Course 8.1Lesson 2 of 3

Classification and IUPAC nomenclature

Organic compounds are open chain or cyclic, and saturated or unsaturated. A functional group sets the chemistry, and a homologous series is a family that differs by a $\text{CH}_{2}$ unit. The IUPAC name is built from the longest chain, a prefix for each branch, a suffix for the principal functional group, and lowest locants.

about 25 min

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

Sorting organic compounds

Acyclic (open chain) compounds have no ring. The chain may be straight or branched, as in propane.
Cyclic (closed chain) compounds have a ring.
- Alicyclic: a ring of carbon atoms that behaves like an open chain, as in cyclohexane.
- Aromatic: a ring with alternate single and double bonds, as in benzene. It is unusually stable.
- Heterocyclic: a ring with an atom other than carbon, such as nitrogen. Pyridine is one.

A tree chart. Organic compounds split into acyclic (open chain) compounds, with propane and but-1-ene as examples, and cyclic (closed chain) compounds. Cyclic compounds split into three boxes: alicyclic, shown by a plain six-membered ring of cyclohexane; aromatic, shown by benzene, a six-membered ring with a circle inside; and heterocyclic, shown by pyridine, a six-membered ring with a circle inside and one nitrogen atom in the ring. A line at the foot says saturated compounds have only single bonds between carbons and unsaturated ones have at least one double or triple bond.

A saturated compound has only single bonds between carbons. An unsaturated one has a double or triple bond. Hydrocarbons fall into alkanes, $\text{C}_n\text{H}_{2n+2}$ , alkenes, $\text{C}_n\text{H}_{2n}$ , and alkynes, $\text{C}_n\text{H}_{2n-2}$ .

Functional groups and homologous series

A functional group is an atom or group of atoms that gives a compound its typical reactions.

Alkene $\text{C=C}$ , alkyne $\text{C}\equiv \text{C}$ , alcohol $\text{OH}$ , ether $\text{C-O-C}$ .
Aldehyde $\text{CHO}$ , ketone $\text{C=O}$ inside a chain, carboxylic acid $\text{COOH}$ , ester $\text{COOR}$ .
Amide $\text{CONH}_{2}$ , amine $\text{NH}_{2}$ , nitrile $\text{CN}$ , halogen compounds such as $\text{Cl}$ , and nitro compounds $\text{NO}_{2}$ .

A homologous series is a family with the same functional group and the same general formula. Each member differs from the next by one $\text{CH}_{2}$ group, which is 14 u. The alkanes are $\text{CH}_{4}$ , $\text{C}_{2}\text{H}_{6}$ , $\text{C}_{3}\text{H}_{8}$ and so on. Chemical properties are alike along a series, and physical properties, such as boiling point, change steadily.

IUPAC names

The IUPAC system gives every compound a name from which you can draw its structure. A name has three parts. A word root gives the chain length. Prefixes name the branches and any group that is not the principal group. A suffix names the principal group, or is -ane, -ene or -yne for a hydrocarbon.

The word roots, for 1 to 10 carbons: meth, eth, prop, but, pent, hex, hept, oct, non, dec.

Naming an alkane

Find the longest continuous chain. The chain may bend on the page, so check every route.
Number it from the end that gives the lowest numbers, the locants, to the branches. If the sets differ, compare them number by number. The set with the smaller number at the first difference wins.
Name each branch as an alkyl group: methyl, ethyl, propyl.
Show repeats with di, tri, tetra, and give a locant for each. Separate locants with commas.
List different branches in alphabetical order, ignoring di and tri. Write one word, with hyphens between numbers and letters.

Example 1. $\text{CH}_{3}\text{CH(CH}_{3}\text{)CH}_{2}\text{CH}_{3}$ is 2-methylbutane. The methyl group is on carbon 2 from one end and on 3 from the other.

Example 2. $\text{(CH}_{3}\text{)}_{2}\text{CHCH}_{2}\text{CH(CH}_{3}\text{)CH}_{2}\text{CH}_{3}$ is 2,4-dimethylhexane, because 2,4 beats 3,5.

Example 3. $\text{(CH}_{3}\text{)}_{2}\text{CHCH(C}_{2}\text{H}_{5}\text{)}_{2}$ is 3-ethyl-2-methylpentane, with ethyl before methyl in the alphabet.

Move the methyl group along the chain. When is the number from the left lower? What happens in the middle?

A saturated ring takes the prefix cyclo, as in methylcyclohexane and 1,2-dimethylcyclohexane.

Alkenes and alkynes. Take the longest chain that contains the multiple bond. End the name in -ene or -yne. Number so that the multiple bond gets the lowest locant. This rule comes before the rule for branches. The locant goes just before the ending.

$\text{CH}_{2}\text{=CHCH}_{2}\text{CH}_{3}$ is but-1-ene, and $\text{CH}_{3}\text{CH=CHCH}_{3}$ is but-2-ene.
$\text{CH}_{3}\text{C(CH}_{3}\text{)=CHCH}_{3}$ is 2-methylbut-2-ene.
$\text{CH}_{2}\text{=CHCH=CH}_{2}$ is buta-1,3-diene, with an extra a for the sound.

A functional group. One group is the principal group. It gives the suffix and the lowest locant. The others become prefixes. The order of seniority, highest first, is:

acid, ester, amide, nitrile, aldehyde, ketone, alcohol, amine, alkene, alkyne.

The suffixes are -oic acid, -oate, -amide, -nitrile, -al, -one, -ol and -amine. Halogens, nitro groups, ethers and alkyl groups are only prefixes: chloro, bromo, nitro, methoxy, methyl.

The final e of the chain name is dropped before a vowel: propan-1-ol, but propanenitrile.
The carbon of $\text{COOH}$ , $\text{CHO}$ , $\text{CONH}_{2}$ and $\text{CN}$ is part of the chain, and is carbon 1.

Examples: $\text{CH}_{3}\text{CH(OH)CH}_{2}\text{CHO}$ is 3-hydroxybutanal, because the aldehyde outranks the alcohol. $\text{CH}_{2}\text{=CHCH}_{2}\text{OH}$ is prop-2-en-1-ol, because the alcohol outranks the alkene. $\text{CH}_{3}\text{CH(CH}_{3}\text{)CH}_{2}\text{CH}_{2}\text{COOH}$ is 4-methylpentanoic acid.

A few old names are still allowed: formic acid for methanoic acid, acetic acid for ethanoic acid, acetone for propanone, toluene for methylbenzene, phenol and aniline.

Try it in Python

The program builds the name of an alkane from its chain length and branches. It tries the numbering from both ends, keeps the lower set, and sorts the branches alphabetically.

Common mistakes

Taking a straight chain instead of the longest chain. A chain can turn a corner.
Choosing the lowest sum of locants. The rule is the lowest at the first point of difference.
Sorting with di and tri. Ethyl comes before dimethyl, because the sort uses ethyl and methyl.
Giving the lowest number to a branch before the principal group. The order is the principal group, then multiple bonds, then branches.

Remember these

Sort by ring or no ring, and by saturated or unsaturated. A functional group sets the chemistry.
A homologous series differs by $\text{CH}_{2}$ , 14 u, from one member to the next.
Longest chain, then lowest locants, then branches in alphabetical order.
Seniority: acid, ester, amide, nitrile, aldehyde, ketone, alcohol, amine, alkene, alkyne.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

Name the family: (a) $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OH}$ ; (b) $\text{CH}_{3}\text{COCH}_{2}\text{CH}_{3}$ ; (c) $\text{CH}_{3}\text{CH}_{2}\text{COOH}$ .