Functional Groups: MDCAT Chemistry notes
Functional Groups for MDCAT: key organic functional groups, their names and general formulas, thiols, carbonyl compounds, esters and oxonium ions.
What a functional group is
A functional group is the atom or group of atoms that gives an organic compound its characteristic chemical properties. Compounds with the same functional group belong to one homologous series. Members of a series share a general formula and differ from one another by $\mathrm{CH_2}$.
Main functional groups
| Class | Group | Group name | Example |
|---|---|---|---|
| Alkene | C=C | Double bond | Ethene |
| Alkyne | C≡C | Triple bond | Ethyne |
| Alkyl halide | −X | Halo | $\mathrm{CH_3Cl}$ |
| Alcohol | −OH | Hydroxyl | Ethanol |
| Ether | R−O−R | Ether (alkoxy) | Dimethyl ether |
| Aldehyde | −CHO | Formyl (aldehydic carbonyl) | Ethanal |
| Ketone | R−CO−R | Carbonyl | Acetone |
| Carboxylic acid | −COOH | Carboxyl | Acetic acid |
| Ester | −COOR | Ester | Ethyl acetate |
| Amine | −NH₂ | Amino | Methylamine |
| Amide | −CONH₂ | Amide | Acetamide |
| Nitrile | −C≡N | Cyano | $\mathrm{CH_3CN}$ |
| Thioalcohol (thiol) | R−SH | Sulphydryl | Ethanethiol |
| Thioether | R−S−R | Thioether | Dimethyl sulphide |
Sulphur compounds are named by analogy with oxygen compounds. Replacing the O of an alcohol by S gives a thioalcohol (thiol), and replacing the O of an ether gives a thioether.
General formulas worth remembering
- Saturated aldehydes and ketones: $\mathrm{C_nH_{2n}O}$
- Saturated monocarboxylic acids and esters: $\mathrm{C_nH_{2n}O_2}$
- Saturated alcohols and ethers: $\mathrm{C_nH_{2n+2}O}$
Because two classes share each formula, these pairs are functional group isomers. Carbolic acid is the old name for phenol, $\mathrm{C_6H_5OH}$. It is not a carboxylic acid.
Hybridization inside groups
- The carbon in −CHO, −COOH and −COOR has a C=O bond, so it is $sp^2$.
- The carbon in −CN is triply bonded, so it is $sp$, and the group is linear.
- The carbon bearing −OH in an alcohol is $sp^3$.
Primary, secondary and tertiary
Alcohols and alkyl halides are classified by how many carbon atoms are attached to the carbon that carries the functional group. In a primary (1°) compound that carbon has one carbon neighbour, as in ethanol $\mathrm{CH_3CH_2OH}$. A secondary (2°) compound has two, as in propan-2-ol. A tertiary (3°) compound has three, as in 2-methylpropan-2-ol. Amines are classified differently, by the number of alkyl groups on the nitrogen: $\mathrm{RNH_2}$ is primary, $\mathrm{R_2NH}$ is secondary and $\mathrm{R_3N}$ is tertiary.
Members of a homologous series show a gradual change in physical properties such as boiling point, and very similar chemical properties, because they share the same functional group.
Onium ions
When an atom with a lone pair accepts a proton or an extra alkyl group, it becomes a positively charged onium ion.
- Oxonium: $\mathrm{H_3O^+}$ or $\mathrm{ROH_2^+}$, where O has three bonds and a positive charge
- Ammonium: $\mathrm{NH_4^+}$
- Phosphonium: $\mathrm{PH_4^+}$
- Sulphonium: e.g. $\mathrm{RSH_2^+}$, which is a sulphur ion, not an oxonium ion
Common MDCAT traps
- R−SH is a thioalcohol (thiol), not a thioether. The thioether is R−S−R.
- The ketone group is called carbonyl. Carboxyl is −COOH, and formyl is −CHO.
- Ethyl acetate contains an ester group, not a carboxyl or an ether group.
- $\mathrm{C_nH_{2n}O}$ fits ketones (and aldehydes), not carboxylic acids or ethers.
- Only $\mathrm{H_3O^+}$ (or $\mathrm{ROH_2^+}$) is an oxonium ion.
Quick revision
- −OH is hydroxyl, −CHO is aldehyde, and C=O in a ketone is carbonyl.
- −COOR is ester and −CN is nitrile (with an $sp$ carbon).
- R−SH is a thioalcohol.
- Phenol is also called carbolic acid.