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1. Structure and basicity
of amines
.

2. Structure of amines
• Name of the Molecule Ammonia
• Molecular Formula 𝑁𝐻3
• Hybridization Type 𝑆𝑃3
• Bond Angle 1070
• Geometry Pyramidal or Distorted Tetrahedral

3. Types of amines
10
20
30

4. Basicity of amines
• Lewis base: A substance that donates a pair of electrons to form
a covalent bond.
• Nitrogen has a lone pair of electrons which can be donated, hence it
acts as Lewis base.
• On comparison with alcohols , ethers, esters ,water , etc. because
nitrogen being less electronegative than oxygen , can better
accommodate the positive charge of the ion.
• 𝑁𝑅2
+
> 𝑂𝑅+ (order of stability)

5. (Group showing +I )

6. Inversion effect of tertiary amine
• If all the group of a tertiary amine are different , the amine will be
chiral.
• Therefore we will be having two enantiomeric forms of the tertiary
amine and theoretically we ought to be able to separate these
enantiomeric forms, but practically it is impossible because the
enantiomers interconvert rapidly.
𝑠𝑝3 𝑠𝑝2 𝑠𝑝3

7. .
• The 𝑠𝑝3
molecule of tertiary amine gets converted into 𝑠𝑝2
due to transfer
of electron from 2S to 2P (i.e pure p orbital), due to comparable sizes of 2S & 2P orbital.
• Energy barrier for this transition state is 24.2 KJ/mol .
• The entire inversion from 𝑆𝑃3 𝑡𝑜 𝑆𝑃2 & 𝑏𝑎𝑐𝑘 𝑡𝑜 𝑆𝑃3 happens at a frequency of 20
billions per second.
• Greater the extent of flipping, more will be the duration for which the
lone pair has changed its state from sp3 to pure p, and hence more will
be the unavailability of the lone pair to the electrophile (as the lone pair
in the hybrid orbital is more concentrated, for the electrophile)
• Thus, the extent of inversion varies inversely with the Lewis base nature
of the amine. Thus, basic nature varies inversely with the extent of
flipping.

8. Structure of Aniline
• Name of the Molecule aniline
• Molecular Formula 𝐶6𝐻5𝑁𝐻2
• Hybridization Type between 𝑆𝑃2 to 𝑆𝑃3 ( lone pair is in an
spx hybrid orbital with high p character)
• Geometry slightly pyramidalized

9. Basicity of aromatic amines
• Aromatic amines are much weaker bases than alkylamines.
• This can be answered on the basis of resonance contributions to the
overall hybrid of an arylamine.
• For aniline the following contributors are important:
• Structures 1 & 2 are the kekule structures that contribute to any benzene
derivative.
• Structure 3-5, however , delocalize the unshared pair of electron of nitrogen over the
ortho & para positions of the ring. This delocalization of electron pair stabilizes the
aniline and makes it less available to a proton. Therefore results in lower basicity
as compared to alkyl amines.

10. PE curve of Ammonia & Ammonium ion Vs Aniline &
Anilinium ion
• Ammonium ion is formed by the protonation of ammonia.
• Lewis dot structure of ammonia & ammonium ion is shown below
In case of aniline & anilinium ion it contains the benzene ring and thus shows hybrids of
kekule structures.

11. .
• This resonance presumably stabilizes both are amine and ion to the same
extent . It lowers the energy content of each by the same number of
kilocalories per mole and does not affect the difference in their energy
contents and thus does not affect the G of the ionization.
• However there are additional structure to be considered, where there is
a double bond between nitrogen and the ring.
• Contribution from these structures is simply a way of indicating the
tendency for nitrogen to share its fourth pair of electrons and to accept a
positive charge , this stabilizes the amine in a way that is not possible for
the ammonium ion.

12. • The net effect of resonance thus lowers the energy content of aniline
more than it lowers the energy content of the anilinium ion.
• This proves that low basicity of aromatic amine is thus due to the fact that
the amine is stabilized by resonance to a greater extent than its conjugate
acid.

13. Effect of substituents on basicity of aromatic
amines
• Electron releasing substituents like -𝐶𝐻3 increases the basicity of aniline and an
electron withdrawing substituent like -X or −𝑁𝑂2decreases the basicity.
• Electron release tends to disperse the positive charge of the anilinium ion and stabilizes the
and relative to the amine.
• Electron withdrawal tends to intensify the positive charge of the anilinium ion and thus
destabilizes the ion relative to the amine.

14. .
• The above groups substituted the ring at para position to amine , the effect of
substituent changes at ortho due to Ortho effect.
• Even electron releasing substituents weaken Basicity when they are ortho
to the amino group and electron withdrawing substituents do so to a much
greater extent from the ortho position than from the meta or para position.
• This is due to the steric hindrance between the ortho substituted group and H
atom of amino group which makes the conjugate acid less stable, hence
decreases the basicity of substituted aniline.
• From both the substituents(i.e. ortho & para) we can consider that an electron
releasing group pushes electrons towards nitrogen and make the fourth pair
more available for sharing with an acid and hence increases basicity, whereas an
electron withdrawing group help pull electrons away from nitrogen anthers make
the fourth pair less available for sharing, hence decreases basicity.

15. Thank you