IMAT Chemistry · Organic Chemistry

Functional Groups: Recognise, Name and Find Them in Biomolecules

A functional group is the small reactive part of a molecule (an OH, a C=O, an NH₂) that decides its family, its name ending and most of its chemistry.

All Organic Chemistry notes

Why this matters

This is the most tested page of the chapter. The ministry papers asked which group is in paracetamol, which molecule has no C=O, what family pentanones belong to, and two questions on amino acids and glycogen; the older papers asked for every group in a drawn vitamin or in a condensed formula.

Concept 1 of 4: The main functional groups and their families

The carbon and hydrogen skeleton is mostly unreactive; the chemistry happens at the oxygen, nitrogen or halogen atoms. So organic chemists sort molecules by these groups. Learn each group by its atoms and by what they are bonded to, because two groups can share the same atoms in a different arrangement.

Definition

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

The carbonyl group C=O is in aldehydes, ketones, carboxylic acids, esters and amides. It is NOT in alcohols, ethers or amines.
R stands for any carbon chain.
Common names still used in IMAT: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), acetic acid (ethanoic acid), methyl acetate (methyl ethanoate), dimethyl ether (methoxymethane).

Family	Group	Name ending	Example
Haloalkane	C-X, where X is F, Cl, Br or I	prefix chloro-, bromo-	Chloroethane $\mathrm{CH_3CH_2Cl}$
Alcohol	$\mathrm{-OH}$ on a saturated carbon	-ol	Ethanol $\mathrm{CH_3CH_2OH}$
Ether	C-O-C, no C=O	alkoxy- prefix, or 'dimethyl ether' style	Methoxymethane $\mathrm{CH_3OCH_3}$
Aldehyde	$\mathrm{-CHO}$ : C=O at the end of a chain	-al	Ethanal $\mathrm{CH_3CHO}$
Ketone	C=O between two carbons	-one	Propanone $\mathrm{CH_3COCH_3}$
Carboxylic acid	$\mathrm{-COOH}$	-oic acid	Ethanoic acid $\mathrm{CH_3COOH}$
Ester	$\mathrm{-COO-}$ between two carbons	alkyl ...-oate	Methyl ethanoate $\mathrm{CH_3COOCH_3}$
Amine	N bonded to C and H, no C=O next to it	-amine	Ethylamine $\mathrm{CH_3CH_2NH_2}$
Amide	$\mathrm{-CONH_2}$ : C=O bonded to N	-amide	Ethanamide $\mathrm{CH_3CONH_2}$
Nitrile	$\mathrm{-C{\equiv}N}$	-nitrile	Ethanenitrile $\mathrm{CH_3CN}$
Thiol	$\mathrm{-SH}$	-thiol	Ethanethiol $\mathrm{CH_3CH_2SH}$

An OH on a benzene ring is a phenol, a separate family that is more acidic than an alcohol.

Practice this conceptself-check · 4 quick reps

Amine or amide: look for the C=O next to the nitrogen

Both contain nitrogen. If the N is bonded directly to a C=O carbon, the group is an amide (as in paracetamol and in proteins); otherwise it is an amine. An amide is not an amine plus a ketone.

Ethers have no C=O at all

An ether is just C-O-C. Aldehydes, ketones, acids, esters and amides all contain C=O, whatever their common names (acetaldehyde, acetone, acetic acid, methyl acetate). In a list asking which molecule lacks C=O, the ether or the alcohol is the answer.

Concept 2 of 4: Finding every functional group in a larger molecule

Large drug and vitamin molecules look complicated, but each group is decided by a single atom and its neighbours. Visit every O and every N in turn and ask what is joined to it. Then visit every C=O and ask what is on either side.

Definition

Work through the molecule atom by atom:

C=O: look at its two other neighbours. H and C: aldehyde. Two C: ketone. OH: carboxylic acid. O-C: ester. N: amide.
O with only single bonds: if it is bonded to a C=O carbon, it is part of an acid or ester (above). Otherwise, bonded to H and C, it is an alcohol; bonded to two C, an ether.
N: next to a C=O carbon, amide; otherwise amine.
Also look for C=C (alkene), a benzene ring (arene), halogens and SH (thiol).
Do not count an OH or O twice: the OH inside COOH belongs to the acid, the O-C inside COO belongs to the ester.

Worked example

List the functional groups in

\mathrm{CH_3COOCH_2CH_2CH(OH)CHO}

Practice this conceptself-check · 4 quick reps

The OH of a COOH group is not an alcohol

In a carboxylic acid the OH is on the C=O carbon, and together they make one group. Listing 'alcohol' as well is double counting. An alcohol needs an OH on a carbon with no C=O.

Concept 3 of 4: Naming compounds that contain a functional group

The main chain must contain the carbon of the functional group, and the group gets the lowest possible number because it matters most. Its family ending replaces the final -e of the alkane name.

Definition

Choose the longest chain that contains the functional group carbon.
Number from the end that gives the functional group the lowest number, then the branches.
Endings: alcohol propan-2-ol, ketone butan-2-one, aldehyde butanal, acid butanoic acid, amine propan-1-amine. Aldehydes and acids need no number: their carbon is always carbon 1.
Esters are named from the alcohol part first, then the acid part with -oate: $\mathrm{CH_3COOCH_2CH_3}$ is ethyl ethanoate.
With two groups, the higher one gets the ending (acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine) and the other becomes a prefix: hydroxy-, oxo-, amino-.

