Chemistry question help please
Hi, please could I have some help on question 6c of this paper. I know when Q is reacted with excess NaOh, a carboxylate salt is formed and alcohol. But then the OH on phenol becomes ONa. Why does the NaOh react with the oh on phenol but not the alcohol. Is it because the oh on the phenol is more reactive because the lone pair of electrons delocalise into the benzene ring?
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!
Original post by anonymous56754
Hi, please could I have some help on question 6c of this paper. I know when Q is reacted with excess NaOh, a carboxylate salt is formed and alcohol. But then the OH on phenol becomes ONa. Why does the NaOh react with the oh on phenol but not the alcohol. Is it because the oh on the phenol is more reactive because the lone pair of electrons delocalise into the benzene ring?
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!
Paper: https://pmt.physicsandmathstutor.com/download/Chemistry/A-level/Past-Papers/CAIE/Paper-4/QP/November%202016%20(v2)%20QP.pdf
Thanks!
Phenols (typical pKa ≈ 10) are a lot more acidic than an alcohol (typical pKa ≈ 16) for reasons that are generally not explained at A level.
Essentially the difference lies in something called resonance stabilisation.
An alkoxide ion lacks any resonance stabilisation because the O^- is not bonded to a (mesomeric) electron-withdrawing group (such as a carbonyl group, nitro group or a benzene ring). This results in poor delocalisation of the negative charge and so the conjugate base is unstable with respect to the conjugate acid (hence the large Ka).
Phenolate ions are better stabilised by delocalisation of the lone pairs into the benzene ring (e.g resonance stabilisation), but the stabilisation is far from brilliant as oxygen is so much more electronegative than the carbon atoms in the ring that most of the charge is still localised on the oxygen (hence why the pKa is still pretty big, despite the stabilisation). This still makes phenols (literally) a million times more acidic than alcohols.
(edited 1 month ago)
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