[szymonwoj]

Hi!

I am having problems with analysis of spectrum that I am attaching below.

http://imageshack.us/photo/my-images/29/83vo.jpg/
http://imageshack.us/photo/my-images/22/ihb3.jpg/

These is a problem from Silverstein, and I manage to identify compound correctly, but I do not understand how one can pinpoint peaks in C13 spectrum for carbons in position 3 and 5. In my opinion it is only possible to mark them as "position 3 or 5" both.

Spectrum contains the answers but not an explanation. Could you help me out?

Simon

[Illuminatus]

Hello Simon,

I hope you agree that carbons 3 and 5 are fairly different in terms of their distance from the functional zone of the lactone. Carbon 3 is two bonds removed from the carbonyl carbon, whereas carbon 5 is two bonds from the ether (oxygen). Oxygen is more electron withdrawing than a carbonyl, and the electronegativity at the locale of carbon 5 is greater than that of carbon 3. Thus carbon 5 has a greater chemical shift.

Note: In effect, carbon 5 also has a small partial negative charge on it whereas carbon 3 could be considered to have a small partial positive charge (the partial charges alternate away from the oxygens which are almost always partial negative); this way of looking at organic molecules can be helpful when dealing with more complex spectra.

Enjoy

[szymonwoj]

I was trapped by my chemical shifts tables and need to find the hard evidence there. I have forgotten to use common sense.

Thank you for your time.

[muse]

Following the correlation in the HETCOR and COSY you can distinguish the two peaks.

[szymonwoj]

Could you elaborate?