Topic 17.5 Amines, Amides, Amino Acids & Proteins Flashcards
Introduction to amines
-Nitrogen containing compounds (similarities with ammonia)
-Three bonding pairs of electrons around nitrogen (distributed in a trigonal pyramidal shape)
-The nitrogen atom has a lone pair of electrons and three bonds to one or more alkyl groups
-If there is one alkyl group the amine is classes as primary, if there are two: secondary, if there are three: tertiary
-Amines occur widely in nature and many drugs (legal and illegal)
Preparation of aliphatic amines: from halogenoalkanes
-Heating the halogenoalkane with ammonia (under pressure, in a sealed container)
OR
-THe halogenoalkane can be mixed with concentrated aqueous ammonia
-CH3Cl + NH3 –> CH3NH2 + HCl
-The reaction involves nucleophilic attack by the lone pair of electrons ammonia (on the electron deficient carbon atom in the halogenoalkane)
-This means it could act as a nucleophile, competing with ammonia in the attack on the halogenoalkane
-CH3Cl + CH3NH2 –> (CH3)NH +HCl the product of this reaction is a secondary amine (eg. dimethylamine)
-Ammonia is used in excess to avoid side-reactions: CH3Cl + 2NH3 –> CH3NH3 + NH4Cl.
Preparation of aliphatic amines: from nitriles
-Nitriles can be reduced to primary amines by reduction (using reducing agent lithium tetrahydidoaluminate)
-Reactants are mixed in dry ether (to ensure there is no water)
-CH3CN + 4[H] –> CH3CH2NH2
([H] represents hydrogen atoms produced by the reagent).
Preparation of aromatic amines
-Made by the reduction of nitrobenzene
-The reducing agent is tin mixed with concentrated hydrochloric acid
-The mixture is heated under reflux
-The reduction is partly achieved through oxidation of tin to tin(II) ions and tin (IV) ions, and partly through the hydrogen produced in the reaction between tin and the acid
-C6H5NH3^+ +6[H] –> C6H5NH2 + H2O
Reactions with water
-The first few in the series of primary aliphatic amines are completely miscible with water
-As the hydrocarbon part of the molecule becomes proportionately larger, the solubility decreases
-They dissolve in water because they can form hydrogen bonds
-They also react with water to form alkaline solutions
CH3NH2 + H2O ⇌ CH3NH3^+ + OH-
NH3 + H2O ⇌ NH4^+ + OH-
Comparing basicities
-The basicity (basic strength) of a base can be quantified using the constant Ka or the constant pKa.
-The pKa of water 7.00
-Extending the hydrocarbon chain causes further, but smaller increases in basicity.
Reactions with acids
All amines react with strong acids to form ionic salts.
Reactions with ethanol chloride
-Addition-elimination reaction (two molecules join together and then a small molecule is eliminated)
-The organic product contains a new functional group- amide- in which a carbonyl group is next to a NH group.
Naming amides
Name contains two words:
-The first is the alkyl group from the amine
-The second indicates the number of carbons in the original acyl chloride.
Paracetamol
-Manufactured in a sequence of reactions, one of which is an addition elimination reaction
-‘Para’ indicates that the two groups attached to the benzene ring are at opposite ends of the ring
-The ‘acetam’ part comes from the old name of ethanamide with used to be called acetamide
-The ‘ol’ part indicates the presence of a hydroxyl group.
Reactions with halogenoalkanes
-The two would react together because a halogenoalkane contains an electron-deficient carbon atom and an amine contains an electron-rich nitrogen atom:
R1NH2 + R2X –> RNHR2 + HX
(where R1 is the alkyl group in the amine, and R2 is the alkyl group in the halogenoalkane)
-The organic product is a secondary amine and inorganic product is a hydrogen halide (often hydrogen chloride).
Reactions with copper(II) ions
-A blue precipitate forms, then with excess butylamine, the precipitate dissolves to give a deep blue
Amides
-Functional group consisting of a carbonyl group joined to an amino acid.
-Solids (except for methanamide, which is a liquid).
-The lower aliphatic amides are soluble in water because they contain two electronegative atoms and polar bonds so can form hydrogen bonds with water.
-The carbon atom is very electron-deficient because it is joined to both nitrogen and oxygen.
Preparation of amides
-Amides can be prepared in a lab by mixing acyl chloride with concentrated aqueous ammonia.
-The lone pair of electrons on the nitrogen of the ammonia molecule is strongly attracted to the electron-deficient carbon atom of the acyl chloride.
-The chlorine of the acyl chloride combines with one of the hydrogen atoms of ammonia to form hydrogen chloride, which appears as misty fumes.
-CH3CH2COCl + NH3 –> CH3CH2CONH2 + HCl
Polyamides
-The formation of polyamides also needs two monomers- a dicarboxylic acid and a diamine.
Nylon
Most familiar polyamide.
-Both of the monomers used in the production of it most common form contain six carbon atoms.
-Common examples of these monomers are hexanedioic acid and hexane-1,6-diamine.
Kevlar
-If the benzene rings take the place of the CH2 groups in the monomers used to make nylon 6,6, and a dioyl chloride is used in place of a dicarboxylic acid, then the monomers and the polymer structure can be shown like this:
This polymer produced is know as Kevlar and has very many uses, the most familiar as body armour (bullet-proof vests).
Amino acids
Over 20 amino acids are found in humans.
-Some of these are synthesised in the body
-Others must be provided in the diet
Acidic and basic properties
Isoelectric point: this is the pH at which the zwitterion (?) exists in aqueous solution.
A low isoelectric point indicates that the molecule is predominantly acidic, while a high value indicates that is predominantly basic.
Salt formation
All amino acids can form salts with acids and bases.
Optical activity of amino acids
Almost all amino acids contain a chiral centre (the C of the CH group) and so are optically active.
The enantiomers rotate the plane of polarisation of polarised light.
If the amino acid is synthesised in the lab, then a racemic mixture is formed.
What is a peptide?
When two amino acid molecules react together, an acid-base reaction occurs. The OH of the COOH group combines with one of the H atoms of the NH2 group to form water.
–> This is a condensation reaction.
The bond that form is a peptide bond and the organic product is a dipeptide.
Hydrolysing proteins
The polypeptide chains can be broken down into their individual amino acids by prolonged heating with hydrochloric acid.
This breaks the peptide bonds between the amino acids.
Using chromatography
A mixture of amino acids produced by hydrolysis of a protein can be spotted onto chromatography paper.
Using a soluble solvent, the individual amino acids will rose to different heights.
As amino acids are colourless, the chromatogram can be sprayed with a developing agent so that the positions of the amino acids van be seen.
