Organic chemists draw the same molecule in several ways, each useful for a different job. Learn to switch between them and to spot the functional group that decides how a molecule reacts.
A molecular formula gives the actual number of each atom (C4H10O); a structural formula shows the arrangement unambiguously (CH3CH2CH2CH2OH); a displayed formula shows every bond; a skeletal formula shows only the carbon skeleton and functional groups, with carbon atoms at corners and line ends and hydrogens on carbon left out. A homologous series shares a general formula and functional group, with each member differing by CH2. The functional group is the atom or group responsible for the characteristic reactions.
Build a molecule in the simulator and switch between its formula types. In skeletal form, count the hydrogens on each carbon so that every carbon makes four bonds.
| Homologous series | Functional group | General formula | Example |
|---|---|---|---|
| alkanes | C–C and C–H only | CnH2n+2 | propane |
| alkenes | C=C | CnH2n | propene |
| halogenoalkanes | C–X | CnH2n+1X | bromoethane |
| alcohols | –OH | CnH2n+1OH | ethanol |
| aldehydes | –CHO | CnH2nO | ethanal |
| ketones | C=O within the chain | CnH2nO | propanone |
| carboxylic acids | –COOH | CnH2n+1COOH | ethanoic acid |
| esters | –COO– | methyl ethanoate | |
| amines | –NH2 | ethylamine | |
| nitriles | –C≡N | propanenitrile |
In a skeletal formula hydrogens on carbon are not drawn, but they are still there: work out how many each carbon needs to make four bonds. Hydrogens on O or N are shown (–OH, –NH2). A structural formula must be unambiguous: C3H7OH is not acceptable because it could be propan-1-ol or propan-2-ol.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the general formula of the alkenes?
Which functional group is present in propanone?
Which formula is a structural formula of propan-2-ol?
Ethanol is classified as:
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
A skeletal formula shows a zig-zag of five carbons with OH on the second carbon. What is its molecular formula?
Which pair have the same empirical formula?
2-methylpropan-2-ol, (CH3)3COH, is classified as:
Which compound contains a nitrile group?
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