Practical infrared
The graph shows the regions of the spectrum where the following types of bonds normally absorb.
Alkanes
Alkanes are build of -CH3 and -CH2- groups. Their IR spectrum displays only
C-C and C-Hbond vibrations.The most useful are the C-H bands. The C-H stretching vibrations originating from CH3 and -CH2- groups are split into four peaksand appear around 2800 - 3000 cm-1. The order of wavenumber inthis broad band is as follows
methyl(asymm. str.), methyl(symm. str.), methylene (asymm. str.), methylene(symm. str.).
Asymmetric vibrations appear always at higher energy than its symmetric counterpart. Since most organic molecules have C-H bonds, most organicmolecules will display those bands in their spectrum.
Alkenes
Besides the presence of C-H bonds, alkenes also show sharp, medium bandscorresponding to the C=C bond stretching vibrationat about 1600-1700 cm-1.
Some alkenes might also show a band for the =C-H bond stretch, appearingaround 3080 cm-1
as shown below. However, this band could be obscured by thebroader bands appearing around 3000 cm-1
Alkines
The most prominent band in alkynes corresponds to the carbon-carbon triple bond. It shows as a sharp, weak band at about 2100 cm-1. The reason it’s weak is because the triple bond is not very polar.
Triple bond and symmetry
Let’s compare spectra 1-octyne and 4-octyne. At 2119 cm-1 the peak of C≡C bond disappears in the case of 4-octyne, because of symmetric position of the group.
Analysing the frequency dependence on force constant
when μ can be regarded as constant as it is in the C-C, C=C, C≡C series.
Bond, type of vibration / Range of absorption / cm-1 / In molecule / Peak at / cm-1C - C / 1200 - 1000
C = C / 1600-1700 / 1-hexene / 1642
C ≡ C / 2100 - 2250 / 1-octine / 2119
Nitriles
In a manner very similar to alkynes, nitriles show a prominent band around 2250 cm-1caused by the CN triple bond. This band has a sharp, pointed shape just like the alkyne C-C triple bond, but because the CN triple bond is more polar, this band is stronger than in alkynes.
Aldehydes and ketones, C=O functional group
Aldehydes and ketones show a strong, stake-shaped band around
1710 - 1720 cm-1. This band is due to thehighly polar C=O bond. Because aldehydes also contain a C-H bond to the sp2 carbon of the C=O bond, they also show a pair of medium strength bands positioned about 2700 and 2800 cm-1. These bands are missing in the spectrum of a ketone because the sp2carbon of the ketone lacks the C-H bond.
Carboxylic acid
A carboxylic acid functional group combines the features of alcohols and ketones because it has both the O-H bondand the C=O bond. Therefore carboxylic acids show a very strong and broad band covering a wide range between 2800 and 3500 cm-1for the O-H stretch. At the same time they also show the stake-shaped band in the middle of the spectrum around 1710 cm-1corresponding to the C=O stretch.
Amines
The most characteristic band in amines is due to the N-H bond stretch, and it appears as a
weak to medium, somewhat broad band (but not as broad as the O-H band of alcohols). This
band is positioned at the left end of the spectrum, in the range of about 3200 - 3600 cm-1.
Primary amines have two N-H bonds, therefore they typically show two spikes that make thisband. Secondary amines have only one N-H bond, which makes them show only one spike. Finally, tertiary amines have no N-H bonds, and therefore this band is absent from the IR spectrum altogether.
Amide functional group
The amide functional group combines the features of amines and ketones becauseit has both the N-H bond and the C=O bond. Therefore amides show a verystrong, somewhat broad band at the left end of the spectrum, in the range between3100 and 3500 cm-1for the N-H stretch. At the same time they also show thestake-shaped band in the middle of the spectrum around 1710 cm-1for the C=Ostretch. As with amines, primary amides show two spikes, whereas secondaryamides show only one spike.
Store
Transfer from Lecture: Vibrational transitions
Fig. 6. The IR spectrum of laboratory air. Double-beam mode produces a straight line, while the single beam is referenced by N2.
Fig. 7. Vapour phase spectrum of hexan.
The number of normal modes in hexan, using 3N-6 rule, is 20. A simpler explanation for bands appear is the so called group frequencies. The molecule consists of CH2 and CH3 groups.
Group frequencies of hexan.
Band position / cm-1 / Chemical group / Vibrational mode2962 / CH3 / asym. str.
2929 / CH2 / asym. str.
2864 / CH3, CH2 / sym. str.
1462 / CH3, CH2 / asym. bend.
1381 / CH3, CH2 / sym. bend.
Fig. 8. Vapour phase spectrum of cyclohexan.
The molecule consists of six CH2 groups.
Group frequencies of cyclohexan.
Band position / cm-1 / Chemical group / Vibrational mode2930 / CH2 / asym. str.
2855 / CH2 / sym. str.
1452 / CH2 / asym. bend.
Fig. 9. Vapour phase spectrum of 1-hexene.
Group frequencies of 1-hexene.
Band position / cm-1 / Chemical group / Vibrational mode3082 / =C-H / asym. str.
2963 / CH3 / asym. str.
2931 / CH2 / asym. str.
2865 / CH3, CH2 / sym. str.
1644 / C=C / str.
1464 / C-H / bend
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