Q42P - Organic Chemistry (Mcmurry)

Question

Some pK_a data for simple dibasic acids are shown. How can you account for the fact that the difference between the first and second ionization constants decreases with increasing distance between the carboxyl groups?

Step-by-Step Solution

Answer

The difference between first and second constant of ionization of dibasic acid decrease with increase in distance needs to be explained.

Step 1: Difference between first and second ionization constant of oxalic acid

There is an attachment of two COOH groups to each other in oxalic acid. There is a maximum repulsion between two negative charges which leads to the production of second ionization. There is no favour for the second ionization. There is a maximum difference between first and second ionization of oxalic acid.

Step 2: Difference between first and second ionization constant of succinic acid

There is a separation of two COOH group by two CH₂ groups and in adipic acid by 4 CH₂ groups. They are separated by CH₂ group intervention by repulsion between two negative charges in adipate and succinate ion. There is a favour in second ionization with increase in intervening number of CH₂ groups. It leads to the small difference between second and first ionization constant in adipic and succinic acid.