pKa predictions for Dihydroxybezenes

User 5508363905

21-04-2006 23:53:41

Is it possible to determine the source of decreasing pKa for para, meta and ortho dihydroxybenzenes? In other words, can you determine from the Marvin calculations what contribution resonance, hydrogen bonding, keto enol tautomerism, etc have on the pKa? ortho dihydroxy benzene has the lowest pKa (8.8) followed by meta(8.9), then para(9.7).

User 851ac690a0

25-04-2006 10:23:37

Dear Carl,

Study of resonance structures, existence of H bond or even tautomerization etc. provides extra information about acid-base properties of molecules and can help to explain differences between pKa values.

The best you can do (in your example) to investigate stabilization of negative charge of conjugated base

Conjugated acid --> Conjugated base + H (+)

Ph-OH --> Ph-O(-) +H(+)

Basically you can consider the next parameters of stabilization

A., stabilization of oxygen anion with delocalization of lone pair electrons into ortho and para position

B., proximity effect of neutral OH. [Interaction of OH with O (-)]

C., electron withdrawing effect of neutral OH group in meta position

D., electron releasing effect of neutral OH group into ortho and para position

1,2 di-OH phenol : A,B,D

1,3 di-OH phenol : A,C

1,4 di -OH phenol : A,D

Acidic pKa is decreased by 'A','B',and 'C' parameters . While pKa is increased by 'D' parameter.