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3.1.2Organic Chemistry

Homologous Series and Functional Groups — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 15 of 42

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Part 1 of 4Homologous Series

3.1.2 Homologous Series and Functional Groups
Definition
  • A homologous series is a family of compounds with the same functional group and the same general formula.
Difference
  • Neighbouring members differ by one CH₂ unit, so their relative molecular masses differ by 14.
Chemistry
  • Members react in similar ways because they share the same functional group, though rates can differ.
Physical trend
  • Boiling point rises along the series because longer chains have stronger London forces between molecules.
Examples
  • Alkanes, alkenes, alcohols and halogenoalkanes are all homologous series you will meet at AS level.

Part 2 of 4Functional Groups

Functional Groups
Definition
  • A functional group is the atom or group of atoms in a molecule that determines its characteristic reactions.
Alkene
  • The C=C double bond makes alkenes unsaturated and gives them their characteristic addition reactions.
Alcohol
  • The –OH group is the hydroxyl group and appears as the suffix –ol or prefix hydroxy–.
Halogenoalkane
  • A halogen atom (F, Cl, Br or I) replaces a hydrogen on an alkane chain and gives the prefix chloro–, bromo– and so on.
Others
  • Aldehydes (–CHO), ketones (C=O), carboxylic acids (–COOH) and esters (–COO–) contain the carbonyl group.
SeriesGeneral formulaFunctional groupSuffix or prefix
AlkaneCₙH₂ₙ₊₂C–C only–ane
AlkeneCₙH₂ₙC=C–ene
AlcoholCₙH₂ₙ₊₁OH–OH–ol
HalogenoalkaneCₙH₂ₙ₊₁X–Cl, –Br, –Ichloro–, bromo–, iodo–

Part 3 of 4Types of Hydrocarbon Skeleton

1
Aliphatic compounds have carbon atoms joined in straight or branched chains, with or without a ring that is not aromatic.
2
Alicyclic compounds are aliphatic compounds in which the carbon atoms form a ring, such as cyclohexane.
3
Aromatic compounds contain a benzene ring with delocalised electrons, as in benzene.
4
Saturated means only single carbon-carbon bonds are present, whereas unsaturated means at least one double or triple bond.
5
The type of skeleton and the functional group together determine both the name and the reactions of a compound.

Part 4 of 4Exam Checklist & Tips

3.1.2 – Key Points for the Exam

  • Homologous series A family of organic compounds with the same functional group and general formula.
  • Functional group The part of a molecule that gives it its typical chemical reactions.
  • General formula An algebraic formula that applies to all members of a homologous series.
  • Aliphatic Describes a carbon skeleton without an aromatic ring.
  • Saturated Containing only single carbon-carbon bonds.

Common Mistakes

  • Saying that members of a series have identical physical properties, when only their chemical properties are similar.
  • Writing the general formula of the alkenes as CₙH₂ₙ₊₂, which belongs to the alkanes.
  • Confusing the functional group with the whole molecule, for example calling the entire alcohol the –OH group.
  • Stating that members differ by CH₄ instead of CH₂.

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