I have read somewhere that $\ce{H2SO4}$ is used to generate $\ce{HBr},$ and at the other place it is written that it is there to protonate the hydroxyl group. Which fact should I believe ?
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No problem with that. But sometimes HBr might react with H2SO4. See my answer to the question here: https://chemistry.stackexchange.com/questions/77198/why-do-we-need-sulfuric-acid-in-creating-alkyl-bromide-and-not-in-creating-alkyl?r=SearchResults – Nilay Ghosh Apr 29 '19 at 03:47
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Yes, we can use $\ce{KBr + H2SO4}$ instead of $\ce{NaBr + H2SO4}$ because they both give $\ce{HBr}$ which is used in the preparation of alkyl bromide. The basic reaction is as follows: $$\ce{ROH + HBr -> RBr + H2O}$$ The above-mentioned compounds $\ce{NaBr}$ and $\ce{KBr}$ are used only for the preparation of $\ce{HBr}$.
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1You haven't answered the main question in the body. Try to add that too. – Zenix Jan 06 '20 at 11:22
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H2SO4 is used to protonate the OH group and form OH2+ group which is a good leaving group.
OH2+ leaves the substrate and forms a carbocation.
Then (Br-) anion attacks on the carbocation so formed.
HBr NaBr KBr all liberate (Br-) anion in the solvent.
So all will give the same product.
H2SO4 has the task to generate H+ that protonates the alcohol.
Your book might have tried to explain it like H2SO4 first forms HBr. Ultimately HBr will dissosiate to give H+ which does the protonation.
Rest over to you. You may believe both or none!