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6.2.3 Functional groups and homologous series

Homologous series members share a functional group

Definition

Homologous series

A family of organic compounds with the same functional group and general formula, in which each member differs from the next by a CH2 unit.

Definition

Functional group

The atom or group of atoms in an organic molecule that is responsible for its characteristic reactions.

  1. Members of the same homologous series have the same functional group and the same general formula.
    1. Each member differs from the next by a CH2\text{CH}_2CH2​ unit, so the chain grows by one carbon atom at a time.
  2. Their physical properties change gradually along the series, so boiling point rises as the chain gets longer.
  3. Their chemical properties are similar, because reactions happen at the functional group while the rest of the chain stays the same.
    1. Once you know how ethanol reacts, you can predict how butan-1-ol reacts, because both contain the −OH-\text{OH}−OH group.
  4. To identify the series a compound belongs to, find its functional group in the name or the displayed formula.
Key Idea

The reactions of an organic compound are set by the general reactions of its functional group, so one set of rules works for a whole series.

Table showing root and stem prefixes for carbon chain lengths from 1 to 10 carbon atoms, from meth- to dec-

Alkanes have single bonds and names ending in -ane

Definition

Alkane

A hydrocarbon in which all the bonds between carbon atoms are single covalent bonds.

Definition

Displayed formula

A two-dimensional formula that shows every atom in a molecule and each bond between the atoms.

  1. Every carbon atom in an alkane forms four single bonds, so alkanes have the general formula CnH2n+2\text{C}_n\text{H}_{2n+2}Cn​H2n+2​.
  2. The prefix of each name gives the number of carbon atoms: meth- is 111, eth- is 222, prop- is 333 and but- is 444.
    1. Methane, CH4\text{CH}_4CH4​: H∣H−C−H∣H\begin{array}{ccccc}&&\mathrm{H}&&\\&&\vert&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{H}\\&&\vert&&\\&&\mathrm{H}&&\end{array}H​−​H∣C∣H​−​H​
    2. Ethane, C2H6\text{C}_2\text{H}_6C2​H6​: HH∣∣H−C−C−H∣∣HH\begin{array}{ccccccc}&&\mathrm{H}&&\mathrm{H}&&\\&&\vert&&\vert&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\&&\vert&&\vert&&\\&&\mathrm{H}&&\mathrm{H}&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H​
    3. Propane, C3H8\text{C}_3\text{H}_8C3​H8​: HHH∣∣∣H−C−C−C−H∣∣∣HHH\begin{array}{ccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\\&&\vert&&\vert&&\vert&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\&&\vert&&\vert&&\vert&&\\&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​H​
    4. Butane, C4H10\text{C}_4\text{H}_{10}C4​H10​: HHHH∣∣∣∣H−C−C−C−C−H∣∣∣∣HHHH\begin{array}{ccccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\\&&\vert&&\vert&&\vert&&\vert&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\&&\vert&&\vert&&\vert&&\vert&&\\&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​H​
Common Mistake
  • Do not leave out any bond in a fully displayed formula, because every carbon atom must show four bonds and every hydrogen atom one.
  • A shortened formula such as CH3CH3\text{CH}_3\text{CH}_3CH3​CH3​ hides the bonds, so it is not a displayed formula.

Alkenes contain a double bond between carbon atoms

Definition

Alkene

A hydrocarbon that contains a double covalent bond between two carbon atoms.

  1. Every alkene contains a double bond between two carbon atoms, C=C\text{C}=\text{C}C=C, as its functional group.
  2. Alkenes with one double bond have the general formula CnH2n\text{C}_n\text{H}_{2n}Cn​H2n​, with two fewer hydrogen atoms than the alkane with the same chain length.
  3. A double bond needs two carbon atoms, so the first four members are ethene, propene, but-1-ene and pent-1-ene.
    1. Ethene, C2H4\text{C}_2\text{H}_4C2​H4​: HH∣∣C=C∣∣HH\begin{array}{ccc}\mathrm{H}&&\mathrm{H}\\\vert&&\vert\\\mathrm{C}&=&\mathrm{C}\\\vert&&\vert\\\mathrm{H}&&\mathrm{H}\end{array}H∣C∣H​=​H∣C∣H​
    2. Propene, C3H6\text{C}_3\text{H}_6C3​H6​: HHH∣∣∣C=C−C−H∣∣HH\begin{array}{ccccccc}\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\\\vert&&\vert&&\vert&&\\\mathrm{C}&=&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\\vert&&&&\vert&&\\\mathrm{H}&&&&\mathrm{H}&&\end{array}H∣C∣H​=​H∣C​−​H∣C∣H​−​H​
    3. But-1-ene, C4H8\text{C}_4\text{H}_8C4​H8​: HHHH∣∣∣∣C=C−C−C−H∣∣∣HHH\begin{array}{ccccccccc}\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\\\vert&&\vert&&\vert&&\vert&&\\\mathrm{C}&=&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\\vert&&&&\vert&&\vert&&\\\mathrm{H}&&&&\mathrm{H}&&\mathrm{H}&&\end{array}H∣C∣H​=​H∣C​−​H∣C∣H​−​H∣C∣H​−​H​
    4. Pent-1-ene, C5H10\text{C}_5\text{H}_{10}C5​H10​: HHHHH∣∣∣∣∣C=C−C−C−C−H∣∣∣∣HHHH\begin{array}{ccccccccccc}\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\\\vert&&\vert&&\vert&&\vert&&\vert&&\\\mathrm{C}&=&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{H}\\\vert&&&&\vert&&\vert&&\vert&&\\\mathrm{H}&&&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\end{array}H∣C∣H​=​H∣C​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​H​
Note

The number in but-1-ene and pent-1-ene shows that the double bond starts at carbon atom 111, at the end of the chain.

