nitrogen compounds

Question 1

for amines, why does boiling point increase with increasing number of carbon atoms?

Answer 1

as the number of carbon atoms & electrons increases, more energy is required to overcome the increasing strength of dispersion forces between the molecules -> bp increases

Question 2

why do primary & secondary amines have higher boiling points than tertiary amines?

Answer 2

tertiary amines have no N-H bond -> unable to form H bonds -> tertiary amine molecules are held together by weaker pd-pd interactions which require less energy to overcome compared to stronger H bonds between molecules of primary & secondary amines

Question 3

why does boiling point increase in the order: ethane, methylamine, methanol?

Answer 3

• methylamine (CH3NH2) has a higher boiling point than its corresponding alkane, ethane, due to stronger intermolecular hydrogen bonding in methylamine compared to only weak dispersion forces in the alkane
• methanol (CH3OH) has a higher boiling point than methylamine as the N-H bond is less polar than the O-H bond, so the intermolecular hydrogen bonds in alcohols are stronger than those between amine molecules, and require more energy to overcome

Question 4

why are amines with increasing length of hydrocarbon chains (>4) and phenylamines virtually insoluble in water?

alipathic amines with four or fewer carbon atoms are readily soluble in water as they form strong hydrogen bonds with water molecules

Answer 4

phenylamines & amines with increasing length of hydrocabon chain (>4) are virtually insoluble in water as

• energy released from hydrogen bonding formed between amines with water is less able to overcome the energy required to overcome the increasing strength of dispersion forces between larger hydrocarbon chains in amines, and existing hydrogen bonding in water
• non-polar large hydrocarbon skeletons also disrupt hydrogen bonding with water molecules
• but these compounds are soluble in organic solvents

Question 5

LO: why does the basicity of amines in the gaseous phase decrease in the order: tertiary amine, secondary amine, primary amine, ammonia?

Answer 5

• (the basicity of amines depends on the availability of the lone pair of electrons on nitrogen for protonation)
• in the gaseous phase, alipathic amines are more basic than ammonia as alkyl groups are electron-donating and increase electron density on the nitrogen atom, making the lone pair of electrons more available to form a dative bond with an acid
• the number of alkyl groups increases in the order: primary, secondary, tertiary amine, so the electron density on the nitrogen atom increases in the same order -> availbility of the lone pair of electrons to form a dative bond with an acid also increases in the same order -> increased basicity

Question 6

LO: why does the basicity of amines in the aqueous medium decrease in the order: ethylamine, ammonia, phenylamine?

Answer 6

• phenylamine is less basic than ammonia as the lone pair of electrons on nitrogen is delocalised into the benzene ring -> decreases electron density on the nitrogen atom, making the lone pair of electrons less available to form a dative bond with a proton -> smaller extent of dissociation in aqueous medium -> less basic
• ethylamine is more basic than ammonia as the electron-donating alkyl group increases electron density on the nitrogen atom, making the lone pair of electrons more available to form a dative bond with a proton -> larger extent of dissociation in aqueous medium -> more basic

Question 7

what affects the basicity of amines in aqueous medium?

Answer 7

• when amines are dissolved in water, an equilibrium is established in which the water donates a hydrogen ion to the amine
• RNH2 + H2O ⇌ RNH3+ + OH-
• Kb = [RNH3+][OH-]/[RNH2]
• larger Kb/smaller pKb = greater extent of dissociation = stronger base

Question 8

how do substituents on the benzene ring affect the basicity of phenylamines?

Answer 8

• electron donating groups increase electron density -> lone pair of electrons on N atom more available for donation to acid -> more basic than phenylamine
• electron withdrawing groups decrease electron density -> lone pair of electrons on N atom less available for doantion to acid -> less basic than phenylamine

Question 9

LO: why are amides neutral

Answer 9

the delocalisation of the lone pair of electrons on the N atom over the C=O bond reduces electron density on the N atom such that the lone pair of electrons on the N atom is not available for protonation

Question 10

how can amines be prepared?

Answer 10

1. nucleophilic sub from halogenoalkanes
2. reduction of nitriles
3. reduction of amides
4. reduction of nitrobenzene

Question 11

how can amides be prepared?

Answer 11

1. condensation of acyl chlorides

carboxylic acids cannot be used in the preparation of amides, as they react with amines/ammonia in an acid base reaction

Question 12

reagents & conditions for reduction of nitrobenzene to form phenylamine?

Answer 12

reagents & conditions: 1. Sn, excess conc HCl, heat. 2. NaOH (aq)

Sn & excess conc HCl reduces nitrobenzene to a ammonium salt, so excess NaOH is added to liberate the free phenylamine

Question 13

reagents & conditions for reaction of amines with acids?

Answer 13

reagents & conditions: mineral acids/carboxylic acids, rt

Question 14

reagents & conditions for nucleophilic sub of amines with halogenoalkane?

primary, secondary & tertiary amines form secondary, tertiary amines & quarternary ammonium salts respectively

Answer 14

reagents & conditions: ethanolic conc. (amine), halogenoalkane, heat in sealed tube

Question 15

reagents & conditions for condensation of amines with acyl chlorides?

Answer 15

reagents & conditions: acyl chloride, amine in excess, rt

Question 16

reagents, conditions & observations for electrophilic sub of phenylamine with aqueous bromine?

alipathic amines do not react with aqueous bromine

Answer 16

reagents & conditions: Br2 (aq), rt
observations: yellow-orange solution decolourises, white ppt formed

Question 17

reagents & conditions for reduction of amides to amines?

Answer 17

reagents & conditions: LiAlH4 in dry ether

Question 18

reagents & conditions for acidic hydrolysis of amides?

Answer 18

reagents & conditions: dilute H2SO4, heat under reflux

Question 19

LO: what are zwitterions and how are they formed?

Answer 19

zwitterions are formed when amino acids undergo an intramolecular acid-base reaction and exist as dipolar ions with no overall electrical charge

Question 20

what are the physical properties of amino acids?

Answer 20

• amino acids are crystalline solids with high melting points, due to strong electrostatic forces of attraction between the dipolar zwitterions and solid lattic structure. a large amount of energy is required to overcome the strong ionic bonds between oppositely charged ions
• amino acids are more soluble in water than in organic solvents, due to the strong ion-dipole interactions between the zwitterions and water molecules

Question 21

LO: how is a peptide bond formed

Answer 21

• peptide bonds are formed from the condensation reaction between the -CO2H and the -NH2 groups on the 2 amino acids through the elimination of a water molecule
• a dipeptide is the result of 2 amino acids condensing together
• a tripeptide results from 3 amino acid molecules condensing together

a protein is formed from the condenstiaon of 50 amino acid molecules or more. (less than 50 is polypeptide)

Question 22

LO: how can proteins be hydrolysed into their constituent amino acids?

Answer 22

proteins can be hydrolysed into their constituent amino acids by an appropriate enzyme or heating under reflux in the presence of aqueous acid or alkali for several hours in the laboratory