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The answer is A. I thought it would be D since the ester group has the highest pka and the more basic functional group would be more prone to protonation. Also, I don't really get how to rank C in terms of atoms that would be protonated first as I don't know the pka of the ether group.

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  • $\begingroup$ Atom C is in nowheresville. By donating one of its "lone pairs" to the ring this oxygen atom gets a positive formal charge. Not good for then accepting a proton. $\endgroup$ – Oscar Lanzi Jun 5 at 21:28
  • $\begingroup$ What is "protonated first" even supposed to mean. Does author imagine they would get a dication? Something like this would break down much earlier. $\endgroup$ – Mithoron Jun 5 at 21:31
  • $\begingroup$ Also if you, say, added conc. H2SO4 to this, there would be protonation equilibria for all these groups, so I suppose properly formulated question would be here which Keq would be highest. $\endgroup$ – Mithoron Jun 5 at 21:35
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In short, none. As Mithoron said, the oxygen atoms can all act as Lewis bases and will all be protonated to an extent. If you rephrase the question to "Which of the atoms has the highest Kb?", well that's another story. Atom D has the lowest Kb, because the carbon in a carbonyl has a slight positive charge. Atom C would be difficult to protonate, the oxygen is already donating a pair to the furan ring to make it aromatic. So the debate is between a carbonyl and a primary alcohol. After consulting some pKa tables I found that protonated ketones have a pKa of around -7.3 while protonated primary alcohols of around -2.4 thus the alcohol is a stronger base (higher pKa means weaker acid thus stronger conjugate base). Finally we can conclude that the primary alcohol -OH group will be the first to accept a proton.

P.S. Fun fact, the quickest to be protonated was none of the ones marked with letter, it would have been the ester's carbonyl group!

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  • $\begingroup$ As far as D is concerned, I think that mesomeric effect may more important then inductive, Not bad for a first answer, though ;) $\endgroup$ – Mithoron Jun 5 at 22:57
  • $\begingroup$ I looked at the pka chart my instructor provided me. On that pka chart, it said the pka of R-OH (A in the molecule) is 16. B (the ketone group) is 20. D (the ester group) is 25. Nothing was given for the ether. A higher pka means its more basic, and thus a higher kb. If it is the highest kb, shouldn't it be D (ester) then, as the pka is higher than the rest. $\endgroup$ – Lat Jun 6 at 18:44
  • $\begingroup$ As I said, those I listed were the pKa values of PROTONATED species. The protonated species with the highest pKa forms the quickest. $\endgroup$ – Valentin Lupu Jun 8 at 0:11
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In the given molecule

  • At , D lone pair on oxygen is in conjugation with C=O group.Consequently it acquires a positive charge,its Kb is less for protoantion to take place.(as shown in scheme 1) (as pointed by MIthoron)
  • At C lone pair on this ring (furan) are delocalised and is AROMATIC (as shown in Scheme 2). Comparatively is kb value is less for protonation to take place.
  • At B carbon of carbonyl carbon is sp2 hybridised and is more withdrawing then sp3 hybridised carbon.(Hybridisation is usefull here) .Hence due to greater inductive polarisation by carbonyl carbon lone pair on oxygen are donated less.
  • At A, lone pair on Oxygen is not in conjugation as in C and D.Further Carbon bonded to alcohol at A is sp3 hybridised and is less withdrawing inductively then carbon at B which is sp2 hybridised.

Summing ,OH group at A has highest Kb value. Its tendency to donate lone paair is comparatively more then B,C or D.

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