Amines

Question 1

What is an amine?

Answer 1

• Amines are organic derivatives of ammonia; where one or more hydrogen atoms of the ammonia molecule has been replaced with alkyl or aryl groups.
• Primary, secondary and tertiary amines refer to the number of substiuents (R-groups) on the nitrogen atom.
• If all four N-bonds have been replaced with an organic R-group (alkyl/aryl groups), the molecule is known as a quaternary ammonium ion. These are postively charged, and form complexes called quaternary ammonium salts, with any nearby negative ions.
• Small amines smell similar to ammonia, with a slightly ‘fishy’ twist.
• Larger amines smell very ‘fishy’.

Question 2

What are the differences in base strength between ammonia, primary aliphatic and primary aromatic amines?

Answer 2

• Amines can accept a proton (H+ ion) thus they are Bronsted-Lowry bases.
• An amines lone pair forms coordinate bonds.
• The strength of a base depends on how readily it will accept a proton; on how availible the nitrogen’s lone pair of electrons is.
• The more availible the lone pair, the more likely the amine is to accept a proton, the stronger the base it will be.
• Alkyl groups release electrons away from the alkyl group and towards the nitrogen atom; the inductive effect, shown as an arrow towards the N.
• Aryl groups withdraw electrons from the nitrgoen atom as the lone pair of electrons overlaps with the delocalised system on the benzene ring; the arrow is towards the benzene ring, from N.
• With aryl groups, the inductive effect means the electron density is greater on the nitrogen atom, thus it is a better elctron donor (more attractive to protons, H+).
• Primary alkylamines are stronger bases than ammonia, secondary alkylamines are stronger bases than primary alkylamines, as there are two inductive effects. Tertiary howeverm are not stronger than secondary, as they are less soluble in water.

Question 3

How are primary aliphatic amines prepared (made)?

Answer 3

• Primary aliphatic amines can be prepared from haloalkanes via heating with ammonia, and also by the reduction of nitriles to amines.

Question 4

What is the reaction of ammonia/consequent amines with bromoethane (haloalkanes)?

(Mechanisms)

Answer 4

• Amines are formed from heating a haloalkane (bromoethane in this instance) with an excess of ammonia.
• It is a nucleophilic substitution reaction.
• A mixture of primary, secondary and tertiary amines and quartenary ammonium salts are produced, as the amines produced are also nucleophiles, thus react with the haloalkane to produce a secondary amine, the secondary amine reacting to produce a tertiary amine, the tertiary amine reacting to produce a quarternary ammonium salt, which can’t react any further as it has no lone pair of electrons.
• The mixture of products can be seperated using fractional distillation.

1. Ammonia attacks the carbon of the haloalkane
2. The halogen (Br^-) fucks off, and an alkyl ammonium salt⁺ is produced.
3. A second ammonia molecule donates its lone pair of electrons to a hydrogen, which breaks of the alkylammonium salt, giving a primary amine and an ammonium salt, NH₄⁺Br^-.
4. The following amines then undergo the same mechanism as ammonia, attacking the C^δ+ of the haloalkane.

NH₃ + RX → [RNH₃]⁺X^-

[RNH₃]⁺X^- + NH₃ → RNH₂ + [NH₄]⁺X^-
primary amine

Question 5

What are the uses of quaternary ammonium salts?

Answer 5

• Quaternary ammonium salts are used industrially in the manufacture of hair and fabric conditions.
• They have a long hydrocarbon chain and a positively charged organic group, forming cations.
• Both wet fabric and wet hair pick up negative charges on their surfaces, thus the positive charges of the cations attract them to the wet surfaces and form a coating that prevents the build-up of static electricity, keeping the surface of the fabric smooth (in fabric conditioner) and prevents ‘flyaway’ hair (in hair conditioners).
• They are called cationic surfactants because in aqueous solution the ions cluster with their charged ends in the water and their hydrocarbon tails on the surface.

Question 6

How are primary aliphatic amines made from reducing nitriles?

Answer 6

Primary aliphatic amines can be prepared from haloalkanes in a two-step process:

1. Haloalkanes react with the cyanide ion in aqueous ethanol. The cyanide ion replaces the halide ion by nucleophilic substiution to form a nitrile:

RBr + CN^-→ R–C≡N + Br

2. Nitriles contain the functional group –C≡N; they can be reduced to primary amines by catalytic hydrogenation; with Ni/H₂.

R–C≡N + 2H₂ → R–CH₂NH_{2<br></br> E.g.}
CH₃≡N + 2H₂ → CH₃CH₂NH₂
ethanenitrile ethylamine

Nickel acts as a catalyst, and catalytic hydrogenation is carried out at high temperature and pressure.

Question 7

How are aromatic amines prepared?
(reaction mixture; equation not required)

Benzene + _____
_____ + _____

Answer 7

Aromatic amines are produced by reducing a nitro compound, such as nitrobenzene.

1. Heat a mixture of a nitro compound (nitrobenzene), tin (Sn) and concentrated hydrochloric acid (HCl) under reflux.
   This makes a salt.

Overall reaction:
C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O
2. Then to turn the salt into an aromatic amine, you need to add an alkali, such as sodium hydroxide solution.

C₆H₅NH₃+Cl + NaOH → C₆H₅NH₂ + H₂O + NaCl