Question

NH2 ~ NH2 14. Which has higher melting point? ( ) 15. Which can be protonated with the 1st H more easily? ( ) NH2 "NH2 16. Rank their leaving group ability of from low to high. (A) OH (B) NH2 (C) F (D) Br

Answer 1

14) Consider, Cis: A and trans: B NH2 NH-NH, (B) (A) Cis isomer (A) has one of the amino group axial and hence is less acceable for hydrogen bonding. Also, isomer-A is less stable as there is more ring strain energy associated with axial amino group. Hence it is expected to have lower melting point. Whereas, trans isomer (B) is more stable and both equatorial amino groups are more accesable for hydrogen bonding. This leads to higher melting point. Litrature melting point of A = -8°C and that of B = 14-15°C. 15) As said earlier, equatorial amino group is more accesable and hence is easy to get protonated first. Thus isomer Bis expected to be first protonated. 16) NH2- < OH- < F. < Br- Strong bases tend to have low ability as a leaving group. This is due to strong bases have lower ability to stabilize the negative charge.