Physical Properties of Amines

Compounds that contain carbon in their molecule in solid, liquid or gaseous state are called Organic Compounds. For Example, Nitrogen is an important class of organic compounds. The important classes of organic compounds containing nitrogen as a part of the functional group is called the Amines.

Amines are a significant class of chemical substances. Amine research is fascinating in a variety of ways. In biology, amino acid chemistry is crucial. It is necessary for all living things. We’ll look at the structure of amino acids and their physical characteristics in this section.

Class of Compounds	General Formula
Primary Amine	RNH2
Secondary Amine	R2NH
Tertiary Amine	R3N

What are Amines?

Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by an alkyl or aryl group.

NH₃ ⇢ RNH₂ ⇢ R₂NH ⇢ R₃N

Amines have a lone pair of electrons on the nitrogen atom. They are essentially ammonia derivatives. In Ammonia, substituent groups such as alkyl or aryl replace one or more hydrogen atoms, resulting in a new family of organic molecules known as Amines. Amino acids, biogenic amine, trimethylamine, and aniline are examples of important amine structures.

Amines constitute an important class of Organic compounds. Synthetic examples include polymers, drugs etc. For example, quinine is an important anti-malarial drug, adrenaline and ephedrine are used for increasing blood pressure, codeine is used as an analgesic, Benadryl is used as an antihistamine drug.

Structure of Amines

Amines are the derivatives of ammonia in which one or more hydrogen atoms has been replaced by alkyl groups. Hence, the structures of amines are similar to ammonia. In ammonia, nitrogen atoms undergo sp³  hybridization forming four sp³ hybrid orbitals. Three of these sp³ hybrid orbitals overlapped with s-orbital of H forms three N—H bonds and the fourth sp³ hybrid orbitals contain a lone pair of electrons.

Examples are Amino acids, biogenic amine, trimethylamine and aniline.

Classification of Amines

The Amines are classified as :

• Primary (1°): Primary amines are those amines if one hydrogen atom of ammonia is replaced by alkyl (R), such amine is called primary amine, which has the general formula: RNH₂.
• Secondary (2°): Secondary amine is that amine if two hydrogen atoms of ammonia or one H atom of  RNH₂ is replaced by another alkyl (R’) is called the Secondary amine with the general formula: R—NH—R’.
• Tertiary (3°): Tertiary amine is that amine if we replace all the three H atoms of ammonia or another H atom of R—NH—R’ by alkyl group (R”)  which gives R—N(R’)R”.

Amines are further classified into two categories:

1. Aliphatic Amines: Aliphatic Amines are those Amines in which the nitrogen atom is directly bonded to one or more alkyl groups. For example, CH₃NH₂  is a Methylamine (1°).
2. Aromatic Amines: Aromatic Amines are of two types-

• Aryl Amines: Aryl Amines are those amines in which the nitrogen atom is directly bonded to one or more which is the same or different aryl groups. Aromatic amines are called arylamines. For example,

• Arylalkyl amines: Those amines in which the nitrogen atom is bonded to the side chain of the aromatic ring is called aryl alkyl amines. This is also called a side chain substituted amines. For example,

Physical properties of Amines

The important physical properties of amines are:

• Physical state and smell: Lower members of the aliphatic amines family such as methylamine, dimethylamine are gases at ordinary temperature and have smelled like ammonia. The higher members with three or more C atoms are mostly liquid has a fishy smell. Most of the amines have an unpleasant odour. Aromatic amines are toxic. Most of the amines in pure form are colourless but they are easily oxidised and they become coloured due to impurities.
• Boiling Points: Amines have higher boiling points than hydrocarbons of comparable molecular mass because amines are polar compounds and with the exception of tertiary amines form associate molecules due to intermolecular hydrogen bonding between nitrogen of one and hydrogen of another molecule as shown below:

Amines have lower boiling points than those of alcohols or carboxylic acids because the electronegativity of nitrogen is lower than that of oxygen and hence O-H bonds present in alcohols and carboxylic acids are more polar than N-H bonds in amines. The hydrogen bonds in alcohols and carboxylic acids are stronger and have higher boiling points. 

