Carbon and its Compounds

What this chapter is about

Carbon is a unique element that forms the basis of all living organisms and countless useful materials around us. This chapter explores why carbon can form an enormous variety of compounds — far more than any other element — and introduces the branch of chemistry dealing with these compounds, called organic chemistry.

You will learn about the special bonding behaviour of carbon, particularly covalent bonding and the ability of carbon atoms to link with each other in chains, branches and rings. The chapter covers important classes of organic compounds including hydrocarbons (saturated and unsaturated), alcohols, carboxylic acids, and their everyday applications.

After studying this chapter, you should be able to write structural formulas of simple carbon compounds, understand why carbon forms so many compounds, explain properties like combustion and addition reactions, and recognise common carbon compounds used in daily life such as ethanol and ethanoic acid.

Key ideas

• Carbon has a valency of four, meaning each carbon atom can form four covalent bonds with other atoms, allowing it to make complex and stable molecules.
• Covalent bonds form when atoms share electrons rather than transferring them; carbon compounds typically have covalent bonds and hence low melting points, do not conduct electricity, and are generally poor conductors of heat.
• Catenation is the unique ability of carbon atoms to bond with other carbon atoms, forming long chains, branched chains, and ring structures.
• Hydrocarbons are compounds containing only carbon and hydrogen; saturated hydrocarbons (alkanes) have single bonds only, while unsaturated hydrocarbons (alkenes and alkynes) have double or triple bonds.
• A homologous series is a family of compounds with the same functional group, similar chemical properties, and a regular difference of CH₂ in molecular formula between successive members.
• Functional groups like –OH (alcohol), –COOH (carboxylic acid), and –CHO (aldehyde) determine the chemical properties of organic compounds.
• Ethanol (C₂H₅OH) is a common alcohol used in beverages, medicines and as a fuel, while ethanoic acid (CH₃COOH) is the acid present in vinegar.

Formulas and facts to remember

• General formula of alkanes: CₙH₂ₙ₊₂ (for example, methane CH₄, ethane C₂H₆, propane C₃H₈).
• General formula of alkenes: CₙH₂ₙ (for example, ethene C₂H₄, propene C₃H₆).
• General formula of alkynes: CₙH₂ₙ₋₂ (for example, ethyne C₂H₂, propyne C₃H₄).
• Combustion of hydrocarbons: Hydrocarbon + Oxygen → Carbon dioxide + Water + Heat. Complete combustion gives a blue flame; incomplete combustion produces soot (carbon) and carbon monoxide.
• Addition reaction: Unsaturated hydrocarbons add hydrogen in presence of a catalyst (nickel or palladium) to become saturated. Example: C₂H₄ + H₂ → C₂H₆ (ethene to ethane).
• Substitution reaction: Saturated hydrocarbons undergo substitution; for example, methane reacts with chlorine in sunlight: CH₄ + Cl₂ → CH₃Cl + HCl.
• Esterification: Alcohol + Carboxylic acid → Ester + Water. This reaction occurs in presence of an acid catalyst.
• Saponification: Fat or oil + Sodium hydroxide → Soap + Glycerol. This is the process of making soap.
• Denatured alcohol: Ethanol made unfit for drinking by adding poisonous substances like methanol or copper sulphate.

Worked examples

Example 1: Writing molecular and structural formula

Question: Write the molecular formula and draw the structural formula of butane, the fourth member of the alkane series.

Solution: Using the general formula CₙH₂ₙ₊₂ with n = 4: Number of hydrogen atoms = 2 × 4 + 2 = 10 Molecular formula: C₄H₁₀

Structural formula (chain structure):

    H   H   H   H

 ·  · 

H — C — C — C — C — H

 ·  · 

    H   H   H   H

Each carbon uses its four bonds: bonding to neighbouring carbons and to hydrogen atoms.

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Example 2: Identifying a homologous series

Question: The compounds CH₃OH, C₂H₅OH, and C₃H₇OH belong to the alcohol family. Show that they form a homologous series.

Solution: Step 1: Check the functional group — all three have the –OH group attached to a carbon chain. Step 2: Find the difference in molecular formula between successive members.

• From CH₃OH to C₂H₅OH: difference is CH₂
• From C₂H₅OH to C₃H₇OH: difference is CH₂

Step 3: The molecular mass increases by 14 units (12 for C + 2 for H₂) for each successive member.

Since they have the same functional group and differ by CH₂, they form a homologous series.

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Example 3: Balancing a combustion equation

Question: Write the balanced equation for the complete combustion of ethanol (C₂H₅OH).

Solution: Unbalanced equation: C₂H₅OH + O₂ → CO₂ + H₂O

Step 1: Balance carbon atoms — 2 carbons on left, so put 2 before CO₂. C₂H₅OH + O₂ → 2CO₂ + H₂O

Step 2: Balance hydrogen atoms — 6 hydrogens on left, so put 3 before H₂O. C₂H₅OH + O₂ → 2CO₂ + 3H₂O

Step 3: Balance oxygen atoms — right side has (2 × 2) + 3 = 7 oxygen atoms. Left side has 1 oxygen in ethanol, so O₂ must provide 6 atoms, meaning 3 molecules. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

This is the balanced equation for complete combustion of ethanol.

Common mistakes

• Thinking carbon forms ionic bonds like sodium → Carbon shares electrons to form covalent bonds because it has four valence electrons, making it easier to share than gain or lose.
• Writing wrong number of bonds for carbon → Always check that each carbon atom makes exactly four bonds in structural formulas.
• Confusing saturated and unsaturated hydrocarbons → Saturated means only single bonds (alkanes); unsaturated means double or triple bonds (alkenes, alkynes).
• Forgetting to balance oxygen atoms last in combustion equations → Balance C first, then H, then count total oxygen needed on the product side.
• Mixing up ethanol and ethanoic acid → Ethanol (C₂H₅OH) is an alcohol; ethanoic acid (CH₃COOH) is a carboxylic acid with sour taste and acidic properties.

Quick revision

• Carbon has valency 4 and forms covalent bonds by sharing electrons.
• Catenation allows carbon to form chains, branches and rings with other carbon atoms.
• Alkanes are saturated (CₙH₂ₙ₊₂); alkenes and alkynes are unsaturated.
• Functional groups like –OH, –COOH, and –CHO give organic compounds their characteristic properties.
• Addition reactions convert unsaturated compounds to saturated; substitution reactions occur in saturated compounds.
• Soaps are sodium or potassium salts of long-chain carboxylic acids, made by saponification.