In these reactions if two reagent is given how to decide who is going to react with the compound.HBr is a strong acid CH3OH is a weak base.So HBr dissociate easily but CH3OH is reacting with the given compound. How is this possible??

closed as unclear what you're asking by Mithoron, Martin - マーチン♦Dec 2 '18 at 20:26

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Who said in the reaction ch3oh has to behave as an acid. It is acting as a nucleophile . It donates oxygen's lone pair to the vacant orbital of the carbonation intermediate forming an oxonium cation which then looses a molecule of H+ to form the product PS: to form the carbonation intermediate the H+ was given by HBr which was given back during the last step of the rxn

No doubt that, HBr is strong and it would disassociate into $$\text{H}^{+}$$ and $$\text{Br}^{-}$$ but methanol is a solvent in the second reaction and it would attack the Carbocation because of higher concentration and it's a better nucleophile (-ve charge density on Oxygen atom is high) and then the Hydrogen atom would deprotonate.

One more question, why in the first reaction options, $$\text{Br}$$ is missing, as $$\text{HBr}/\text{H}_{2}\text{O}$$ is $$\text{HOBr}$$ in reaction.