Amines and Amino Acids (Senior 5 Chemistry): free lessons | Ganzaa

Amines and Amino Acids

Lessons

Senior 5 · Chemistry

Amines and Amino Acids

4 lessons

Lesson 1 of 4

Classifying, Naming and the Physical Properties of Amines

25 min

Why does lemon juice take the smell off fish?

Leave fresh sambaza from Lake Kivu in the sun for a few hours and the "fishy" smell grows strong. Squeeze lemon juice over cooked fish and the smell fades. Both observations have one chemical explanation. As fish spoils, bacteria convert a compound in its flesh, trimethylamine N-oxide, into trimethylamine, $\mathrm{(CH_3)_3N}$ : a volatile organic base with a strong fishy odour. The citric acid in lemon juice turns the amine into a salt, and salts are ionic, so they do not evaporate and cannot reach your nose.

Trimethylamine belongs to the amines, the organic relatives of ammonia. In this lesson you will classify and name amines and explain their physical properties from their structure. Their reactions, including that reaction with acid, come in the next lesson.

Definition

Amine: an organic compound formed when one or more hydrogen atoms of ammonia, $\mathrm{NH_3}$ , are replaced by alkyl or aryl groups. The functional group of a primary amine is the amino group, $\mathrm{-NH_2}$ . Example: ethanamine, $\mathrm{CH_3CH_2NH_2}$ .

Primary, secondary and tertiary amine (1°, 2°, 3°): an amine with one, two or three carbon atoms bonded directly to the nitrogen atom. Examples: $\mathrm{CH_3NH_2}$ (1°), $\mathrm{(CH_3)_2NH}$ (2°), $\mathrm{(CH_3)_3N}$ (3°).

Quaternary ammonium ion: a nitrogen bonded to four carbon groups and carrying a positive charge, e.g. tetramethylammonium, $\mathrm{(CH_3)_4N^{+}}$ . It has no lone pair left.

Aliphatic and aromatic (aryl) amine: in an aryl amine the nitrogen is bonded directly to a benzene ring, as in phenylamine (aniline), $\mathrm{C_6H_5NH_2}$ . In phenylmethanamine, $\mathrm{C_6H_5CH_2NH_2}$ , a $\mathrm{CH_2}$ separates the ring from the nitrogen, so it is an aliphatic amine.

Classifying amines

Count the carbon atoms joined to the nitrogen. This is a different rule from alcohols, where you count the carbons joined to the carbon that carries the OH. Figure 9.1 shows ammonia and the three classes built from it.

Figure 9.1: Replacing the hydrogens of ammonia one by one gives primary, secondary and tertiary amines. Each still has a lone pair on nitrogen.

The rule matters with branched groups. 2-Methylpropan-2-amine, $\mathrm{(CH_3)_3C{-}NH_2}$ , has a tertiary carbon skeleton, yet only one carbon is bonded to nitrogen, so it is a primary amine. The corresponding alcohol, $\mathrm{(CH_3)_3C{-}OH}$ , is a tertiary alcohol.

Naming amines (IUPAC)

Situation	Rule	Example
Primary amine	Take the alkane name of the longest chain carrying N, drop the final "e", add "-amine"; number the carbon carrying N as low as possible	$\mathrm{CH_3CH(NH_2)CH_3}$ : propan-2-amine
Secondary or tertiary amine	The longest carbon group is the parent amine; the other groups on N are prefixes with the locant N	$\mathrm{CH_3NHCH_2CH_3}$ : N-methylethanamine
Two identical groups on N	Write N,N- and "di"	$\mathrm{(CH_3)_2NCH_2CH_3}$ : N,N-dimethylethanamine
Two amino groups	Keep the "e" of the alkane, add "-diamine"	$\mathrm{H_2N(CH_2)_6NH_2}$ : hexane-1,6-diamine
A higher-priority group is present (acid, ester, amide, aldehyde, ketone, alcohol)	The amino group becomes the prefix "amino-"	$\mathrm{H_2NCH_2COOH}$ : aminoethanoic acid; $\mathrm{H_2NCH_2CH_2OH}$ : 2-aminoethanol
Nitrogen on a benzene ring	Phenylamine (IUPAC also accepts aniline or benzenamine)	$\mathrm{C_6H_5NH_2}$

Worked example

Problem. Name each amine and classify it as 1°, 2° or 3°: (a) $\mathrm{CH_3CH_2CH_2CH_2NH_2}$ ; (b) $\mathrm{CH_3CH_2CH(NH_2)CH_3}$ ; (c) $\mathrm{(CH_3CH_2)_2NCH_3}$ ; (d) $\mathrm{C_6H_5NHCH_3}$ .

