Introduction to Organic Chemistry

Two ideas organise the whole topic: homologous series and functional group. Learn them first, then use them to connect alkanes, alkenes, alcohols and carboxylic acids.

1. Definition

A. Homologous series

A homologous series is a family of organic compounds with the same functional group and same general formula, where successive members differ by CH₂. Members have similar chemical properties and show a gradation in physical properties as their molecules become larger.

B. Functional group

A functional group is an atom or group of atoms responsible for the characteristic chemical reactions of a homologous series (e.g. C = C in alkenes).

C. Hydrocarbon

A hydrocarbon is an organic compound containing only carbon and hydrogen.

2. Key Ideas

• Successive members in a homologous series differ by CH₂.
• Same functional group → similar chemical properties.
• More carbon atoms (bigger molecules) → generally higher boiling point and viscosity.
• Saturated = only C-C single bonds (alkanes). Unsaturated = has C = C (alkenes).
• Names are built from a carbon-number prefix + a series suffix.

3. Detailed Explanations

A. Homologous series (the checklist)

B. Core structural features

C. Gradation of physical properties

As n increases in a homologous series:

• boiling point generally increases (larger molecules → stronger intermolecular forces),
• viscosity generally increases (larger molecules flow less easily).

D. Naming basics

4. Common Mistakes

• Forgetting CH₂ difference between successive homologues.
• Calling “homologous series” the functional group (they are not the same).
• Writing melting point “always increases smoothly” (it does not; write “generally increases”).
• Using “saturated/unsaturated” for alcohols (these terms are mainly used for hydrocarbons at this level).

5. Exam Tips

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

   They are carbon compounds containing hydrogen and do not belong to an excluded class.

   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

   Subtract the smaller molecular formula from the larger one.

   Reason

   Successive members of a homologous series must differ by CH₂.

   Working

   C₃H₈ - C₂H₆ = CH₂.

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

Challenge 5

Saturated vs unsaturated hydrocarbon

Minimal support

Evidence transfer

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: Fix the wrong definitionExtension

Question: A student writes: “A homologous series is a group of compounds with the same general formula.” Explain why this is incomplete (2 points).

Show Answer

Missing point 1: they must have the same functional group (that explains similar chemical properties).

Missing point 2: successive members differ by CH₂ (a key defining feature).

Final: Same general formula is not enough; must include same functional group and CH₂ difference.

8. Quiz