Resonance ,inductive effect -----(Organic )
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Resonance occurs when a molecule can be represented by multiple Lewis structures and the actual structure is a hybrid of all the contributing structures. A resonance hybrid is the average structure between all the canonical or contributing Lewis structures. Delocalization of electrons through resonance leads to decreased potential energy and increased stability compared to any single contributing structure. Electron donating groups cause positive resonance or inductive effects while electron withdrawing groups cause negative resonance or inductive effects. Resonance and inductive effects influence acidity, basicity, and reactivity.
Resonance ,inductive effect -----(Organic )
- 1. 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)
- 2. 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
- 3. 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.
- 4. 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.
- 5. 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
- 6. 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.
- 7. 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
- 8. 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
- 9. 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
- 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
- 11. 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).
- 12. 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:
- 13. 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)
- 14. 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.
- 15. 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).
- 16. 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
- 17. 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).
- 18. 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
- 19. 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).
- 20. Class exercise 6 Account for the order acidity in the following compounds. CH3CCH2COOH CH3CH2COOH O HC CCH2 COOH H2 C = CHCH2 COOH (i) (ii)
- 21. 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)
- 22. Class exercise 7 Which of the following two amines is more basic and why? CCl3CH2CH2CH2NH2 or CCl3CH2CH2NH2
- 23. 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.