Chapter 19 - Nitrogen Compounds

Question 1

How do boiling points differ with increasing number of carbon atoms in nitrogen compounds, specifically amines?

Answer 1

As the number of carbon atoms and electrons increase, more energy is required to overcome the increasing strength of dispersion forces between molecules. Thus, boiling points increase with increasing number of carbon atoms.

Question 2

How do the boiling points of primary, secondary and tertiary amines differ?

Answer 2

Primary and secondary amines have higher boiling points than tertiary amines. Since tertiary amines have no N-H bond, they are unable to form hydrogen bonds. Their molecules are held together by weaker pd-pd interactions which require less energy to overcome as compared to stronger hydrogen bonds between molecules of primary and secondary amines.

Question 3

How do the boiling points of amines compare to its corresponding and alkane and alcohol respectively?

Answer 3

Alkane: amine has a considerably higher boiling point due to the presence of a stronger intermolecular hydrogen bonding compared to only weak dispersion forces in the alkane.
Alcohol: amine has a lower boiling point since the N-H bond is less polar than the O-H bond. Hence, the intermolecular hydrogen bonds in alcohols are stronger than those between amine molecules and require more energy to overcome.

Question 4

How do amides behave in room temperature and why?

Answer 4

Amides are white crystalline solids at room temperature (apart from methanamide which is liquid). Amides can form extensive intermolecular hydrogen bonds as a result of the polar nature of the N-H bond, and these hydrogen bonds result in relatively high melting and boiling points.

Question 5

Describe and explain the solubility of amines in water. (2)

Answer 5

1) aliphatic amines with 4 carbon atoms or less are readily soluble in water due to their ability to form strong hydrogen bonds with water molecules.
2) Phenylamines and amines with increasing length of hydrocarbon chain are virtually insoluble in water because the energy released from hydrogen bonding formed between these amines with water is less able to overcome the energy required to overcome the increasing strength of dispersion forces between larger hydrocarbon chains, as well as the existing hydrogen bonding in water. In addition, the non-polar hydrocarbon skeletons also disrupt the hydrogen bonding with water molecules. (They do dissolve in organic solvents though)

Question 6

Why can amides dissolve in water well?

Answer 6

They can form extensive hydrogen bonds with water.

Question 7

Why are amines basic?

Answer 7

Amines contain a lone pair of electrons on the nitrogen atom that can form coordinate (dative covalent) bond with a proton. Hence, amines can act as proton acceptors/Bronsted-Lowry bases. They can also act as Lewis bases by donating the electron pair to an acid.

Question 8

What are the 2 factors affecting the basicity of amines?

Answer 8

1) Inductive effect: Presence of electron-donating groups such as alkyl groups increase the electron density on the nitrogen atom and increase availability of the lone pair of electrons on N for donation to an acid.
2) Steric hindrance: Bulky groups attached to N hinder the donation of its lone pair to form a dative bond with an acid.

Question 9

Compare the basicity of different classes of amines in the gaseous phase and explain.

Answer 9

From strongest to weakest base,
Tertiary > Secondary > Primary > Ammonia
Aliphatic amines are more basic than ammonia because alkyl groups are electron-donating and they can increase the electron density on the nitrogen atom, making the lone pair of electrons more available to form a dative bond with an acid.

Question 10

Compare the basicity of different classes of amines in the aqueous phase and explain.

Answer 10

It is the same except tertiary amines are less basic than secondary and primary amines, so from strongest to weakest base, secondary > primary > tertiary > ammonia
This is because:
1) the bulky alkyl groups attached directly to the N atom hinder its donation of its lone pair to form a bond with a proton.
2) the resulting conjugate acids are not as effectively solvated through hydrogen bonding by the surrounding water molecules as compared to the conjugate acids of primary and secondary amines. Thus, the conjugate acids of tertiary amines are less stable, resulting in tertiary amines to ionise in water to a smaller extent.

Question 11

How does the basicity of phenylamines compare to ammonia and primary amines and why?

Answer 11

From strongest to weakest base, primary > ammonia&raquo_space; phenylamine
In phenylamine, the lone pair of electrons on nitrogen is delocalised into the benzene ring. This decreases the electron density on the nitrogen atom, making the lone pair of electrons less available to form a dative bond with a proton.

Question 12

How does the basicity of substituted amines (substituent attached to the benzene ring) differ from phenylamines? (2)

Answer 12

1) electron donating groups increase electron density on the benzene ring, making the lone pair of electrons on N more available for donation to an acid. Hence, it is more basic than phenylamine.
2) electron withdrawing groups decrease electron density on the benzene ring, making the lone pair of electrons on N less available for donation to an acid. Hence, it is less basic than phenylamine.

Question 13

How does the basicity of amides compare to other nitrogen compounds and why?

Answer 13

Amides give neutral solutions in water. The lone pair of electrons on the nitrogen atom delocalises over the C=O bond, which reduces the electron density on N such that the lone pair is not available for protonation.
Thus, it is a weaker base than all other nitrogen compounds (not sure about amino acids).