Alcohols contain the hydroxyl group and end in -ol

Definition

Alcohol

An organic compound containing the hydroxyl group, OH, bonded to a carbon atom.

  1. Every alcohol contains the hydroxyl group, −OH-\text{OH}−OH, bonded to a carbon atom, as its functional group.
  2. Alcohols with one −OH-\text{OH}−OH group have the general formula CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH}Cn​H2n+1​OH.
  3. The -ol ending shows an alcohol, and the number before it shows which carbon atom carries the −OH-\text{OH}−OH group.
    1. Methanol, CH3OH\text{CH}_3\text{OH}CH3​OH: H∣H−C−O−H∣H\begin{array}{ccccccc}&&\mathrm{H}&&&&\\&&\vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&&&\\&&\mathrm{H}&&&&\end{array}H​−​H∣C∣H​−​O​−​H​
    2. Ethanol, C2H5OH\text{C}_2\text{H}_5\text{OH}C2​H5​OH: HH∣∣H−C−C−O−H∣∣HH\begin{array}{ccccccccc}&&\mathrm{H}&&\mathrm{H}&&&&\\&&\vert&&\vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&\vert&&&&\\&&\mathrm{H}&&\mathrm{H}&&&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​O​−​H​
    3. Propan-1-ol, C3H7OH\text{C}_3\text{H}_7\text{OH}C3​H7​OH: HHH∣∣∣H−C−C−C−O−H∣∣∣HHH\begin{array}{ccccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&&&\\&&\vert&&\vert&&\vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&\vert&&\vert&&&&\\&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​O​−​H​
    4. Butan-1-ol, C4H9OH\text{C}_4\text{H}_9\text{OH}C4​H9​OH: HHHH∣∣∣∣H−C−C−C−C−O−H∣∣∣∣HHHH\begin{array}{ccccccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&&&\\&&\vert&&\vert&&\vert&&\vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&\vert&&\vert&&\vert&&&&\\&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​O​−​H​
Common Mistake
  • Do not call the −OH-\text{OH}−OH group a hydroxide, because it is covalently bonded to carbon and is not a hydroxide ion, OH−\text{OH}^{-}OH−.
  • In a fully displayed formula, draw the bond between the oxygen and hydrogen atoms as well as the bond from carbon to oxygen.

Carboxylic acids contain the carboxyl group

Definition

Carboxylic acid

An organic compound containing the carboxyl group, COOH.

  1. Every carboxylic acid contains the carboxyl group, −COOH-\text{COOH}−COOH, as its functional group.
    1. In this group, one carbon atom has a double bond to one oxygen atom and a single bond to an −OH-\text{OH}−OH group.
  2. The carbon atom in −COOH-\text{COOH}−COOH counts in the name, so ethanoic acid has two carbon atoms in total.
    1. Methanoic acid, HCOOH\text{HCOOH}HCOOH: O∥H−C−O−H\begin{array}{ccccccc}&&\mathrm{O}&&&&\\&&\Vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\end{array}H​−​O∥C​−​O​−​H​
    2. Ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3​COOH: HO∣∥H−C−C−O−H∣H\begin{array}{ccccccccc}&&\mathrm{H}&&\mathrm{O}&&&&\\&&\vert&&\Vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&&&&&\\&&\mathrm{H}&&&&&&\end{array}H​−​H∣C∣H​−​O∥C​−​O​−​H​
    3. Propanoic acid, C2H5COOH\text{C}_2\text{H}_5\text{COOH}C2​H5​COOH: HHO∣∣∥H−C−C−C−O−H∣∣HH\begin{array}{ccccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{O}&&&&\\&&\vert&&\vert&&\Vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&\vert&&&&&&\\&&\mathrm{H}&&\mathrm{H}&&&&&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​O∥C​−​O​−​H​
    4. Butanoic acid, C3H7COOH\text{C}_3\text{H}_7\text{COOH}C3​H7​COOH: HHHO∣∣∣∥H−C−C−C−C−O−H∣∣∣HHH\begin{array}{ccccccccccccc}&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&\mathrm{O}&&&&\\&&\vert&&\vert&&\vert&&\Vert&&&&\\\mathrm{H}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{C}&-&\mathrm{O}&-&\mathrm{H}\\&&\vert&&\vert&&\vert&&&&&&\\&&\mathrm{H}&&\mathrm{H}&&\mathrm{H}&&&&&&\end{array}H​−​H∣C∣H​−​H∣C∣H​−​H∣C∣H​−​O∥C​−​O​−​H​
Exam technique
  • When a question asks for a fully displayed formula, draw every atom and every bond, including each C−H\text{C}-\text{H}C−H and O−H\text{O}-\text{H}O−H bond.
  • Then count the bonds: four on each carbon atom, two on each oxygen atom and one on each hydrogen atom.
Self review
  • What do members of a homologous series have in common?
  • Why do members of the same homologous series react in similar ways?
  • Why is ethene the first member of the alkene series?
  • Draw the fully displayed formula of propan-1-ol.
  • How many carbon atoms are there in a molecule of butanoic acid?
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A functional group is the atom or group of atoms responsible for an organic compound's characteristic reactions. For example, the −OH-\text{OH}−OH group gives alcohols their typical chemical reactions.

A homologous series is a family with the same functional group and general formula. Consecutive members differ by CH2\text{CH}_2CH2​, have similar chemical properties, and show gradual changes in physical properties such as boiling point.

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What two features do all members of a homologous series share?

6.2.3 Functional groups and homologous series Revision Guide

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Revision notes for OCR GCSE Chemistry 6.2.3 Functional groups and homologous series: explanations and worked examples.

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