Amino Acids, Amides & Chirality

What is an α-amino acid?

An organic molecule containing an amine group and a carboxylic acid group bonded to the same carbon atom

What is the general formula of an α-amino acid?

RCH(NH₂)COOH

What does the R group represent in an amino acid?

A variable side chain that differs between amino acids

Why can amino acids act as both acids and bases?

They contain both a carboxylic acid group and an amine group

How does the carboxylic acid group behave in reactions?

It donates a proton and acts as an acid

How does the amine group behave in reactions?

It accepts a proton and acts as a base

What is a zwitterion?

A dipolar ion with both a positive and a negative charge in the same molecule

What charges are present in an amino acid zwitterion?

+NH₃⁺ on the amine and COO⁻ on the carboxyl group

At what pH do amino acids exist mainly as zwitterions?

At the isoelectric point

What is the isoelectric point?

The pH at which the overall charge of the amino acid is zero

What is an amide?

A compound containing a C=O group bonded to a nitrogen atom

How are amides formed from amines?

By nucleophilic addition–elimination with acyl chlorides

What functional groups are present in amides?

A carbonyl group and an NHR group

What is the general structure of a primary amide?

RCONH₂

What is an N-substituted amide?

An amide with an extra carbon chain bonded to the nitrogen

What is another name for secondary amides?

N-substituted amides

How are N-substituted amides named?

The prefix names the group on nitrogen and the suffix names the carbonyl-containing chain

What reaction type produces amides from acyl chlorides?

Nucleophilic addition–elimination

What is a chiral centre?

A carbon atom bonded to four different groups

Why do molecules with chiral centres lack symmetry?

The four different groups prevent a line of symmetry

How is a chiral centre often indicated in structures?

With an asterisk (*)

What is optical isomerism?

A type of stereoisomerism caused by different spatial arrangements of atoms

What causes optical isomerism?

The presence of a chiral centre

What are optical isomers?

Non-superimposable mirror-image molecules

What is another name for optical isomers?

Enantiomers

How do enantiomers differ in their effect on light?

They rotate plane-polarised light in opposite directions

What type of light is used to test optical activity?

Plane-polarised monochromatic light

Are all amino acids optically active?

Yes, except glycine

Why is glycine not optically active?

Its R group is hydrogen, so it lacks four different groups on the carbon

Why are most amino acids optically active?

They contain a chiral carbon atom

What happens when a solution contains chiral amino acids?

It rotates plane-polarised light