Physical Properties of Amines – Solubility, Melting and Boiling Point

Physical Properties of amines: Have you ever heard the word “amino acid”? Yes, it is a compound that is the building block of proteins in living organisms. Amines are involved in the formation of amino acids. Many vitamins are made of amino acids. Serotonin is an important amine that functions as one of the major neurotransmitters in the brain. Amines are a significant class of organic compounds. Amine research is fascinating in a variety of ways. Amine chemistry is critical in biology. It is essential to all living things. Amines may be liquid, solid and gas at room temperature. To learn more about amines, read the below article.

Define Amines

Amines may be regarded as the derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl or aryl groups.

General Physical Properties of Amines

General physical properties of amines are:

1. Physical state, colour, and odour: Lower aliphatic amines are gases, higher members are solids, and the intermediate members are liquids under ordinary temperature and pressure. Lower aliphatic amines smell like ammonia; higher amines are fishy-smelling liquids. Lower arylamines are liquids that have an unpleasant odour. Higher arylamines are odourless solids with a low melting point. Pure amines are colourless, but take on colour over time. Aromatic amines are generally toxic.

2. Solubility: Lower aliphatic amines are soluble in water because these compounds form hydrogen bonds with water. 

However, the solubility of amines in water decreases with increasing molecular weight. Amines with six or more carbon atoms are almost insoluble in water. Aryl amines are insoluble in water. This is due to the larger hydrocarbon part in the molecule. Due to the hydrophobic nature of the hydrocarbon part, aryl amine molecules do not form hydrogen bonds with water. Both alkyl amines and aryl amines are soluble in organic solvents such as benzene, ether, alcohol, etc.

3. Boiling points: The boiling points of amines are higher than the non-polar hydrocarbons of comparable molar mass. This is because amines are polar compounds and capable of forming intermolecular hydrogen bonding.

Comparison of boiling points of different groups of compounds

The boiling points of amines are lower than those of alcohol and carboxylic acids of comparable molecular mass. This is because of the fact that hydrogen bonds in amines are weaker due to the lower electronegativity of nitrogen compared to oxygen in carboxylic acids and alcohols.

4. Melting points: Melting points of aryl amines increases with an increase in the molecular mass of the compound.

5. Toxic nature: Aryl amines are highly toxic substances. These compounds can destroy red blood cells, suppress heart muscle activity, and lead to mental instability.

Chemical Properties of Amines

Because of the presence of an unshared electron pair on the \({\rm{N}}\) atom, amines act as nucleophiles. The amines are reactive due to the difference in electronegativity between the \({\rm{N}}\) and \({\rm{H}}\) atoms and exhibit the following reactions:

A. The reaction of an amine with acids: On account of their basic nature, amines form salts with strong acids.

B. Exhaustive alkylation of amines: Amines are alkylated with alkyl halide on heating. The primary amine is converted into secondary and tertiary amines and finally into a quaternary ammonium salt. The process is known as exhaustive alkylation.

C. Acylation of amines: The aliphatic amines form substituted amides when treated with an acid chloride or acid anhydride.

D. The reaction of amines with nitrous acid: 

Nitrous acid has a different reaction with each of the three classes of amines (which is generally prepared by the action of mineral acid on sodium nitrite).

A. Aliphatic diazonium salts are formed when primary aliphatic amines react with nitrous acid. Even in cold temperatures, these salts are too unstable to isolate. They decompose as soon as they are formed, resulting in a mixture of different products, including alcohol. Ethanol, for example, is formed from ethanamine.

B. Pure methanol is not formed when methanamine is treated with nitrous acid.

C. Secondary amine reacts with nitrous acid to form nitroso amine.

E. Oxidation of amines: The oxidation products of the three classes of aliphatic amines differ depending on the nature of the oxidizing agent and the nature of the alkyl group. When aliphatic amines are exposed to air, they usually produce \({\rm{N}}-\)oxides.

F. Reaction of amines with aldehydes: Primary amines react with the aldehyde to form compounds known as Schiff’s base.

G. Carbylamine reaction: The carbylamine reaction is the formation of a foul-smelling isocyanide (carbylamine) by heating a primary amine with chloroform and ethanolic \({\rm{KOH}}\). This reaction is also known as the isocyanide test.

Amide

Amide is a compound with the general formula \({\rm{RC}}( = {\rm{O}}){\rm{N}}{{\rm{R}}^\prime }{{\rm{R}}^{\prime \prime }}\) where \({\rm{R,R}}\) and \({\rm{R}}\)″ represent organic groups or hydrogen atoms. In organic chemistry, an amide is also referred to as an organic amide or carboxamide. The amide group is referred to as a peptide bond when it is part of the main chain of a protein and an isopeptide bond when it is on a side chain, such as the amino acids asparagine and glutamine.

Properties of Amide

Bonding:

The lone pair of electrons on the nitrogen atom is delocalised on the carbonyl group and thus forms a partial double bond between nitrogen and carbon. In fact, the \({\rm{O, C}}\) and \({\rm{N}}\) atoms have molecular orbitals that are occupied by delocalised electrons and form a conjugate system.

