CHEMISTRY A LEVEL(FORM SIX) NOTES – ORGANIC CHEMISTRY 1.2- AMINES

ORGANIC CHEMISTRY 1.2- AMINES

i) STRUCTURES OF AMINES

Amines are derivatives of ammonia in which one or more hydrogen have been replaced by alkyl group or aryl group.

General formula:

If two hydrogens have been replaced we have;

2° amine

If 3 hydrogens have been replaced we have;

3° amine

NOTE:

Quaternary salt of amine

ii) NOMENCLATURE

IUPAC SYSTEM

iii) PREPARATION OF AMINES

1. ALKYLATION OF AMMONIA WITH HALOALKANES

Amine can be obtained by reaction of alkyl halide with excess ammonia.

Why excess ammonia?

So as to prevent the formation of 2° amine or 3° amine. Therefore, this is not suitable for preparation of amine because the substitution of hydrogen does not stop at the first stage.

2. REDUCTION OF NITROALKANES (R – NO2)

1° amines can be obtained by reduction of nitroalkane. The reducing agent is LiAlH4 or Sn/HCl or Fe/HCl.

3. REACTION OF AMIDES WITH LiAlH4

1° amides can be reduced to 1° amines.

4. AMMONIATION OF CARBONYL COMPOUND FOLLOWED BY REDUCTION WITH HYDROGEN IN THE PRESENCE OF NICKEL

5. REDUCTION OF NITRILES

6. REACTION OF ALCOHOLS WITH AMMONIA

Catalytic ammoniation of alcohol gives a mixture of amines.

7. HOFFMAN’S DEGRADATION OF AMIDES

It is important reaction in the conversion of amide to amine with one carbon less.

(i) By Hoffman’s degradation reaction

Solution:

4. PROPERTIES OF AMINES

a) PHYSICAL PROPERTIES OF AMINES

Boiling point and melting point

Amines have high boiling and melting points due to their ability to form hydrogen bonding as compared to hydrocarbons.

(i) Its boiling point is lower than alcohol (R – OH)

Reason: In oxygen, there are 2 lone pairs which increase its chance of hydrogen bonding.

It also has lower boiling point than carboxylic acid.

1° amines have higher boiling point than 2° amines. For 3° amines, there is no possibility of H–bonding since Nitrogen is not attached to hydrogen.

(ii) Solubility:

Lower aliphatic amines are soluble in water.

Reason: They can form hydrogen bond with water.

As the molecular mass increases, solubility decreases.

Reason: The long chain of hydrocarbon (R) is insoluble since it is hydrophobic. As the alkyl group becomes bigger, solubility decreases.

Amines containing 6 or more carbons do not dissolve in water.

E.g: aniline is insoluble due to this reason.

b) CHEMICAL PROPERTIES OF AMINES

1. Basic character

Amines are the most important organic bases. Aliphatic amines are slightly stronger Lewis bases than ammonia. Lone pair on the nitrogen is more readily available to accept a proton in amines than it is in ammonia.

R R— Positive inductive effect
| |
R-NH₂ R-N-H R-N-R

2° amine is the strongest base among the three.

The stability of hydrated ion decreases from 1° amine to 3° amine.

Reason: 3° amines form a very unstable ammonium salt. Although it can easily donate a lone pair of electrons as compared to 1° and 2° amines, it forms a very unstable hydrated ion.

Note:

The inductive effect on alkyl group tends to make 3° amines more basic while hydration effect tends to make 1° amine more basic. As a result of the combined effect of these two, the observed order of the base strength will be;

Methylammonium chloride

Q. Between the following amines, which is stronger base?

Reason: In Aniline there is negative mesomeric effect since lone pair is withdrawn from the amine group.

Mesomerism of aniline

For aromatic compounds, the lone pair is less readily available for protonation (addition of proton).

EFFECT OF SUBSTITUENT GROUP ON THE BASICITY OF ANILINE

· All activators make aniline more basic while deactivators make aniline less basic.

Electrons supplying groups

Electrons withdrawing groups

2. Alkylation (reaction with haloalkanes)

Example:

Quaternary salt

3. Reaction with ketones and aldehydes

Both aliphatic and aromatic 1° amines react with aldehydes and ketones to form a base known as Schiff’s bases.

