Resonance
• When a compound can be represented by
more than one Lewis structure and actual
structure is hybrid of all these structure,
the conpound is said to possess resonance
• Also called mesomerism
• Actual structure of molecule is called
resonance hybrid
• Canonical structure (resonance structure)

Resonance or Mesomerism
1. All the properties of a compound
cannot be explained by single
structure.
2. Canonical structures or resonance
contributing structures–differ in position
of electrons.
3. Delocalisation of electrons leads to decrease in
potential energy of molecule.
canonical structures of
benezene
Resonance hybrid
structure of benezene

Resonance or Mesomerism
4. Resonance hybrid is more stable than
canonical structures.
5. Resonance structures are imaginary.
6. Resonance energy = Actual energy of hybrid–energy of
most stable contributing structure.
7. Resonance is measure of stability.

Rules for Drawing Resonance Structure
1. The relative positions of nuclei should remain
unchanged.
2. All the canonical forms do not contribute equally i.e
Benzene.
3. All the contributing structure should be bonafied
Lewis structures.
4. The electrons always move away from a negative
charge.
5. Arrows should be drawn to indicate the direction of
the movement of electrons.

Resonance Effect
• The increase in electron density at one position
with corresponding decrease at another
position is called resonance effect.
• Types of Resonance Effect
• +Resonance
• - Resonance
• Groups attached which donate electron shows
+ resonance effect while groups attached
which withdraw electrons shows - resonance

Types of Resonance
For substituted benzene
NH2 NH2 NH2
+
–
+
–
etc.
+R effect of –NH2 group.
N
O O
N
O O
N
O O
–
etc.
+
+
–
+
–
+
+
– –
–R effect of –NO2 group.

Inductive Effect
Permanent dipole induced in one bond by another is called
Inductive effect, when there is a covalent bond b/w unlike
atoms
The bonding electron pair is nearer to more electronegative
atom the bond is said to b polarized.
c-c-cl
.Types of inductive effect
– I effect=3NH, CN, COOH, F, Cl, Br, I,
RO
+I effect R3C, R2C, RCH2, CH3
Electron withdrawing groups creates –I
effect
Electron donating groups creates + I
effect

Features of Inductive Effect
1. Chloroacetic acid is a stronger acid
than acetic acid because
Cl - CH2 C
OH
O
Cl-CH2 - C
O
O
H
+
+
-I
CH3-C
OH
O
CH3-C
O
O
H
+
+
+I
Ka = 1.4 × 10-3
Ka = 1.75 × 10-5

Features of Inductive Effect
2. The larger is the electron-withdrawing
effect of a group, the greater is the
–I (inductive) effect.
F CH2 COOH Br CH2 COOH
-32.5×10 -31.3×10Ka
3. Inductive effect is additive
Cl3CCOOH Cl2CHCOOH
Ka
-12.3×10 -25.4×10

Features of Inductive Effect
4. Since this effect is transmitted
through a chain it becomes less
effective with distance
ClCH2CH2COOH ClCH2CH2CH2COOH
Ka
-48.32×10 -53.02×10

Class exercise 1
The hybridization of carbon
atoms C — C single bond in
vinylacetylene is
(a) sp3
- sp3
(b) sp - sp2
(c) sp2
- sp (d) sp3
- sp
H2C CH — C CH
1 2 3 4
Vinylacetylene
Solution :
Hence answer is (c).

Class exercise 2
Allyl isocyanide has
(a) 9 bonds and 4 bonds
(b) 8 bonds and 5 bonds
(c) 8 bonds, 5 bonds and 4 non-bonding electrons
(d) 9 bonds, 2 bonds and 2 non-bonding electrons
π
π
π
π
σ
σ
σ
σ
H2C CH — CH2 — N C
Allyl isocyanide
+ –
The compound has 3 π bonds and
one lone pair, i.e. two non-bonding
electrons. It also contains 9 σ-
bonds. Hence answer is (d).
Solution:

Class exercise 3
Among the following which has the
most acidic -hydrogen?α
CH3CCH2CHO
O
CH3
CCH2
CCH3
O O
CH3
CCH2
COOCH3
O
(d) CH3CHO
(a) (b)
(c)

Solution
H3C — C — CH2 C — CH3
O O
H3C — C — CH2 C — OCH3
O O
H3C CH
O
H3C — C — CH2 — CH
O O
α
two e-withdrawing keto
groups.
α
keto and ester group.
α
one aldehyde group.
α
one keto and one
aldehydic carbonyl group.

Solution
Since e-withdrawing nature of
gas varies as aldehyde > keto > ester
Then most acidic a-H atom is present in
C O
H3C C — CH2 — CH
O O
Hence answer is (a).

Class exercise 4
The decreasing order of acidity among
phenol, p-methylphenol, m-nitrophenol
and p-nitrophenol is
(a) m-nitrophenol, p-nitrophenol, phenol,
p-methylphenol
(b) p-nitrophenol, m-nitrophenol, phenol,
p-methylphenol
(c) p-methylphenol, phenol, m-nitrophenol,
p-nitrophenol
(d) phenol, p-methyl phenol, p-nitrophenol,
m-nitrophenol

Solution
OH OH
CH3
OH
NO2
OH
NO2
+I –I –I, –R
Electron withdrawing groups increase acidic strength
while electron donating group decreases the same.
So the proper decreasing order of acidic strength is
OHOH
NO2
OH
NO2
OH
CH3
> > >
Hence answer is (b).

Class exercise 5
In the following compounds, the order
of basicity is
H
N
N
N
H H
N
O
(I) (II) (III) (IV)
(a) I > IV > II > I (b) II > I > IV > III
(c) III > I > IV > II (d) IV > I > III > II

Solution
N
H
N N
O
H
N
H
sp3
I
sp2
II
sp3
IV
sp2
III
Between I and IV, IV is less basic because of the –I
effect of oxygen atom.
II is more basic than III as the lone pair on N-atom in
III is not available for protonation as it is involved in
resonance.
Therefore, the correct order is I > IV > II > III
Hence answer is (a).

Class exercise 6
Account for the order acidity in the
following compounds.
CH3CCH2COOH CH3CH2COOH
O
HC CCH2
COOH H2
C = CHCH2
COOH
(i)
(ii)

Solution
(i) H3C C CH2 COOH > CH3 CH2 COOH
O
α α
(a) (b)
In compound (a), electron-withdrawing keto group increases
the acidic strength by decreasing the O — H bond strength,
while no such effect is there in compound (b).
Carbon atoms attached to triple bond is sp hybridised and
more electron-withdrawing than sp2
hybridised carbon atom.
Hence, such order in acidic strength is observed.
HC C CH2 COOH > CH2
CH CH2 COOH(ii)

Class exercise 7
Which of the following two amines is
more basic and why?
CCl3CH2CH2CH2NH2 or CCl3CH2CH2NH2

Solution
Electron-withdrawing groups decrease
the charge density on N-atom of
organic amines and hence decrease the
basic strength. In Cl3C CH2 CH2 CH2
NH2, the electron withdrawing — CCl3 is
far apart from — NH2 group as
compared to Cl3C CH2 CH2 NH2.
Hence, the former is more basic in
nature.