Class 10 Carbon Compounds — Homologous Series Explained

Carbon and Its Compounds is the chapter where Class 10 students meet organic chemistry for the first time, and the idea that makes the whole chapter manageable is the homologous series. Once you see that carbon compounds arrive in orderly families rather than as hundreds of unrelated names, the formulae stop being something to memorise and start being something you can derive in the exam hall.

Why carbon forms so many compounds

Two properties do all the work.

• Catenation — carbon atoms bond strongly to other carbon atoms, forming long chains, branched chains and rings. Because the carbon–carbon bond is short and strong, these chains are stable. Silicon, just below carbon in the same group, catenates far less and its chains break easily.
• Tetravalency — carbon has four electrons in its outermost shell, so it forms four covalent bonds. That lets it bond to four other atoms, which is why so many different arrangements are possible.

Carbon also forms strong bonds with hydrogen, oxygen, nitrogen, sulphur and the halogens, which is where all the functional groups come from.

Saturated and unsaturated

A saturated carbon compound has only single bonds between carbon atoms — these are the alkanes. An unsaturated compound contains at least one double bond (alkenes) or triple bond (alkynes). Unsaturated compounds are more reactive, and they burn with a sooty, yellow flame because the higher proportion of carbon does not get enough oxygen for clean combustion. Saturated hydrocarbons burn with a clean blue flame in a good air supply.

What a homologous series actually is

Four consequences follow, and every one of them is examinable:

1. Successive members differ by one CH₂ unit, so their molar masses differ by a fixed amount — 14.027 g/mol (12.011 for carbon plus 2 × 1.008 for hydrogen).
2. All members show similar chemical properties, because the functional group is the same and the functional group decides the chemistry.
3. Physical properties change gradually down the series — melting point, boiling point and density rise steadily as the chain gets longer and the molecules attract each other more.
4. All members can be written from a single general formula, so you never have to memorise individual formulae.

The general formulae you must know

Two of these have a starting point students often get wrong. An alkene needs two carbons to hold a C=C bond, so the series begins at C₂H₄ — there is no one-carbon alkene. For the same reason there is no one-carbon alkyne. A ketone needs the C=O to sit between two carbon atoms, so the ketone series begins at three carbons, with propanone.

Worked example 1 — the first four alkanes, and the 14 g/mol step

Worked example 2 — the same step in the alcohols

Worked example 3 — carboxylic acids, and a warning about similar masses

How physical properties change down a series

As the chain lengthens, the molecules become bigger and the forces between them grow stronger, so more energy is needed to separate them. That is why the first few alkanes are gases at room temperature, the middle members are liquids, and the long-chain members are waxy solids. Solubility in water moves the opposite way for alcohols and acids: methanol and ethanol mix freely with water because the –OH group forms hydrogen bonds, but as the hydrocarbon chain grows it dominates the molecule and solubility falls.

Chemical properties, by contrast, barely change. Ethanoic acid and butanoic acid both turn blue litmus red, both react with sodium carbonate to give carbon dioxide, and both form esters with alcohols — because both carry –COOH.

Mistakes Class 10 students make

Where this appears in your exam