Nomenclature of Alkyl Halides: MDCAT Chemistry notes
Nomenclature of Alkyl Halides for MDCAT: primary, secondary and tertiary halides, IUPAC names, common names and uses of chloroform, freons and halothane.
What alkyl halides are
Alkyl halides (haloalkanes) are halogen derivatives of alkanes: one hydrogen of an alkane is replaced by a halogen. General formula $\mathrm{C_nH_{2n+1}X}$ or R–X, where X = F, Cl, Br or I.
Classification: primary, secondary, tertiary
Look only at the carbon that carries the halogen and count how many other carbons are attached to it.
| Type | C–X carbon attached to | Examples |
|---|---|---|
| Primary (1$^\circ$) | one carbon | $\mathrm{CH_3CH_2Cl}$; 1-chlorobutane; isobutyl chloride; neopentyl chloride $\mathrm{(CH_3)_3CCH_2Cl}$ |
| Secondary (2$^\circ$) | two carbons | 2-chloropropane; 2-chlorobutane |
| Tertiary (3$^\circ$) | three carbons | 2-chloro-2-methylpropane (tert-butyl chloride) |
Methyl halides ($\mathrm{CH_3X}$) have no carbon on the C–X carbon and are usually grouped with primary halides. There is no "quaternary" alkyl halide: a carbon carrying X has only three bonds left for carbons. Branching elsewhere in the molecule does not matter; neopentyl chloride is heavily branched but still primary.
IUPAC naming
- Choose the longest chain containing the carbon bearing the halogen.
- Number it so the halogen (and other substituents together) get the lowest locants.
- Name halogens as prefixes: fluoro, chloro, bromo, iodo. Arrange all prefixes alphabetically.
Worked examples:
- $\mathrm{(CH_3)_3CCH_2Br}$: the longest chain has three carbons; Br on C1, two methyls on C2 → 1-bromo-2,2-dimethylpropane.
- $\mathrm{CH_3CHClCH_2CH_3}$ → 2-chlorobutane (secondary).
- $\mathrm{(CH_3)_2CHCH_2Cl}$ → 1-chloro-2-methylpropane (isobutyl chloride, primary).
Worked example: the four isomers of $\mathrm{C_4H_9Cl}$
| Structure | IUPAC name | Common name | Class |
|---|---|---|---|
| $\mathrm{CH_3CH_2CH_2CH_2Cl}$ | 1-chlorobutane | n-butyl chloride | 1$^\circ$ |
| $\mathrm{CH_3CHClCH_2CH_3}$ | 2-chlorobutane | sec-butyl chloride | 2$^\circ$ |
| $\mathrm{(CH_3)_2CHCH_2Cl}$ | 1-chloro-2-methylpropane | isobutyl chloride | 1$^\circ$ |
| $\mathrm{(CH_3)_3CCl}$ | 2-chloro-2-methylpropane | tert-butyl chloride | 3$^\circ$ |
Common names are formed by naming the alkyl group and then the halide (methyl chloride, ethyl bromide, isopropyl iodide).
Common names of polyhalogen compounds
| Formula | Common name | IUPAC name |
|---|---|---|
| $\mathrm{CH_3Cl}$ | Methyl chloride | Chloromethane |
| $\mathrm{CH_2Cl_2}$ | Methylene chloride | Dichloromethane |
| $\mathrm{CHCl_3}$ | Chloroform | Trichloromethane |
| $\mathrm{CCl_4}$ | Carbon tetrachloride | Tetrachloromethane |
| $\mathrm{CHI_3}$ | Iodoform | Triiodomethane |
Important uses
- Chloroform: industrial solvent; formerly an anaesthetic; starting material for chlorodifluoromethane, which is used as a refrigerant and to make tetrafluoroethene, the monomer of Teflon. It is not an embalming agent; formalin (aqueous formaldehyde) preserves tissue.
- Freons: chlorofluorocarbons (CFCs) such as $\mathrm{CCl_2F_2}$. They are non-toxic, non-flammable, easily liquefied gases, used as refrigerants and aerosol propellants. They damage the ozone layer.
- Halothane: $\mathrm{CF_3CHBrCl}$, 2-bromo-2-chloro-1,1,1-trifluoroethane, an inhaled general anaesthetic. It is a halogen derivative of ethane.
Common MDCAT traps
- Neopentyl chloride is primary, not tertiary or "quaternary".
- $\mathrm{CH_2Cl_2}$ is methylene chloride; chloroform is $\mathrm{CHCl_3}$.
- "1-bromo-2,2,2-trimethylethane" is not a valid name; always use the longest chain.
- Halothane is based on ethane, not methane or ethanol.
- Freons are refrigerants and aerosol propellants, not insecticides or fire extinguishers; their ozone-damaging effect is why they are now restricted.
- Chloroform is used in making refrigerants and Teflon, but formalin, not chloroform, is the embalming fluid.
Quick revision
- Alkyl halide = alkane with one H replaced by X.
- 1$^\circ$, 2$^\circ$, 3$^\circ$ depends on the C–X carbon only.
- Chloroform = trichloromethane.
- Freons = CFCs, used as refrigerants.
- Halothane = anaesthetic ethane derivative.
- There is no quaternary alkyl halide.
- Methylene chloride = $\mathrm{CH_2Cl_2}$ = dichloromethane.