Química Orgânica 1 · Caminhos de substituição
Ionize a primary benzylic bromide
Condições: Benzyl bromide (bromomethylbenzene) warmed in water with a little acetone as cosolvent and no added nucleophile. Follow the SN1 branch to benzyl alcohol. One water is the nucleophile and a second water is the final proton acceptor. Ph is an unchanged phenyl group. With a good nucleophile in a polar aprotic solvent, benzyl bromide reacts by SN2 instead; that route is not requested here.
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Ionize the benzylic bromide
Water is a weak nucleophile but a highly polar solvent that stabilizes the separating ions, so the leaving group departs first.
As setas curvas
- The C–Br pair leaves with bromine, giving a benzyl cation and bromide.
Why can a primary cation form?
The empty p orbital of the benzylic carbon lines up with the π system of the ring. The positive charge is shared by that carbon and the ortho and para ring carbons, which makes this primary cation roughly as stable as a secondary or even tertiary alkyl cation.
Water captures the cation
Water is present in huge excess, so it rather than bromide is the main nucleophile. The reacting carbon carries two H atoms and the phenyl group, so the product has no stereocenter.
As setas curvas
- A lone pair on water oxygen bonds to the empty orbital of the carbocation.
Deprotonate the oxonium ion
The alkyloxonium ion (pKa approximately −2) and hydronium (pKa approximately −1.7) are acids of similar strength, and water is present in large excess, so the proton passes to solvent.
Nota: The two oxonium O–H protons are equivalent; either can be transferred.
As setas curvas
- A lone pair on the second water takes a proton from the oxonium oxygen.
- The oxonium O–H pair stays on oxygen, which becomes neutral.
Benzyl alcohol, hydronium and bromide
The benzylic carbon now carries OH from water: benzyl alcohol. The second water became hydronium, and with bromide it accounts for the HBr formed. The phenyl ring was never changed; it only stabilized the cation while it existed.
O que você vai praticar
Explain why a primary benzylic halide can ionize in water, then complete water capture and deprotonation to the alcohol.
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