THE GRIGNARD REAGENT REACHES THE FRESHMAN
0
W. BERNARD KING Iowa State College, Ames, Iowa
JOHN A. BEEL Colorado State College of Education, Greeley, Colorado 't
H o w do you know that the atoms in organic molecules are arranged as you claim them to be? Why do you insist that alcohol must be written H H H-LA-OH
AA
rather than H H-Lo-
\
I-
stand the nature of the problem. For example, the following information concerning Grignard's discovery in 1900 will set the stage for the performance of the experiment. Victor Grignard, a famous French chemist, discovered in 1900 that metallic magnesium suspended in diethyl ether reacts readily with methyl iodide and similar compounds (the alkyl halides), according to the equation,
H
-
H
A-H
Mg
A
+ H-&-I
ether
I H
or simply CIHBO?"These are questions, and logical ones, that the inquiring student asks of his instructor when int,roduced to organic chemistry in his general chemist.ry course. To help satisfy such curiosity, the experiment described in this article has been devised, and though far from ideal, it may provide adeWate answers to students' questions concerning the stmctul.e of molecules. As a preface to the suggested experiment, a brief historical background will help the student better under-
H H-h-Mg-I
4
methylmagne~iurn
iodide
~h~ ~ , j reagents, ~ ~ of ~ which ~ d methylmagnesium iodide is typical, are very reactive and enter into a wide , ~ e t y of reactions. ~h~~ yield hydrocarbons when brought in contact with water, alcohols, organic acids, and substances in general containing a hydroxyl group. Methylmagnesium iodide gives methane. H
H-&-M~-I
I
+ H20
H H H-A-Mg-I
I
H
+ RoH
-
H
H-A-H
I
+ Ho-Mg-1
H H
H-&-H
I
+ Ro-Mg-I
H
H
I I
H-C-Mg-I
+ RCOOH
H
H H-A-H
I
+ moo-Mg-1
H
500 m ~ .
~i-pe
I.
nppmtus ior ~
.
t
After the historical introduction, the student should be encouraged to review and thoroughly understand the following principles. The problem given will help him in his review. The percentage compasition of a compound is determined by an appropriate analysis or synthesis. The weight can be determined by various means caneratins a d conacting c ethane cram h ~ hdide ~ ~ end .A I C ~ O ~ O L~ ~ i ~ ~such as the vapor density method (weight of 22.4 liters) 146
MARCH, 1955
and the molal lowering of the freezing point (for watersoluble solutes, grams of solute in 1000 grams of water that causes a depression of the freezing point of 1.86"). From the percentage composition and the molecular weight, the number of each kind of atom in the molecule can be calculated. The valences of carbon, oxygen, and hydrogen are, respectively, 4, 2 , and 1. In a structural formula for an organic molecule containing these elements, one must show four valence bonds for each carbon atom, two valence bonds for each oxygen atom, and one valence bond for each hydrogen atom. Problem: A certain organic compound upon analysis was found t o contain 37.50 per cent carbon, 50.00 per cent oxygen, and 12.50 per cent hydrogen. One liter of the vapor corrected to standard conditions weighs 1.42 grams. Question: What is the molecular weight? What is the molecular formula? Draw the structural formula. Is there any other feasible structure for this molecule if the above mentioued valence requirements for C, 0, and H a r e fulfilled? In preparation for the actual experiment t o determine the volume of methane obtainable from methvlrnagnrsium iodide when it rencts with ethyl alwhol,
