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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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Session l.jpg
Session

GENERAL ORGANIC CHEMISTRY - 4



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Session Objectives

  • Resonance or Mesomeric effect

  • Inductive effect

  • Electromeric effect

  • Hydrogen bond

  • Hyper conjugation

  • Steric effect


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Resonance or Mesomerism

  • All the properties of a compoundcannot be explained by single structure.

  • Canonical structures or resonancecontributing structures–differ in positionof electrons.

  • Delocalisation of electrons leads to decrease in potential energy of molecule.


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Resonance or Mesomerism

  • Resonance hybrid is more stable thancanonical structures.

  • 5. Resonance structures are imaginary.

  • Resonance energy = Actual energy of hybrid–energy of most stable contributing structure.

  • Resonance is measure of stability.


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Rules for Drawing Resonance Structure

  • The molecule should be planar.

  • It contains an alternating system of single and double bonds(a conjugated system).

  • The relative positions of nuclei should remain unchanged (e.g. tautomerism).

  • The negative charge must preferably lie on the most electronegative atom.

  • The charge needs to be preserved in all the resonating structures.

  • The electrons always move away from a negative charge.

  • Arrows should be drawn to indicate the direction of the movement of electrons.


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+R or +M effect

–R or –M effect

Types of Resonance


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Types of Resonance

For substituted benzene

+R effect of –NH2 group.

–R effect of –NO2 group.


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Inductive Effect

  • Permanent effect in saturated carbon chain compounds.

  • Group attached to carbon chain shouldhave tendency to release or withdraw electrons.

Types of inductive effect

+ I effect effect –electron donating groups

e.g., CH3 , C2H5

– I effect effect –electron withdrawing groups

e.g., - NO2 , –CN


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Features of Inductive Effect

  • Chloroacetic acid is a stronger acid than acetic acid because

Ka = 1.4 × 10-3

Ka = 1.75 × 10-5


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F CH2 COOH Br CH2 COOH

Ka

Cl3CCOOH Cl2CHCOOH

Ka

Features 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


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Features of Inductive Effect

  • Since this effect is transmitted through a chain it becomes less effective with distance

  • ClCH2CH2COOH ClCH2CH2CH2COOH

  • Ka


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Electromeric Effect

Temporary effect which is observed in presence of reagents involving transfer of electrons in an unsaturated system.


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Electromeric Effect

Addition of HBr to an alkene


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+

H

H

H

H

H

H

H

+

+

H – C = C

H – C = C – H

H – C – C

H C = C

+

H

H

H

H

H

H

H

H

Hyperconjugation 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


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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.


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1–butene

2–butene

Significance of Hyperconjugation

More stable




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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

Solution :

Hence answer is (c).


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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

Class exercise 2

Solution:

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).


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Among the following which has the most acidic -hydrogen?

(a)

(b)

(c)

(d) CH3CHO

Class exercise 3



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Since e-withdrawing nature of

gas varies as aldehyde > keto > ester

Then most acidic a-H atom is present in

Solution

Hence answer is (a).


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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


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Solution

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).


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In the following compounds, the order of basicity is

(I) (II) (III) (IV)

(a) I > IV > II > I (b) II > I > IV > III

(c) III > I > IV > II (d) IV > I > III > II

Class exercise 5


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Solution

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).



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(i)

(ii)

Solution

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.


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Class exercise 7

Which of the following two amines is more basic and why?

CCl3CH2CH2CH2NH2 or CCl3CH2CH2NH2


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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.


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Do you think

can show tautomerism?

Class exercise 8

Solution:

Yes. The tautomeric form is CH3COCH3.



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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.


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Which hydrogen is maximum acidic in the following compound?

Class exercise 10

Solution:

Carboxylic hydrogen is maximum acidic.