Question Video: Causes of Hypsochromic Shifts in UV Absorption Spectrum

Video Transcript

Which is the following would result in a hypsochromic shift in the UV spectrum? A) Dehydration of an alcohol to form a double bond. B) Addition of an amino group. C) Cyclization of an aliphatic compound. Or D) saturation of a double bond.

This question asks us about a hypsochromic shift in a UV spectrum. So let’s remind ourselves what that means. A hypsochromic shift occurs when the band position in a spectrum moves to shorter wavelength. This is sometimes called a blue shift. If a band moves in the opposite direction to higher wavelength or lower frequency, this is called a bathochromic shift. The question is, which sort of changes in the chromophore would cause a hypsochromic or bathochromic shift?

The wavelength of light that a chromophore absorbs is affected by how conjugated the molecule is. If we reduce the amount of conjugation in our chromophore, we induce a hypsochromic shift in the UV spectrum. Conversely, if we increase the amount of conjugation in our chromophore, we cause a bathochromic shift. These shifts are cause to the change in the energy levels of the molecular orbitals as we change the amount of conjugation.

So let’s look at each of our potential answers in turn and see whether they would induce a chromic shift. Answer A suggests dehydration of an alcohol to form a double bond. Forming a double bond increases the conjugation of our molecule. So this will not cause a hypsochromic shift. So this is not the right answer.

Next, we have the addition of an amino group. The addition of an amino group to our chromophore also means that we’re adding a lone pair, since nitrogen contains a lone pair of electrons. This lone pair addition will also cause a bathochromic shift. So this is not a correct answer.

Cyclization of an aliphatic compound. An aliphatic compound does not contain any double bonds, so does not have conjugation. And simply cyclizing this linear molecule is not necessarily going to add any double bond character. An example could be the cyclization of hexane into cyclohexane. This means that cyclization of on aliphatic compound is unlikely to really change the UV spectrum. So we can rule this out.

Finally, we come to saturation of a double bond. By saturating a double bond, we are reducing the amount of conjugation in our chromophore. This would indeed lead to a hypsochromic shift in the UV spectrum. So this is a correct answer to our question.

It should also be noted that simply changing the bonding in a molecule is not the only way to cause bathochromic or hypsochromic shifts. The type and polarity of the solvent used to record the spectrum may also have an effect. But from this list of answers, the one which would result in a hypsochromic shift is the saturation of a double bond.