Question 14

How can amines be prepared? (4)

Answer 14

1) nucleophilic substitution of halogenoalkanes
2) reduction of nitriles
3) reduction of amides
4) reduction of nitrobenzene

Question 15

How are amides prepared? (1)

Answer 15

Condensation of acyl chlorides

Question 16

What are the reagents and conditions needed for the reduction of nitrobenzene to form amines? (2 steps)

Answer 16

1. Sn, excess concentrated HCl, heat

2. NaOH (aq)

Question 17

What are the 3 things amines can behave as?

Answer 17

1) bases (donate lone pair to a proton/acid)
2) good nucleophiles (lone pair can attack the partial positive end of a polarized bond)
3) excellent ligands with transition metals

Question 18

How can amines be separated from other organic compounds?

Answer 18

Add acid such that water soluble ionic salts are formed. The ionic salts will be soluble in water as they can form a favourable ion-dipole interaction with water molecules. They can be regenerated from their corresponding salts by using strong bases.

Question 19

What are the 4 types of reactions amines can undergo?

Answer 19

1) acid-base
2) nucleophilic substitution (with halogenoalkanes) for primary, secondary and tertiary
3) condensation (act as nucleophile to react with acid chlorides to form amides) for primary and secondary
4) electrophilic substitution of phenylamines

Question 20

Why can phenylamines undergo electrophilic substitution without catalysts (unlike benzene)?

Answer 20

The lone pair of electrons on the nitrogen atom is delocalised into the benzene ring, which greatly increases the electron density in the ring, making it much more susceptible to electrophilic attack than benzene. Hence, the presence of the -NH2 group highly activates the benzene ring towards electrophilic substitution.

Question 21

What are the reagents, conditions and observations of the electrophilic substitution of phenylamine?

Answer 21

Reagents and conditions: aqueous bromine, room temperature
Observations: yellow-orange solution decolourise, white ppt formed (white fumes of HBr may not be observed as HBr is dissolved in aqueous solution)

Question 22

Why are amides unable to act as a nucleophile like amines?

Answer 22

The lone pair of electrons on the nitrogen atom is delocalised into the adjacent C=O bond, reducing the electron density on the nitrogen atom. It makes amides very steady and least reactive out of the carboxylic acid derivatives.

Question 23

What are the 2 reactions amides can undergo?

Answer 23

1) reduction of amides to amines

2) hydrolysis of amides to form carboxylic acids

Question 24

What are the reagents and conditions needed for amides to be reduced?

Answer 24

LiAlH4 in dry ether

Question 25

What are the reagents and conditions needed for the hydrolysis of amides? (2)

Answer 25

1) acid hydrolysis: dilute H2SO4, heat under reflux

2) base hydrolysis: dilute NaOH, heat under reflux, followed by acidification with dilute H2SO4

Question 26

Define zwitterions.

Answer 26

They are amino acids that exist as dipolar ions with no overall electrical charge, only in aqueous solution and in the solid state.

Question 27

Why do amino acids have high melting points?

Answer 27

They are solids with high melting points because there are strong electrostatic forces of attraction between the dipolar zwitterions in the solid lattice structure. Large amounts of energy is required to overcome the strong ionic bonds between oppositely charged ions.

Question 28

Why are amino acids more soluble in water than in organic solvents?

Answer 28

There is strong ion-dipole interaction between the zwitterions and water molecules.

Question 29

What is the isoelectric point (pI) of an amino acid/protein?

Answer 29

It is the pH at which the overall net charge on the amino acid is 0 and it exists primarily as the neutral zwitterion. It depends on the nature of the side chain.

Question 30

How do we determine whether amphoteric amino acids behave more as cations or anions? (2)

Answer 30

1) when pH &laquo_space;pI, the amino acid exists predominantly as cations.
2) when pH&raquo_space; pI, the amino acid exists predominantly as anions.

Question 31

What are peptide bonds and how are they formed?

Answer 31

They are amide bonds between amino amino acids. They are formed from the condensation reaction between the -CO2H and the -NH2 groups on two amino acids through the elimination of a water molecule.

Question 32

What are polypeptides and protein molecule?

Answer 32

Peptides made from the condensation of up to about 50 amino acid molecules are known as polypeptides while anything larger is classified as a protein molecule.

Question 33

Define proteins.

Answer 33

Proteins are naturally occurring polypeptides of molecular weigh > 5000 and are formed when over 10^5 amino acids condense together, all linked by peptide bonds.

Question 34

How can proteins be hydrolysed into their constituent amino acids? (2)

Answer 34

1) appropriate enzymes

2) heat under reflux in the presence of aq acid/alkali for several hours in the lab.

Question 35

During reduction of nitrobenzene to phenylamine, why must the acidic mixture be neutralised before phenylamine can be separated? (2)

Answer 35

1) ensure all excess concentrated sulfuric acid has been neutralised
2) ensure all phenylamine has its amine group in deprotonated form. The deprotonated form is insoluble in NaOH and forms a separate layer from NaOH, allowing easier separation.