6.2.2 Amino acids, amides and chirality Flashcards
what is the general formula for amino acids?
RCH(NH2)COOH
how does the COOH group in amino acids react?
- reacts with bases (e.g. metals, metal carbonates, metal hydroxides) to form a salt
how does the NH2 group in amino acids react?
- reacts with acids (e.g. HCl, H2SO4, HNO3) to form a salt
what does amphoteric mean?
- amino acids are amphoteric and can react as both an acid and a base
what is a chiral carbon atom?
- has four different groups attached
- so have no plane of symmetry and so have a non-superimposable mirror image
what are optical isomers?
- non-superimposable mirror images
- amino acids are examples of this
- also known as enantiomers
what effects do enantiomers have on light?
- each enantiomer rotates plane polarised light by equal amounts in opposite directions
how can chiral carbon atoms be identified?
- must have 4 different groups attached and so 4 single bonds
- benzene or alkenes cannot have chiral carbon atoms
what is the relationship between chiral carbon atoms and stereoisomers?
- if a molecule has more than 2 chiral centres then it will have 4 stereoisomers
- in general, n chiral centres will have 2^n isomers
what is a zwitterion?
- the acidic carboxyl group and the basic amine group in the amino acid can interact to form n internal salt, known as a zwitterion
- a proton is transferred from the carboxyl group to the amide group
- a zwitterion contains both a positive and negative charge - so has no overall charge
when does an amino acid exist as a zwitterion?
- at a specific pH volume called the isoelectric point
- this will vary dependent on R-groups
what is the effect on zwitterions when the pH changes from the isoelectric point?
- if the pH decreases: acts as a base and accepts a proton
- if the pH increases: acts as an acid and donates a proton
what is the functional group of an amide?
- an acyl group RCO attached to a nitrogen
what is the structure of a primary amide?
- the nitrogen atom has 2 Hs
- general formula: RCONH2
what is the structure of a secondary amide?
- the nitrogen atom has 1 H on
- general formula: RCONHR’
how are amides made?
- the reaction of an acyl chloride with ammonia
what does N mean in the name of a compound?
- N indicates that the functional group stated next to it is bonded directly to the nitrogen atom
summarise the reactions of an acyl chloride
- RCOCl + NH3 -> amide
- RCOCl + RNH2 -> secondary amide
- RCOCl + H2O -> carboxylic acid
- RCOCl + ROH -> ester
How are amides hydrolysed?
- in acidic conditions:
- carboxylic acid formed
- in alkaline conditions:
- carboxylate salt formed
What is the isoelectric point?
- the pH at which the amino acid exists as a zwitterion
