Chapter 27.2

Question 1

What is an amino acid?

Answer 1

An organic compund with COOH, NH2, an ‘R group’ and a hydrogen atom around a central carbon atom. There are 20 common amino acids in the body that can be built into proteins and these differ by the functional (R) group.

Question 2

What is an alpha amino acid?

Answer 2

An amino acid where the amine group (NH2) group is attached to the central carbon atom - the alpha carbon atom.

Question 3

How can amino acids react in general?

Answer 3

They undergo reactions similar to both carboxylic acids and amines as there is a COOH group and an NH2 group.

Question 4

What types of amino acids are there other than alpha?

Answer 4

There are amino acids which are less common, where the amine group is connected to the beta carbon atom (the third carbon atom) or the y-carbon atom (the fourth carbon atom).

Question 5

How do amino acids react using the amine group?

Answer 5

The amine group is basic and so reacts with acids to make salts. This reaction can happen with HCl. Here the cation (H+) is attracted by the lone pair on the nitrogen atom (a nucleophile) and bonds to it, making N+. The Cl- then ionically bonds to the whole amino acid.

Question 6

How does the carboxylic acid group in amino acids react in general?

Answer 6

It can react with alkalis to form salts. It can react with alcohols to form esters.

Question 7

How does the carboxylic acid group react with aqueous alkalis?

Answer 7

Amino acids react with sodium/potassium hydroxide to form a salt and water. The H of COOH leaves the amino acid leaving O-. The cation (Na+ or K+), which has dissociated (as it is in aqueous form) from OH-, then ionically bonds to the O- on the amino acid.

Question 8

How are amino acids systematically named?

Answer 8

The carboxylic acid takes priority so glycine (where R group is just an H atom) becomes aminoethanoic acid.

Question 9

How does the carboxylic acid group react with alcohols in basic terms?

Answer 9

When heated with an alcohol in the presence of concentrated sulphuric acid, amino acids are easily esterified, producing an ester and H2O.

Question 10

What are amides?

Answer 10

Molecules with O=C-N in (basically a peptide bond). They are common in natural and since dipeptides and polypeptides are amides.

Question 11

How are amides formed in general?

Answer 11

Acyl chlorides bond with ammonia or an amine.

Question 12

How does the carboxylic acid group react with alcohols in detailed terms?

Answer 12

The acidic conditions (H+ from H2SO4) ‘protonate’ the NH2 so it becomes NH3+. The OH of COOH becomes O- and H+. The alcohol splits into OH- and a cation. The cation then bonds to the amino acid’s O- to form the ester. The OH- of the alcohol and the H+ of the amino acid bond to form H2O.

Question 13

How are amides categorised?

Answer 13

Primary, secondary and tertiary, depending on how many carbon atoms the nitrogen atom is bonded to.

Question 14

What is the bond in an amide called and what is it?

Answer 14

It is called a secondary amide linkage and it comprises O=C-N- AND any Hs that N is directly bonded to.

Question 15

What is an acyl chloride?

Answer 15

The organic compound RCOCl, where R is a side chain and -COCl is a functional group. -COCl is where a central carbon atom is double-bonded to an oxygen atom and bonded normally to a chlorine atom.

Question 16

How are primary amides formed in detail?

Answer 16

The acyl chloride reacts with two ammonia molecules (2 NH3). One of the H atoms of NH3 join the other NH3 atom, forming NH4+. The chlorine atom in the acyl chloride is substituted for the remaining NH2-. The NH4+ and the Cl- then ionically bond, leaving an amide and NH4+Cl+.

Question 17

How are secondary/tertiary amides formed in detail?

Answer 17

The process is the same as for primary amides but the two ammonia molecules have one or two H atoms replaced by CH3 molecules. This means that multiple carbon atoms will be bonded to the nitrogen atom in the amide.

Question 18

What are stereoisomers?

Answer 18

Compounds with the same structural formula but different arrangement of atoms.

Question 19

What is optical isomerism

Answer 19

A type of stereoisomerism found in molecules that contain a chiral centre.

Question 20

What are chiral centres?

Answer 20

Atoms that can bond to four other atoms (they have a valency of four) and therefore are chiral. In organic chemistry, the chiral centre will be a carbon atom bonded to four different atoms or groups of atoms.

Question 21

How does optical isomerism work?

Answer 21

The presence of a chiral carbon atom in a molecule leads to the existence of two non-superimposable (you can’t put one on top of the other so that they match) mirror image structures. These two molecules are known as optical isomers or enantiomers. For each chiral carbon atom, there is always one pair of optical isomers.

Question 22

Which amino acids are chiral?

Answer 22

All but glycine which has 2 H atoms around central/alpha carbon.

Question 23

What is chirality?

Answer 23

Where an atom (not just carbon) acts as a chiral centre by bonding to four different atoms or groups of atoms, thereby having an oppositie enantiomer.

Question 24

What are quick signs for whether an atom is chiral?

Answer 24

No double bonds, no similar groups

Question 25

How are optical isomers drawn?

Answer 25

One is drawn to show the 3D tetrahedral arrangement of the four groups around the central atom. Then the other isomer is drawn as a mirror image.