hybridisation of nitrogen in primary, secondary and tertiary amines vs quaternary ammonium salts.
- BOTH n hybrid orbitals = 4
- BOTH sp3 hybridisation
- AMINES are trigonal pyramidal
-AMMONIUM SALT is tetrahedral - AMINES bond angle = 107
-AMMONIUM SALT = 109.5 - AMINES = 1 lone pair of e
- AMMONIUM SALT = no lone pairs
equilibrium equation
keq = Ka reactant acid / Ka reactant product
at equilibrium weaker acid is generated
what does the keq value indicate?
- keq > 0 = favourable reaction
- keq<0 = unfavourable reaction
what can pka tables be used to predict?
ionisation
what affects the avaliability of the nitrogens lone pair?
EWG
EDG
what affects the aqueous solubility of a compound containing N?
the number of hydrocarbon substituents bonded to the N
what affects the steric hindrance at the lone pair?
the number and size of substituents
what does the geometry affect?
protonation
what does the steric hindrance to the lone pair of electrons cause?
decrease in basicity.
compare the pka of an anilinium ion to a methylammonium ion.
the pKa of anilinium ion is 4.63
whereas
pKa of methyl ammonium is 10.66
why are arylamines (aniline) less basic than alkylamines?
- the nitrogen’s lone pair of electrons are delocalised by interaction with aromatic ring pi electron system
- less available to bond with H+
arylamines are stabilized how?
with respect to alkylamines because of five resonance structures which can be drawn.
what do EWG and EDG on the aromatic ring do?
increase or decrease basicity of the substituted analine.
sp2 hybridised nitrogen properties
- n of hybrid orbitals =3
- sp2 hybridised
- geometry = trigonal planar
- bond angles = 120
- the lone pair can be in an sp2 hybrid orbital or in a non-hybridised p orbital.
explain lone pair in unhybridised sp2 orbital.
-pi excessive
- N is electron donating group due to resonance effect (donation of lone pairs)
- lone pair is part of pi cloud of aromatic electrons
- lone pair cannot be protonated
explain lone pair in hybridised sp2 orbital.
- pi deficient
- N is electron withdrawing group due to inductive effect. (electronegativity)
- lone pair not part of pi cloud of aromatic electrons
- lone pair can be protonated.
EWG and EDG effect on heterocycles
electron density
hence their basicity
the effect of EWG and EDG is linked to their position on the ring (especially if the ring is aromatic)
the case of conjugation/aromaticity on heterocycles.
substituted heterocycles can exist in different tautomeric forms.
what is guanadine?
nitrogen rich functional group characteristic of the proteinogenic amino acid L-arginine.
characteristics of guanidine.
- strong base due to resonance delocalisation of the positive charge present on its conjugate acid (guanidinium cation)
- guanidine is flat and all notrogen atoms are sp2 hybridised
- high pka = guanidine derivatives can mimic inorganic metal cations
- can be used as anti-hypertensives due to blockage of ion channels.
what are free amine bases?
uncharged organic molecules - not very water soluable
what are quaternity ammonium salts?
charged organic molecules - relatively water soluable
common quaternary ammonium salts
- hydrochlorides
- hydrobromides
- carboxylates (succinates, tartrates, citrates)
- methanesulphonates (mesylates)
selection criteria for quaternary ammonium salts
- solubility
- pH of resulting solution
- strength of ionic couple
- ease of formulation
