Classification of Hydrocarbons

1.0What are Hydrocarbons?

Hydrocarbons are organic compounds that are entirely made up of only two kinds of atoms – carbon and hydrogen. Typically, hydrocarbons are colourless gases that have very weak odours. Hydrocarbons can feature simple or relatively complex structures and can be generally classified into four subcategories, namely alkanes, alkenes, alkynes, and aromatic hydrocarbons. The study of hydrocarbons can provide insight into the chemical properties of other functional groups and their preparation. 

2.0Types of Hydrocarbons

• Saturated Hydrocarbons: In these compounds, carbon-carbon atoms and carbon-hydrogen atoms are held together by single bonds. These single-bonded compounds are the simplest hydrocarbons. These types of hydrocarbons don’t have double or triple bonds. In terms of hybridization, they have $S{p}^3$ hybridised carbon atoms with no $S{p}^2$ or Sp hybridised carbon atoms. They are together called alkanes which have a general formula of $C_n{H}_{2n+2}$. For example, $C{H}_4{C}_3{H}_6.$
• Unsaturated Hydrocarbons: These compounds consist of a single, double or triple bond between carbon-carbon atoms. The double-bonded compounds are called alkenes, and the triple-bonded compounds are called alkynes. The general formula for alkenes is $C_n{H}_{2n}$, and for alkynes, the general formula is $C_n{H}_{2n-2}$.
• Cycloalkanes: These hydrocarbons possess one or multiple carbon rings. The hydrogen atom is attached to the carbon ring.
• Aromatic Hydrocarbons: They are also called arenes. Arenes are compounds which consist of at least one aromatic ring.
• Aliphatic Hydrocarbons: They are straight chain structures having no rings in them.
• Alicyclic Hydrocarbons: They are hydrocarbons having a ring structure in them. The carbons atoms can be Sp, $S{p}^2$, or $S{p}^3$ hybridised.

3.0Classification of Hydrocarbons

Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are classified into two broad categories: Aliphatic (Acyclic) and Alicyclic, based on their structure and bonding. Each category is further divided into specific subtypes.

Aliphatic / Acyclic Hydrocarbons

These are open-chain hydrocarbons that may be either straight or branched. They do not contain any cyclic structure.

(a) Saturated Hydrocarbons

• Also called Alkanes.
• Contain only single bonds between carbon atoms.
• General formula: $C_n{H}_{2n+2}.$
• Example: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈).
• They are relatively less reactive and undergo substitution reactions.

(b) Unsaturated Hydrocarbons

• Contain double bonds (alkenes) or triple bonds (alkynes) between carbon atoms.
• General formula for alkenes: $C_n{H}_{2n}$
• General formula for alkynes: $C_n{H}_{2n-2}$
• Example: Ethene (C₂H₄), Propyne (C₃H₄).
• More reactive than alkanes, undergoing addition reactions.

2. Alicyclic Hydrocarbons

These are cyclic hydrocarbons that contain carbon atoms arranged in a ring structure. Depending on the atoms present and the type of bonding, they are classified into two types:

(a) Homocyclic Compounds

• Rings are formed only by carbon atoms.
• Can be further classified into:

1. Aromatic Compounds
2. • Possess a conjugated π-electron system (alternating double bonds).
   • Follow Hückel’s rule (4n + 2 π electrons).
   • Highly stable due to resonance.
   • Example: Benzene (C₆H₆), Toluene, Naphthalene.
2. Non-Aromatic Compounds
3. • Cyclic but do not have conjugated double bonds.
   • Lack resonance stability.
   • Example: Cyclohexane, Cyclopentane.
3. Anti-Aromatic Compounds
4. • Cyclic compounds with conjugated π-electrons but follow the 4n rule instead of Hückel’s rule.
   • Highly unstable due to electron delocalization.
   • Example: Cyclobutadiene, Cyclopentadienyl cation.

(b) Heterocyclic Compounds

• Rings contain carbon atoms along with one or more heteroatoms (such as nitrogen, oxygen, or sulfur).
• Can be aromatic or non-aromatic.
• Examples: Pyridine (aromatic), Furan (aromatic), Tetrahydrofuran (non-aromatic).

4.0Properties of Hydrocarbons

Hydrocarbons show both physical and chemical properties that vary based on saturation and structure.

