Examining the IR pattern for C=O vibrations

Infrared (IR) spectrometry is an excellent diagnostic tool for identifying, or lack of, a carbonyl functional group(s). Carbonyl stretching vibration absorbs between 1900-1600 cm-1—a region where few other functional groups absorb. In addition, the carbonyl vibration is typically intense and thus easy to spot.

The gas phase FT-IR spectra, shown below, represent five carbonyl bands differing in the neighbouring atoms: acid halide, carboxylic acid, ester, ketone and amide. The general pattern can be used to suggest a carbonyl group based on the wavenumber.

TIP: conjugated carbonyl groups and hydrogen bonding will typically shift the C=O band to a lower frequency. Ring strain and heteroatoms will shift the C=O band to a higher frequency.

Examining the IR pattern for C=O vibrations

Infrared (IR) spectrometry is an excellent diagnostic tool for identifying, or lack of, a carbonyl functional group(s). Carbonyl stretching vibration absorbs between 1900-1600 cm-1—a region where few other functional groups absorb. In addition, the carbonyl vibration is typically intense and thus easy to spot.

The gas phase FT-IR spectra, shown below, represent five carbonyl bands differing in the neighbouring atoms: acid halide, carboxylic acid, ester, ketone and amide. The general pattern can be used to suggest a carbonyl group based on the wavenumber.

TIP: conjugated carbonyl groups and hydrogen bonding will typically shift the C=O band to a lower frequency. Ring strain and heteroatoms will shift the C=O band to a higher frequency.

Examining IR spectra for the presence of OH

Infrared (IR) spectrometry can serve as a simple method to gather information on the presence of OH. Characteristic OH absorptions occur around the ranges of 3700-3200 cm-1 (OH stretching) and 1200-1000 cm-1 (OH bending). In the case of hydrogen-bonded OH, the band in the region 3700-3200 cm-1 generally appears broad and sometimes can go unnoticed.

Below is the IR spectrum for butanoic acid. The presence of the two bands at 3582 cm-1 and 1150 cm-1 suggest that an OH group is present.

 

 

TIP: An incorrect interpretation may occur for samples that contain impurities with OH groups, for example, water.

Examining IR spectra for the presence of OH

Infrared (IR) spectrometry can serve as a simple method to gather information on the presence of OH. Characteristic OH absorptions occur around the ranges of 3700-3200 cm-1 (OH stretching) and 1200-1000 cm-1 (OH bending). In the case of hydrogen-bonded OH, the band in the region 3700-3200 cm-1 generally appears broad and sometimes can go unnoticed.

Below is the IR spectrum for butanoic acid. The presence of the two bands at 3582 cm-1 and 1150 cm-1 suggest that an OH group is present.

 

 

TIP: An incorrect interpretation may occur for samples that contain impurities with OH groups, for example, water.

Does my Unknown Compound contain Chlorine?

Like bromine, compounds that contain chlorine atoms have a distinct ion pattern on a mass spectrum. The A+2 peak for a monochlorinated compound will be at almost one-third the intensity to the 35Cl peak due to the presence of 37Cl isotope. A compound with two chlorine atoms will show distinct A+2 and A+4 peaks with an approximate ratio of 10:6:1.

The following EI mass spectra are for 3-chloropropanenitrile and (1E)-1,2-dichlorobut-1-ene with nominal masses at 89 and 110 Da, respectively. The top MS, for 3-chloropropanenitrile, shows 2 identifiable ion clusters at m/z 49/51 and 89/91 that contain chlorine. The ion peak at m/z 54 does not contain chlorine.

 

The MS for (1E)-1,2-dichlorobut-1-ene shows 2 ion clusters at m/z 75/77 and 110/112/114. Only the ion cluster at m/z 110 is dichlorinated.

TIP: Check the fragment ion peaks too for the distinct pattern especially when the molecular ion peak is not visible.

Does my Unknown Compound contain Chlorine?

Like bromine, compounds that contain chlorine atoms have a distinct ion pattern on a mass spectrum. The A+2 peak for a monochlorinated compound will be at almost one-third the intensity to the 35Cl peak due to the presence of 37Cl isotope. A compound with two chlorine atoms will show distinct A+2 and A+4 peaks with an approximate ratio of 10:6:1.

The following EI mass spectra are for 3-chloropropanenitrile and (1E)-1,2-dichlorobut-1-ene with nominal masses at 89 and 110 Da, respectively. The top MS, for 3-chloropropanenitrile, shows 2 identifiable ion clusters at m/z 49/51 and 89/91 that contain chlorine. The ion peak at m/z 54 does not contain chlorine.

 

The MS for (1E)-1,2-dichlorobut-1-ene shows 2 ion clusters at m/z 75/77 and 110/112/114. Only the ion cluster at m/z 110 is dichlorinated.

TIP: Check the fragment ion peaks too for the distinct pattern especially when the molecular ion peak is not visible.

Comparing Spectra of the Starting Material to the Unknown Product

For synthetic reactions where rearrangement, derivatization, or cyclization has occurred, a common task is to compare the NMR spectra between the starting material and the product. The similar peaks indicate a structural region where change has not occurred whereas the unique peaks indicate a region where change has occurred. Arguably, this becomes a peak mapping task as opposed to structure elucidation which is geared towards the unique pieces.

A cyclization reaction for the starting material, N-(2,6-dimethylphenyl)-N2,N2-dimethylalaninamide is shown below. The dotted circle represents the unknown region of the product. The CH3 groups circled in red indicate the similar regions between the starting material and the product.

 

The unknown product, N,N-dimethyl-1-(8-methyl-1,4-dihydro-2H-3,1-benzoxazin-2-yl)ethanamine, is shown below. The blue circle indicates the point of the cyclization and thus the area where an elucidator must piece together the experimental data to complete the structure.

 

Comparing Spectra of the Starting Material to the Unknown Product

For synthetic reactions where rearrangement, derivatization, or cyclization has occurred, a common task is to compare the NMR spectra between the starting material and the product. The similar peaks indicate a structural region where change has not occurred whereas the unique peaks indicate a region where change has occurred. Arguably, this becomes a peak mapping task as opposed to structure elucidation which is geared towards the unique pieces.

A cyclization reaction for the starting material, N-(2,6-dimethylphenyl)-N2,N2-dimethylalaninamide is shown below. The dotted circle represents the unknown region of the product. The CH3 groups circled in red indicate the similar regions between the starting material and the product.

 

The unknown product, N,N-dimethyl-1-(8-methyl-1,4-dihydro-2H-3,1-benzoxazin-2-yl)ethanamine, is shown below. The blue circle indicates the point of the cyclization and thus the area where an elucidator must piece together the experimental data to complete the structure.