explain why a thioester
absorbs at a lower frequency than an ester in IR spectroscopy. Use inductive
and resonance arguments in your explanation.
In order to explain why a thioester absorbs at a lower frequency than an ester in IR spectroscopy, we need to consider the differences in the molecular structure and the electronic properties of these two functional groups. Let's break it down step by step.
First, let's consider the concept of inductive effects. These effects arise from the movement of electrons through sigma bonds in a molecule. In the case of an ester, the carbonyl group consists of a carbon atom double-bonded to an oxygen atom. Oxygen is more electronegative than carbon, which means it attracts electrons towards itself. This creates a partial positive charge on the carbon atom and a partial negative charge on the oxygen atom.
Now, let's compare this to a thioester. In a thioester, the carbonyl group consists of a carbon atom double-bonded to a sulfur atom. Sulfur is less electronegative than oxygen, which means it attracts electrons less towards itself compared to oxygen. As a result, there is a smaller inductive effect in a thioester compared to an ester.
Next, let's consider resonance effects. Resonance occurs when the electrons in a molecule are delocalized, resulting in electron density being spread across multiple atoms. In an ester, resonance can occur through the oxygen atom, which can donate electron density to the carbonyl carbon. This delocalization of electrons stabilizes the ester's molecular structure.
In contrast, a thioester has a larger sulfur atom in place of oxygen. Sulfur has a larger atomic size and less electronegativity compared to oxygen. These differences make resonance less effective in thioesters, reducing the stabilization of the molecular structure through electron delocalization.
So, how does all of this affect the absorption frequency of these functional groups in IR spectroscopy? When a molecule interacts with infrared radiation, it causes vibrations of the chemical bonds within the molecule. These vibrations result in absorption of specific frequencies of infrared light.
The presence of an ester or thioester functional group in a molecule influences its vibration frequencies. Due to the stronger inductive effect and more effective resonance in esters, the carbonyl group in an ester tends to absorb at higher frequencies compared to a thioester. This means that the absorption peak for an ester will appear at a higher wave number (or lower wavelength) in an infrared spectrum compared to a thioester.
In summary, the lower frequency of absorption of thioesters compared to esters in IR spectroscopy can be explained by the weaker inductive effect and less effective resonance in thioesters, which result from the lower electronegativity and larger atomic size of sulfur compared to oxygen.