Chemistry. GENERAL ORGANIC CHEMISTRY - 4 Session.
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Transcript of Chemistry. GENERAL ORGANIC CHEMISTRY - 4 Session.
Chemistry
GENERAL ORGANIC CHEMISTRY - 4
Session
Session Objectives
1. Resonance or Mesomeric effect
2. Inductive effect
3. Electromeric effect
4. Hydrogen bond
5. Hyper conjugation
6. Steric effect
Session Objectives
Resonance or Mesomerism
1. All the properties of a compoundcannot be explained by single structure.
2. Canonical structures or resonancecontributing structures–differ in positionof electrons.
3. Delocalisation of electrons leads to decrease in potential energy of molecule.
canonical structures of benezene
Resonance hybridstructure of benezene
Resonance or Mesomerism
4. Resonance hybrid is more stable thancanonical 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 molecule should be planar.
2. It contains an alternating system of single and double bonds(a conjugated system).
3. The relative positions of nuclei should remain unchanged (e.g. tautomerism).
4. The negative charge must preferably lie on the most electronegative atom.
5. The charge needs to be preserved in all the resonating structures.
6. The electrons always move away from a negative charge.
7. Arrows should be drawn to indicate the direction of the movement of electrons.
Types of Resonance
C C OH — C — C–
O H
(+M or +R effect of –OH group)
+
+R or +M effect
–R or –M effect
C C CH O — C — C CH — O–
(–R effect of –CHO group)
+
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
1. Permanent effect in saturated carbon chain compounds.
2. Group attached to carbon chain shouldhave tendency to release or withdraw electrons.
Types of inductive effect
+ I effect effect –electron donating groupse.g., CH3 , C2H5
– I effect effect –electron withdrawing groupse.g., - NO2 , –CN
Features of Inductive Effect
1. Chloroacetic acid is a stronger acid than acetic acid because
Cl - CH2 C
OH
OCl-CH2 - C
O
O
H++
-I
CH3-C
OH
OCH3-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
Electromeric Effect
Temporary effect which is observed in presence of reagents involving transfer of electrons in an unsaturated system.
X Y X Y
in presence of reagent + –
in absence of reagent
Electromeric Effect
R
C
H
CH
HC — C
R
H
H
H
R — CH — CH3
Br
C — CH3
R
H
HBr12
+ –
Br – + H+
Addition of HBr to an alkene
Hyperconjugation or no bond resonance
(a) Involves and bond orbitals
(b) More the number of hyperconjugative structures, more will be the stability of ion or molecule
H – C – C
H H
H H
+ H – C = C – H
H
H H
+
H C = C
H H
H H
+H H
H H+
H – C = C
Structure of ethyl carbonium ion
Hyperconjugation or no bond resonance
(c) The number of hyperconjugative structures in an alkene is obtained by the number of C — H bonds attached to the carbon bonded directly to the double bonded carbon atoms.
CH CH
H
H
CH2 CH CH
H
H
CH2
–
+
CH CH
H
H
CH2
–+
CH CH
H
H
CH2
–
+
Significance of Hyperconjugation
H3C — CH — CH CH2H3C — CH CH — CH2
H+
(2 hyperconjugative structures)1–butene
H3C — CH CH — CH3 H3C — CH — CH
H
CH2
–
+
(6 hyperconjugative structures)2–butene
More stable
Relative strength of organic acids
R - C
OH
O
R - CO
O
H+
R - C
O
O
Resonance structures
+
Class Exercise
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 CH1 2 3 4
Vinylacetylene
Solution :
Hence answer is (c).
Class exercise 2Allyl 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
H2C CH — CH2 — N C
Allyl isocyanide
+ –
The compound has 3 bonds and one lone pair, i.e. two non-bonding electrons. It also contains 9 -bonds. Hence answer is (d).
Solution:
Class exercise 3
Among the following which has the most acidic -hydrogen?
CH3CCH2CHO
O
CH3CCH2CCH3
O O
CH3CCH2COOCH3
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
two e-withdrawing keto groups.
keto and ester group.
one aldehyde group.
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
NN
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 CCH2COOH H2C = CHCH2COOH
(i)
(ii)
Solution
(i) H3C C CH2 COOH > CH3 CH2 COOH
O
(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 > CH2CH 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.
Class exercise 8
Do you think
can show tautomerism?
CH3CH CH2
OH
Yes. The tautomeric form is CH3COCH3.
Solution:
Class exercise 9
Which one will be more acidic ?
OH
CH3NO2
CH3
OH
CH3C
CH3
N
I II
Solution
Because of steric inhibition of resonance conjugate base of I will not be stabilised by resonance. But for II there is no such steric inhibition of resonance.
Class exercise 10
Which hydrogen is maximum acidic in the following compound?
NO2
O — H
H — C C
COOH
Solution:
Carboxylic hydrogen is maximum acidic.
Thank you