Draw the structure of a compound of molecular formula C4H8O that has a signal in its 13C NMR spectrum at > 160 ppm. Then draw the structure of an isomer of molecular formula C4H8O that has all of its 13C NMR signals at < 160 ppm.
The Correct Answer and Explanation is:
To draw the structures and provide explanations for the given compounds, let’s break down the molecular formula C4H8O into its possible structural configurations, focusing on their 13C NMR spectra.
1. Compound with a 13C NMR signal at >160 ppm:
For a compound with molecular formula C4H8O and a signal at >160 ppm in the 13C NMR spectrum, we are looking for a structure where a carbonyl group (C=O) is present. The carbonyl carbon will appear at >160 ppm because it is highly deshielded, typically in the range of 160-220 ppm. A typical compound of this type would be an α,β-unsaturated carbonyl compound like but-3-en-2-one (methyl vinyl ketone).
Structure of But-3-en-2-one (C4H8O):
iniCopyEdit CH2=CH-CO-CH3
- The carbonyl group (C=O) at position 2 is the deshielded carbon responsible for the 13C NMR signal at >160 ppm (around 200 ppm).
- The C=C bond is also conjugated with the carbonyl group, which slightly shifts the carbonyl signal downfield.
In the 13C NMR spectrum, the signal from the carbonyl carbon will be observed at >160 ppm, and other carbons will appear at lower ppm ranges.
2. Isomer with all 13C NMR signals at <160 ppm:
For the second compound, we need a structure with no carbonyl group, as carbonyl carbons give signals above 160 ppm. A saturated alcohol or ether could be a potential isomer. One possible isomer is butanol (butan-1-ol), a saturated alcohol.
Structure of Butanol (C4H8O):
markdownCopyEdit CH3-CH2-CH2-OH
- In this structure, all carbons are attached to hydrogens and a hydroxyl group (-OH).
- The 13C NMR signals will be in the range of 0-70 ppm, as none of the carbons are deshielded by a carbonyl group or other highly electronegative groups.
- The C-O signal will be around 60-70 ppm, which is typical for carbons attached to an oxygen atom in alcohols.
Summary of NMR Signals:
- But-3-en-2-one (α,β-unsaturated ketone): The signal at >160 ppm corresponds to the carbonyl carbon.
- Butanol (saturated alcohol): All signals are below 160 ppm, with no carbonyl group contributing a high-shielding signal.
By understanding how functional groups like carbonyls influence the chemical shifts in the NMR spectrum, we can accurately predict and explain the different chemical environments in these isomers of C4H8O.
