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What is a Leaving Group?

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University Course Reader · STEM

Because bromide is a good leaving group, the substitution proceeds readily under mild conditions.

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Overview

A leaving group is the atom or group that breaks off a molecule during a reaction, taking both electrons of its bond. The reaction cannot go ahead until that group has left, so one that leaves readily is called a good leaving group. Iodide is large, so the charge spreads out and it leaves readily. Hydroxide has the same charge on one small atom, is less stable, and under ordinary conditions does not leave. Acid turns it into water, which leaves readily.
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Overview

A leaving group is the bit of a molecule that breaks off in a reaction and takes the shared electrons with it. It leaves easily if it can carry those electrons comfortably, and drags its feet if it can't. Iodide, huge and unbothered by an extra charge, is out the door in a flash. Hydroxide, one small oxygen clutching a full negative charge, has to be coaxed. An acid's proton turns it into water, and water leaves happily. Fine on its own is free to go. 😎

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Detail

A leaving group is the atom or group that breaks off a molecule during a reaction, and it takes both electrons of the bond that held it. Warm bromoethane with sodium hydroxide and the bromine breaks away as bromide, a negative ion, and hydroxide takes its place. Bromide is the leaving group, and it is negative because it took both electrons it shared with the carbon. This is a substitution reaction, one group replacing another; SN1 and SN2 are the two routes such reactions take. On either route the reaction cannot go ahead until the leaving group has left, so chemists call a group that leaves readily a good leaving group. Whether a group leaves readily comes down to one question: is it stable holding those two electrons? Bromide is. It is a large ion, the charge spreads out, and it leaves readily. Hydroxide is not. The same charge sits on one small oxygen atom, so under ordinary conditions it does not leave. That is why an alcohol keeps its OH. If the OH group broke away it would take both electrons with it and become hydroxide, the unstable ion, so under ordinary conditions it stays attached. Add hydrobromic acid and the OH gains a proton first, so the group that would break away is now water, neutral and stable. Water leaves readily, and the acid's bromide takes its place.
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Detail

A leaving group is the piece of a molecule that breaks off in a reaction and takes the two electrons of its old bond with it. Because it takes both, it usually leaves carrying a negative charge, and the question is whether it can carry that charge without complaint. Iodide can. It is a large ion and the charge spreads across all of it, so it leaves in a flash. Iodomethane reacts readily where fluoromethane barely reacts at all, because fluoride is small, hates the charge and stays put. Hydroxide is worse still: one small oxygen, a full negative charge, no interest in leaving. So how does anyone swap out an OH? You change what leaves. Add acid, and a proton parks on the oxygen, so what would have left as hydroxide now leaves as water, which is neutral and perfectly comfortable on its own. Same atoms, one proton, and now it goes. Shoving does not help a leaving group that won't go; making it fine afterwards does. A leaving group goes readily when it will be all right once it has left, and one that won't go becomes one that will the moment you arrange that. 😎

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Analogy

A leaving group is like an employee who leaves a company. How readily the employee leaves depends on how well they will manage on their own. An employee with savings and another job lined up resigns easily. An employee with nothing to fall back on tends to stay, until a generous severance package makes leaving affordable. A leaving group is the same. It leaves with the electrons of its old bond. It leaves readily if it is stable with those electrons, and under ordinary conditions it does not leave if it is not. When a chemist adds acid, the group gains a proton and becomes water, which is stable on its own, and it leaves. The acid is the molecule's severance package.
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Analogy

A leaving group is the bit that breaks off a molecule in a reaction, and it behaves like the friend on your sofa. He leaves the moment he can afford a place of his own, and not a day before, and no amount of hinting moves him while he can't. A leaving group goes readily if it will be stable once it has left, electrons and all, and it stays put if it won't. Iodide has a place of its own already, so it's gone in a flash. Hydroxide can't afford the deposit and doesn't budge. Adding acid is lending it the deposit: one proton turns hydroxide into water, water is fine on its own, and your sofa is empty by Friday. 😎
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Formal definition — The same term, explained the usual way

A leaving group is a molecular fragment that departs with the pair of electrons from a heterolytically cleaved bond, typically in a nucleophilic substitution or elimination reaction. Leaving-group ability correlates inversely with basicity: good leaving groups are weak bases, the conjugate bases of strong acids, such as iodide, bromide, chloride, water, tosylate and triflate; poor leaving groups are strong bases such as hydroxide, alkoxide, amide and hydride. Among the halides, ability increases down the group (I⁻ > Br⁻ > Cl⁻ >> F⁻). A poor leaving group can be converted into a good one, for example by protonating a hydroxyl group so that it departs as water, or by converting it to a sulfonate ester. Because departure of the leaving group is rate-limiting in SN1 and E1 reactions and part of the rate-determining step in SN2, leaving-group ability strongly influences reaction rate.

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