Whats the order of increasing pka? Explain

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Al E. Share
Feb 8, 2018

We're considering three protonated amines.

To determine the strength of the of the base, in this case, we would evaluate how stable the positive charge was among the three.

Consider pyridinium,

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The charge can be stabilized through resonance across the arene, but the resonance forms wouldn't be very significant because they all break the octet rule.

Consider the second molecule,

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The charge can be stabilized through resonance throughout the cycloalkene. However, it is in the same situation as the first molecule, the resonance forms all break the octet rule.

Consider the third molecule,

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The charge can be stabilized throughout the cycloalkene, but it has another resonance form that doesn't break the octet rule.

So under that much quantitative analysis (which wasn't too much), I would wager that the resonance in the third molecule makes it the least acidic (most basic).

Again, since pyridinium exhibits more resonance than the second molecule, I would wager it is less acidic (more basic) than the second molecule.

Hence,

#pK_a: 2<1<3#, #"A"#.

To be sure, more basic molecule have higher #pK_a# values because since it is the inverse logarithm of the amount of deprotonation that takes place in solution, the less it deprotonates, the higher the #pK_a#.

I'm open to feedback if I made a mistake!

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