Beta-carotene, with its system of 11 conjugated double bonds, absorbs light with wavelengths in the blue region of the visible spectrum while allowing other visible wavelengths — mainly those in the red-yellow region - to be transmitted.
This is why carrots are orange. However, this molecule also absorbs at nm. We have been talking in general terms about how molecules absorb UV and visible light — now let's look at some actual examples of data from a UV-vis absorbance spectrophotometer.
The basic setup is the same as for IR spectroscopy: radiation with a range of wavelengths is directed through a sample of interest, and a detector records which wavelengths were absorbed and to what extent the absorption occurred. Notice also that the convention in UV-vis spectroscopy is to show the baseline at the bottom of the graph with the peaks pointing up. Wavelength values on the x-axis are generally measured in nanometers nm rather than in cm -1 as is the convention in IR spectroscopy.
Peaks in UV spectra tend to be quite broad, often spanning well over 20 nm at half-maximal height. The cinnamaldehyde example far right shows that extended conjugation further lowers the absorption frequency, although not to the same degree. Incorporation of the carbonyl group in a small ring 5, 4 or 3-membered , raises the stretching frequency. The increase in frequency ranges from 30 to 45 cm -1 for a 5-membered ring, to 50 to 60 cm -1 for a 4-membered ring, and nearly cm -1 for a 3-membered ring.
This shift also occurs in the presence of the previous conjugative lowering of the stretching absorption. Examples of this effect are shown below. Changing an alkyl substituent of a ketone for an electron releasing or withdrawing group. This effect, which may shift the carbonyl stretching frequency up or down, is particularly important when an alkyl substituent is replaced by a hetero atom such as N, O or X halogen.
Such cases will be discussed as carboxylic acid derivatives. The following examples show the influence of a strongly electron withdrawing group -CCl 3 and a conjugatively electron donating group -OCH 3. To understand the functioning of these factors consider the following analyses:.
Conjugation extends the dipolar character of the carbonyl group to the double bond or aromatic ring so that the beta-carbon atom shares the positive character of the carbonyl carbon. As illustrated by the following resonance equation, this not only explains conjugate addition reactions of nucleophiles, but also suggests that the carbonyl double bond has slightly more single bond character than does an unconjugated function.
When this happens, the C-C bonds of the ring assume greater p-character and the C-O sigma bond has correspondingly greater s-character. The double bond of the carbonyl group is therefore shorter and stronger, and exhibits a larger stretching frequency. The stretching frequency is therefore decreased, as noted in the right hand example below.
Electron withdrawing groups have an opposite influence, and increase the stretching frequency of the carbonyl group. Trichloroacetaldehyde left below provides a good example. Starting with the spectrum of cyclohexanone, infrared spectra of six illustrative ketones will be displayed below on clicking the " Toggle Spectra " button. The difference between liquid film and solution spectra is shown for cyclohexanone, but all other compounds are examined as liquid films.
The carbonyl stretching absorption is colored blue, and characteristic overtones near cm -1 , which are only prominent in liquid phase spectra, are colored orange. The influence of heteroatom substituents on the reactivity of carbonyl functions toward nucleophiles was discussed earlier with respect to carboxylic acid derivatives.
A useful relationship exists between the reactivity of these derivatives and their carbonyl stretching frequencies. Thus, the very reactive acyl halides and anhydrides absorb at frequencies significantly higher than ketones, whereas the relatively unreactive amides absorb at lower frequencies. These characteristics are listed below.
Infrared spectra of many carboxylic acid derivatives will be displayed in the figure below the table by clicking the appropriate buttons presented there. In acyl chlorides a lower intensity shoulder or peak near cm -1 is due to an overtone interaction. Cyclic anhydrides also display two carbonyl stretching absorptions, but the lower frequency band is the strongest. In concentrated samples this absorption is often obscured by the stronger amide I absorption.
Hydrogen bonded association shifts some of these absorptions, as well as the prominent N-H stretching absorptions. N-H stretch : to cm Compare the UV absorption spectrum of 1-butene to 1,3-butadiene.
Representation of overlap of p-orbitals of a double bond. Ultraviolet absorption spectrum of 1-butene. Representation of the four p-orbitals in 1,3-butadiene.
Using representations of the p-orbitals in which the dark color indicates the positive region of the wave function and a light color indicates the negative region of the wave function, draw all of the possible ways in which the wave functions of the four p-orbitals can overlap with each other.
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