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Why is a strong acid like $\ce{H2SO4}$ used for imine formation here instead of a weaker acid like TsOH? I thought that if the pH was too low, the amine molecule would get protonated to an ammonium, which are not nucleophilic? enter image description here

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  • $\begingroup$ In short, this reaction has two steps: 1. Nucleophilic attack of nitrogen on the carbonyl carbon 2. elimination of water in acidic conditions $\endgroup$ – Gaurang Tandon Feb 11 '18 at 11:21
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You need to protonate the oxygen in order to actívate the molecule towards nucleophilic attack by the nitrogen of Ethyl amine. pH plays a very important rule here. Too low pH gives an efficient protonation of the oxygen, but it also protonates the nitrogen of the Ethylamine(As ethylamine is weak base due to the lone pair of electrons on nitrogen in it), so the production of the imine results inefficient due to less amount of the desired reagent. On the other hand, at higher pH values, the oxygen does not get protonated enough to proceed the reaction as the electronegativity of oxygen dominates it from giving electrons to a low concentration of protons( As pH is high). So there is an optimum pH between pH 4.5 and 5, for this reaction to take place. So dil H2SO4 can give this pH level while weaker acids cannot. Also when you look at the mechanism for we need to dehydrate to get imine. Dil H2SO4 is good dehydrating agent.
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For all the factors above diluted strong acids are better than weak acids.

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    $\begingroup$ Diluted H2SO4 isn't good dehydrating agent. $\endgroup$ – Mithoron Feb 11 '18 at 16:55
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You can use any acid, since it will be immediately neutralized by the amine.

The p$K_a$ of $\ce{H2SO4}$ is $-9$ while that of TosOH is $-6.5$, but both will form $\ce{EtNH3^+}$ which has a p$K_a$ between 10 and 11.

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