EXAM L 9a. Practical Infrared Explain the wavenumber shift in C≡C

L 9a.
Practical Infrared
Explain the wavenumber shift in C≡C, C=C and C-C bonds. Use the
following equation:
2 
The bands are in decreasing energy order in a way like this C≡C,
C=C and C-C.
The reuced mass is the same for all the three chemical groups,
only the k value difference can cause the wavenumber shift.
The C≡C frequency is the highest about 2100 cm-1, the strongest
bond having the highest k value.
The propane has four characteristic C-H band about 2800 cm-1. What
is the sequence of the bands in energy?
The energy decreases as the wavenumber decreases. The order
of vibrations in this broad band is as follows: asymmetric
stretching methyl, symmetric stretching methyl, asymmetric
stretching methylene, symmetric stretching methylene.
The C≡C stretch band at 2119 cm-1 in 1-octyne disappears in 4-octyne,
explain this experimental fact.
Compare bands C≡C stretch at 2119 cm-1 and C≡N stretch at
2249 cm-1. The latter band is more intense than the C≡C. Explain this
experimental fact.
Hint: polarity
The two characteristic spectral range of carboxylic acids.
Compare the number of peaks arising from N-H vibrations in
molecules containing primary, secondary and tertiary amin group.
Primary amines have two N-H bonds, therefore they typically
show two spikes that make this band. 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.