Organic Nitrogen Compounds

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Organic Nitrogen Compounds ­ Answers
1. Amines contain the group NR3, . There are three types of amine:
Primary amine: Only one carbon atom bonded to the nitrogen atom and, therefore, has an ­NH2 group.
Ammonia can be regarded as the simplest amine. E.g. methylamine, CH3CH2.
Secondary amine: Two carbon atoms directly joined to the nitrogen atom. All have an >NH group. E.g.
dimethylamine, (CH3)2NH.
Tertiary amine: three carbon atoms and no hydrogen atoms attached directly to the nitrogen atom. E.g.
trimethylamine, (CH3)3N.
2. The nitrogen atom in all amines has three -bonds and a lone pair of electrons. The four pairs of electrons are
arranged in a tetrahedron around the nitrogen. Therefore, the three bonding pairs are arranged pyramidally with
the H-N-H bond angle less than the tetrahedral angle. This is because the lone pair/bond pair repulsion is greater
than the bond pair/bond pair repulsion.
3. Amines can be named by:
Adding amine to the stem of the alkyl group, e.g. C2H5NH2 is ethylamine.
Adding the prefix amino- to the alkane from which the amine is derived, e.g. C2H5NH2 is aminoethane.
4. Physical properties of amines:
Boiling point: Methylamine is a gas at room temperature and pressure. Ethylamine boils
around room temperature and the next members of primary amines are liquids. Their boiling
temperatures are higher than those of the parent alkane because hydrogen bonding occurs
between amine molecules.
Smell: Amines have a characteristic fish-like smell, As they get bigger they tend to smell more
`fishy'.
Solubility: Amines are water-soluble because they form hydrogen bonds with water molecules.
The nitrogen is - and the hydrogen atom attached to it is +.
5. Amines can be prepared by reacting ammonia with a halogenoalkane:
Reagents: Aqueous alcoholic ammonia
Conditions: Heating the mixture in a sealed tube
Nucleophile: Ammonia (NH3)
Equation: C2H5Cl + 2NH3 C2H5NH2 + NH4Cl
Excess ammonia results in the primary amine, but excess of the halogenoalkane will yield the secondary
amine.
6. A nitrile or an amide can be reduced by warming with a solution of lithium tetrahydridoaluminate (III) in dry ether
followed by hydrolysis of the adduct with dilute acid.
CH3CN + 4[H] CH3CH2NH2
CH3CONH2 + 4[H] CH3CH2NH2 + H2O
This is an example of the reduction of a polar bond by the nucleophilic H- ion in LiAlH3.
7. Ammonia and amine molecules have a lone pair of electrons on the nitrogen atom that can be used to form a
dative covalent bond with an H+ ion; therefore amines are Bronsted-Lowry bases as they are proton acceptors.
The ability of an amine to act as a base depends on how well the lone pair on the N atom can accept H+. The
higher the electron density of the lone pair on the N, the better it will be able to accept H+, so the stronger the
base. The more alkyl groups that are substituted onto the N atom in place of H atoms, the more electron density
is pushed on the N atom, so the higher the electron density on the lone pair on the N, so the stronger the base. In
general, the order of base strength is:
Tertiary > secondary > primary > ammonia > aromatic
8. The base reactions of amines are similar to those of ammonia:
Reaction with water: Amines react reversibly with water, e.g. C2H5NH2 + H2O C2h5NH3+ + OH-. As the OH-
ions are produced, the solution is alkaline. The ­C2H5 group pushes electrons slightly towards the nitrogen
atom making it more - than it is in ammonia, so ethylamine is a stronger base than ammonia.
Reaction with acid: When an amine reacts with a strong acid, a salt similar to an ammonium salt is formed.
E.g. ethylamine and dilute HCl produce ethylammonium chloride: C2H5NH2 + HCl C2H5NH3+Cl-.
9. A nucleophile is something, which is attracted to, and then attacks, a positive part of another molecule or ion. All
amines contain an active lone pair of electrons on the electronegative nitrogen atom. It is these electrons, which
are attracted to positive parts of other molecules or ions.

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Amines have a lone pair on the N atom, so they can act as nucleophiles:
Reactions with halogenoalkanes: The H's on the N can be successively replaced by the R group from the
halogenoalkanes, right through to quaternary ammonium salts. This complicates the reaction, because any
amine produced in such a reaction can react with more halogenoalkane and undergo further substitution.…read more

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The pulling of the lone pair of electrons from the nitrogen atom by the benzene ring makes the nitrogen atom
less - and, therefore, less effective as a base and as a nucleophile:
Phenylamine as a base: Phenylamine is a weak base, so it reacts reversibly with water
C6H5NH2 + H2O C6H5NH3+ + OH-
Phenylamine reacts with strong acids to form phenylammonium salts. E.g.…read more

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Physical properties of amides:
Melting points: Amides have high melting points for their size, because they can form H bonds.
The hydrogen atoms in the ­NH2 group are sufficiently positive to form a hydrogen bond with a
lone pair on the oxygen atom of another molecule.
Solubility: Apart from those with a large hydrophobic group, amides are water-soluble due to the
molecules' ability to form several hydrogen bonds with water.
23.…read more

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H bonds that can form between the ­OH groups that occur on alternate carbon
atoms and water molecules.
33. Amino acid molecules contain at least one ­NH2 group and one ­COOH group. Amino acids have the general
formula:
Those amino acids that are the building blocks of proteins have the ­NH2 group attached to the carbon atom next
to the ­COOH group. These are called -amino acids, e.g. glycine, NH2CH2COOH.
34.…read more

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Nitriles contain the -C N group. They used to be called cyanides. The names of nitriles depend on the longest
carbon chain, including the carbon of the CN group. Some nitriles contain another group as well as ­CN.
43. Simple nitriles are prepared by the reaction between a halogenoalkane and potassium cyanide. E.g. the product
of the reaction between bromoethane and potassium cyanide is propanenitrile:
C2H5Br + KCN C2H5CN + KBr
This is a nucleophilic substitution reaction.…read more

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