Chemistry. Session. GENERAL ORGANIC CHEMISTRY - 4. Session Objectives. Session Objectives. Resonance or Mesomeric effect Inductive effect Electromeric effect Hydrogen bond Hyper conjugation Steric effect . Resonance or Mesomerism.
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GENERAL ORGANIC CHEMISTRY - 4
–R or –M effectTypes of Resonance
For substituted benzene
+R effect of –NH2 group.
–R effect of –NO2 group.
Types of inductive effect
+ I effect effect –electron donating groups
e.g., CH3 , C2H5
– I effect effect –electron withdrawing groups
e.g., - NO2 , –CN
Ka = 1.4 × 10-3
Ka = 1.75 × 10-5
F CH2 COOH Br CH2 COOH
KaFeatures of Inductive Effect
2. The larger is the electron-withdrawing effect of a group, the greater is the –I (inductive) effect.
3. Inductive effect is additive
Temporary effect which is observed in presence of reagents involving transfer of electrons in an unsaturated system.
Addition of HBr to an alkene
H – C = C
H – C = C – H
H – C – C
H C = C
HHyperconjugation or no bond resonance
(b) More the number of hyperconjugative structures, more will be the stability of ion or molecule
Structure of ethyl carbonium ion
(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.
2–buteneSignificance of Hyperconjugation
The hybridization of carbon atoms C — C single bond in vinylacetylene is
(a) sp3 - sp3 (b) sp - sp2(c) sp2 - sp (d) sp3 - sp
Hence answer is (c).
(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 electronsClass exercise 2
The compound has 3 p bonds and one lone pair, i.e. two non-bonding electrons. It also contains 9 s-bonds.
Hence answer is (d).
(d) CH3CHOClass exercise 3
gas varies as aldehyde > keto > ester
Then most acidic a-H atom is present inSolution
Hence answer is (a).
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
Electron withdrawing groups increase acidic strength while electron donating group decreases the same.
So the proper decreasing order of acidic strength is
Hence answer is (b).
(I) (II) (III) (IV)
(a) I > IV > II > I (b) II > I > IV > III
(c) III > I > IV > II (d) IV > I > III > IIClass exercise 5
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).
(ii)Class exercise 6
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.
Which of the following two amines is more basic and why?
CCl3CH2CH2CH2NH2 or CCl3CH2CH2NH2
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.
can show tautomerism?Class exercise 8
Yes. The tautomeric form is CH3COCH3.
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.
Carboxylic hydrogen is maximum acidic.