Functional groups and IUPAC names
A functional group gives a compound its typical properties. IUPAC names join a word root for the carbon count with an ending for the group.
Common functional groups
| Group | Class | Name ending | Example |
|---|---|---|---|
| C=C (double bond) | Alkene | -ene | Ethene, H₂C=CH₂ |
| C≡C (triple bond) | Alkyne | -yne | Ethyne, HC≡CH |
| -X (F, Cl, Br, I) | Haloalkane | Prefix chloro-, bromo- | Chloromethane, CH₃Cl |
| -OH (hydroxyl) | Alcohol | -ol | Methanol, CH₃OH |
| -CHO (at the end of the chain) | Aldehyde | -al | Ethanal, CH₃CHO |
| C=O inside the chain (keto group) | Ketone | -one | Propanone, CH₃COCH₃ |
| -COOH (carboxyl) | Carboxylic acid | -oic acid | Ethanoic acid, CH₃COOH |
| -COOR | Ester | -oate | Methyl ethanoate, CH₃COOCH₃ |
IUPAC rules for alkanes
- 1Find the longest continuous chain of carbon atoms. Its word root gives the parent name.A 6-carbon chain gives hexane
- 2Name each side group (alkyl group) by changing -ane to -yl.-CH₃ is methyl; -C₂H₅ is ethyl
- 3Number the chain from the end that gives the side groups the lowest numbers.
- 4Write the number, a hyphen, then the group name joined to the parent name.3-ethylhexane: no space between ethyl and hexane
- 5For different groups, list them in alphabetical order.3-ethyl-3-methylhexane
- 6★For identical groups, use di, tri, tetra. These prefixes are ignored in alphabetical order.2,2-dimethylbutane: numbers are separated by a comma
★ key step
Worked example: name these alkanes
Name: (a) CH₃-CH(CH₃)-CH₂-CH₂-CH₃ (b) CH₃-CH(CH₃)-CH(CH₃)-CH₂-CH₃ (c) CH₃-CH₂-CH(C₂H₅)-CH₂-CH₃
- (a) The longest chain has 5 carbons, so the parent is pentane. There is one -CH₃ group.
- Numbering from the left puts it on carbon 2; from the right it would be 4. Use the lower number, 2.
- (b) The longest chain has 5 carbons. There are two -CH₃ groups, on carbons 2 and 3 from the left (3 and 4 from the right).
- Two identical groups take the prefix di, with the numbers 2,3.
- (c) The longest chain has 5 carbons, with one -C₂H₅ group on carbon 3 from either end.
(a) 2-methylpentane (b) 2,3-dimethylpentane (c) 3-ethylpentane
Naming alkenes, alkynes and other groups
- Alkenes: change -ane to -ene. Give the double bond the lower of its two carbon numbers: CH₂=CH-CH₂-CH₃ is 1-butene; CH₃-CH=CH-CH₃ is 2-butene.
- Alkynes: change -ane to -yne: HC≡C-CH₃ is propyne; HC≡C-CH₂-CH₃ is 1-butyne.
- Alcohols: replace the final e of the alkane by -ol: methanol, ethanol, 2-propanol.
- Aldehydes: replace the e by -al: HCHO is methanal; CH₃CHO is ethanal.
- Ketones: replace the e by -one: CH₃COCH₃ is propanone.
- Carboxylic acids: CH₃COOH is ethanoic acid. Count the carbon in -COOH too.
Worked example: name these compounds
Name: (a) CH₃CH₂CH₂OH (b) CH₃CH₂CHO (c) CH₃CH₂COOH (d) CH₃CH₂Cl
- (a) 3 carbons (prop) and an -OH group (-ol): propanol.
- (b) 3 carbons, counting the C of -CHO (prop), and an aldehyde (-al): propanal.
- (c) 3 carbons, counting the C of -COOH (prop), and a carboxylic acid (-oic acid): propanoic acid.
- (d) 2 carbons (eth) with -Cl as a prefix: chloroethane.
Propanol, propanal, propanoic acid, chloroethane
The labels on nail polish remover (propanone, or acetone) and on a bottle of vinegar (ethanoic acid) use these names. IUPAC names mean a chemist in Chennai and one in Tokyo know exactly which compound is meant.
1 marks: “Write the IUPAC name of HC≡C-CH₂-CH₃.” 1-butyne.
2 marks: “Identify the functional group and give the IUPAC name of (i) CH₃CH₂CH₂CHO and (ii) CH₃COCH₂CH₃.” (i) Aldehyde group (-CHO): butanal. (ii) Ketone group (C=O inside the chain): butanone.
3 marks: “Give the IUPAC rules for naming a branched alkane, and use them to name CH₃-C(CH₃)₂-CH₂-CH₂-CH₃.” Choose the longest chain (5 C: pentane); name the side groups (two methyls); number from the end nearer the groups (both on C2); use di for identical groups. Name: 2,2-dimethylpentane.
Monkeys Eat Peeled Bananas: meth, eth, prop, but. Then pent, hex, hept, oct.