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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: daveylf on May 30, 2005, 05:41:16 AM

Title: how different substituents on the benzene ring affect the acidity?
Post by: daveylf on May 30, 2005, 05:41:16 AM

hi , i need help with explaining why the acidity of phenols is affectd by different substituents on its benzene ring. i know that ortho and para groups make it more acidic as does meta but not as much. is that what is meant by the question, i dont really understand what the question is asking

Title: Re:how different substituents on the benzene ring affect the acidity?
Post by: Grafter on May 30, 2005, 07:22:56 AM

There are two different ways of making phenols more acidic with substituents. Electron withdrawing groups, and mesomeric withdrawing groups.

Electron withdrawing groups, (eg Cl) will pull electron density out of the ring, which will in turn pull electron density away from the oxygen of the OH, making it more acidic.

If you place a substituent on the ortho or para positions that withdraws charge mesomerically (eg NO₂), you can draw out canonical structures that show the delocalisation of oxygen's lone pair into the nitro group. This drastically decreases the proton affinity for the phenol. Mesomeric stabilisation only occurs on the ortho and para positions, as conjugation is lost when delocalising charge into a meta substituent. This is easily observed by the construction of the relevant canonicals.