Section 13.8 Amines and amides

Slightly relevant fact: amines and amides get their name from am- in ammonia

Highly irrelevant fact: ammonia gets its name from sal ammoniac, an outdated word for ammonium chloride NH₄Cl. The name came from sal ammoniacus, Latin for ‘salt of Ammon’. The word ammoniacus came into the Latin vocabulary from Greek, in the form of the Greek word ammōniakos (αμμωνιακος), meaning ‘of Ammon’.

Shockingly pointless fact: What’s Ammon and why is ammonium chloride named after it, you ask? The salt found by the Greeks near the temple of Jupiter Ammon at Siwa in Egypt was in fact ammonium chloride. As far as the Greeks were concerned this was just some random salt – they didn’t know it contained nitrogen or hydrogen or chlorine. They just named it ammōniakos after the temple.

What are amines and how are they named?

• Amines are the organic versions of ammonia
• An alkyl group takes the places of one, two or three of the hydrogens
• They are called primary, secondary and tertiary amines, respectively

• Smaller (lower) primary amines are called things like methylamine (CH₃–NH₂) and ethylamine (CH₃–CH₂–NH₂).

• Simple secondary and tertiary amines are also easy to name. Dimethylamine is CH₃–NH–CH₃ and trimethylamine is CH₃–N(CH₃)–CH₃

• Bigger amines have names beginning with amino. For example,

CH₃–CH(NH) –CH₂–CH₂–CH₃ is called 2-aminopentane.

• Amines with low RMM are gases or volatile liquids
• Volatile amines have strong smells like ammonia
• Ethylamine and trimethylamine smell like decaying fish
• 1,4-diaminobutane and 1,5-diaminopentane are called putrescine and cadaverine because they are given off when flesh rots

Properties of amines

• Amines are similar to ammonia but the alkyl groups modify their properties
• Most of their behaviour can be explained by the lone pair of electrons on the N

The lone pair of electrons explains why amines and ammonia are:

· very soluble in water

· a base

· a Ligand

· a nucleophile

Solubility of amines

• Amines form hydrogen bonds with water
• Small amines are soluble in water
• Larger amines are less soluble because their long alkyl groups disrupt the hydrogen bonding in water

Amines as bases

• The lone pair on the N can take part in dative covalent bonding
• An amine can donate a pair of electrons to an H⁺
• By generously donating two electrons, the amine is an H⁺ acceptor and acts as a base
• The amine in question becomes an alkylammonium ion, with one extra hydrogen than normal and a +1 positive charge
• If the amine has nicked a proton from water, an OH^- is left over, causing the solution to be alkaline
• Solutions of amines are alkaline
• Amines react with acids (usually the oxonium ion H₃O⁺) to form alkylammonium ions. Since the lone pair is playing the dative covalent bonding game, it can’t interact with everything else so much. Therefore, alkylammonium ions lose their smell
• The result is, adding acids to amines takes away their smell

Amines as ligands

• The lone pair on the N can take part in dative covalent bonding
• An amine can donate a pair of electrons to an H⁺

There should be a paragraph on amines as nucleophiles but I haven’t written it yet. It’s on page 332 of Chemical Ideas.

There should also be sections on

What are amides?

Hydrolysis of amides and

Condensation polymers involving the NH₂ group

but I couldn’t be bothered to write them when I made this page.

Hopefully I’ll update it sometime. Watch this space. It’s all on p332-334 of Chemical Storylines.

At least I’ve given you the answers to the questions so you can pretend you did them for homework.

Problems for 13.8

1 Name the amines with the following structures.

a		Ethylamine
b		Dimethylamine
c		2-aminopropane
d		Ethyldimethylamine
e		Cyclohexylamine

2 Draw structures for the following amines.

a	Propylamine
b	Phenylamine
c	Diethylmethylamine
d	Butylethylmethylamine
e	3-aminopentane
f	2,4-diaminopentane

3 Draw structures for the products formed when

2-aminopropane reacts with:

a	Hydrochloric acid
b	Ethanoyl chloride
c	Chloroethane

5 a Which of the reactions of amine described in Section 13.8 could not be undergone by a tertiary amine such as trimethylamine? Briefly explain your answer.

Tertiary amines cannot react with acyl chlorides to form amides because the amines are not bonded to any hydrogen atoms. Acyl chlorides can replace a hydrogen atom bonded to a nitrogen atom, liberating HCl, but they cannot replace an alkyl group.

b Explain, in terms of intermolecular forces and with the aid of a diagram, why butylamine is soluble in water.

The carbon chain in a butylamine molecule is terminated by an amine group, which has dipoles. The nitrogen atom has a partial negative charge due to its electronegativity being much higher than the hydrogen atoms it is bonded to.

Butylamine can dissolve in water by forming hydrogen bonds with water. Oxygen atoms in water hydrogen-bond to hydrogen atoms on the amine group. The nitrogen atom in the amine group hydrogen-bonds to hydrogen atoms in water molecules.

4 Write equations for the reactions of the following pairs of substances:

a		→
b		→
c		→
d		→
e		→
f		→

6 Complete the following reaction schemes by inserting the structures of the missing reactants or products, or by writing the reaction conditions on the arrow.

a
b
c
d

b Explain, with an equation, why a deep blue colour is formed when butylamine is added to copper(II) sulphate solution.

4C₄H₉NH_{2 (aq)} + Cu²⁺ _(aq) + 2H₂O _(l) → [Cu(C₄H₉NH₂)₄(H₂O)₂]²⁺ _(aq)

Copper sulphate solution is a source of aqueous copper(II) ions. Four butylamine molecules will each form a dative covalent bond to a single copper(II) ion using the electrons in the lone pairs on their nitrogen atoms. In doing so, butylamine molecules are acting as ligands.

The complex formed is a deep blue colour because the butylamine ligands create new energy levels for the copper atoms electrons in d-orbitals. These electrons can be excited to the new energy levels by photons of light.

When the excited electrons return to a lower energy level, they emit photons of light. These photons have the same energy as the difference in energy between the two electronic energy levels. The photon energy dictates its wavelength and therefore its colour, in this case deep blue.