Basicity:

Amides are very weak bases when compared to amines. The conjugate acid of an amine has a \({\rm{pKa}}\) of approximately \(9.5\), whereas the conjugate acid of an amide has a \({\rm{pKa}}\) of approximately \(0.5\). As a result, amides do not have as obvious acid-base properties in water. The carbonyl withdrawing electrons from the amine explains this relative lack of basicity.

Hydrogen Bonding and Solubility

Due to the higher electronegativity of oxygen, carbonyl \({\rm{(C = O)}}\) is a stronger dipole than the \({\rm{N – C}}\) dipole. The presence of a \({\rm{C = O}}\) dipole and, to a lesser extent, an \({\rm{N – C}}\) dipole allows the amides to act as \({\rm{H}}\)-bond acceptors. In primary and secondary amides, the presence of \({\rm{NH}}\) dipoles enables the amides to also act as \({\rm{H}}\)-bond donors. In this way, amides can participate in hydrogen bonds with water and other protic solvents; the oxygen atom can take up hydrogen bonds from water, and \({\rm{NH}}\)-hydrogen atoms can give off hydrogen bonds. Such interactions make the water solubility of amides greater than that of the corresponding hydrocarbons. These hydrogen bonds also play an important role in the secondary structure of proteins. The solubilities of amides and esters are approximately comparable. Amides are usually less soluble than comparable amines and carboxylic acids because these compounds can give up and take up hydrogen bonds.

Summary

Amines may be regarded as the derivatives of ammonia in which one or more hydrogen atoms have been replaced by alkyl or aryl groups. Amines may be gaseous, liquid and solid at room temperature. Lower aliphatic amines are soluble in water, whereas arylamines are insoluble in water. The boiling points of amines are higher than the non-polar hydrocarbons of comparable molar mass. The boiling points of amines are lower than those of alcohol and carboxylic acids of a comparable molecular mass. Melting points of arylamines increases with an increase in the molecular mass of the compound.

FAQs on Physical Properties of Amines

Q.1. What are the physical properties of amines?
Ans: The physical properties of amines are as follows:
i. Physical states of amine: The lower members of aliphatic amines are gases with a fish-like ammoniacal odour that vanishes in higher members. The higher amines are liquids, and still higher ones are solids.
ii. Solubility: Lower aliphatic amines are soluble in water because these compounds form hydrogen bonds with water. 
iii. Boiling points: The boiling points of amines are higher than the non-polar hydrocarbons of comparable molar mass. The boiling points of amines are lower than those of alcohol and carboxylic acids of a comparable molecular mass.
iv. Melting points: Melting points of aryl amines increases with an increase in the molecular mass of the compound.
v. Toxic nature: Aryl amines are highly toxic substances. These compounds can destroy red blood cells, suppress heart muscle activity, and lead to mental instability.

Q.2. What are the properties of amines?
Ans: Amino compounds can bind to hydrogen, giving them water solubility and high boiling points. The general structure of an amine is a nitrogen atom with a single pair of electrons and three substituents. However, nitrogen can bind to four substituents, leaving a positive charge on the nitrogen atom.

Q.3. What are the chemical properties of amines?
Ans: The chemical properties of amines are as follows:
i. Reaction of amine with acids: On account of their basic nature, amines form salts with strong acids.
ii. Exhaustive alkylation of amines: Amines are alkylated with alkyl halide on heating. The primary amine is converted into secondary and tertiary amines and finally into a quaternary ammonium salt. The process is known as exhaustive alkylation.
iii. Acylation of amines: The aliphatic amines form substituted amides when treated with an acid chloride or acid anhydride.
iv. Reaction of amines with aldehydes: Primary amines react with the aldehyde to form compounds known as Schiff’s base.
v. Carbylamine reaction: The carbylamine reaction is the formation of a foul-smelling isocyanide (carbylamine) by heating a primary amine with chloroform and ethanolic \({\rm{KOH}}\).

Q.4. What are the physical and chemical properties of amides?
Ans: Among all acid derivatives, amide has the lowest reactivity for nucleophilic acyl substitution reactions. The basicity of amides is much lower than that of amines. The amide behaves as both a weak base and a weak acid. The amide can be hydrolyzed by boiling with water, acid or alkali.
Amides generally have high boiling points and melting points. These properties and their solubility in water are caused by the polarity of the amide groups and hydrogen bonds.

Q.5. What are amines used for?
Ans: Following are the uses of amines:
i. Biological compounds such as adrenaline and ephedrine contain the secondary amino group.
ii. These compounds are used to increase blood pressure. 
iii. Benadryl contains a tertiary amino group. This is an antihistamine. 
iv. Novocaine is a synthetic amino compound. It is used as an anaesthetic in dentistry.
v. The diazonium salt is used to prepare useful aromatic compounds, such as dye.
vi. Quaternary ammonium salt is used as a surfactant.

Q.6. What are the acid-base properties of amines?
Ans: Amines react with acids to accept a hydrogen ion, making them a base according to the Bronsted-Lowry definition. This gives the amine a positive charge. Amines also acted as bases according to Lewis’s definition. The amino group has a lone pair of electrons when it forms three bonds.

We hope this article on ‘Physical Properties of Amines’ has helped you. If you have any queries, drop a comment below, and we will get back to you.