Reaction Mechanism Exercise # 3

REACTION MECHANISM Exercise # 3 1. [Matrix Match Type Questions] Part # I Match the column I with column II. Column-I (reaction) Column-II (Mechanism) Cl C Ph H (+) OH C + SOCl2 Ph (+) H CH 3 (A) CH3 OH C + SOCl2 Ph (+) H CH3 (B) Pyridine CH 3 H 5C 6 (p) SN1 Cl (q) C H (–) SN2 (r) SNi (s) E2 – (C) CH 3 OH Cl CH 3 C +HCl  C H5C6 (+) H C 6H 5 H (±) CH3 (D) 2. Match the column I with column II. Column-I (Substrate) (A) Column-II (Relative rate of solvolysis in 50% aqueous ethanol at 45°C) Cl (B) Cl (p) 7700 (q) 1 (r) 91 (s) 1,30,000 H3C (C) H3C (D) Ph Part # II Cl Cl [Comprehension Type Questions] Comprehension # 1 Nucleophilic substitution reactions generally expressed as Nu– + R — L  R— Nu + L– Where Nu–  Nucleophile ; R—L  substrate ; L  leaving group The best leaving groups are those that become the most stable ions after they depart. Since most leaving groups leave as a negative ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be (A) electron-withdrawing to polarize the carbon (B) stable once it has left (not a strong base) (C) polarisable- to maintain partial bonding with the carbon in the transition state (both SN1 and SN2). This bonding helps to stabilise the transition state and reduces the activation energy. 1. Among the following which is feasible ? (A) X– + CH3—CH2—H  CH3—CH2—X + H– (B) X– + CH3—OH  CH3—X + OH  CH3—X + H2O (C) (D) X– + CH3—CH3  CH3—X + CH 3 2. Among the following which is false statement ? (A) The weaker the base after the group departs, the better the leaving group (B) A reactive leaving group would raise the energy of the product, driving the equilibrium towards the reactants (C) Relative leaving group ability may vary with change of solvent (D) Better leaving group only increases SN2 rate, not SN1. 3. CH 3 Br CH 3 F (I) CH 3 OH (II) CH 3 OSO 2 CF3 (III) (IV) The correct order of decreasing reactivity of the above compounds towards CH3O– in an SN2 reaction is : (A) I > IV > II > III 4. Cl– (B) IV > I > II > III CH3 O – (I) (II) CH 3 S – (II) (C) IV > I > III > II (D) IV > II > I > III I– (IV) The correct order of increasing leaving group capability of above anoins (A) III < IV < II < I (B) II < III < I < IV (C) II < IV < III < I (D) I < III < II < IV Comprehension # 2 Nucleophilic aliphatic substitution reaction is mainly of two types : SN1 and SN2. The SN1 mechanism is a two step process. Reaction velocity of SN1 reaction depends only on the concentration of the substrate. Since product formation takes place by the formation of carbocation, optically active substrate gives (+) and (–) forms of the product. In most of the cases the product usually consits of 5-20% inverted product and 80-95% racemised species. The more stable the carbocation, the greater is the proportion of racemisation. In solvolysis reaction, the more nucleophilic the solvent, the greater is the proportion of inversion. 1. Which one of the following compound will give SN1 reaction predominantly ? CH3 (A) H5C6——– Br (B) H 3C Br CH3 (C) H 3C — Br (D) All of these REACTION MECHANISM 2. Which of the following compounds will give SN1 and SN2 reactions with considerable rate ? CH3 I. C6H5—CH2—Br II. CH2=CH—CH2—Br III. CH3—CH(Br)CH3 IV. H3C CH3 Br Select the correct answer from the codes given below (A) I, II and III (B) I, II and IV (C) II, III and IV 3. (D) I, III and IV For the given reaction R1 R1 R——–X HOH R——–OH R2 R2 Which substrate will give maximum racemisation ? H2C CH3 (A) H5C6——–Br CH3 Br (B) C2H5 C2H5 Br Br H5C6——– —OCH3 H5C6——– (C) —NO2 (D) + CH3 NH3 EXERCISE - 3 : PART # I 1. A  (r), B  (q), C  (s), D  (p) 2. A  (q), B  (r), C  (s), D  (p) PART # II Comprehension # 1 : 1. C 2. D 3. B 4. B Comprehension # 2 : 1. A 2. A 3. C