Functional Groups, Homologous Series and IUPAC Names – Introduction to Organic Chemistry (Senior 5 Chemistry) | Ganzaa

Introduction to Organic Chemistry

Lessons

Senior 5 · Chemistry

Introduction to Organic Chemistry

4 lessons

Lesson 3 of 4

Functional Groups, Homologous Series and IUPAC Names

25 min

One name, one structure

A bottle in a school laboratory store is labelled "isopropyl alcohol". A second label, on another shelf, reads "propan-2-ol". They are the same liquid, used as a disinfectant and to clean electronic parts. The first name is a traditional one, learned by heart; the second is a systematic name, built by rules from the International Union of Pure and Applied Chemistry (IUPAC), so that any chemist can draw the structure from the name, and write exactly one name for any structure.

In Senior 3 you named simple compounds with one functional group. In this unit and the ones that follow you will meet molecules with two or three groups, such as lactic acid in sour milk, $\mathrm{CH_3CH(OH)COOH}$ . This lesson gives the full rules.

Definition

Functional group: the atom or group of atoms that gives a compound its characteristic chemical reactions. Example: –OH in alcohols.

Homologous series: a family of compounds with the same functional group and the same general formula, in which each member differs from the next by $\mathrm{CH_2}$ ; its members show a gradual trend in physical properties and have similar chemical properties. Example: the alcohols, $\mathrm{C_nH_{2n+1}OH}$ .

Principal functional group: in a compound with more than one functional group, the one highest in the priority order (Table 3.1). It alone is named by the suffix; all other groups are named by prefixes.

The families and their priority

Table 3.1: Homologous series in Senior 5, in order of priority for naming (highest first)

Series	Functional group	Named as suffix	Named as prefix	Example
carboxylic acid	–COOH	-oic acid	carboxy-	propanoic acid, $\mathrm{CH_3CH_2COOH}$
acid anhydride	–CO–O–CO–	-oic anhydride	–	ethanoic anhydride, $\mathrm{(CH_3CO)_2O}$
ester	–COO–R	alkyl -oate	alkoxycarbonyl-	methyl ethanoate, $\mathrm{CH_3COOCH_3}$
acyl chloride	–COCl	-oyl chloride	–	ethanoyl chloride, $\mathrm{CH_3COCl}$
amide	–CONH₂	-amide	carbamoyl-	ethanamide, $\mathrm{CH_3CONH_2}$
nitrile	–C≡N	-nitrile	cyano-	propanenitrile, $\mathrm{CH_3CH_2CN}$
aldehyde	–CHO	-al	oxo-	propanal, $\mathrm{CH_3CH_2CHO}$
ketone	C–CO–C	-one	oxo-	propanone, $\mathrm{CH_3COCH_3}$
alcohol	–OH	-ol	hydroxy-	propan-2-ol, $\mathrm{CH_3CH(OH)CH_3}$
amine	–NH₂	-amine	amino-	propan-1-amine, $\mathrm{CH_3CH_2CH_2NH_2}$
alkene, alkyne	C=C, C≡C	-ene, -yne (in the stem)	–	propene, propyne
ether	C–O–C	–	alkoxy-	methoxyethane, $\mathrm{CH_3OCH_2CH_3}$
halogenoalkane	–F, –Cl, –Br, –I	–	fluoro-, chloro-, bromo-, iodo-	2-bromopropane
alkane	C–C and C–H only	-ane	alkyl- (methyl, ethyl…)	propane

General formulae worth knowing: alkanes $\mathrm{C_nH_{2n+2}}$ ; alkenes and cycloalkanes $\mathrm{C_nH_{2n}}$ ; alkynes $\mathrm{C_nH_{2n-2}}$ ; alcohols and ethers $\mathrm{C_nH_{2n+2}O}$ ; aldehydes and ketones $\mathrm{C_nH_{2n}O}$ ; carboxylic acids and esters $\mathrm{C_nH_{2n}O_2}$ . Notice that some pairs of series share a general formula. Lesson 4 shows why: they are functional-group isomers.

The IUPAC rules, step by step

Find the principal group (the highest in Table 3.1). It gives the suffix.
Find the parent chain: the longest chain of carbons that contains the principal group (and, as far as possible, the C=C or C≡C bonds). The carbon of –COOH, –CHO, –CN, –COCl and –CONH₂ is counted in the chain and is always C1. The chain need not be drawn in a straight line.
Number the chain from the end that gives the lowest numbers, in this order of importance: (a) the principal group; (b) multiple bonds; (c) all prefixes together, at the first point of difference.
Name the substituents as prefixes with their locants. Put them in alphabetical order (bromo before chloro before methyl). Use di-, tri-, tetra- for repeats, but ignore these when alphabetising.
Assemble: prefixes + stem + (-en- or -yn- with locant) + suffix with locant. Commas separate numbers; hyphens separate numbers from words. Drop the final "e" of the stem before a vowel (propan-1-ol, propanal) but keep it before a consonant (ethane-1,2-diol).

Figure 3.1: Reading a systematic name. The chain is numbered from the end that gives the principal group (–OH) the lowest number.

Table 3.2: Common alkyl groups

Group	Formula	Group	Formula
methyl	$\mathrm{CH_3{-}}$	1-methylethyl (isopropyl)	$\mathrm{(CH_3)_2CH{-}}$
ethyl	$\mathrm{CH_3CH_2{-}}$	butyl	$\mathrm{CH_3CH_2CH_2CH_2{-}}$
propyl	$\mathrm{CH_3CH_2CH_2{-}}$	phenyl	$\mathrm{C_6H_5{-}}$

Worked example

Problem. Name (a) $\mathrm{CH_3CH(CH_3)CH(OH)CH_3}$ ; (b) $\mathrm{CH_3COCH_2CH(OH)CH_3}$ .