E.g:

– Lewis acid is a substance that accepts a lone pair of electrons.

4. Reaction of 1° amines with chloroform (carbylamines reaction)

· Both aliphatic and aromatic amines on heating with chloroform and alcoholic KOH give isocyanides (carbylamines).

· The isocyanide form has offensive smell. It is used as identification test for 1° amine.

Carbylamines (offensive smell)

This reaction is a test for 1° amine.

(i) Convert;

5. Acylation

N-methylethanamide

N-phenylethanamide

6. Reaction with Nitrous acid

Note: It is used as a distinguishing test between different classes of amines.

· Aliphatic 1° amine reacts with HONO to form alcohol and nitrogen gas (you will see bubbles).

E.g:

– For aromatic amine react with HNO2 in cold (below 5ºC) to form diazonium salt.

· Aromatic 1° amines react with nitrous acid to give N-nitrosoamine which separates out as a yellow oily liquid.

It is a distinguishing test since we see yellow oil liquid.

Example:

– For aromatic 3° amines react with HONO to give green colour p-nitrosoamine.

This is a distinguishing test since there is a green colored product.

Reaction of amines with metal ions Ag⁺ & Cu²⁺

Reaction of aromatic amine due to benzene ring

1. Reaction with bromine water

· Aniline undergoes halogenations even in the absence of catalyst. With bromine it gives 2,4,6-tribromoaniline (white solid).

Here benzene ring is highly activated due to the presence of amine.

To obtain mono substituted derivative, aniline should be first acylated then halogenated. After halogenation, the acyl group is removed by hydrolysis to obtain mono substituted aniline.

2. Nitration

Direct nitration is not possible because aniline gets oxidized. Amine group should be protected by acylation, then nitration and finally hydrolysis.

3. Sulphonation

We have different products due to H-bonding. At 180°C, H-bonding will not be effective hence SO₃H will be at para position.

4. Diazonium salt

· It is used to form many other products.

· Reaction is nucleophilic substitution.

H/W

3. Why is phenylamine more soluble in HCl than in H₂O?

Phenylamine is more soluble in HCl since they will form a stable hydrated ion but it is less soluble in water due to the large hydrophobic part i.e. the benzene ring.

4. Define the term “base.” Write down structural formula of the following compounds and arrange them in order of increasing basic strength;

Methylamine, ammonia, ethylamine, ethanamide, phenylamine. Give reasons.

5. Give structural formula of the organic products that are obtained when;

(a) Ethylamine

(b) Phenylamine

(c) Phenylmethylamine

(d) N-methylphenylamine are treated with;

(i) Chloromethane

(ii) A mixture of NaNO₂ and HCl

(iii) Benzoylchloride

ANSWERS

Phenylamine is more soluble in HCl since they will form a stable hydrated ion but it is less soluble in water due to the large hydrophobic part i.e. the benzene ring.

4. A base is a substance which releases OH^– as the only negative ion.

QUESTIONS

1. An aromatic compound contains 80% carbon, 9.6% hydrogen and nitrogen. Its relative molecular mass is 135 g/mol. It is slightly basic and forms salts but gives no other reaction with nitric (III) acid. Identify the structure of this compound.

2. An organic base A contains 61% carbon, 15.3% hydrogen, 23.7% nitrogen. When treated with nitric (III) acid, A yields an alcohol B and nitrogen gas is evolved. The alcohol B contains 60% carbon, 13.33% hydrogen and on careful oxidation yields compound C, which has a vapor density of 29. Compound C forms an oxime with hydroxylamine (H₂N-OH) but does not react with Fehling’s solution. Suggest structures for the compounds A, B and C and indicate the cause of the above reactions.

3. How does the introduction of nitro group at –2- and –4- positions of phenylamine affect its basicity? Give reasons.

(a) Give the conditions under which the following reactions proceed

(b) Write the structural formula for each of the organic compounds formed when compound M reacts with each of the following;

ANSWERS:

∴ Structure of the compound is C₉H₁₃N

3. The nitro group attached to position –4– will make phenylamine more basic than the nitro group attached to position –2.

Reason: This is because in –2-nitrophenylamine there might be hydrogen bonding between the hydrogen and oxygen hence basicity will decrease.