A Visual Manifestation of the Norrish Type I Reaction The Cyclohexanone Sunburn Dosimeter Felix A. Carroll,' GeoMey F. Strwse, and Jon M. Hain Davidson College, Davidson. NC 26036 The most successful laboratory experiments are those that allow students to correlate molecular chance with macroscopir phenomena. If the experiment ran be made relevant to the students' everyday lives, it is all the more meanindul. In the course of some research on organic photochem~stry ( I ) , we have observed reactions that not only adapt well to laboratory experiences hut also can he applied outside the laboratory. The organic reagent is inexpensive, and the use of water as the reaction medium eliminates the need for toxic or flammable solvents. Alinhatic ketones exhibit several tvves .. of vhotochemical reactions (2). Two intramolecular processes are the Norrish e reacTvve I reaction or n cleavare and the Norrish T v ~ I1 tion, also known as p cleavage (3).The @ cleavage reaction proceeds through abstraction of a hydrogen atom, preferably on a carbon y to the carhonyl, as shown in eq 1. The resulting diradical can then produce cyclobutanol or the P cleavage products (alkene and enol), or it can transfer the hydrogen back to the y carbon, allowing racemization of a chiral center (4).
stabilized by transferring vibrational energy to solvent molecules so that carbonvl loss does not occur. In the case of cyclohexanone, for example, a cleavage in solution produces a diradical which can proceed by either of two pathways to isomeric ground state products. The first is an m-hydrogen abstraction pathway that produces an enal, and the second is an &hydrogen abstraction pathway that produces a ketene. The ratio of ketene to e n d varies with temperature and with the pattern of substituents on the cylohexane ring (6).
In alcoholic solvents the ketene is trapped by nucleophilic addition, forming an ester. In aqueous solutions water adds t o form a carboxylic acid.2
' Alrthor to whom correspondence should be sent.
The products can also undergo further photochemical reaction
(7).
The a cleavage reaction is simpler. The photoexcited ketone dissociates between the carbonyl carbon and a carbon a to it. In the gas phase the resulting diradical may be vibrationally excited and can undergo additional processes, including carbonyl loss (5).
Cycloalkanones with small rings do not exhibit @ cleavage reactions since abstractable hydrogens are not conformationally accessible to the excited carhonyl group. Moreover in solution the diradical produced by a cleavage can be
84
Journal of Chemical Education
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Figure 1. Solar irradiation of 0.1 M cyciohexanone in water wmainlng 0.5 M NaCi. pH values are estimates from indicator sbips graded in 0.5 pH unit inc~ements.
Figure 2. pH changes in the laboratory irradiation of 0.11 M cyclohexanone in B~UBOUS0.05 M NaCl solution.The pH was initially adjusted to 8.4 with NaOH solution. Irradiation of aqueous cyclohexanone solutions thus produces hex-5-enal and hutylketene. T h e latter reacts with water to form caproic acid, which lowers the p H of the solution. If the solution is initially basic, the change in pH upon irradiation is rapid and can he followed by p H meter or indicator strips. The reaction can be carried out using solar irradiation or by using a UV light source in the laboratory.
Solar Irradiation-The Sunburn Dosimeter A 40-mL solution of 0.1 M distilled cyclohexanone in 0.5 M aqueous NaCl is adjusted to pH 8 with dilute sodium hydroxide solution and is placed intoa 25- X 200-mm Pyrex test tube. The test tube is placed outdoors on a white cardboard refledor mounted so that the test tuhe is nearly perpendicular to the sun's rays. The pH is read initially and then is read again approximately every 10 min with indicating pH paper.3,' The indicator strip is immersed in the solution for 2 min before each reading.
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Laboratory Irradiation A 50-mL solution of cyclohexanone (0.1M) and sodium chloride5 (0.05 M) in water is adjusted to pH 1.8 with dilute sodium hydroxide solution and is placed into a 25- X 200-mm Pyrex test tube. A small stir bar is placed in the test tube and a pH electrode is inserted into the ~olution.~The test tuhe isplacedapproximately30 cmfrom a 450 W medium pressure mercury vapor lamp contained in a quartz irradiation well. The solution is stirred and the pH is read approximately every 2 min during irradiation. Analysis of the Results Ruth t hr lal~uratorvand field irradiations produce significant pH changes in a matter oiminutes. Figure I shows the change in pH for snlutions of cyclohexanone in aqncous solution rxposed ro sunlight, with pH monitorrd by indicator itrioa. F'ieure 2 shows the rhanen in r)H with time fur a solution irraziated with the light"from'a 450-W medium nressure mercurv . vanor . lamn. .. with DH monitored continuously by pH meter. The two curves reflect the pH changes associated with the titration of a strong base with an excess of weak acid.7 A very small percent conversion of cyclohexanone to c a ~ r o i cacid can produce pH changes of the magnitude observed. Carrying out the pH change in sunlight not only allows students to follow the Norrish Type I cleavage of cyclohexanone, hut also it allows them to monitor the intensity of sunlight. The pH changes could he read both on sunny and on cloudy days to determine the effect of scattered light on overall UV intensity. Thus, the reaction can serve as a solar actinometer. A solar photochemical experiment also provides an opportunity to discuss with students the effects of sunlight on
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Fiaure 3. Relative suniiaht intensitv 1- .- 4 and erythema action spectrum (- - -) [lehscale] andcyc ohexanone absorplmn ( -1 5 X 10~'Msoul on. right sca el n the UV-B region. 290-320 nm skin. The cyclohexanone system is particularly useful in this regard, since the portion of the cyclohexanone absorption spectrum that is accessible through Pyrex vessels closely matches the erythema (skin reddening) action spectrum (Fig. 3) (10). The action spectrum, in turn, is related to the absorotion of lieht - bv.nucleic acids and proteins in the skin (11).w i t h some experience, students c o d d calibrate the pH chanees observed with their own epidermal tanningburning respinse, thus allowing the cyclohexanone system to function as a "sunburn do~imeter."~ Acknowledgment Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Additional support was provided by the National Science Foundation. Llterature CHed F A . I I , , ~.I \ I . L P C . H H . \ I ~ . .l c r l h r i z l n ~ l t U.lllln~~ctnrsr.a...ln .~~~ PI..,
