Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: CrimpJiggler on November 04, 2011, 03:13:26 PM

Title: Acidity of aromatic acids
Post by: CrimpJiggler on November 04, 2011, 03:13:26 PM
Heres meta methoxy benzoic acid:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.safcglobal.com%2Fmedium%2Fstructureimages%2F99%2Fmfcd00002499.png&hash=a7aa916ce5cbdc8e160d0de10270f33e)
and heres para methoxy benzoic acid:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg.guidechem.com%2Fcasimg%2F100-09-4.gif&hash=011717bfcca9e7538dbc4da5939faae8)
m-methoxybenzoic acid is a stronger acid than its para substituted analogue. I'm a bit confused about why this is. From what I've read, its because at the meta position, only the methoxy groups inductive withdrawing effects apply whereas at the para position, it also acts as a mesomeric (resonance) donator. Why does the methoxy groups resonance effects only work when its at the para position? Why doesn't it act as a resonance donator when its at the meta position?
Title: Re: Acidity of aromatic acids
Post by: fledarmus on November 04, 2011, 04:49:57 PM
Try drawing resonance structures for your para-substituted analog. Since you are interested in the donating effects, focus on resonance structures that involve moving the lone pairs on your methoxy oxygen into the benzene ring. How is the carboxylic acid involved?

Try the same thing with the meta-substituted analog. Do you see any differences?
Title: Re: Acidity of aromatic acids
Post by: CrimpJiggler on November 06, 2011, 07:39:38 AM
**** my organic chem is a bit rusty, I don't know how to draw this resonance structure. Heres my attempt for the meta isomer:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg28.imageshack.us%2Fimg28%2F2921%2Fresonance.png&hash=968f2b53d3a78da9452434465b987c83)
what am I doing wrong there?