EP - ENGINEERING PROTEINS

Carboxylic Acids and Amines

Amino Acids and Proteins

Protein Building

Chemical Equilibrium

Reaction Rates

Enzymes

Carboxylic Acids

Amines and Amides

Amides

Carboxylic Acids

Carboxylic acids contain the carboxyl group

They are named after the alkane with the same number of carbon atoms.  The ending -ane is changed to -anoic acid.  Some examples are:

Formula	Systematic Name	Common Name
HCO2H	Methanoic acid	Formic acid
CH3CO2H	Ethanoic acid	Acetic acid
C2H5CO2H	Propanoic acid	-
CO2H	Benzoic acid	-
HO2C-CO2H	Ethanedioic acid	Oxalic acid

The C=O and the OH of the carboxyl group are so close that they modify each other's properties, hence carboxylic acids have many reactions which are different from those of both alcohols and carbonyl compounds.  Aromatic carboxylic acids have many properties in common with aliphatic ones.

The C in the -CO₂H group is always given the number 1 when naming substituted carboxylic acids, e.g.:

	2-hydroxypropanoic acid
	3-aminopropanoic acid

Carboxylic acids are soluble in water because they can form hydrogen bonds with water.

Reactions of Carboxylic Acids

1. Water.

   In water carboxylic acids form weak acid solutions.  An equilibrium is set up:

   CH₃CO₂H  +  H₂O    H₃O⁺  +  CH₃CO₂^-

   Carboxylic acids are stronger acids than alcohols and phenols.  It is the stability of the anion formed from the acid which decides how strong the acid is.  If the negative charge on the oxygen can be shared with other atoms, the anion will be more stable and more of it will be made.  The electric charge is spread out by delocalisation.

   

   Benzoic acid is stronger than ethanoic acid because the negative charge on the carboxylate ion is delocalised in the benzene ring and hence the ion is stabilised more readily.

2. Bases.

   Carboxylic acids form salts with metals, alkalis and metal carbonates.

   CH₃CO₂H  +  NaOH    CH₃CO₂Na  +  H₂O

   Here sodium ethanoate, CH₃CO₂Na, is formed.

   The evolution of carbon dioxide from carbonates and hydrogencarbonates is used as a test to distinguish carboxylic acids from phenols.

3. Esterification.

   Carboxylic acids react with alcohols in the presence of an acid catalyst, e.g. concentrated hydrochloric acid, to form esters.

   CH₃CO₂H  +  C₂H₅OH    CH₃CO₂C₂H₅  +  H₂O

   (Ethyl ethanoate, CH₃CO₂C₂H₅, formed)

4. Phosphorus Pentachloride.

   Carboxylic acids react vigorously with phosphorus pentachloride to produce an acyl chloride.

   CH₃CO₂H  +  PCl₅    CH₃COCl  +  POCl₃  +  HCl
5. Reduction.

   Carboxylic acids are reduced to alcohols by lithium tetrahydridoaluminate (lithium aluminium hydride), LiAlH₄.

   CH₃CO₂H    CH₃CH₂OH

Amines and Amides

Amines are organic derivatives of ammonia.  They are derived from ammonia by the replacement of hydrogen atoms in ammonia by alkyl or aryl groups.  If one hydrogen is replaced, the amine is a primary amine; if two hydrogens are replaced, a secondary amine results; if three are replaced, a tertiary amine results.

The alkyl groups in secondary and tertiary amines need not all be the same.

The prefix amino is sometimes used to indicate the NH₂ group, e.g.:

      3-aminopentane

An aromatic amine has the nitrogen atom directly attached to the benzene ring.

     phenylamine (aniline)

The nitrogen atom in ammonia has a lone pair of electrons on it.  The lone pair of electrons are responsible for ammonia being:

• very soluble in water
• a base
• a ligand
• a nucleophile

Amines similarly have a lone pair of electrons and have these properties too.

1. Solubility in Water

   Amines can form hydrogen bonds with water.

   

   Due to this strong attraction between amine molecules and water molecules, amines with small alkyl groups are soluble.  Amines with larger alkyl groups are less soluble because the alkyl groups disrupt the hydrogen bonding in water.

2. Amines as Bases

   The lone pair of electrons on the nitrogen can take part in dative bonding.  It can accept a hydrogen ion, hence it acts as a base.

   C₂H₅NH₂(aq)  +  H₂O(l)    C₂H₅NH₃⁺(aq)  +  OH^-(aq)

   The presence of hydroxide ions makes the solution alkaline.

   As amines are bases, they can react with acids to form salts.

   C₂H₅NH₂(aq)  +  HCl(aq)    C₂H₅NH₃⁺ Cl^- (ethylammonium chloride)
3. Amines as Ligands

   Ammonia is an effective ligand because the lone pair of electrons on the nitrogen can dative bond to transition metal.  Amines behave in a similar way.

   Cu(H₂O)₆²⁺  +  4C₄H₉NH₂   [Cu(C₄H₉NH₂)₄(H₂O)₂]²⁺  +  2H₂O
4. Amines as Nucleophiles

   Ammonia can act as a nucleophile with the lone pair of electrons attacking electrophiles such as halogenoalkanes.

   CH₃Cl  +  NH₃    CH₃NH₂  +  HCl

   Amines can act in a similar way:

   CH₃Cl  +  CH₃NH₂    (CH₃)₂NH  +  HCl

   Dimethylamine, (CH₃)₂NH, formed here.

   Amines will also react with acyl chlorides to form an amide.

   CH₃NH₂  +  >      +  HCl

   is an example of a secondary amide.

Amides

Amides contain the group:      

Primary amides are formed when ammonia is reacted with an acyl chloride:

  +  NH₃      +  HCl

Secondary amides are formed when an amine is reacted with an acyl chloride:

  +  CH₃NH₂      +  HCl

Amides are hydrolysed when refluxed with either acid or alkali.

CH₃CONH₂  +  H₂O  +  HCl    CH₃CO₂H  +  NH₃⁺Cl^-

CH₃CONH₂  +  NaOH    CH₃CO₂Na  +  NH₃