Question

18. What is the major organic product of the following reaction? Tips Br a. 1 b.2 c. 3 d. 4 19. What would be the effect on the following substitution reaction of changing the solvent from methanol to acetone? Br + NaBr + Nal * —- Y - Nile a, the change in reaction rate cannot be predicted without more information b. the reaction rate would stay the same c. the reaction rate would decrease d. the reaction rate would increase 20. What order of reagents would be best to carry out the following synthesis? a. 1) PBry; 2) NaSCH, b. 1) HBr; 2) NaSCH, c. 1) TSCI; 2) NaSCH, d. a orb 21. What is the major product of the following reaction? Tight d. 4

Answer 1

18. NBS is known for allylic bromination i.e it replaces hydrogen on the carbon adjacent to double bond with bromine . NBS can be used to substitute Br₂  as it provided low concentration of Br₂  & it doesn't compete with bromination of double bond. Therefore , the correct answer is 'a'. The mechanism is shown in the image.

® Ler + hore N-BY & HBr - Br-Brt bror + , Homolytic clavage: Bo-Brt ho Br-Br 2 Br. Initiation lis zur Bro ( Ó + HBr Pologation (1) (iii) The radical then react with Bre, girre the Product & the Br radical es regenerated Br Bror I + Br. Prologation (2) o is repeated til NBS is consumed.

Ans 19. Since methanol is a polar protic solvent & acetone is non polar organic solvent . As polar solvents are used for solvating anions by Hydrogen bonding , therefore, the rate is going to decrease  on going from methanol to acetone. The correct option is 'c'.

Ans 20. NaSCH₃ is a strong nucleophile & a weak base . Therefore, it reacts with alkyl halide by an SN2 mechanism with inversion of configuration. In first step, nucleophilic substitution reaction of alcohol with both PBr₃ & HBr is possible to give alkyl halide followed by SN2 reaction by NaSCH_{3.
me e o Ansao HBO NaSCH}

The correct option is d.

Ans 21. Here, reaction proceeds by free radical bromination . Hence, the correct ans is 'a'.