Chapter 16: Nitrogen Compounds
Chapter 16: Nitrogen Compounds
Chapter 16
Nitrogen Compounds
Before we dive into the chemical intricacies of amines, let us look at some of its important physical properties.
| Boiling point | Solubility |
|---|---|
|
(i) For a particular class of amines (eg. comparing primary amines), boiling point generally increases with increasing size of electron cloud
(ii) Among isomeric amines, boiling point increases from tertiary amine to primary amine
(iii) Amines have lower boiling points than alcohols of similar size of electron cloud
(iv) Amines have higher boiling point than alkanes of similar electron cloud size
|
(i) Short chain amines are soluble in water and other highly polar solvents
(ii) As the non-polar alkyl groups in amines increases, the solubility of amines in water decreases
(iii) Phenylamine is only sparingly soluble in water
|
| Boiling point |
|---|
|
(i) For a particular class of amines (eg. comparing primary amines), boiling point generally increases with increasing size of electron cloud
(ii) Among isomeric amines, boiling point increases from tertiary amine to primary amine
(iii) Amines have lower boiling points than alcohols of similar size of electron cloud
(iv) Amines have higher boiling point than alkanes of similar electron cloud size
|
| Solubility |
|---|
|
(i) Short chain amines are soluble in water and other highly polar solvents
(ii) As the non-polar alkyl groups in amines increases, the solubility of amines in water decreases
(iii) Phenylamine is only sparingly soluble in water
|
All about Amines
Students must know that amines are Lewis bases in gaseous phase. Its N atom is able to donate its lone pair of electrons to an empty orbital of another atom.
However, amines are Bronsted-Lowry bases in an aqueous solution. It accepts a proton from a water molecule to produce OH- ion, by using its lone pair of electrons to form a dative bond with the proton dissociated from the water molecule.
Since amines partially ionise in aqueous solutions, they are weak Bronsted-lowry bases.
(Recall the 3 theories of acids & bases - Arrhenius, Bronsted-Lowry, Lewis)
To compare basicity of amines,
Any group that increases the availability of lone pair of electrons on N atom for donation will increase the basicity of the amine, vice versa.
Checklist when comparing basic strength of nitrogen compounds:
❏ Mention that basic strength of organic compound depends on availability of lone pair of electrons on N atom to donate to a proton
❏ Consider the presence of electron-donating groups and electron-withdrawing groups and the effects on the basicity of the organic compound
Relative basicities of the various amines in aqueous solution:
| Phenylamine | Ammonia | Ethylamine | |
|---|---|---|---|
| Basicity in aqueous medium | 
|
||
| Reason | Lone pair of electrons on the N atom partially delocalises into the benzene ring, reducing the availability of lone pair of electrons to be donated to a proton. | Ethyl group is electron-donating, increasing the electron density at the N atom. Hence, the availability of lone pair of electrons to be donated to a proton increases. | |
| Basicity in aqueous medium | |
|---|---|
| Phenylamine |
|
| Ammonia | |
| Ethylamine | |
| Reason | |
|---|---|
| Phenylamine | Lone pair of electrons on the N atom partially delocalises into the benzene ring, reducing the availability of lone pair of electrons to be donated to a proton. |
| Ammonia | |
| Ethylamine | Ethyl group is electron-donating, increasing the electron density at the N atom. Hence, the availability of lone pair of electrons to be donated to a proton increases. |
Amino Acids & Proteins
Students need to be clear of what amino acids are and their characteristics.
Properties:
1. Amino acids contain both –NH₂ (amino) group and –COOH (carboxyl) group
2. This is how an α-amino acid looks like:
Where R is either a H or an alkyl group
(Do note that for the ‘A’ Level syllabus, you are only required to know about α-amino acids)
3. All α-amino acids have at least one chiral carbon atom (α-carbon), thus able to have enantiomers (optically active). (Recall Enantiomerism)
Exception: Glycine (aminoethanoic acid)
4. Since amino acids show the properties of amines (basic) and carboxylic acids (acidic), amino acids are therefore amphoteric.
Acid-Base Properties
Since amino acids contain both –CO₂H and –NH₂ groups, what is the pH of amino acids?
Well this depends on the structure of the amino acid. More specifically, it depends on the number of –CO₂H and –NH₂ groups present.
| Number of –CO₂H and
–NH₂ groups |
Neutral, Acidic or Basic? | Colour of Litmus |
|---|---|---|
| –CO₂H = –NH₂ | neutral | purple |
| –CO₂H > –NH₂ | acidic | red |
| –CO₂H < –NH₂ | basic | blue |
| Number of –CO₂H and
–NH₂ groups |
|---|
| –CO₂H = –NH₂ |
| Neutral, Acidic or Basic? |
| neutral |
| Colour of Litmus |
| purple |
| Number of –CO₂H and
–NH₂ groups |
|---|
| –CO₂H > –NH₂ |
| Neutral, Acidic or Basic? |
| acidic |
| Colour of Litmus |
| red |
| Number of –CO₂H and
–NH₂ groups |
|---|
| –CO₂H < –NH₂ |
| Neutral, Acidic or Basic? |
| basic |
| Colour of Litmus |
| blue |
pI is the isoelectric point where the amino acid has no net charge. It is the pH at which the amount of positive and negative charges on an amino acid exactly balances each other. It is also known as the point where the amino acid is electrically neutral and is not necessarily pH 7.
Note: Amino acids can be a buffer since it is able to remove small amounts of H⁺ and OH⁻, rendering the pH almost unchanged.
Now that we are clear of what amino acids are, let us take a look at zwitterions:
Zwitterions
Zwitterion is a particle containing both a positive and a negative ion, and it has no net electrical charge. In other words, it is a neutral particle.
How it is formed: Acid-base reaction occurs internally in the amino acid, whereby H⁺ ion from the acidic –COOH group moves to the basic –NH₂ group.
Physical Properties:
- Crystalline solids with strong ionic bonds between the zwitterions. Thus, it has a high melting point. (Recall Chemical Bonding for explanation)
Worked Example 1:
A sodium salt of glutamic acid, monosodium glutamate (MSG), is a flavour enhancer that occurs naturally in tomatoes, mushrooms and some other vegetables.
(i) Draw the structural formula of the zwitterion formed from glutamic acid.
Answer:
Recall what a zwitterion is and how it is formed. (Zwitterion is a particle containing both a positive and a negative ion, and it has no net electrical charge. H⁺ ion from the acidic –COOH group moves to the basic –NH2 group. )
(ii) Suggest a likely pH for an aqueous solution of glutamic acid.
Answer: 3
(iii) Explain with the aid of relevant equations, how glutamic acid can act as a buffer on adding separate small amounts of:
(a) Dilute HCl
Answer:
pH remains relatively constant as most of the added HCl is removed by the reservoir of glutamic acid.
(b) Dilute KOH
Answer:
pH remains relatively constant as most of the added KOH is removed by the reservoir of glutamic acid.
(iiii) (a) Glutamic acid is a compound of many proteins and peptides. Draw the structural formula of a dipeptide formed between glutamic acid and glycine, NH₂CH₂CO₂H.
Answer:
(b) Name the functional group which is present in the dipeptide but not in the amino acids.
Answer: Peptide