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O LevelChemistryCambridgeRevision Notes11. Organic Chemistry11.1 Formulae, Functional Groups & TerminologyHomologous Series

Homologous Series (Cambridge O Level Chemistry)

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Caroline

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Homologous Series

  • This is a series or family of organic compounds that have similar features and chemical properties due to them having the same functional group
  • The functional group is a group of atoms which are bonded in a specific arrangement that is responsible for the characteristic reactions of each member of a homologous series

Table of Compounds & their Functional Groups

homologous-series-table

Examiner Tip

Make sure you can identify the functional group for each homologous series.

General Formulae

  • This type of formula tells you the composition of any member of a whole homologous series of organic compound
  • For example, all of the alkanes have the general formula CnH2n+2, where n represents the number of carbon atoms
  • This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkane
  • General formulae can be used to work out the formula of a compound from different homologous series if the number of carbon atoms present is known

General Formula of Common Homologous Series

Homologous Series General Formula

Alkanes

CnH2n+2

Alkenes

CnH2n

Alcohols

CnH2n+1OH

Carboxylic Acids

CnH2n+1COOH

Worked example

What is the formula of an alcohol that contains 5 carbon atoms?

Answer

  • Number of carbons = 5
  • Number of hydrogens (excluding in the functional group) = 2 x 5 + 1 = 11
  • Formula = C5H11OH

Worked example

A compound has the formula C12H24. To which homologous series does this compound belong to?

Answer

  • There are 12 carbon atoms, so n = 12
  • There are twice the number of hydrogen atoms than carbon atoms = 2n
  • Therefore the general formula of the compound is CnH2n which means this compound is an alkene

General Characteristic of Homologous Series

  • All members of a homologous series have:
    • The same general formula
    • Same functional group
    • Similar chemical properties
    • Gradation in their physical properties, such as melting and boiling point
    • The difference in the molecular formula between one member and the next is CH2
    • These characteristics are shown below for ethanol and propanol, which belong to homologous series, alcohols

Table of Characteristics of Ethanol and Propanol

11-1-2-characteristics-of-ethanol-and-propanol

Examiner Tip

Make sure you learn the general formula for each homologous series.

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.