Why is bromine a better nucleophile than chloride

It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. I am a student of biochemistry. My organic chemistry professor was giving a lecture and he talked about halohydrin formation reactions. At one point, he said that "water attacks instead of bromide because water is a better nucleophile than bromide". He repeated that five times or so. A classmate argued that, because water is the solvent in the reaction and is in greater proportion, it will attack faster.

But that still doesn't mean water is a better nucleophile, right? Bromide itself has a negative net charge, while water has no net charge, only 2 free electrons. That is not true. As you noted bromine has a negative charge and also lower electronegativity than oxygen and is therefore the better nucleophile even under these aqueous conditions.

Halohydrins are not even usual the result of bromination in water. Reusch's online Organic Chemistry book. Under many circumstances, I would agree that bromide is a better nucleophile than water. It's negatively charged and, due to its size, bromine is more polarizable than oxygen. However, in water, the bromide anion is very stable, reducing its nucleophilicity. I suspect that bromide is still more nucleophilic than water even considering solvent effect , but combined with the large excess of water, it is more likely that water opens the bromonium ion in the halohydrin formation.

Since there is so much more water in solution the chances of water attacking is more likely than the negative Br attacking. Once water attacks the carbocation with the lone pair on oxygen it forms a positive charge. Resonance effects also come into play when comparing the inherent nucleophilicity of different molecules.

The reasoning involved is the same as that which we used to understand resonance effects on basicity. If the electron lone pair on a heteroatom is delocalized by resonance, it is inherently less reactive - meaning less nucleophilic, and also less basic. An alkoxide ion, for example, is more nucleophilic and more basic than a carboxylate group, even though in both cases the nucleophilic atom is a negatively charged oxygen. In the alkoxide, the negative charge is localized on a single oxygen, while in the carboxylate the charge is delocalized over two oxygen atoms by resonance.

The nitrogen atom on an amide is less nucleophilic than the nitrogen of an amine, due to the resonance stabilization of the nitrogen lone pair provided by the amide carbonyl group.

Steric hindrance is an important consideration when evaluating nucleophility. For example, tert -butanol is less potent as a nucleophile than methanol. This is because the comparatively bulky methyl groups on the tertiary alcohol effectively block the route of attack by the nucleophilic oxygen, slowing the reaction down considerably imagine trying to walk through a narrow doorway while carrying three large suitcases!

It is not surprising that it is more common to observe serines acting as nucleophiles in enzymatic reactions compared to threonines - the former is a primary alcohol, while the latter is a secondary alcohol. Explain your choice. What is a nucleophile? Protonation states and nucleophilicity The protonation state of a nucleophilic atom has a very large effect on its nucleophilicity. Periodic trends and solvent effects in nucleophilicity There are predictable periodic trends in nucleophilicity.

Moving horizontally across the second row of the table, the trend in nucleophilicity parallels the trend in basicity: The reasoning behind the horizontal nucleophilicity trend is the same as the reasoning behind the basicity trend: more electronegative elements hold their electrons more tightly, and are less able to donate them to form a new bond.

Relative nucleophilicity in a protic solvent This of course, is opposite that of the vertical periodic trend for basicity, where iodide is the least basic. Protic solvent molecules form very strong ion-dipole interactions with the negatively-charged nucleophile, essentially creating a 'solvent cage' around the nucleophile: In order for the nucleophile to attack the electrophile, it must break free, at least in part, from its solvent cage.

Relative nucleophilicity in a polar aprotic solvent The reason for the reversal is that, with an aprotic solvent, the ion-dipole interactions between solvent and nucleophile are much weaker: the positive end of the solvent's dipole is hidden in the interior of the molecule, and thus it is shielded from the negative charge of the nucleophile.

Resonance effects on nucleophilicity Resonance effects also come into play when comparing the inherent nucleophilicity of different molecules. Steric effects on nucleophilicity Steric hindrance is an important consideration when evaluating nucleophility. What is a weak nucleophile? Weak nucleophiles are neutral and don't bear a charge. So let's look at the 4 examples I started with yesterday.

Example 1 uses NaCN a strong nucleophile. Codou Stefani Pundit. Is water a good Nucleophile? Because there isn't a full negative charge, water isn't going to be as good a nucleophile as a negative ion like OH - , and so the reaction is slower. Reita Slany Pundit. Is I a strong or weak nucleophile? In general, good bases are also good nucleophiles. Therefore, strong bases such as negatively charged oxygens and nitrogens will also be strong nucleophiles. Assunta Lecointe Pundit.

What is a good Nucleophile but weak base? Explanation: In general, good bases are also good nucleophiles. But weak bases can also be good nucleophiles. Umbelina Vyalba Pundit. Is F or Cl A better leaving group? A leaving group , LG, is an atom or a group of atoms that is displaced as stable species taking with it the bonding electrons.

Typically the leaving group is an anion e. Cl - or a neutral molecule e. Sandee Voit Pundit. Which is better leaving group Cl or I? Good leaving groups are weak bases.

They're happy and stable on their own. The weaker the base, the better the leaving group. Laszlo Bendito Teacher. Does Nucleophilicity increase down group? Nucleophilicity increases down the group. Reason- size increases down the group and thus the ease to loose electron increases. Nucleophilicity decreases across a period from left to right. Reason- as the atomic no. Fructuosa Feigenbaum Teacher. Is iodine or fluorine a better Nucleophile?