Physical Properties

• State: Lower alkanes are gases; higher alkanes are liquids or solids
• Boiling and melting points: Increase with molecular weight
• Solubility: Insoluble in water, soluble in organic solvents
• Density: Generally less than water

Chemical Properties

Alkanes:

• Combustion: Produce CO₂ and H₂O
  $C{H}_4+2O_2\to C{O}_2+2H_{2}O$
• Substitution reactions with halogens (free radical mechanism)

Alkenes:

• Addition reactions: Halogens, hydrogen halides, water (hydration)
• Polymerization to produce plastics

Alkynes:

• Addition reactions similar to alkenes
• Acidic hydrogen in terminal alkynes reacts with metals

Aromatic hydrocarbons:

• Electrophilic substitution reactions: Nitration, sulfonation, halogenation

5.0Preparation of Hydrocarbons

Preparation of Hydrocarbons – Alkanes

From Alkenes and Alkynes

• Alkanes are prepared by catalytic hydrogenation of alkenes/alkynes.
• In presence of Ni, Pt, or Pd catalysts, hydrogen adds across the multiple bond.
  $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2\stackrel{{\mathrm{H}}_2/Ni}{\to }\mathrm{C}{\mathrm{H}}_3-\mathrm{C}{\mathrm{H}}_3$
• This reaction is known as the Sabatier-Senderens reaction.

From Alkyl Halides

• Alkyl halides can be reduced to alkanes using:
• 1. Zn/Protic solvents
  2. Coupling reactions (e.g., Wurtz reaction – yields alkanes with even carbons).
  3. Reducing agents (LiAlH₄, NaBH₄, NaNH₂).
     $R-X\stackrel{[H]}{\to }R-H$

From Aldehydes and Ketones

• Conversion to alkanes via reduction methods:
• • Clemmensen Reduction (Zn/Hg in HCl).
  • Wolff–Kishner Reduction (NH₂NH₂/KOH).

From Carboxylic Acids

• Decarboxylation reactions yield alkanes:
• • Kolbe’s Electrolysis.
  • Heating with soda lime.

Preparation of Hydrocarbons – Alkenes

General Formula: CₙH₂ₙ

By Elimination Reactions (β-elimination)

• E2 mechanism
• • One-step, second-order.
  • Favoured in aprotic solvents.
  • Less substituted alkene (Hofmann product).
• E1 mechanism
• • Two-step, first-order.
  • Favoured in protic solvents.
  • More substituted alkene (Zaitsev product).

From Alcohols (Dehydration)

• Acid catalysed dehydration using conc. H₂SO₄ or Al₂O₃.

From Alkyl Halides (Dehydrohalogenation)

• Treatment with alcoholic KOH removes HX to give alkenes.

Addition Reactions

• Hydration of alkenes gives alkanes (reverse route).

Preparation of Hydrocarbons – Alkynes

From Dihalides

• Vicinal or geminal dihalides undergo double dehydrohalogenation with alcoholic KOH to form alkynes.
  $\mathrm{C}{\mathrm{H}}_2\mathrm{Br}-\mathrm{C}{\mathrm{H}}_2\mathrm{Br}\stackrel{alc.KOH}{\to }\mathrm{CH}\equiv \mathrm{CH}$

From Alcohols

• Alcohols → Alkyl halides → Alkynes (via dehydrohalogenation).

From Calcium Carbide

• CaC₂ reacts with water to give acetylene (ethyne).
  $\mathrm{Ca}{\mathrm{C}}_2+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{H}}_2+\mathrm{Ca}(\mathrm{OH}{)}_2$

Preparation of Hydrocarbons – Aromatic (Benzene)

From Ethyne

• Ethyne undergoes trimerisation at 600 °C in presence of red-hot iron to give benzene.

From Phenol

• Phenol reduced by Zn dust gives benzene.

From Aniline

• Aniline undergoes diazotisation followed by reduction to form benzene.

6.0Uses of Hydrocarbons

Hydrocarbons have widespread industrial and domestic applications.

Alkanes:

• Fuel: LPG, natural gas, petrol, diesel
• Raw materials in plastics and synthetic fibers

Alkenes:

• Manufacture of polymers: Polyethylene, polypropylene
• Production of alcohols, aldehydes, acids

Alkynes:

• Welding and cutting torches (C₂H₂)
• Manufacture of synthetic rubber and pharmaceuticals

Aromatic hydrocarbons:

• Solvents: Benzene, toluene
• Precursors for dyes, drugs, detergents, and explosives