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11-3 Additional Methods for Oxidizing Alcohols 547 -- Part 5
The pi bond between carbon and nitrogen is less than a full C " O double bond because of the dipolar resonance structure.
They are in the same part of the spectrum.
Simple carboxylic esters absorb around 1735 cm-1.
In a five-membered ring or smaller, these higher-frequency absorptions can be seen.
In a small ring, the angle strain on the carbonyl group forces more electron density into the C " O double bond, resulting in a stronger bond.
N single bond stretch is not useful for structure determination.
Carbon-nitrogen double bonds absorb in the same region as C double bonds, around 1660 cm-1, but the C " N bond gives rise to stronger absorptions because of its greater dipole moment.
The C " N stretch is similar to a carbonyl in intensity absorption.
The triple bond of a nitrile is the most recognized carbon-nitrogen bond.
nitrile triple bonds are more polar than C triple bonds, so they produce stronger absorptions.
The regions of the IR spectrum correspond with bonds to hydrogen, followed by triple bonds, double bonds, and single bonds between heavier elements.
The stretching absorptions of nitrile triple bonds are more intense than those of alkyne triple bonds.
Triple bonds, double bonds, and single bonds are the next highest.
There are three compounds that have the infrared spectrum.
There are at least one functional group in each compound.
The major peaks above 1600 cm-1 should be assigned to the functional group in each compound.
There are hundreds of characteristic absorptions in Appendix 2.
Appendix 2A is visually organized while Appendix 2B is organized by functional groups.
When using this table, remember that the numbers are approximate and they don't give ranges to cover all the unusual cases.
As a result of ring strain and other factors, frequencies change.
The ability to identify functional groups is the most useful aspect of IR.
The carbon skeleton and alkyl groups in the compound are not given much information by IR.
In Chapter 13, we will see that the aspects of the structure can be determined by NMR.
An expert can only determine a structure based on the IR spectrum.
In the interpretation of IR spectrum there can be misunderstandings.
A strong absorption at 1680 cm-1 might be the result of an amide, an isolated double bond, a conjugated ketone, a conjugated aldehyde, or a carboxylic acid.
Familiarity with other regions of the spectrum allows us to determine which functional groups are present.
In some cases, we can't be certain of the functional group without additional information.
The peaks in the fingerprint region are dependent on complex vibrations involving the entire molecule, and it is highly unlikely for any two compounds to have the same spectrum.
It shows the functional groups in the compound.
It is possible to confirm the identity of a compound by comparing it to a known sample.
It's usually strong problems until you feel confident.
C O stretching between 1000 and 1200 cm-1 is shown by ethers, esters, and alcohols.
Many students don't know how much information they can get from the spectrum.
IR and other information will be used to determine the entire structure in Chapter 13.
If you want to get the most out of the IR spectrum, concentrate on getting as much information as you can.
There are several solved problems in this section.
The major, most reliable, features will be the focus of the analysis.
You can study this section by looking at each spectrum and writing down your proposed functional groups.
Look at the solution and compare it to yours.
The structures of these compounds are shown at the end of the section.
At 1714 cm-1, there is a car bonyl absorption.
The weak absorption at 3400 cm-1 is probably an overtone of the strong C " O absorption.
An acid is eliminated by H stretch.
The compound appears to be saturated if H absorption above 3000 cm-1 is taken into account.
A simple ketone is what the compound is.
The absorption at 1650 cm-1 is so intense that it is likely a carbonyl group.
There is a carbonyl group at low frequencies.
This is probably a saturated amide.
The stretch above 3000 cm-1 suggests that the nitrile is saturated.
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