amines and amino acids (A2)

Question 1

name this compound

Answer 1

N-ethyl-N-methyl phenyl amine

phenyl is used in the root name and not as a branch because its larger

Question 2

give the 3 functional groups of amines

Answer 2

primary amines = R-NH₂

secondary amines = R₁NHR₂

tertiary amines = R₁NR₂R₃

therefore primary, secondary and tertiary amines have different functional groups

Question 3

describe the physical properties of amines

Answer 3

• fishy smell
• 1º and 2º amines that are also short-chained are water soluble due to the polar N^𝛿--H^𝛿​+ bond which can form H bonds with water molecules
• 3º amines are insoluble in water because they do not have the polar N^𝛿--H^𝛿​+ bond
• 1º and 2º amines have BP similar to alcohols
• 3º amines have a lower BP than alcohols because they lack H bonds

Question 4

describe the ways amines can be synthesised

Answer 4

• nucleophilic substitution of haloalkanes with ammonia
  
  • problem is that you will get a mixture of different amines
  • you will not get a 100% yield if you are trying to synthesise a particular amine
  • also further distillation required to separate products
• reduction of nitriles to make primary amines
  
  • RCN + 4[H] → RCH₂NH₂
  • reducing agent = LiAlH₄ in dry ether (because it reacts vigorously with water)
• reduction of nitrobenzenes makes aromatic amines
  
  • C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + H₂O
  • because 6 moles of [H] are needed, a very strong reducing agent is required - Sn + HCl
  • also NaOH (aq) and heat are needed

Question 5

describe the chemical properties of amines

Answer 5

• weak bases - only partly dissociate in water
  
  • RNH₂ + H₂O ⇌ RN⁺H₃ + O^-H
  • the react with acids to form salts
  • RNH₂ + HCl ⇌ RN⁺H₃Cl^- (ammonium chloride salt)
  • they react with carboxylic acids to produce ammonium salts
  • RCOOH + RNH₂ ⇌ RNH₃OOCR
• can acts as nucleophiles due to the LP on N
  
  • nucleophilic substitution to produce 2º, 3º and 4º alkyl salts (which are crystalline ionic solids at room temp)
  • excess haloalkane needed for this to happen
  • react with acyl chlorides to form amides via nucleophilic addition elimination

Question 6

in order of strength of basicity:

primary amines > ammonia > aromatic amines

explain why

Answer 6

• primary amines are stronger bases than ammonia because the alkyl group on the primary amine pushes e- density towards the N atom so the LP on N is more available to accept a H+
• the LP on N in aromatic amines is delocalised into the aromatic ring = less e- density around the N atom = LP less available to accept a H+

Question 7

why are 3º amines weaker bases than 1º and 2º amines?

Answer 7

3º are the weakest bases due to steric hindrance (the 3 R groups prevent the LP on the N atom from accepting H+)

Question 8

describe the uses of amines

Answer 8

synthesis of drugs

make asodyes and indicators

make condensation polymers

4º alkyl ammonium salts can be used as cationic sufactants in detergents, fabric softener and hair conditioner - they coat the surface of hair/fabric with positive charges and prevent electrostatic forces with negatively charged e-

Question 9

give the general formula of 2-amino acids

Answer 9

amino group not always on carbon 2 but at a-level we focus more on 2-amino carboxylic acids

Question 10

describe the physical properties of amino acids

Answer 10

• all amino acids except glycine(2-amino ethanoic acid) show optical isomerism
  
  • natural amino acids are mostly - enantiomers
• medium and short chain amino acids are water soluble as both the NH₂ and COOH are polar groups
• higher BP than carboxylic acids/aldehydes/ketones with similar Mr due to twice as many H bonds/dipole-dipole forces between amino acid molecules and ionic bonds between zwitter ions (ionic bonds stronger than intermolecular forces)

Question 11

describe how amino acids can be synthesised

Answer 11

• most common way is via the hydrolysis of proteins
  
  • protein + (n-1) H₂O → amino acids
  • using HCl or enzymes as catalysts
  • • enantiomers will be produced only
• many different ways to synthesise amino acids in a lab depending on the starting product

Question 12

describe the chemical properties of amino acids

Answer 12

• each functional group keeps its separate chemical properties
  
  • e.g amine group has basic and nucleophilic properties
  • NH₂ become N⁺H₃ in low pH
  • amine group can be acylated to become amide
  • carboxyl group has acidic properties
  • COOH becomes COO^- in high pH
  • carboxyl group can be esterified
  • therefore amino acids are amphoteric (they can act as both bases and acids)
  • also be careful about R groups as they may also participate in reactions
• less acidic than most carboxylic acids and less basic than most amines
• zwitter ion character occurs due to the interaction between the two functional groups
  
  • LP on N accepts the H atom from the carboxyl group within the same molecule
  • this forms a zwitter ion which has both a positive charge (on the N atom) and a negative charge (on the O of the carboxyl group)
• amino acids undergo condensation polymerisation to form proteins (secondary polyamides)

Question 13

what is the isoelectric point

Answer 13

the pH value at which an amino acid precipitates (amino acid molecules become zwitter ions and form ionic bonds between the zwitter ions resulting in a solid)