Amines, amides, amino acids and proteins

Question 1

Amine Functional Group

Answer 1

Nitrogen compounds in which the hydrogen atoms in ammonia are replaced by alkyl or aryl groups

• The number of hydrogen atoms replaced determines whether it is primary, secondary or tertiary
• The nitrogen always has a lone pair attached
• Simple aliphatic amines are named as the alkyl or aryl group e.g. ethylamine
• When there are two or more functional groups, the prefix, amino is used
• Aromatic Amines can be formed when phenol groups replace the H atoms e.g. Phenylamine

Question 2

Properties of Amines

Answer 2

Solubility:

• simpler amines (shorter hydrocarbon chain) are readily soluble in water as they can hydrogen bond with it
• phenylamine has a large non-polar benzene ring so is only slightly soluble - there are intermolecular forces between molecules

State

• methylamine and ethylamine are gases at room temperature and smell like ammonia
• propylamine and butylamine are liquids at room temperature

Base Strength of Alkylamines

• as the hydrocarbon chain gets longer, the strength of the base increases - when more alkyl groups are added, there will be more electrons, so electron density increases and the electrons in the functional groups go closer to the nitrogen
• any value greater than 7.00 indicates a basicity greater than that of water
• the strength of a base is governed by the availability of the lone pair of electrons on the N atom to accept a proton
• secondary amines are stronger bases than primary amines because more electrons are pushed towards the nitrogen

Question 3

Preparation of Aliphatic Amines

Answer 3

Halogenoalkanes are heated in a sealed flask with excess ammonia in ethanol

1. The ammonia nucleophile attacks the partially positive carbon
2. The initial product is a salt of the amine
3. The free amine can be liberated from the salt by adding dilute sodium hydroxide solution
4. The electrons from one of the N-H bonds move to the N and form a bond with the OH^-
5. Products include water, halogen and the amine

Ammonia must be in excess, so that only primary amines are produced

Question 4

Amides

Answer 4

Nitrogen compounds derived from carboxylic acid, where the -OH group is replaced by -NH₂ or -NHR where R is an alkyl group

-C(O)NH-

• simple amides are named with the suffix -amide following the prefix of the relevant alkane
• If there is an alkyl group attached to the nitrogen atom, it is given the prefix N- e.g. N-methyl propanamide

Question 5

Preparation of Aromatic Amines

Answer 5

An amine with an aromatic ring attached

1. React a benzene ring with concentrated nitric acid and concentrated sulfuric acid in conditions of 50-60°C to add the NO₂ group onto the ring
2. Reduce it by boiling under reflux with tin and concentrated hydrochloric acid
3. To get the final product, separate by steam distillation

The tin reacts with the hydrochloric acid forming hydrogen gas and the product of the reaction of this hydrogen gas with the nitro group complexes with the tin chloride pulling the reaction to the right hand side.

The tin complex formed is broken down by mild basic hydrolysis, warming with dilute sodium hydroxide solution, to yield the final product.

Question 6

Reaction of Amine and Water

Answer 6

Aliphatic amines are quite strong bases (accept a proton), stronger than ammonia and readily from alkylammonium ions in water

The strength of the base depends on what the chain looks like and the availability of a lone pair

CH₃NH₂(aq) + H₂O(l) ⇔ CH₃NH₃⁺(aq) + OH^-(aq)

The solution produced is alkaline

Question 7

Reaction of Amine and Acids

Answer 7

Since amines are bases they are more soluble in hydrochloric acid than in water, and react with acid to form the ionic salt, alkylammonium chloride

C₄H₉NH₂ + HCl → C₄H₉NH₃⁺Cl^-

The lone pair accepts a proton to form a substituted ammonium salt, forming a white smoke which settles as a white solid​

The fishy smell of the amine disappears but reappears on the addition of sodium hdyroxide

