Introduction to Organic Chemistry

Organic compounds can look complicated, but you can sort them by a small number of structural features. First learn to recognise the family, then use its shared pattern to predict formulae and reactions.

Families, groups and hydrocarbons

A homologous series is a family

Members of a homologous series share a general formula, have similar chemical properties, and show trends in physical properties. Successive members differ in molecular formula by CH₂: one extra carbon and two extra hydrogens.

Members share the same functional group when the series has one. For example, alcohols contain the hydroxyl group, -OH. Alkanes are a homologous series too, but have no characteristic functional group: their shared structure is a saturated, open-chain hydrocarbon.

A functional group is part of the structure

A functional group is an atom or group of atoms responsible for characteristic chemical reactions. The carbon–carbon double bond, C = C, is the characteristic feature of alkenes.

“Alcohols” is a family name; “hydroxyl group” names a feature in each alcohol molecule. Keep those two levels distinct.

A hydrocarbon contains only carbon and hydrogen

Methane, CH₄, is a hydrocarbon. Ethanol, C₂H₅OH, is organic but is not a hydrocarbon because it also contains oxygen. Carbon oxides and carbonates are treated as inorganic compounds. Containing carbon alone does not identify a compound’s family.

Use both the formula and the structure

A molecular formula counts atoms. A structural formula also shows how they are connected. A general formula expresses the atom-count pattern for a family in terms of n, the number of carbon atoms.

For example, the open-chain alkanes follow CₙH₂ₙ₊₂. At n = 3, this gives C₃H₈. Their carbon atoms are connected by single bonds. Open-chain alkenes with one carbon–carbon double bond follow CₙH₂ₙ; at n = 3, this gives C₃H₆.

The formula is a useful check, but the structure tells you where the bonds and groups are. Do not classify an unfamiliar structure solely by counting carbon atoms.

Connect structure to behaviour

What members share, and what changes

A CH₂ difference is a comparison of formulae, not a reaction in which one molecule turns into another by adding a free CH₂ particle.

Recognise the whole functional group

A carbon–oxygen double bond, C = O, is called a carbonyl group. A hydroxyl group is -OH. Notice how those features are connected in each family.

In the map, R and R′ represent carbon-containing parts of the molecule; they need not be identical. Methanoic acid has H in place of the acid’s R group. Its esters can also have H at that position. These are still acids and esters because the characteristic group is unchanged.

The -OH in a carboxyl group is not a separate alcohol group. Look at its neighbour: in an acid it is attached to the carbon that is also double-bonded to oxygen. In an ester, that H is replaced by a carbon-containing group.

Why physical properties vary

Larger members of a series generally have stronger intermolecular attractions, so their boiling points tend to rise. Liquid members often become more viscous: they resist flow more. Melting points can show a less smooth pattern because molecular packing also matters.

These are physical changes in properties across a series, not new functional groups. Boiling separates molecules; it does not normally break the covalent bonds within them. See Alkanes for a boiling-point graph.

Build the simplest names

These patterns introduce unbranched names. Some structures also need a position number, such as but-1-ene and but-2-ene, because the double bond can occur at different positions. The family lessons develop structures and naming further. There is no methene: a carbon–carbon double bond needs two carbon atoms.

Avoid three shortcuts

• Organic does not mean hydrocarbon: ethanol contains oxygen as well as carbon and hydrogen.
• Family is not group: alcohols are a family; -OH is a structural feature.
• The letters OH alone are not enough: distinguish an alcohol’s hydroxyl group from an acid’s complete carboxyl group.

Explain what the structure shows

6. Worked Examples

Modelled example 1

Organic or not?

Core

Problem

Which of the following are organic compounds in this syllabus context: CH₄, CO₂, CaCO₃ and C₂H₅OH?

Study the worked solution

1. Identify the usual organic compounds

   Method

   Select CH₄ and C₂H₅OH.

   Reason

   Methane is an alkane and ethanol is an alcohol: both belong to organic families. Hydrogen content is not a universal definition of an organic compound.

   Working

   Organic: CH₄ and C₂H₅OH.

2. Apply the syllabus exclusions

   Method

   Exclude the carbon oxide and carbonate.

   Reason

   Carbon oxides and carbonates are not counted as organic compounds in this syllabus context.

   Working

   Not organic here: CO₂ and CaCO₃.

Guided practice 2

Spot successive members

About 5 min

Problem

Are C₂H₆ and C₃H₈ successive members of the same homologous series? Show the key check.

Compare the two molecular formulae

Formula difference

CH2CHC2H2

Homologous series

AlkanesAlkenesAlcohols

Check my attempt

Hints

Hint 1: subtract formulae

Compare the carbon and hydrogen subscripts separately.

