Calculating Chemical Shifts for OCH3 Benzene Ring

[MusicMonkey]

Can someone please explain to me how to calculate chemical shifts only given a few constants? For example the compound is as follows:

a benzene ring except in positions 1 and 4 there is an OCH3 group.

given only the values of:
alkane CH3 = 0.9
alkane CH2 = 1.3
alkane CH = 1.4

O=C-CH3 --> 2.1
-C(triple bond)C-H -->2.5
R-CH2-X --> 3-4
>C=C<H --> 5-6
>C=C<CH3 --> 1.7
Ph-H --> 7.2
Ph-CH3 --> 2.3
R-CHO --> 9-10
R-COOH --> 10-12
R-OH --> 2-5
Ar-OH --> 4-7
R-NH2 --> 1.5-4

The final answer is supposed to be for the H's delta 7.2 and for the H's attached to CH3O delta 3.6. Please help I don't understand how to do any of the chemical shift calculations at all! Thank you in advance.

 

[chem_tr]

It is somewhat complex, but we know that the compound includes phenyl H (Ph-H), and Ph-X-CH₃. Therefore, the entries R-CH₂X and Ph-H would be useful for you.

 

[ravenprp]

Calculating chemical shifts for a compound involves using the chemical shifts of known functional groups and substituents and applying them to the specific molecule being analyzed. In this case, the compound is a benzene ring with an OCH3 group attached at positions 1 and 4.

First, let's identify the functional groups present in the compound and their corresponding chemical shifts. The benzene ring itself will have a chemical shift range of 7-8 ppm, as it is an aromatic system. The OCH3 group will have a chemical shift of around 3.5-4 ppm, as it is an electron-withdrawing group.

Next, we need to consider the effect of the OCH3 group on the chemical shifts of the hydrogens attached to it. The CH3 group will have a chemical shift of around 0.9 ppm, as it is an alkane CH3 group. However, the presence of the OCH3 group will slightly deshield the hydrogens, resulting in a slightly higher chemical shift of around 1.0-1.2 ppm.

Similarly, the CH2 group will have a chemical shift of around 1.3 ppm, but the presence of the OCH3 group will slightly deshield the hydrogens, resulting in a slightly higher chemical shift of around 1.4-1.6 ppm.

Now, let's consider the effect of the OCH3 group on the hydrogens attached to the benzene ring. The hydrogens at positions 1 and 4 will be deshielded by the OCH3 group, resulting in a higher chemical shift of around 7.2 ppm. This is because the OCH3 group is an electron-withdrawing group, which pulls electron density away from the benzene ring, making it less shielded and resulting in a higher chemical shift.

Overall, the final answer for the hydrogens attached to the OCH3 group will be around 3.6 ppm (1.0-1.2 ppm for the CH3 group and 1.4-1.6 ppm for the CH2 group), and the hydrogens attached to the benzene ring will have a chemical shift of 7.2 ppm.

It is important to note that these chemical shift values are approximations and can vary depending on the specific chemical environment of the molecule. It is always best to consult a reference table or use a chemical shift prediction tool for a more accurate calculation.