Testing the Effectiveness of Sunscreens

Purpose

Determine the effectiveness of suntan lotions in screening out UV radiation using the benzopinacol reaction.

Introduction:

The sun produces all forms of electromagnetic energy. Some of this energy is visible light while most falls outside the range of wavelengths that are visible to the human eye. For example, the sun produces infrared, radio and ultraviolet radiation that our eyes cannot detect. Ultraviolet radiation makes up a small fraction of sunlight but it a much more energetic radiation than visible light.
Ultraviolet (UV) radiation is often divided into three different ranges; UV-A, UV-B, and UV-C. UV-A radiation constitutes about 90-95% of the UV radiation that reaches the Earth’s surface. It has the longest wavelengths (315-400 nm) and the lowest energy in inducer of tanning in humans. UV-A can also damage the skin the furthest and is the primary inducer of tanning humans. UV-A can also damage eyes and lead to an increase in cataracts. UV-B rays have a medium wavelength (290-315 nm) and are partially absorbed by the ozone layer. UV-B radiation does not penetrate the skin as far as UV-A rays do but they contain much more energy. UV-B is responsible for activating the synthesis of vitamin D. UV-B radiation is the primary cause of sunburn and can cause cellular damage to the skin and eyes. UV-C radiation is the highest energy ultraviolet wavelength (200-290 nm) but most of it is absorbed by the ozone layer in the Earth’s upper atmosphere. Recent satellite readings show that the ozone layer has been damaged and is thinning. This may allow more UV light to reach the Earth’s surface UV radiation is seen as a threat to animals and plants since UV radiation can cause changes in the chemical bonds of sensitive compounds.
When your skin absorbs UV radiation, two effects occur. First, melanin (substance located in protective cells) absorbs as much UV radiation as possible and is oxidized to a darker color, giving a tan in a few hours. The second effect is the destruction of the keratinocyte cells (responsible for producing new cells), or even worse, a rearrangement or chemical change in the DNA within the keratinocyte cells. If the keratinocyte cells are destroyed, the nervous system sensors are also damaged resulting in pain and increase in the blood circulation in the area and producing the characteristic red skin of sunburn. If the DNA is damaged, mutations may occur resulting in age spots, wrinkling, and cancer. The higher the level of UV exposure, the more likely the keratinocyte cells will be damaged.
To limit the amount of exposure to harmful UV radiation, sunscreens are recommended. Suntan and sunblock lotions are two different products. Sun blocks contain compounds like titanium dioxide or zinc oxide that completely prevent all light from reaching the skin. Suntan lotions contain compounds that absorb UV radiation and reduce the amount of UV radiation that is absorbed by the skin. The ability of a sunscreen to protect the user from UV radiation is defined as the Sun protection factor (SPF).　
The SPF is the ratio of the amount of UV radiation required to produce pinkness in skin covered by the sunscreen (assessed 24 hours after exposure) to the amount of UV radiation required to produce a similar level of pinkness in unprotected skin. If a sunscreen reduces the effect of sunlight on skin by 50%, it would have a SPF of 2; if by 75%, the SPF would be 4. A sunscreen with a SPF of 30 reduces the UV radiation exposure by 96.7%.
Less UV penetration protects skin cells from the damaging effects of UV light. While the lotion limits damage to a cell’s DNA, it also prevents the UV from reacting melanin-containing cells and therefore prevents tanning. The more protection given by the suntan lotion, the higher its SPF.
Suntan lotions contain compounds that effectively absorb UV radiation. The compounds absorb most of the UV radiation, thus decreasing possible skin and cellular damage. Suntan lotions are normally clear and contain aromatic organic compounds. Benzene rings in aromatic compounds are very effective at absorbing UV radiation. Some common molecules used as active ingredients in suntan lotions are shown in Figure 1.

In this experiment, the effectiveness of suntan lotions will be analyzed using the reaction to produce benzopinacol. Benzophenone (two phenyl groups attached to a ketone) absorbs ultraviolet light to produce an excited state. This excited molecule abstracts a proton from the solvent (isopropyl alcohol) to form a radical that then dimerizes to benzopinacol (Figure 2).

In benzophenone, the most loosely held electrons are the two pairs of nonbonded electrons on the carbonyl oxygen. These electrons have the highest energy are therefore the most easily excited. One of these electrons is excited by the UV light into the lowest unoccupied excited state, which is the carbon from the carbonyl group. The activated state abstracts a proton from the isopropyl alcohol solvent to form a diphenyl hydroxyl radical. The isopropyl alcohol radical loses another proton to form acetone. Two diphenyl hydroxyl radicals will dimerize to produce benzopinacol.
The reaction proceeds nearly 100% completion and the produce is insoluble in isopropyl alcohol. Yields of > 90% are possible. The more UV light that reaches the solution of benzophenone and isopropyl alcohol, the greater the amount of benzopinacol precipitate formed. Therefore, the difference in yield between a vial coated with suntan lotion and one without will be proportional to the reduction in absorbed UV radiation caused by the suntan lotion.