[madscientist]

Hi all,

Have a question which I've attempted to answer, if possible could you let me know if its correct or not.

Question reads:

Propose a structure for the compound which fits the following ¹HNMR data:

C₈H₉Br

3H doublet at 2.0 d, J = 7Hz  (d is a lower case delta indicating shift)
1H quartet at 5.0 d, J = 7Hz
5H singlet at 7.3 d

My working so far is:

(1-bromoethyl) benzene: 

The 5H singlet at 7.3 d indicates a mono-substituted arene.  This 5H aromatic group actually has three types of nonequivalent hydrogens which would be predicted to show three different signals with splitting patterns including a 1H triplet, 2H triplet and a 2H doublet.  Accidental overlapping of signals from the three nonequivalent aromatic ring protons causes them to appear as a single unsplit signal (5H singlet) at 7.3 d

The 1H quartet at 5.0 d indicates an alkyl halide.  Alkyl halides usually show a chemical shift between 2.5-4.0 d.  In this case the higher ppm value of the shift (5.0 d) is caused by the mildly electronegative neighbouring aromatic ring.

The 3H doublet at 2.0 d has the same coupling constant as the 1H quartet at 5.0 d indicating that the protons which cause these signals are adjacent in the molecule.



Do you think (if this is right) that this is enough info to explain why this structure fits the ¹HNMR data?

any help would be much appreciated,

madscientist

P.S. why do electronegative groups affect the shift shown by protons on neighbouring carbons?

[Dan]

That looks fine to me. As you say, you would expect the 5H on the aromatic ring to be more complicated than a singlet, but I can't think of another way to get 5 nearly equivalent H.

The shift to higher ppm caused by electronegative groups is a result of a reduction in diamagnetc shielding. The electrons around a nucleus will align to oppose the magnetic field of the spectrometer, partially shielding the nucleus from the applied magnetic field. The greater the electron density around the nucleus, the greater the diamagnetic shielding - so electronegative groups pull electron density away, reducing diamagnetic shielding, resulting in a greater chemical shift.

[madscientist]

Thanks for that Dan, much appreciated.  Good explaination of the electronegative effect too.

cheers,

madscientist