Homologous Series vs Functional Groups

This is an extension of the HSC Chemistry course relevant to all aspects of the organic chemistry module. 

HSC Chemistry Syllabus

  • Investigate the nomenclature of organic chemicals, up to C8, using IUPAC conventions, including simple methyl and ethyl branched chains including:

– alkanes
– alkenes
– alkynes
– alcohols (primary, secondary and tertiary)
– aldehydes and ketones
– carboxylic acids
– amines and amides
– halogenated organic compounds

    Homologous Series vs Functional Groups

    The terms "homologous series" and "functional group" can often be confused because they are usually discussed in conjunction.


    What are Functional Groups?

    A functional group is a group of atoms that characterise an organic compound. These groups determine both the chemical and physical properties of the compounds they belong to and play a crucial role in their identification and nomenclature. 

    Examples of Functional groups: 

    • Alkenes: Characterised by the C=C double bond
    • Alcohols: Denoted by R-OH groups where an OH is connected to a carbon chain
    • Carboxylic acids: Contains a COOH group with a C-OH single bond and a C=O.
    • EstersConsists of a C=O carbonyl carbon, with an adjacent oxygen connecting two carbon chains. 

    What is a Homologous Series?

    A homologous series is a series of organic compounds that all possess the same functional group. 

    Key Characteristics:

    • Members of the series differ only by a single constant unit, typically a `CH_2` bond. 
    • They share a general molecular formula
    • They usually display similar chemical properties. 

    The boiling point (BP) and melting point (MP) within a homologous series, often rises with increasing molecular mass. This change is due to the lengthening of the carbon chain. For example, as alcohol chain length increases, so does the boiling point. This can be attributed to the formation of more dispersion forces. 

    Examples of Homologous Series

    • Alkanes: Comprise of only carbon-to-carbon and carbon-to-hydrogen single bonds. They lack unique functional groups and follow the general formula `C_nH_{2n+2}`


    Homologous series of alkanes


    • Alkenes: Contain a C=C double bond and follow the general formula `C_nH_{2n}`

    Homologous series of alkenes


    • Alkynes: Contain a C ≡C triple bond and follow the general formula `CnH_{2n-2}`


    • Alcohols: Feature an O-H hydroxyl group and follow the general formula `C_nH_{2n+2}O`

    alcohol homologous series


    • Aldehydes: These have a terminal carbonyl carbon and follow the general formula  `C_nH_{2n}O`

    aldehyde homologous series



    • Ketones: Like aldehydes, they contain a carbonyl carbon but not at the terminal position. They share the general formula  `C_nH_{2n}O` with aldehydes. 


    • Carboxylic Acids: Recognised by the COOH group and follow the formula `C_nH_{2n}O_2`  

    carboxylic acid homologous series


    • Esters: Contain a carbonyl carbon bonded to an oxygen, which is then connected to another alkyl group. They share the same general formula as carboxylic acids  `C_nH_{2n}O_2`  



    • Amines: Contain a singly bonded nitrogen in place of a carbon, with a formula of  `C_nH_{2n+3}N`


    • Amides: Similar to amines, they have nitrogen but are distinguished by an adjacent carbonyl carbon. They follow a general formula of `C_nH_{2n+1}NO`