C₄H₉NH₃⁺ + OH^- → C₄H₉NH₂ + H₂O

The addition of hydroxide turns the salt back into an amine

Question 8

Preparation of Amides

Answer 8

Amides can be made by reacting acyl chlorides with ammonia or with ammines

The by-product is hydrogen chloride gas

Ethanoyl chloride and ammonia → ethanamide

Propanoyl chloride + ethylamine → N-ethyl propanamide

Question 9

Forming Proteins

Answer 9

Proteins are condensation polymers of amino acids - they are made up of lots of amino acid joined by peptide (amide bonds) and the chain is put together by condensation reactions and broken by hydrolysis reactions

Water is also formed

A peptide bond is a formed when a molecule of water is removed from two glycine amino acids

When two amino acids join, a dipeptide is formed.

Further condensation reactions occur between dipeptides and the amino acid molecules to produce polypeptides and evenutally proteins

Question 10

Breaking up Proteins

Answer 10

Harsh conditions are required to break up or hydrolyse proteins:

• concentrated hydrochloric acid
• heating under reflux

Water is then added to reform the initial amino acids

Separating a mixture of amino acids

After hydrolysis, there will be a mixture of amino acids

Paper chromatography is used to separate them

• Only soluble substances will move

1. Spot the amino acids onto chromatography paper and run in a suitable solvent
2. Dry and spray with ninhydrin - turns amino acids purple/brown​
3. Measure the distance moved by the spot and solvent and calculate RF value
4. Compare RF value to data book and determine amino acid

The stationary phase is the thing the spots are moving on (paper)

The mobile phase is the thing moving the spots (solvent)

Question 11

Reaction of Amine and Acyl Chloride

Answer 11

Amines react vigorously with acyl chlorides to form N-substituted amides

The lone pair on the N atom of the amines, attacks the C^ς+ of the acyl chloride

Methylamine + ethanoyl chloride → N-methylethanamide

Butylamine + ethanoyl chloride → N-butylethanamide

To get an amide, react with ammonia

Question 12

Reaction of Amine and Halogenoalkane

Answer 12

A primary amine reacts with a halogenoalkane to produce a mixture of the salts or a secondary amine and tertiary amine

Amines react with the ς+ carbon atoms in the C-X bond of the halogenoalkanes in a nucleophilic substitution reaction.

The protonated amine formed in the first step then loses a proton to form the secondary amine

C₂H₅NH₂ + C₂H₅Cl → (C₂H₅)₂NH₂⁺Cl^-

(C₂H₅)₂NH₂⁺Cl^- + C₂H₅Cl → (C₂H₅)₃NH⁺Cl^- + HCl

The lone pair on the nitrogen of the secondary amine is more reactive than the lone pari on the primary amine because of the inductive effect, so the secondary amine can react with the halogenoalkane to form a tertiary amine

Question 13

Dicarboxylic Acids and Diols

Answer 13

This reaction forms polyesters and water

Question 14

Dicarboxylic Acids and Diamines

Answer 14

Makes polyamides and water

Question 15

Amino Acids

Answer 15

Has two functional groups: amino group (NH₂) and a carboxyl group (COOH)

General structure: ethanoic acid + amine group

Naming Amino Acids

1. Longest carbon chain including carboxyl group
2. Number carbon atoms in the chain
3. Write down position of any NH₂ groups and name ‘amino’
4. Write down any other functional groups

Chirality

Most amino acids except glycine will rotate polarised light as they are chiral since they have a central carbon atom attached to four different groups

Acid Base Behaviour

In aqueous solution, the carboxyl groups ionise producing H⁺, while amino groups act as a base and accept hydrogen ions. Amino acids form ions in aqueous solution that have both a negative and positive charge, called a zwitterion

The pH determines whether or not the acid or amine group changes

• At a higher pH alkaline, OH^- ions remove H⁺ ions to form a negative ion
• At a lower pH acid, H⁺ ions react with the zwitterion producing a positive ion

The isoelectric point is the pH at which amino acids exist as zwitterions