Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Can someone me

  1. Apr 9, 2007 #1
    Can someone plz help me...

    I have a few questions to ask.. if someone can help I will very thankfull..
    1st: Can someone explain to me the para, ortho, and meta effects of subsituents on the acidity of benzoic acid..
    2nd: Can HNMR be used to distinguish ortho-ethyl benzaldehyde, meta-ethyl benzaldehyde, and para-ethyl benzaldehde..
  2. jcsd
  3. Apr 10, 2007 #2
    If the group is an electron donating group (+R/+I), then the acidity of benzoic acid decreases. Opposite when an ewg is present (-R/-I).

    In the ortho and para position, the groups can partake in resonance, and the resonance effect is greater than the inductive effect. Hence, the +R/-R groups effect the acidity more than the +I/-I groups. If a group is present on the ortho position, and it can form H-bonds, it will do so with th -COOH group present.

    In the meta position, the groups do not undergo resonance with the benzene ring and can only show their +I/-I effect.
  4. Apr 10, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes, NMR can be used to determine the differences between ortho, meta and para substitution. You can see that there will be one ortho proton remaining for ortho-ethylbenzaldehyde. This proton will be split by the others in the benzene ring but should be far enough from the others to be completely resolved. Integtration will give a 1:4 ratio with the other aromatic protons. Para-substitution will yield only two unique aromatic protons by introducing a C2 symmetry element. Both ortho protons will be identical (no ortho-ortho coupling) and both meta protons will be identical (no meta-meta coupling). The result is a pair of resolved doublets that integrate 1:1. You can see the spectra for yourself at the following site:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook