Q2P NMR spectra for two compounds ar... [FREE SOLUTION]

Chapter 18: Q2P (page 920)

NMR spectra for two compounds are given here, together with the molecular formulas. Each compound is a ketone or an aldehyde. In each case, show what characteristics of the spectrum imply the presence of a ketone or an aldehyde, and propose a structure for the compound.

Short Answer

Expert verified

Structure obtained from the given spectrum is,

$C_{9} H_{10} O$ has five elements of unsaturation. $1 H$ doublet with very small coupling constant is observed at chemical shift value 9.7 indicates aldehydic hydrogen next to data-custom-editor="chemistry" $CH$ . data-custom-editor="chemistry" $5 H$ multiple peaks at chemical shift value 7.2-7.4 indicates a monosubstituted benzene. $1 H$ multiplet at chemical shift value 3.6 and $3 H$ doublet at chemical shift value 1.4 indicates $C H C H_{3}$ . Combining all the fragments together, we get the proposed structure to be 2-phenylpropanal.

Step by step solution

Step-1. Information about the structure from given spectrum:

$C_{9} H_{10} O$ has five elements of unsaturation. $1 H$ doublet with very small coupling constant is observed at chemical shift value 9.7 indicates aldehydic hydrogen next to $C H$ . $5 H$ multiple peaks at chemical shift value 7.2-7.4 indicates a monosubstituted benzene. $1 H$ multiplet at chemical shift value 3.6 and $3 H$ doublet at chemical shift value 1.4 indicates $C H C H_{3}$ .

Step-2. Structure determination:

By combining all the fragements from the data obtained from the given spectrum we have,

The only structure possible from the data and fragments will be,

The splitting of the hydrogen on carbon-2 which is next to the aldehyde in the obtained structure looks like a quartet due to splitting from adjacent methyl group. A closer examination of the peaks shows that each peak of the quartet is split into two peaks and this is due to splitting from the aldehydic hydrogen. The aldehyde hydrogen and the methyl hydrogens are not equivalent so it is expected that coupling contants are not equal.