Which one is more nucleophilic in strength $\ce{Br-}$ or $\ce{OH-}$?
I am confused since $\ce{OH-}$ is more electronegative than $\ce{Br-}$ but also a strong base which contradicts for a better nucleophile .
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2 Answers
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Like most concepts that are not captured by a single number, nucleophilicity is not a simple (or at least, relatively simple) concept like acidity.
Nucleophilicity depends a lot on orbital alignment and interaction, in addition to thinking about charges. It's also confusing because nucleophility is not always married to the stability of the adduct. For example, iodide is a great nucleophile, but alkyl iodides are also highly reactive.
In your specific case, since the two nucleophiles have the same charge, you should look at other factors. Bromide is significantly larger than hydroxide, and it is a much better nucleophile, at least when we're considering carbon electrophiles. Indeed, we see bromide used in nucleophilic catalysis but not hydroxide.
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Better nucleophile, as per definition, are those which can donate their electron pair to (and form a bond with) a CARBON. Although, we could see usually, nucleophilicity and basicity (ability to donate LP to a proton) go side by side, there are cases where it is not true.
So, for OH-, it is better nucleophile than Br- in an aprotic medium, due to higher charge density on O atom. While when it come to basicity in aqueous medium, OH- ions are heavily hydrated due to extensive H-bonding, making it less likely to donate its LP to a Carbon, hence less basic than Br-.
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$\begingroup$ This answer is wrong because the reality is much more complicated. It depends strongly on if your electrophile is soft or hard. Hard electrophiles will react much better with hydroxide. Carbon electrophiles are generally soft, so bromide is the better nucleophile. $\endgroup$– ZheCommented Apr 17, 2017 at 22:03
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1$\begingroup$ Do you really mean that hydroxide is a weaker base than bromide? Bromide is pretty well solubilized by water. $\endgroup$ Commented Apr 17, 2017 at 22:47
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$\begingroup$ @Zhe if br- is a better nucleophile than why doesn't it react with methyl alcohol to give methylbromine indeed when methyl alcohol is treated with h+( acidic medium ) dehydration takes place and finally br- attacks to give methylbromine .why br- does not attack methylalcohol directly to remove oh-despite being a better nucleophile ( br-) $\endgroup$ Commented Apr 18, 2017 at 5:22
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$\begingroup$ Because hydroxyl is a terrible leaving group. You can't compare two reactions with two different nucleophiles when you're also using two different electrophiles. $\endgroup$– ZheCommented Apr 18, 2017 at 13:37
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$\begingroup$ @Zhe Okay, accepted. So taking DMSO as a solvent: R-Cl + OH- -----> ? And, then R-Cl + Br- -----> ? I believe, Cl- is fairly GOOD LG... Please comment. $\endgroup$ Commented Apr 21, 2017 at 19:22