Question

For this question, you can ignore part (5). (10 points each) 3.11. The Cannizzaro reaction is a disproportionation that takes place in strongly basic solution and converts aromatic aldchydes to the corresponding benzyl alcohol and sodium benzoate. Several mechanisms, all of which involve a hydride transfer as a key step, have been postulated. On the basis of the following information, formulate one or more mechanisms that would be consistent with all the data provided. Indicate the significance of each observation with respect to the mechanism(s) you postulate. I. When the reaction is carried out in D,O, the benzyl alcohol contains no deuterium in the methylene group. benzoate contain 1O. 2. When the reaction is carried out in H218O, both the benzyl alcohol and sodium 3. The overall reaction rate is given by the expression 4. The rates of substituted benzaldehydes are correlated by a Hammet LFER with ?-+3.76. 5. The solvent isotope effect kpo/k120 -1.90 Optional challenge (no points): In a way similar to how we proved ku/ ko 7 as the theoretical limit for primary KIEs in class, now use the same equations to prove that the theoretical limit for secondary KIEs is ku/ ko 1.4 You will need to use the change in bond bending frequency shown on page 429 of the textbook.

Answer 1

0 0 晶 8
In the above mechanism, the first step is a nucleophilic acyl substitution on an aldehyde. First, hydroxide attacks a carbonyl. The resulting tetrahedral intermediate then rearranges and eliminates water and transfers hydride to attack another carbonyl. In the final step of the reaction, the carboxylate and alkoxide ionsare formed. Alkoxide ion acquires a proton (here, deuterium) from the solvent (D₂O).
8 궈 0 Na

(1) Thus from the mechanism we can say that the H in methylene group of benzyl alcohol come from hydroxide ion(NaOH) (not from D₂O), so it has no D in methylene group.