Solution.

Four-carbon chain, N on C1, one carbon on N: butan-1-amine, primary.
Four-carbon chain; numbering from the right puts N on C2 (not C3): butan-2-amine, primary, because still only one carbon is bonded to N.
Three groups on N: two ethyl and one methyl. The longest group, ethyl, is the parent: ethanamine. The other groups are N-substituents: N-ethyl-N-methylethanamine, tertiary.
Parent phenylamine with a methyl on N: N-methylphenylamine, secondary (two carbons, one from the ring and one from the methyl group, are bonded to N).

Worked example

Problem. Draw and name the four amines with molecular formula $\mathrm{C_3H_9N}$ , and classify each.

Solution. Place the three carbons in every possible way around one nitrogen.

All three carbons in one chain, N at the end: $\mathrm{CH_3CH_2CH_2NH_2}$ , propan-1-amine (1°).
Chain of three, N on the middle carbon: $\mathrm{CH_3CH(NH_2)CH_3}$ , propan-2-amine (1°).
An ethyl and a methyl on N: $\mathrm{CH_3CH_2NHCH_3}$ , N-methylethanamine (2°).
Three methyls on N: $\mathrm{(CH_3)_3N}$ , N,N-dimethylmethanamine, also called trimethylamine (3°).

Answer: four structural isomers: two primary, one secondary, one tertiary.

Physical properties

State and smell. Methanamine, N-methylmethanamine, N,N-dimethylmethanamine and ethanamine are gases at room temperature; larger amines are liquids or solids. The small amines smell like ammonia or fish, and the diamines released as flesh decays, putrescine (butane-1,4-diamine) and cadaverine (pentane-1,5-diamine), smell of rot.

Boiling point. Nitrogen is more electronegative than carbon, so the N–H bond is polar and amines with an N–H bond form hydrogen bonds between molecules. Table 9.1 compares molecules of almost the same molar mass, so that London (dispersion) forces are about equal.

Compound	Type	$M$ / $\mathrm{g\,mol^{-1}}$	Boiling point / °C
butane, $\mathrm{CH_3CH_2CH_2CH_3}$	alkane	58.1	−0.5
N,N-dimethylmethanamine, $\mathrm{(CH_3)_3N}$	3° amine	59.1	3
N-methylethanamine, $\mathrm{CH_3NHCH_2CH_3}$	2° amine	59.1	37
propan-1-amine, $\mathrm{CH_3CH_2CH_2NH_2}$	1° amine	59.1	48
propan-1-ol, $\mathrm{CH_3CH_2CH_2OH}$	alcohol	60.1	97

Table 9.1: Boiling points of compounds with molar mass close to 59 g mol⁻¹ (values from PubChem, rounded).

Read the table from top to bottom:

The tertiary amine has no H on nitrogen, so it cannot hydrogen-bond to itself; only permanent dipole and London forces act, and it boils only a little above butane.
The secondary amine has one N–H per molecule and the primary has two, so the primary amine forms more hydrogen bonds and boils highest of the amines. Its unbranched chain also gives slightly stronger London forces.
The alcohol boils far higher still: O is more electronegative than N (3.44 against 3.04 on the Pauling scale), so O–H···O hydrogen bonds are stronger than N–H···N.

Solubility in water. All amines, even tertiary ones, can accept a hydrogen bond from water through the nitrogen lone pair, and 1° and 2° amines can donate one as well. Small amines are therefore very soluble: butan-1-amine mixes with water in all proportions. As the hydrocarbon part grows, solubility falls: hexan-1-amine dissolves only about 12 g per dm³, and phenylamine, with its large benzene ring, about 36 g per dm³ at 20 °C.

Where amines occur naturally

In decaying fish and meat: trimethylamine, putrescine and cadaverine.
As hormones and neurotransmitters: adrenaline (epinephrine, a 2° amine), dopamine and histamine (1° amines) carry signals in the body.
As alkaloids in plants: nitrogen bases such as nicotine (tobacco) and quinine (cinchona bark, used against malaria). Many are toxic or strongly active as drugs.
In every protein: as amino acids, the subject of Lesson 3.

