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Question

Arrange the following in the decreasing order of basicity:

guanidine



A

I > II > III

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B

I > III > II

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C

III > II > I

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D

II > I > III

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Solution

The correct option is B

I > III > II


Guanidium

The above is the conjugate acid of guanidine called the guanidinium cation. This planar, symmetric ion consists of three amino groups each bonded to the central carbon atom with a covalent bond of order 4/3. It is a highly stable +1 cation in aqueous solution due to the efficient resonance stabilization of the charge and efficient solvation by water molecules. As a result, its pKa is 13.6, meaning that guanidine is a very strong base in water; in neutral water, it exists almost exclusively as guanidinium. The amide has just the one nitrogen in conjugation with the carbonyl group, whereas urea has two such. This becomes clearer when urea gets protonated at the oxygen atom (why?):

Uronium

So, the order is:

I > III > II


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