Physical Properties of Amines

Lecture 7

Nitrogen Compounds

1. Introduction

2 Nomenclature of Amines

3 Physical Properties of Amines

4 Preparation of Amines

1. Reactions of Amines
2. Arendiazonium Salts
3. Uses of Amines and their Derivatives

Organic nitrogen compounds:

• amines, amino acids, amides and nitro compounds

Amines are organic derivatives of ammonia.

• We can think them as substituted ammonia molecules in which one, two or three of the ammonia hydrogens have been replaced by an organic group.

• According to the number of organic groups (both alkyl or aryl groups) directly bonded to the nitrogen atom, amines are classified as:

Examples of 1° amines:

Examples of 2° amines:

Examples of 3° amines:

 

 

Addition of the fourth organic group to tertiary amine yields a quaternary ammonium ion

 

 

Aromatic amines are formed when one or more hydrogen atoms in the ammonia are replaced by phenyl or substituted phenyl groups

e.g.

If the substituent is present on the nitrogen, it is designated by the prefix N.

 

Nomenclature of Amines

 

• 1° amines are named as alkylamines

e.g.

• 2° and 3° amines are named in the same general way
• If the organic groups attached to the nitrogen are the same, use the prefixes ‘ di- ’ or ‘ tri ’
• If they are not the same, use the prefixes ‘ N - ’ or ‘ N,N- ’ to designate the substituents

e.g.

 

Physical Properties of Amines

• Lower members of amines are gases and have a characteristic ‘ammonia’ odour.

• The higher members are liquids with a distinctive ‘fishy’ odour

Amine	Formula	Boiling point (°C)	Melting point (°C)	Density at 20°C (g cm–3)
Primary amines: Methylamine Ethylamine Propylamine Butylamine Pentylamine Hexylamine Phenylmethylamine Phenylamine	CH3NH2 CH3CH2NH2 CH3(CH2)2NH2 CH3(CH2)3NH2 CH3(CH2)4NH2 CH3(CH2)5NH2 C6H5CH2NH2 C6H5NH2	–6.3 16.6 48.6 77.8	–92.5 –84 –83 –50.5 –55 –19 –6.2	— — 0.719 0.741 0.761 0.768 0.981 1.022
Secondary amines: Dimethylamine Diethylamine N -Methylphenylamine Diphenylamine	(CH3)2NH2 (CH3CH2)2NH2 C6H5NHCH3 (C6H5)2NH	7.4 56.3	–96 –48 –57 52.8	— 0.707 0.989 —
Tertiary amines: Trimethylamine Triethylamine N,N-Dimethylphenylamine Triphenylamine	(CH3)3NH2 (CH3CH2)3NH2 C6H5N(CH3)2 (C6H5)3N	3.5 89.4	–117 –115 2.5	— 0.728 0.956 —

Boiling Point and Melting Point

1° or 2° amines can form strong hydrogen bonds with each other

Þ the b.p. is higher than those of alkanes but lower than those of alcohols of comparable molecular masses

∵ O – H bond is more polar than N – H bond as oxygen is more electronegative Þ stronger hydrogen bonds are formed between alcohol molecules

3° amines have lower b.p. and m.p. than 1° and 2°

all hydrogen in 3° amines are replaced by organic groups Þ no hydrogen bond can be formed

Density

Molecular mass of amines ­ Þ densities ­

Solubility

• The solubility of amines in water:
1° amines > 2° amines > 3° amines

∵ The intermolecular hydrogen bonds formed between 2° or 3° amines and water molecules decrease

• Amines are soluble in common organic solvents, e.g. hexane, ethanol and propanone

Preparation of Amines

Hydrogen reduces a nitro compounds to a 1° amine in the presence of nickel catalyst

Example:

Reduction of Nitriles

Reaction with Hydrogen

Hydrogen reduces a nitrile to a 1° amine in the presence of nickel catalyst

Example:

Reduction of Amides

Hofmann Degradation

• Amides with no substituent on the nitrogen react with Br₂ in NaOH (or Cl₂ in NaOH) to give amines

• The amines formed have one carbon less than the parent amides

Preparation of Phenylamine

Reactions of Amines

Basicity of Amines

• Amines are far weaker bases than hydroxide ions but are stronger bases than water

• The lone pair electrons on nitrogen atom can accept a proton from a water molecule Þ amines are basic

Amines dissolve in mineral acids such as HCl to form salts

e.g.

