Explain the reaction :
(a) `ROH + NaBr rarr`
(b) `ROH + HBr rarr`
(c)

Explanation

(a) No reaction. Extremely weak base `Br^(Ө)` cannot displace very strong base `overset (Ө) O H`.
(b) This reaction occurs in two steps.
`ROH + HRr rarr ROH_2^(oplus) + Br^(Ө) rarr RBr + H_2 O`
Displacement is on the onium ion of `ROH` whose good leaving group property is due to the weekly basic `H_2 O`.
`{:(underset(ddot)overset(overset(O)(||))(N)--,underset(ddot)overset(ddot)(O): ,,("Ambident nucleophile"),("Two nucleophilic centres N and O")),(underset(underset("centre")("nucleophilic"))("Less EN"),underset(underset("centre")("nucleophilic"))("More EN"),,["EN of O gt N"],):}`
Rule : If there is more positive charge on the carbonium formed, it will attack at the more `EN` nucleophilic centre and vice versa.