Warning

Classify by nitrogen, not by carbon. A common exam error is to call $\mathrm{(CH_3)_3CNH_2}$ a tertiary amine. Count the carbons on N: one, so it is primary. Also, "secondary" or "tertiary" in an amine name does not change the suffix: every amine name ends in "-amine".

Safety. Volatile amines are flammable and irritate the eyes and lungs; phenylamine is toxic and is absorbed through the skin. Handle them in a fume cupboard with gloves and goggles. CDC (NIOSH) first aid for phenylamine: eye contact, irrigate with water immediately; skin, wash promptly with soap and water; breathed in, move to fresh air and give respiratory support; swallowed, get medical attention immediately.

Tip

To name a secondary or tertiary amine, find the longest carbon group on N first and make it the "-amine". Everything else on the nitrogen gets "N-" in front: N-ethyl-N-methylpropan-1-amine has a propyl parent, an ethyl and a methyl on nitrogen.

Confident Chemistry: How to Name Primary, Secondary and Tertiary Amines. Watch how the longest carbon group becomes the parent amine and the other groups on nitrogen receive the locant N.

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Practice

Classify each as 1°, 2° or 3°: (a) $\mathrm{CH_3CH_2NHCH_2CH_3}$ ; (b) $\mathrm{(CH_3)_2CHNH_2}$ ; (c) $\mathrm{C_6H_5N(CH_3)_2}$ .
Name: (a) $\mathrm{CH_3CH_2CH_2CH_2CH_2NH_2}$ ; (b) $\mathrm{CH_3CH_2N(CH_3)_2}$ ; (c) $\mathrm{H_2NCH_2CH_2NH_2}$ .
Give the structural formula of (a) N-ethylpropan-1-amine; (b) 3-aminopropanoic acid.
Draw and name every primary amine with formula $\mathrm{C_2H_7N}$ or $\mathrm{C_3H_9N}$ . (There are three in total.)
Explain why ethanamine (bp 17 °C) boils far higher than propane (bp −42 °C), although their molar masses are almost equal.
Propan-1-amine boils at 48 °C, but N,N-dimethylmethanamine, an isomer, boils at 3 °C. Explain the difference.
A student predicts that because trimethylamine cannot hydrogen-bond to itself, it must be insoluble in water. Evaluate this prediction.

Show answersHide answers

(a) 2° (two ethyl groups on N); (b) 1° (one carbon on N); (c) 3° (ring carbon and two methyls).
(a) pentan-1-amine; (b) N,N-dimethylethanamine; (c) ethane-1,2-diamine.
(a) $\mathrm{CH_3CH_2CH_2NHCH_2CH_3}$ ; (b) $\mathrm{H_2NCH_2CH_2COOH}$ .
$\mathrm{CH_3CH_2NH_2}$ ethanamine; $\mathrm{CH_3CH_2CH_2NH_2}$ propan-1-amine; $\mathrm{CH_3CH(NH_2)CH_3}$ propan-2-amine.
Ethanamine has polar N–H bonds and forms N–H···N hydrogen bonds between molecules; propane has only weak London forces, so much less energy is needed to separate its molecules.
The primary amine has two N–H bonds and forms hydrogen bonds; the tertiary amine has no N–H, so only weaker dipole and London forces hold its molecules together.
The prediction is wrong. Water can donate a hydrogen bond to the lone pair on trimethylamine's nitrogen, so the amine is very soluble. Hydrogen bonding to itself needs an N–H; hydrogen bonding with water needs only the lone pair.

Sources and credits

Boiling points and solubilities: compound records in PubChem, US National Library of Medicine (public-domain data). Electronegativities: Pauling scale.
First aid: NIOSH Pocket Guide to Chemical Hazards: Aniline, US Centers for Disease Control and Prevention.
Image: Rwanda sambaza2.jpg by Antoshananarivo, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: How to Name Primary, Secondary and Tertiary Amines, Confident Chemistry, embedded from YouTube.
The text, Figure 9.1, the tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why does lemon juice take the smell off fish?

2Classifying amines

3Naming amines (IUPAC)

4Physical properties

5Where amines occur naturally

Why does lemon juice take the smell off fish?

Classifying amines

Naming amines (IUPAC)

Physical properties

Where amines occur naturally