Methylamine will partially dissociate into ions

The basic strength of amines is shown by the value of base dissociation constant (K_b)

 

pK_b is used for convenience as K_b is small for weak bases
pK_b = –log K_b, the smaller the pK_b value, the stronger is the base

Name	Formula	pKb
Ammonia	NH3	4.75
Primary aliphatic amines: Methylamine Ethylamine Propylamine Butylamine	CH3NH2 CH3CH2NH2 CH3(CH2)2NH2 CH3(CH2)3NH2	3.36 3.27 3.16 3.39
Aromatic amines: Phenylamine	C6H5NH2	9.38

 

The basicity of amines is governed by the availability of the lone pair electrons on the nitrogen atom

• Primary aliphatic amines are the most basic

electron-releasing alkyl groups enhance the availability of the lone pair electrons on the nitrogen atom while ammonia does not

• Aromatic amines are weaker bases than ammonia

the lone pair electrons on the nitrogen atom are delocalized over the benzene ring making it less available to a proton

Acylation of amines

• 1° and 2° amines react readily in cold with ethanoyl chloride or anhydride to form N -substituted amides

Examples:

• 1° and 2° amines react with benzoyl chloride in the presence of alkalis to form N -substituted amides

Examples:

 

N-nitrosoamines

Examples:

As Drugs

Amine derivatives are commonly used as drugs such as painkillers (analgesics), transquillizers and antihistamines

1. Acetaminophen (also known as paracetamol)

• used to relieve pains

• less harmful to the stomach compared with aspirin

2. Chlorpheniramine

• helps to relieve allergic disorders caused by cold, insect bites and stings

• present in some over the counter drugs such as Coricidin, Coltaline, Piriton and Dristan

3. Chlorpromazine

• sedative effect without inducing sleep

• used to relieve anxiety, excitement, restlessness and even metal disorders

Контрольні питання до лекції 7.

1. Напишіть структурні формули сполук. Розташуйте сполуки за збільшенням їх основності. Дайте пояснення. П -хлоранілін, 2-бром-4-хлоранілін, о -толуїдин

2. Вкажіть правильну схему синтезу трет-бутанолу з ізобутилену. Напишіть реакції. А) ; В) ; С) ; D) .

3. Встановити будову сполуки за брутто-формулою та продуктами хімічного перетворення: С₇Н₉N, з соляною кислотою утворює сіль, після послідовної взаємодії з нітритною кислотою і нітритом калію в присутності міді утворює п-нітротолуол. Напишіть реакції.

4. Запропонуйте схему синтезу м -метилфенолу з хлорбензолу

5. Запропонуйте схему синтезу барвника з азо- і діазокомпоненти. Назвіть сполуки.

Lecture 7

Nitrogen Compounds

1. Introduction

2 Nomenclature of Amines

3 Physical Properties of Amines

4 Preparation of Amines

1. Reactions of Amines
2. Arendiazonium Salts
3. Uses of Amines and their Derivatives

Organic nitrogen compounds:

• amines, amino acids, amides and nitro compounds

Amines are organic derivatives of ammonia.

• We can think them as substituted ammonia molecules in which one, two or three of the ammonia hydrogens have been replaced by an organic group.

• According to the number of organic groups (both alkyl or aryl groups) directly bonded to the nitrogen atom, amines are classified as:

Examples of 1° amines:

Examples of 2° amines:

Examples of 3° amines:

 

 

Addition of the fourth organic group to tertiary amine yields a quaternary ammonium ion

 

 

Aromatic amines are formed when one or more hydrogen atoms in the ammonia are replaced by phenyl or substituted phenyl groups

e.g.

If the substituent is present on the nitrogen, it is designated by the prefix N.

 

Nomenclature of Amines

 

• 1° amines are named as alkylamines

e.g.

• 2° and 3° amines are named in the same general way
• If the organic groups attached to the nitrogen are the same, use the prefixes ‘ di- ’ or ‘ tri ’
• If they are not the same, use the prefixes ‘ N - ’ or ‘ N,N- ’ to designate the substituents

e.g.

 

Physical Properties of Amines

• Lower members of amines are gases and have a characteristic ‘ammonia’ odour.

• The higher members are liquids with a distinctive ‘fishy’ odour