Chapter 9 - Hydrocarbons

Hydrocarbons are compounds containing only carbon and hydrogen. They provide the carbon skeletons from which many other organic compounds are derived. This chapter applies hybridisation, electronic effects and reaction-mechanism ideas to alkanes, alkenes, alkynes and aromatic hydrocarbons.

Classification

• Alkanes are saturated hydrocarbons containing only single bonds.

• Alkenes contain at least one carbon-carbon double bond.

• Alkynes contain at least one carbon-carbon triple bond.

• Aromatic hydrocarbons contain specially stabilised cyclic electron systems such as benzene.

Alkanes

Students study nomenclature, chain isomerism and preparation through hydrogenation, reduction of alkyl halides, Wurtz reaction, decarboxylation and Kolbe electrolysis.

Important reactions include combustion, controlled oxidation, halogenation, isomerisation, aromatisation and pyrolysis.

Free-radical halogenation proceeds through initiation, propagation and termination.

Conformations of ethane are shown using Newman and sawhorse projections. The staggered conformation is more stable than the eclipsed form because it has lower torsional strain.

Alkenes

A double bond contains one sigma and one pi bond. The weaker, exposed pi bond makes alkenes more reactive than alkanes.

Preparation methods include dehydration of alcohols, dehydrohalogenation of alkyl halides, dehalogenation of vicinal dihalides and partial reduction of alkynes.

Important reactions include hydrogenation, halogenation, addition of hydrogen halides, hydration, oxidation, ozonolysis and polymerisation.

Markovnikov’s rule predicts the major product of addition to an unsymmetrical alkene. In the presence of peroxide, HBr can add in the anti-Markovnikov direction through a radical mechanism.

Ozonolysis breaks the double bond to form carbonyl compounds and can reveal the original position of unsaturation.

Alkynes

A triple bond contains one sigma and two pi bonds. The carbon atoms are sp hybridised and the bond is linear.

Students study preparation, addition reactions, oxidation and polymerisation. Terminal alkynes show acidic character because their conjugate bases place negative charge on an sp-hybridised carbon with high s-character.

Aromatic hydrocarbons

Benzene is planar and contains delocalised pi electrons. Its carbon-carbon bonds are equivalent because the molecule is a resonance hybrid.

Benzene commonly undergoes electrophilic substitution:

• Nitration

• Sulphonation

• Halogenation

• Friedel–Crafts alkylation

• Friedel–Crafts acylation

An existing substituent affects both ring reactivity and the position of further substitution. Groups may be ortho-para directing or meta directing.

The chapter also discusses the toxicity and carcinogenicity of certain aromatic hydrocarbons.

What can students study on Teachoo?

• Nomenclature and classification

• Preparation reactions

• Free-radical halogenation

• Ethane conformations

• Markovnikov’s rule

• Peroxide effect

• Ozonolysis

• Terminal-alkyne acidity

• Benzene reactions

• Directive influence

• Organic conversions

• NCERT solutions, MCQs and case-based questions

Common mistakes

• Treating every addition as Markovnikov addition

• Applying the peroxide effect to HCl or HI

• Forgetting reaction conditions

• Confusing substitution with addition in benzene

• Predicting directive influence without identifying the existing group

• Memorising conversions without tracking the carbon skeleton

Frequently asked questions

What are hydrocarbons?

They are organic compounds made only of carbon and hydrogen.

Why are alkenes more reactive than alkanes?

Alkenes contain a weaker and more exposed pi bond that is readily attacked.

What is Markovnikov’s rule?

It predicts the major orientation of addition of an unsymmetrical reagent to an unsymmetrical alkene, based on intermediate stability.

Does the peroxide effect occur with every hydrogen halide?

No. At this level it is associated with HBr under radical-forming conditions.

Why are terminal alkynes acidic?

Their conjugate bases place negative charge on sp-hybridised carbon, which holds electrons relatively close to the nucleus.

Why does benzene prefer substitution?

Substitution permits restoration of aromatic stability, whereas addition would destroy it.

How should hydrocarbon reactions be revised?

Make separate reaction maps for alkanes, alkenes, alkynes and benzene. Organise each map by preparation, substitution, addition, oxidation and conversion.

Learn Hydrocarbons with Teachoo for reaction mechanisms, named rules, benzene chemistry, conversions and complete NCERT solutions.