• Comparison of boiling points of different groups of compounds: 

Compound	Molar Mass	Boiling point (K)
Methanoic acid	46	374
Ethanol	46	351.5
Ethanamine	45	290
Propane	44	231

In Isomeric Amines, the intermolecular association is more in primary amines than in secondary amines. Tertiary amines do not have intermolecular associations due to the absence of hydrogen atoms available for hydrogen bond formation. Hence, primary and secondary amines have the highest boiling point whereas tertiary amines have the lowest boiling points.

Thus, the order of Boiling points of isomeric amines is: 

Primary>Secondary>Tertiary

Example:    CH₃CH₂CH₂NH₂                                  (C₂H₅)₂NH                                                      C₂H₅N(CH₃)₂

Propylamine (Primary)                Diethylamine (Secondary)                            Ethyldimethylamine (Tertiary)

                  b.p. : 323 K                                    b.p. : 329.3 K                                                   b.p. : 310.5 K

• Solubility: The lower aliphatic amines are soluble in water because they are capable of forming hydrogen bonds with water.  However, Solubility decreases with increases in the molar mass of amines due to an increase in the size of the hydrophobic alkyl part. The higher amines containing six or more carbon atoms are insoluble because of weaker hydrogen bonds the solubility of amines in water is less than that of alcohols. For example, butan-1-amine is less soluble than butan-1-ol. The amines are also soluble in less polar solvents like ether, alcohol etc.

Aromatic Amines are insoluble in water because larger hydrocarbon part tends to retard the formation of hydrogen bonds. It is quite soluble in organic solvents such as ether, alcohols. For example, Aniline is insoluble in water. 

• Melting Points: The melting points of aryl amines increases with an increase in the molecular mass of the compound.
• Toxic Nature: Aryl amines are highly toxic substances as these compounds can destroy red blood cells and lead to mental instability.

Sample Questions 

Question 1: Why does methylamine have a lower boiling point than methanol?

Answer: 

Methylamine has a lower boiling point than methanol because Methylamine is polar and form intermolecular hydrogen bonds and its tendency to form intermolecular hydrogen bonds is less than methanol which is a highly electronegative oxygen atom.

Question 2: Trimethylamine and n-propylamine have the same molecular mass, the former boils at a lower temperature (276 K) than the latter (322 K). Why?

Answer: 

As n-propylamine has two hydrogen atoms on nitrogen atoms and form intermolecular hydrogen bonding. Therefore, its boiling point is high whereas, trimethylamine does not have hydrogen atoms on the nitrogen atom and it do not undergo hydrogen bonding. Therefore, its boiling point is low.

Question 3: Explain Tertiary amines do not undergo acylation.

Answer: 

Acylation reactions are those amines containing replaceable hydrogen atoms that react with acid chloride to form substituted amides. In tertiary amines, there is no replaceable H atom. Hence, it does not react with acetyl chloride and does not undergo acylation.

Question 4: Arrange the following in the increasing order of boiling points C₂H₅NH_2, C₂H₅OH, (CH₃)₃N.

Answer: 

The increasing order of boiling points is  (CH₃)₃N < C₂H₅NH₂ < C₂H₅OH

Question 5: Why are amines insoluble in water?

Answer: 

Amines are insoluble in water because the large hydrocarbon part attached to it is hydrophobic and secondly it includes that hydrogen bonding does not occur and more hydrocarbons get attached to it and become more hydrophobic.

Question 6: Why do primary amines have lower solubility than alcohols?

Answer: 

Primary amines have lower solubility than alcohols because alcohols have high polarity as compared to amines.

Question 7: Is amine flammable?

Answer: 

No, the amine is not extremely flammable at room temperature and amines have high boiling liquids that are combustible.