The language of organic chemistry

A hydrocarbon is a compound of hydrogen and carbon only. Organic compounds are sorted into homologous series — families that:

• share the same functional group (the bit of the molecule that decides how it reacts),
• fit one general formula, and
• have a steady trend in properties as the chain gets longer.

You represent molecules in several ways. For propane:

Empirical formula = simplest whole-number ratio of atoms (CH₂ for C₂H₄). Isomerism = same molecular formula, different arrangement of atoms (e.g. butane vs methylpropane, both C₄H₁₀). You name compounds with up to six carbons using IUPAC rules.

Reactions of organic compounds are classified as substitution, addition or combustion — no mechanisms needed.

What makes a homologous series?

Crude oil & fractional distillation

Crude oil is a mixture of hydrocarbons (mostly alkanes). It is separated by fractional distillation in a column that is hot at the bottom, cool at the top. Each fraction is a group of hydrocarbons that condense over a similar boiling range:

The trend down the column: longer chains → higher boiling point, more viscous (thicker), and darker in colour. A fuel is a substance that releases heat energy when burned.

Burning fuels — and the pollutants

Complete combustion (plenty of oxygen) gives only carbon dioxide + water:

Incomplete combustion (too little oxygen) also makes carbon monoxide (CO) and soot (carbon particulates):

• CO is poisonous — it reduces the capacity of the blood to carry oxygen.
• Soot blackens surfaces and causes breathing problems.
• Sulfur impurities in the fuel burn to SO₂, which causes acid rain.
• In hot car engines, nitrogen and oxygen from the air react to form oxides of nitrogen (NOₓ) — these also cause acid rain.

Watch out: CO₂ and water are the only products of complete combustion. CO and soot are signs of incomplete combustion. SO₂ comes from sulfur in the fuel; NOₓ forms from air in the engine.

Reading a sooty flame

Cracking — supply meets demand

Distillation makes too much of the long-chain fractions and not enough useful short-chain ones. Cracking breaks long-chain alkanes into shorter alkanes + alkenes:

Catalytic cracking conditions: a silica or alumina catalyst at 600–700 °C. The alkenes produced are valuable feedstock for making polymers.

Why crack? It balances supply and demand — turning surplus heavy fractions into the petrol-range alkanes and reactive alkenes that industry actually wants. Notice an alkene is always formed so the atoms balance.

Alkanes — saturated & unreactive

Alkanes are saturated hydrocarbons — only single C–C bonds, so they hold the maximum number of hydrogens. General formula:

Reaction with halogens: alkanes are unreactive, but with chlorine or bromine in UV light they undergo substitution (one H is swapped for a halogen):

Your turn — an alkane formula

Alkenes — the C=C double bond

Alkenes contain the functional group >C=C<. That double bond makes them unsaturated — they hold fewer hydrogens than the matching alkane. General formula:

Don't mix them up: an alkane (CₙH₂ₙ₊₂) is saturated with single bonds and does substitution; an alkene (CₙH₂ₙ) is unsaturated with a C=C and does addition, where the double bond opens up.

The bromine water test

Alkenes undergo addition with bromine — the C=C opens and a bromine atom adds to each carbon, making a colourless dibromoalkane:

This is the test that tells an alkane from an alkene. Shake each with orange bromine water:

Other additions: alkenes also add hydrogen (with a nickel catalyst → an alkane) and steam (with a phosphoric acid catalyst → an alcohol, e.g. ethene + steam → ethanol).

Which tube decolourises?

Alcohols — the –OH group

Alcohols contain the functional group –OH (hydroxyl). The first four are methanol, ethanol, propanol, butanol. Ethanol (in drinks, fuels and solvents) is the one you study in detail:

Three reactions of ethanol:

• Combustion — burns in air: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
• With sodium — fizzes, giving hydrogen gas (like a very weak acid).
• Oxidation — to ethanoic acid, either by microbial oxidation (air + microbes) or by heating with potassium dichromate(VI) in dilute sulfuric acid.

Making ethanol — two routes

Ethanol can be manufactured two different ways. You must compare the conditions:

Why those fermentation conditions? No air stops the ethanol oxidising to vinegar; ~30 °C is the optimum for the yeast enzymes (too cold = slow, too hot = the enzymes denature).

Which route is this?

Carboxylic acids — the –COOH group

Carboxylic acids contain the functional group –COOH (carboxyl). The first four are methanoic, ethanoic, propanoic, butanoic acid. Ethanoic acid is the acid in vinegar:

Carboxylic acids are weak acids (only partly ionise in water), but they still show typical acid reactions:

• With metals (e.g. magnesium) → a salt + hydrogen gas.
• With metal carbonates → a salt + water + carbon dioxide (fizzing).

Family ↔ functional group

Tap a family on the left, then its functional group on the right.

Esters — the sweet smells

An alcohol + a carboxylic acid react (with an acid catalyst, e.g. concentrated sulfuric acid) to make an ester + water. The ester functional group is –COO–:

Naming: the alcohol gives the first part (ethyl), the acid gives the second (ethanoate) → ethyl ethanoate. Esters are volatile with distinctive smells — used in food flavourings and perfumes.

Name that ester

Addition polymers

In addition polymerisation, many alkene monomers join — the C=C double bonds open up and link into one long chain, with no other product:

Other addition polymers: poly(propene), poly(chloroethene) (PVC) and poly(tetrafluoroethene) (PTFE). You can work a repeat unit ↔ its monomer either way.

Disposal problems: addition polymers are inert and do not biodegrade (they fill landfill for centuries), and burning them makes toxic gases (e.g. HCl from PVC, and carbon monoxide).

Monomer → repeat unit

Condensation polymers

In condensation polymerisation the monomers each have two functional groups. A dicarboxylic acid + a diol join to make a polyester — and a small molecule, water, is released each time a link forms:

Addition vs condensation: addition uses one monomer (an alkene) and makes nothing else; condensation uses two monomers (each with two groups) and releases a small molecule (water). Some polyesters, called biopolyesters, are biodegradable.

Which polymerisation?

Saturated or unsaturated?

Tap a molecule, then tap the box it belongs in. (Look for a C=C.)

Section 4 at a glance

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