Name with a functional group

\text{branches} + \text{stem} + \text{locant} + \text{group ending}

example4-methylpentan-2-ol
locantthe carbon of the group; omitted for aldehydes and acids

Worked example

Name

\mathrm{CH_3CH(CH_3)CH_2CH(OH)CH_3}

Practice this conceptself-check · 4 quick reps

Esters are named alcohol part first

\mathrm{CH_3COOCH_2CH_3}

the acid part is

\mathrm{CH_3COO}

(ethanoate) and the alcohol part is

\mathrm{CH_2CH_3}

(ethyl). The name is ethyl ethanoate. Reading the formula left to right and naming the first part first gives the wrong ester.

Concept 4 of 4: Functional groups in biological molecules

The big molecules of life are built from small units joined by the same groups you have just learned. Amino acids join through amide links, sugars through C-O-C links, and fats through ester links. Knowing the group tells you how the molecule is made and how it is broken down.

Definition

Amino acids carry an amine group and a carboxylic acid group on the same carbon. In water they exist as zwitterions ( $\mathrm{NH_3^+}$ and $\mathrm{COO^-}$ ), so they dissolve well in water, poorly in hexane, and act as both acids and bases.
Proteins are chains of amino acids joined by peptide bonds, which are amide groups.
Carbohydrates contain only C, H and O. Glucose $\mathrm{C_6H_{12}O_6}$ has five OH groups and an aldehyde group (in its open form); fructose has a ketone group instead.
Polysaccharides are long chains of sugar units. A homopolysaccharide uses one kind of unit (starch, glycogen and cellulose are all made of glucose only).
Triglycerides (fats and oils) are triesters of glycerol with three fatty acids.

Molecule	Made from	Linking group	Key fact
Protein (polypeptide)	Amino acids	Peptide bond (amide)	Every amino acid has NH₂ and COOH; cysteine also has a thiol SH
Starch	Glucose	Glycosidic link (C-O-C)	Energy store in plants
Glycogen	Glucose	Glycosidic link (C-O-C)	Energy store in animals (liver and muscle), not in plants
Disaccharide (sucrose, lactose)	Two monosaccharides	One glycosidic link	Sucrose is glucose + fructose
Triglyceride (fat, oil)	Glycerol and three fatty acids	Three ester groups	Insoluble in water; soluble in non-polar solvents

Practice this conceptself-check · 4 quick reps

Glycogen is a homopolysaccharide stored by animals, not plants

Glycogen is a branched chain of glucose units only, so it is a homopolysaccharide, not a heteropolysaccharide and not a disaccharide. Animals store glycogen; plants store starch.

Amino acids are water-soluble ions, not oily molecules

Because each amino acid carries both an acidic and a basic group, it exists as a zwitterion in water. That makes it dissolve well in water and badly in hexane, and gives it both acid and base behaviour.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Naming compounds that contain a functional group
Name with a functional group
$\text{branches} + \text{stem} + \text{locant} + \text{group ending}$

Reference tables (2)

The main functional groups and their families11 rows

Family	Group	Name ending	Example
Haloalkane	C-X, where X is F, Cl, Br or I	prefix chloro-, bromo-	Chloroethane $\mathrm{CH_3CH_2Cl}$
Alcohol	$\mathrm{-OH}$ on a saturated carbon	-ol	Ethanol $\mathrm{CH_3CH_2OH}$
Ether	C-O-C, no C=O	alkoxy- prefix, or 'dimethyl ether' style	Methoxymethane $\mathrm{CH_3OCH_3}$
Aldehyde	$\mathrm{-CHO}$ : C=O at the end of a chain	-al	Ethanal $\mathrm{CH_3CHO}$
Ketone	C=O between two carbons	-one	Propanone $\mathrm{CH_3COCH_3}$
Carboxylic acid	$\mathrm{-COOH}$	-oic acid	Ethanoic acid $\mathrm{CH_3COOH}$
Ester	$\mathrm{-COO-}$ between two carbons	alkyl ...-oate	Methyl ethanoate $\mathrm{CH_3COOCH_3}$
Amine	N bonded to C and H, no C=O next to it	-amine	Ethylamine $\mathrm{CH_3CH_2NH_2}$
Amide	$\mathrm{-CONH_2}$ : C=O bonded to N	-amide	Ethanamide $\mathrm{CH_3CONH_2}$
Nitrile	$\mathrm{-C{\equiv}N}$	-nitrile	Ethanenitrile $\mathrm{CH_3CN}$
Thiol	$\mathrm{-SH}$	-thiol	Ethanethiol $\mathrm{CH_3CH_2SH}$

An OH on a benzene ring is a phenol, a separate family that is more acidic than an alcohol.

Functional groups in biological molecules5 rows

Molecule	Made from	Linking group	Key fact
Protein (polypeptide)	Amino acids	Peptide bond (amide)	Every amino acid has NH₂ and COOH; cysteine also has a thiol SH
Starch	Glucose	Glycosidic link (C-O-C)	Energy store in plants
Glycogen	Glucose	Glycosidic link (C-O-C)	Energy store in animals (liver and muscle), not in plants
Disaccharide (sucrose, lactose)	Two monosaccharides	One glycosidic link	Sucrose is glucose + fructose
Triglyceride (fat, oil)	Glycerol and three fatty acids	Three ester groups	Insoluble in water; soluble in non-polar solvents

Watch out for (6)

Amine or amide: look for the C=O next to the nitrogen
→ The main functional groups and their families
Ethers have no C=O at all
→ The main functional groups and their families
The OH of a COOH group is not an alcohol
→ Finding every functional group in a larger molecule
Esters are named alcohol part first
→ Naming compounds that contain a functional group
Glycogen is a homopolysaccharide stored by animals, not plants
→ Functional groups in biological molecules
Amino acids are water-soluble ions, not oily molecules
→ Functional groups in biological molecules

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