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Topic: Deducing structure by MW and spectroscopy readings  (Read 2213 times)

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Offline alrob8a

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Deducing structure by MW and spectroscopy readings
« on: October 17, 2011, 10:42:38 PM »
This is my assignment question, which I've answered. Just hoping someone could weigh in and tell me if my reasoning is on track.

Q:

Compund X has a MW of 74. The IR spectrum of X contains a strong band between 1650 and 1800cm-1 but no bands at 1600cm-1 or 3500cm-1. X does not react with 2,4-DNP or with aq NaHCO3. The 13C NMR spectrum consists of three peaks - one above 80ppm and two below 80ppm. The 1H NMR spectrum consists of 2 peaks of equal areas. Deduce the structure of X showing all your reasoning


My Answer:


MW of 74 equates to formulas of C4H10O, or C3H6O2

IR absorption between 1650 and 1800cm-1 indicates a C=O bond.
No absorption at 1600cm-1 indicate absence of C=C bonds (occuring in the 1600-1650cm-1 spectrum). X is therefore not an alkene.
No absorption at 3500cm-1 indicates absence of O-H bonds (occuring in the 3300-3500cm-1 spectrum). X is therefore not an alcohol.

X has 1 degree of unsaturation in the form of a C=O, therefore follows the molecular formula of CnH2n

Since there is no reaction with 2,4-DNP or with aq NaHCO3, X cannot be an aldehyde or ketone, or an acid.

113C NMR analysis tells us there are 3 carbon in the compound.
1 peak above 80ppm indicates an sp2 C, confirming presence of C=O.
2 peaks below 80ppm indicates 2 sp3 C.

The 1H NMR spectrum consisting of 2 peaks of equal areas tells us a 1:1 ratio of hydrogen in 2 different chemical environments.

Compound X must follow the formula C3H6O2. Since X has no O-H bonds, is not an aldehyde or ketone, the 2 O must be part of an ester functional group.

(pardon crude structure here)

      H     O            H
       |    ||            |
H -- C -- C -- O -- C -- H
       |                   |
      H                   H


So how did I go?

Thanks heaps!
Alister

Offline fledarmus

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Re: Deducing structure by MW and spectroscopy readings
« Reply #1 on: October 18, 2011, 08:00:28 AM »
Looks good!  :)

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