Amines 2 Flashcards
what makes amines reactive
Difference in electronegativity between nitrogen and hydrogen atoms and
the presence of unshared pair of electrons over the nitrogen atom makes
amines reactive. The number of hydrogen atoms attached to nitrogen
atom also decides the course of reaction of amines
Moreover, amines behave as nucleophiles due to the presence
of unshared electron pair.
how to seperate amines from other organic cpds
Amine salts are soluble in water but insoluble in organic solvents
like ether. This reaction is the basis for the separation of amines from
the non basic organic compounds insoluble in water.
RNH3+X- + OH⁻ → R-NH2 + H2O+ X⁻
what shows the basic nature of amines
The reaction of amines with mineral acids to form ammonium salts
shows that these are basic in nature. Amines have an unshared pair
of electrons on nitrogen atom due to which they behave as Lewis base.
what factors affect the basicity of amines
Besides inductive effect, there are other effects like
solvation effect, steric hinderance, etc., which affect the basic strength
of amines.
Basicity of amines is related to their structure.
Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine.
alkylaminea are more basic than ammonia. why
i) Due to the electron releasing nature of alkyl group, it (R) pushes electrons towards nitrogen and thus makes the unshared electron
pair more available for sharing with the proton of the acid.
ii) Moreover, the substituted ammonium ion formed from the amine gets stabilised
due to dispersal of the positive charge by the +I effect of the alkyl group.
ammonia is more basic than aryl amines aka aniline.
It is because in aniline or other arylamines, the -NH2 group is attached directly to
the benzene ring. It results in the unshared electron pair on nitrogen atom to be in conjugation with the benzene ring and thus making it less available for protonation.
aniline is a resonance
hybrid of the following five structures.
On the other hand, anilinium ion obtained by accepting a proton
can have only two resonating structures (kekule).
We know that greater the number of resonating structures, greater is the stability. Thus you can infer that aniline (five resonating
structures) is more stable than anilinium ion.
effect of substituents of arylamines
In case of substituted aniline, it
is observed that electron releasing groups like –OCH3
, –CH3
increase
basic strength whereas electron withdrawing groups like –NO2
, –SO3H,
–COOH, –X decrease it.
order of basicity in gaseous phase
The order of basicity of amines in the gaseous
phase follows the expected order:
tertiary amine > secondary amine > primary amine > NH3
what change happens to ammonium ions in water/aq phase
In the aqueous phase, the substituted ammonium cations get stabilised
not only by electron releasing effect of the alkyl group (+I) but also
by solvation with water molecules.
The greater the size of the ion,
lesser will be the solvation and the less stabilised is the ion. The order of stability of ions are as follows:
primary> secondary>tertiary
Greater is the stability of the substituted ammonium cation, stronger
should be the corresponding amine as a base. T
steric hinderance as a factor
Secondly, when the alkyl group is small, like –CH3
group, there is no steric hindrance to
H-bonding. In case the alkyl group is bigger than CH3 group, there will be steric hinderance to H-bonding.
Therefore, the change of nature of
the alkyl group, e.g., from –CH3
to –C2H5
results in change of the order
of basic strength.
order of basicity in amines in aquoes phase
i)ethyl amines
ii)methyl amines
Thus, there is a subtle interplay of the inductive
effect, solvation effect and steric hinderance of the alkyl group which decides the basic strength of alkyl amines in the aqueous state.
i) 2>3>1
ii) 2>1>3
