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For oxygen nucleophiles, e.g. $\ce{ROH}$, when reacting with a carbonyl group to form acetal, the acid catalyst used is protonated alcohol.

for nitrogen nucleophiles, e.g. $\ce{RNH2/RR'NH}$, when reacting with a carbonyl group to form imine/enamine, the acid catalyst used is $\ce{NH4+}$ instead of protonated alcohol.

According to both reaction mechanisms, both catalysts are just acting as $\ce{H+}$ donators & acceptors, so why 2 different acid catalysts need to be used? Can't I use protonated alcohol all the time?

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    $\begingroup$ Where did you get the idea that NH4+ is the preferred catalyst? I find Ti(IV) species such at Ti(OiPr)4 the most effective. $\endgroup$ – Waylander Jul 2 '20 at 12:22
  • $\begingroup$ @Waylander from 'Organic chemistry as a 2nd language 2nd semester', it told me ammonium ion is generally used as the catalyst. And sorry that I am just a beginner, when I look up the structure of Ti(OiPr)4 in Google, I don't know where is the ionizable proton. $\endgroup$ – 234ff Jul 2 '20 at 12:33
  • $\begingroup$ Titanium(IV) is a Lewis acids. $\endgroup$ – Mathew Mahindaratne Jul 2 '20 at 12:48
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    $\begingroup$ @234ff You need a better textbook $\endgroup$ – Waylander Jul 2 '20 at 13:17
  • $\begingroup$ Seems all my talking under earlier question was in vain... $\endgroup$ – Mithoron Jul 2 '20 at 16:58

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