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Chemistry

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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Chemistry

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  1. Chemistry

  2. Session GENERAL ORGANIC CHEMISTRY - 4

  3. Session Objectives

  4. Session Objectives • Resonance or Mesomeric effect • Inductive effect • Electromeric effect • Hydrogen bond • Hyper conjugation • Steric effect

  5. 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.

  6. 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.

  7. 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.

  8. +R or +M effect –R or –M effect Types of Resonance

  9. Types of Resonance For substituted benzene +R effect of –NH2 group. –R effect of –NO2 group.

  10. 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

  11. Features of Inductive Effect • Chloroacetic acid is a stronger acid than acetic acid because Ka = 1.4 × 10-3 Ka = 1.75 × 10-5

  12. 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

  13. Features of Inductive Effect • Since this effect is transmitted through a chain it becomes less effective with distance • ClCH2CH2COOH ClCH2CH2CH2COOH • Ka

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

  15. Electromeric Effect Addition of HBr to an alkene

  16. + 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

  17. 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.

  18. 1–butene 2–butene Significance of Hyperconjugation More stable

  19. Relative strength of organic acids

  20. Class Exercise

  21. 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).

  22. 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).

  23. Among the following which has the most acidic -hydrogen? (a) (b) (c) (d) CH3CHO Class exercise 3

  24. Solution

  25. 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).

  26. 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

  27. 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).

  28. 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

  29. 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).

  30. Account for the order acidity in the following compounds. (i) (ii) Class exercise 6

  31. (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.

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

  33. 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.

  34. Do you think can show tautomerism? Class exercise 8 Solution: Yes. The tautomeric form is CH3COCH3.

  35. Which one will be more acidic ? Class exercise 9

  36. 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.

  37. Which hydrogen is maximum acidic in the following compound? Class exercise 10 Solution: Carboxylic hydrogen is maximum acidic.

  38. Thank you

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