H (stability of radicals)

Alkenes

General Formula

$$C_nH_{2n}$$

Geometrical Isomerism

Requires:

1. Restricted rotation (C=C)
2. Different groups on each sp² carbon

cis: Same groups on same side trans: Same groups on opposite sides

Preparation

1. Dehydration of alcohols:

   $$R-CH_2-CH_2OH \xrightarrow{H_2SO_4, \Delta} R-CH=CH_2 + H_2O$$

   (Saytzeff’s rule: More substituted alkene preferred)

2. Dehydrohalogenation:

   $$R-CH_2-CH_2X \xrightarrow{\text{alc. KOH}} R-CH=CH_2 + HX$$

3. Dehalogenation:

   $$R-CHX-CHX-R' \xrightarrow{Zn} R-CH=CH-R' + ZnX_2$$

Electrophilic Addition Mechanism

Example: HBr addition

1. π electrons attack H⁺ → carbocation forms
2. Br⁻ attacks carbocation → product

Markovnikov’s Rule: In addition of HX to unsymmetrical alkene, H goes to carbon with more H atoms.

$$CH_3CH=CH_2 + HBr \rightarrow CH_3CHBr-CH_3$$

Anti-Markovnikov (Peroxide Effect)

With peroxides (HBr only):

$$CH_3CH=CH_2 + HBr \xrightarrow{R_2O_2} CH_3CH_2CH_2Br$$

Important Reactions

Reaction	Reagent	Product
Hydrogenation	H₂/Ni	Alkane
Halogenation	X₂	Vicinal dihalide
HX addition	HX	Haloalkane
Hydration	H₂O/H⁺	Alcohol
Ozonolysis	O₃, Zn/H₂O	Aldehydes/Ketones

Ozonolysis

$$\text{Alkene} \xrightarrow{O_3} \text{Ozonide} \xrightarrow{Zn/H_2O} \text{Carbonyl compounds}$$

Used to locate C=C position.

Alkynes

General Formula

$$C_nH_{2n-2}$$

Preparation

1. From vicinal dihalides:

   $$R-CHX-CHX-R' \xrightarrow{\text{alc. KOH, }\Delta} R-C\equiv C-R'$$

2. From calcium carbide:

   $$CaC_2 + 2H_2O \rightarrow HC\equiv CH + Ca(OH)_2$$

Acidic Nature

Terminal alkynes are weakly acidic (sp hybridization, 50% s-character).

$$HC\equiv CH + Na \rightarrow HC\equiv C^-Na^+ + \frac{1}{2}H_2$$

Acidity order: Alkynes > Alkenes > Alkanes

Important Reactions

Reaction	Reagent	Product
Complete hydrogenation	H₂/Ni (excess)	Alkane
Partial hydrogenation	H₂/Lindlar’s catalyst	cis-Alkene
Partial hydrogenation	Na/liq. NH₃	trans-Alkene
Addition of HX (one mole)	HX	Vinyl halide
Hydration	H₂O/H₂SO₄/HgSO₄	Carbonyl compound

Aromatic Hydrocarbons (Benzene)

Structure

• 6 carbon, 6 hydrogen, planar hexagonal
• All C-C bonds equal (1.39 Å)
• Delocalized π electrons (resonance)

Aromaticity (Hückel’s Rule)

A compound is aromatic if:

1. Cyclic
2. Planar
3. Completely conjugated
4. (4n + 2) π electrons (n = 0, 1, 2, …)

Electrophilic Aromatic Substitution

General Mechanism:

1. Generation of electrophile
2. Attack on benzene ring (arenium ion formed)
3. Loss of H⁺ (aromaticity restored)

Important Reactions

Reaction	Reagent	Product
Halogenation	X₂/FeX₃	Halobenzene
Nitration	HNO₃/H₂SO₄	Nitrobenzene
Sulphonation	H₂SO₄ (fuming)	Benzenesulphonic acid
Friedel-Crafts Alkylation	RX/AlCl₃	Alkylbenzene
Friedel-Crafts Acylation	RCOCl/AlCl₃	Ketone

Directing Effects

ortho-para Directors (Activating - except halogens): $-OH, -NH_2, -OR, -NHCOR, -R, -X$ (halogens)

meta Directors (Deactivating): $-NO_2, -CN, -CHO, -COR, -COOH, -SO_3H$

Practice Problems

1. Give the mechanism of chlorination of methane.

2. Predict the product: $CH_3CH=CHCH_3 + HBr \rightarrow ?$

3. What happens when propyne is treated with dilute H₂SO₄ in presence of HgSO₄?

4. Arrange in order of reactivity towards electrophilic substitution: Benzene, Toluene, Nitrobenzene, Phenol.

Further Reading

• Basic Principles
• Halogen Compounds