Hint 2: check the general formula

For n = 2 and n = 3, test CₙH₂ₙ₊₂.

View solution step by step

1. Find the formula difference

   Method

   Compare the carbon and hydrogen counts separately.

   Reason

   Successive members of a homologous series must differ by CH₂.

   Working

   3-2 = 1 extra carbon and 8-6 = 2 extra hydrogens: a CH₂ difference.

2. Confirm the series

   Method

   Identify both compounds as alkanes.

   Reason

   Both fit the alkane general formula CₙH₂ₙ₊₂.

   Working

   Yes; they are successive members of the alkane homologous series.

Common misconception 3

Functional group identification

Find and correct the mistake

Learner claim

Ethanol has formula C₂H₅OH. A learner says, “Its homologous series is -OH and its functional group is alcohol.” Correct the two labels.

Separate the structural feature from the family name

Functional group

−OHalcohol

Homologous series

alcohols−OH

Check my attempt

View solution step by step

1. Name the functional group

   Method

   Identify -OH as the functional group.

   Reason

   A functional group is an atom or group of atoms in the molecule responsible for characteristic reactions.

   Working

   Functional group: -OH (hydroxyl).

2. Name the homologous series

   Method

   Identify the family as alcohols.

   Reason

   A homologous series is the family of compounds sharing a functional group and general formula.

   Working

   Homologous series: alcohols.

Examiner practice 4

Naming from carbon count + series

3 marks

Examination question

Name the alkane with three carbon atoms and give its structural formula in one line. [3 marks]

Write the name and structural formula

Write your answer

View solution step by step

1. Choose the carbon prefix

   1 mark

   Method

   Use the prefix prop-.

   Reason

   Prop- represents a chain containing three carbon atoms.

   Working

   Three carbons → prop-.

2. Add the series suffix

   1 mark

   Method

   Add -ane to name the compound propane.

   Reason

   The question identifies the compound as an alkane.

   Working

   prop- + -ane → propane.

3. Write the structural formula

   1 mark

   Method

   Show the three carbon atoms connected by single bonds with complete hydrogen groups.

   Reason

   Each carbon in an alkane forms four covalent bonds.

   Working

   CH₃-CH₂-CH₃.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the prefix, complete name and structural formula.

Mark points

1. 1 mark: Uses prop- for three carbon atoms.
2. 1 mark: Names the alkane propane.
3. 1 mark: Gives CH3−CH2−CH3.

Calculate my mark

Guided practice 5

Saturated vs unsaturated hydrocarbon

About 6 min

Combine the evidence

A hydrocarbon has molecular formula C₄H₈ and decolourises bromine water. Identify its homologous series and justify your answer using both pieces of evidence.

Connect formula and test observation

General formula match

CnH2n (alkene)CnH2n+2 (alkane)

Bromine-water result indicates

a C=C bondonly C−C bonds

Check my attempt

Hints

Hint 1: formula

Substitute n = 4 into the alkane and alkene general formulae.

Hint 2: test

Recall which carbon–carbon bond reacts with bromine water and removes its colour.

View solution step by step

1. Use the molecular formula

   Method

   Match C₄H₈ to CₙH₂ₙ.

   Reason

   When n = 4, the alkene general formula gives eight hydrogen atoms.

   Working

   2n = 8, so C₄H₈ fits the alkene series.

2. Use the chemical test

   Method

   Use bromine-water decolourisation as evidence of unsaturation.

   Reason

   At this level, an alkene’s C = C bond reacts with bromine and removes its colour.

   Working

   The hydrocarbon is an alkene: both its formula and bromine-water result support a C = C bond.

Next step

Try Fix the wrong definition independently.

7. Mind Stretchers

Mind stretcher 1: Identify families from connected groupsExtension

Classify each structure and identify the feature that supports your decision:

• A: CH₃CH₂CH₂OH
• B: CH₃CH₂COOH
• C: CH₃COOCH₃

A learner says A and B must be members of the same homologous series because both end in OH. Explain the error. Then give the molecular formula of the alcohol with one extra CH₂ compared with A.

Show answer

• A is an alcohol: its -OH is attached to a saturated carbon.
• B is a carboxylic acid: it contains the complete -C(= O)OH group. The OH is attached to a carbonyl carbon, not an alcohol-type saturated carbon.
• C is an ester: -C(= O)O⁻ links to another carbon-containing group.

A and B belong to different families; sharing the letters OH does not establish a common functional group. A has formula C₃H₈O. Adding one C and two H atoms gives C₄H₁₀O for the next alcohol by carbon count. This counts atoms; it does not by itself specify where the OH is attached in an isomer.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.