The carbonyl stretching vibrations of esters and amides occur at 1735 and 1670 cm^-1, respectiv

The carbonyl stretching vibrations of esters and amides...

Key Concepts

Infrared Spectroscopy

Infrared spectroscopy is a technique used to study the vibrational transitions in molecules. It involves measuring the absorption of infrared light, which causes molecular bonds to stretch or bend. The specific frequency at which a bond absorbs IR radiation can provide detailed information about its environment and structure, making IR a valuable tool in organic and analytical chemistry.

Carbonyl Stretching Vibrations

Carbonyl stretching vibrations are a key feature in infrared spectroscopy for compounds containing a carbonyl group (C=O). The strength and frequency of the carbonyl stretch are influenced by the bond order, the masses of the atoms involved, and the surrounding electronic environment, providing insights into the molecular structure and bonding characteristics.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across a molecule, which can lower the effective bond order of functional groups like carbonyls. In systems where lone pairs or ?-electrons can be delocalized into adjacent bonds, the resulting decrease in double bond character can shift the vibrational frequency observed in IR spectroscopy.

Electron Donation from Heteroatoms

The ability of certain atoms, such as nitrogen in amides, to donate electron density through their lone pairs plays a crucial role in modifying the carbonyl group's characteristics. This electron donation via resonance reduces the double bond character of the carbonyl, leading to a lower stretching frequency compared to systems such as esters, where the electron donating capability is less pronounced.

Transcript

00:02 This is the answer to chapter 13, problem number 57 from the smith organic chemistry textbook.

00:10 And in this problem, we are asked, explain why a ketone carbonyl typically absorbs at a lower wave number than an aldehyde carbonule.

00:20 So 1715 versus 1730 wave number.

00:25 And so obviously i've drawn a lot of this already.

00:30 So i tried to upload this previously and it didn't work.

00:34 So rather than redraw all of this, i've erased a couple parts that i'll draw back in and i'll talk about what's drawn here.

00:41 And so the answer to this question is going to be found by looking at the charge -separated resonance structure for each of these two molecules.

00:51 So i've drawn a simple ketone and a simple aldehyde here.

00:54 So just a methyl group for each.

00:58 And so in the charge separated resonance structure for the ketone, there are two methyl groups, one to either side of the carbonyl carbon.

01:09 And so these carbons are going to act as electron donating groups.

01:15 And so they're going to donate electron density towards that positive charge in this resonance structure.

01:21 And so this is going to stabilize.

01:24 This resonance structure.

01:26 So that's a somewhat stable resonant structure.

01:31 And then so looking at the aldehyde, we only have one methyl group that's able to do that...