Solution. (a) Principal group –OH (suffix -ol). Parent chain: four carbons, so "butan". Numbering from the right puts –OH on C2 (from the left it would be C3); the methyl group is then on C3. Name: 3-methylbutan-2-ol.

(b) Two groups: C=O (ketone) and –OH. The ketone is higher in priority, so the suffix is -one and –OH becomes "hydroxy". Five carbons: "pentan". Number from the left so that C=O gets 2 (not 4); –OH is then on C4. Name: 4-hydroxypentan-2-one.

Worked example

Problem. Name (a) $\mathrm{ClCH_2CH_2CHBrCH_3}$ ; (b) $\mathrm{CH_3CH(NH_2)COOH}$ ; and (c) write the structural formula of 3-methylbut-2-en-1-ol.

Solution. (a) Only prefixes (no suffix group), four carbons. From the Cl end the locants are {1, 3}; from the other end {2, 4}. At the first point of difference 1 < 2, so number from the Cl end. Alphabetical order: 3-bromo-1-chlorobutane.

(b) –COOH is principal and its carbon is C1; the amino group is on C2: 2-aminopropanoic acid (the amino acid alanine).

(c) "but": four carbons; "-1-ol": OH on C1; "-2-en": C=C between C2 and C3; "3-methyl": a methyl group on C3: $\mathrm{HOCH_2CH{=}C(CH_3)CH_3}$ .

Warning

Three classic slips. (1) Taking the horizontal chain as the longest: "2-ethylbutane" is really 3-methylpentane, because the ethyl branch extends the chain. (2) Alphabetising by di- or tri-: in 2,2-dimethyl-3-ethylpentane the order is wrong; it is 3-ethyl-2,2-dimethylpentane (e before m). (3) Giving a substituent a lower number than the principal group: in $\mathrm{CH_3CH(Cl)CH_2CH_2OH}$ , –OH must be on C1: 3-chlorobutan-1-ol.

Tip

Think of the principal group as the head of the family: it takes the surname (the suffix) and decides where counting starts. Every other group is a first name (a prefix), listed in alphabetical order.

Science Ready: Naming Every Organic Functional Group Using IUPAC Conventions (28 min). Watch for the order of priority when a molecule has two functional groups, and how the suffix and prefixes are chosen. The video is made for the HSC course in Australia; its rules are the same IUPAC rules used here.

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Practice

Name: (a) $\mathrm{CH_3CH_2CH(CH_3)CH_2CH_3}$ ; (b) $\mathrm{CH_3CH{=}C(CH_3)CH_3}$ ; (c) $\mathrm{HOCH_2CH_2OH}$ ; (d) $\mathrm{CH_3CH(OH)CH_2CHO}$ .
Write structural formulae for: (a) 2,2-dimethylpropane; (b) pent-3-en-2-one; (c) 2-chloropropanoic acid; (d) 4-methylpentan-2-ol.
Lactic acid is $\mathrm{CH_3CH(OH)COOH}$ . Name its two functional groups and give its systematic name.
Explain why "3-methylbut-3-ene" is wrong, and give the correct name.
Give the systematic name of $\mathrm{HOCH_2CH_2COCH_3}$ , explaining which group is named by the suffix.
Write the general formula of the alkynes and the molecular formula of the alkyne with five carbon atoms.
Which two homologous series in Table 3.1 share the general formula $\mathrm{C_nH_{2n}O_2}$ ? Give one example of each with formula $\mathrm{C_3H_6O_2}$ .

Show answersHide answers

(a) 3-methylpentane; (b) 2-methylbut-2-ene; (c) ethane-1,2-diol; (d) 3-hydroxybutanal.
(a) $\mathrm{C(CH_3)_4}$ ; (b) $\mathrm{CH_3COCH{=}CHCH_3}$ ; (c) $\mathrm{CH_3CHClCOOH}$ ; (d) $\mathrm{CH_3CH(OH)CH_2CH(CH_3)CH_3}$ .
Hydroxyl (–OH) and carboxyl (–COOH); 2-hydroxypropanoic acid.
The chain must be numbered to give the double bond the lowest number: $\mathrm{CH_2{=}C(CH_3)CH_2CH_3}$ is 2-methylbut-1-ene.
The ketone outranks the alcohol, so it takes the suffix -one and the lowest locant: $\mathrm{CH_3COCH_2CH_2OH}$ is 4-hydroxybutan-2-one.
$\mathrm{C_nH_{2n-2}}$ ; $\mathrm{C_5H_8}$ .
Carboxylic acids and esters: propanoic acid, $\mathrm{CH_3CH_2COOH}$ , and methyl ethanoate, $\mathrm{CH_3COOCH_3}$ (or ethyl methanoate, $\mathrm{HCOOCH_2CH_3}$ ).

Sources and credits

Written by Ganzaa, following the IUPAC recommendations for the nomenclature of organic chemistry (Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013, IUPAC).
Video: Naming Every Organic Functional Group Using IUPAC Conventions // HSC Chemistry, Science Ready, embedded from YouTube.
The text, diagram, tables, worked examples and Practice items were written and drawn by Ganzaa.

1One name, one structure

2The families and their priority

3The IUPAC rules, step by step

One name, one structure

The families and their priority

The IUPAC rules, step by step