Gene R. Ziegler University of California Berkeley 94720
The Preparation of Naphthalene-a-d An experiment in organic chemistry
Physical properties, bonding, and the chemistry of alkanes are dealt with in the first weeks of the typical organic chemistry course. This experiment is designed to help coordinate the laboratory work with these early lectures. In the preparation of a hydrocarbon via the Grignard reagent from the corresponding bromide (eqn. ( I ) ) , the Griguard reagent may be considered as the salt of the weak hydrocarbon acid RH. The stronger acid, water, will displace RH from its salt. RM!:Br
+ HOH
-
RH
+ MgX(0H)
Using a-bromonaphthalene as the halide is instructive for the following reasons: (1) It is a liquid a t room temperature and the product, naphthalene, is a solid even though it has a much lower molecular weieht. This observation easily lends itself to a discussion of the relationship of symmetry and melting points. (2) By employing deuterium oxide (DzO) as the strong acid it is possible to prepare specifically deuterated naphthalene-ry-d.' Xaphthalene-a-d has astrong C-D band in the infrared which allows an introduction to infrared analysis and the simple effects of isotopic substitution. (3) The product may be purified by elution chromatography and thus provides experience with this important technique. u
-
The Experiment
Ethyl iodide (1.0 g, 0.0064 mole), magnesium chips (l.Og, 0.042mole) and20mlof dry2ethyletherareadded to a 100-ml round bottom flask. and theflaskisattached to a reflux condenser equipped with a calcium chloride drying tube at the top. The flask and condenser are shaken vigorously for a few minutes until the reaction begins. After the ether is refluxing, a-bromonaphthalene (4.2 g, 0.02 mole) is added. In most cases the vigorous reaction will contiuue; if not, shaking may again be required. Reflux will be maintained by the heat of reactiou for about 45 miu during which time an alumina column (using a 25-ml burette) may be prepared. a Then 1 ml of D 2 0 (>99%D) is added through the top of the condenser. The mixture is refluxed on the steam bath for 5 min to ensure complete quenching of the Grignard reagent. The mixture is cooled to O°C and ether (20 ml) and dilute hydrochloric acid (25 ml) are added. The two liauid ~ h a s e sare ~ o u r e dinto a se~aratorv funnel. The 'lower water layer is discarded 'and thk upper ether layer is dried over anhydrous magnesium sulfate and transferred into a 100-ml round bottom
flask. The ether is removed by attaching the flask to a water aspirator and swirling to prevent bumping. A boiling chip may be added. The solvent should not be evaporated a t an elevated temperature as the naphthalene is very volatile. The impure yellow crystals obtained in this manuer are dissolved in a minimum amount of benzene and the solution placed on the chromatography column. Elution of the naphthalene is accomplished with low boiling petroleum ether. The fractions collected are evaporated at water aspirator pressure in the same way the ethyl ether was removed. The off-white solid is recrystalized by heatirlg a minimum amount (-20 of to boiling and slowly adding the solid. Results
The yield of naphthalene-a-d is usually about 50'%. Since the last traces of water should have been removed by the Grignard generated from the ethyl iodide, the per cent of deuterium iucorporated is quite high (>go% by mass spectroscopy). This experiment provides a built-in monitor of the student's laboratory ability since poor technique will be reflected not only in a low percentage yield but also a low deuterium content. Comparison of the infrared spectra of naphthalene and naphthalene-a-d in the 3500-1700 cm-' range shows the C-D baud located at 2260 em-' and the C-H band at 3080 cm-1. A rough calculation of the A-B stretching frequency may be obtained4from
where lc is the force constant and p is the reduced mass, (M*Ms)/(M* M,). The ratio of C-H/C-D stretching frequencies should be 1/2 = 1.41 using the ratio of the reduced masses as 2. Experimentally we find 1.36. This low value is due mainly to two factom: (1) the reduced mass is a little less than two, (2) in the calculation of the stretching frequency, a harmonic oscillator is assumed while the potential function is actually anharmonic.
+
'STREITIVIESER, JR., A,, LAWLER, R. G.,
AND
PERRIN, C.,
J . Am. Chem. Soc., 87,5383 (1965). Commercially available Baker and Adamson anhydrous ether was found satisfactarv. a Directions for preparing an alumina column may be found in organic laboratory SILVERSTEIN, 11. M., AND BASSLER, G. C., "Spectr~metric Identification of Organic Compounds," John Wiley & Sons, New YO&,
1964.
Volume 44, Number 10, October 1967
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609
