JOURNAL OF CHEMICAL EDUCATION
THE WURTZ REACTION1 HARRY W. DAVIS, W. R. GILKERSON, and H. H. HERNANDEZ University of South Carolina, Columbia, South Carolina
INMANY
textbooks of elementary organic chemistry, one of the first reactions presented to the student is the Wurtz reaction. The application of this reaction to t,he preparation of a hydrocarbon containing an odd number of carbon atoms requires the use of two different alkyl halides. However, when two different alkyl halides, R X and R'X, are used together, three hydrocarbons are produced, R-R and R'-R' from reactions of like molecules together, and RR' from reaction of unlike molecules together. The question is frequently raised by the student as to the proportions in which the three hydrocarbons are produced. Most of the texthooks do not attempt to answer this qnestion. But one2,in referring to the preparation of n-heptane by using n-butyl iodide and n-propyl iodide, says: "The reaction affords a mixture of which the unsymmetrical product, n-heptane, can be expected to constitute no more than one-third." However, application of the laws of probability leads to a differentconclusion. Let us suppose that we use n molecules of RX and n molecules of R'X in a Wurtz reaction and let us assume furthermore that the two halides are of equal reactivities. Then the number of combinations forming each hvdrocarbon is eiven in column two of Table 1.
-
Presented before the South Carolina Section of the American Chemicd Society in joint session with the South Carolina Aeadomy of Science in Columbia, South Carolins, on April 23, 1949. 2 FIESEE,L. F., AND MARY FIESER, "Organic Chemistry," I).C. H ~ a t h& Co., Roston. 1944, pp. 38and 39.
Also given is the ratio of each number of combinations to the total nnmber of combinations and the limit of that ratio as n approaches infinity. The number of molecules in any workable quantity of halide is so large that n may be considered infinite. This leads then to the conclusion that the composition of the hydrocarbon mixture produced by such a reaction should be as follows: R-R, I/&; R-R', R'-R', I/,. In order to test this prediction an experiment was done in which 1 mol of n-butyl bromide was mixed with 1 mol of n-hexyl bromide and reacted with 2.4 gram-atoms of sodium in the conventional manner." The crude yield was carefully fractionated through a 1.5 X 65 cm. column packed with helices, with the following results: 14.3 g. of material considered to be 1-hexene and hexane (disproportionation by-products; no effort was made to retain any butene and butane formed); 21.7 g. (0.19 mol) n-octane; 42.3 g. (0.30 mol) ndecane; 33.3 g. (0.20 mol) n-dodecane. The mixture produced by the Wurtz reaction proper totals 0.69 mol of hydrocarbon which is proportioned as follows: n-octane, R-R, 27.5 per cent; d e c a n e , R-R', 43.5 per cent; n-dodecaue, R'-R', 29.0 per cent. This result is in general aereement with the above prediction and sh'ows defi&ely that more of the product is a ROBERTSON, G. R., "Laboratory Practice of Organic Chemistry," Rev. ed., T ~ Maomillan P Co., New York, 1943, p. 287.
TABLE 1 Hudroearbm
RR' R'-R'
No. of combinations
Ratio to total combinatim
-
Limit of mtio, n-
8
pH AND HYDROLYSIS OF A DOUBLY WEAK SALT KURT EISEMANN Yeshiva University, New York, New York
THE
problem of calculating the pH and hydrolysis of doubly weak salts (i. e., salts of a weak acid and a weak base with different ionization constants) is customarily shunned on account of its comparative complexity. Below an analysis of the order of magnitude of the quantities involved gives a very close approximation and a simplifying rule for solution of specific cases. The Problem. It is required to calculate the pH and percentage hydrolysis of a 0.1 M NH4CN solution.* Notation. C = Concentration of salt originally introduced
-4 = Acid ionization constant = 7 X 10-lo
B
b z
= = = = =
y
=
W
a
=
0.1 M
KHs.HzO and' the cycle of dissociation is repeatedly gone through in diminishing amounts until the relative amounts of ions finally adjust themselves so as to satisfy (11, (21, and (3). Since confusion is likely to arise in visualizing the mutual reactions and arriving a t a clear concept of the quantities takimg part, it is strongly recommended that all such reactions be represented graphically in the following manner, showing the over-all resulting dissociation t,oget,herwith the mutual relat,ions of ion concentrations:
Base ionization constant = 1.8 X 10Water ion product = lo-" F i n d acid anion concentration = [CN-] Final base cation concentration = [NHd+] Final lHfl Final [OH-]
/
Chemical Equilibrittm. The following relations hold between the final ion concentrations at. equilibrium:
These are three equations with four unknowns (x,y, a, b), requiring a fourth relation for solution. In order that the action of hydrolysis may be dealt with in the more familiar concept of ionic dissociation, consider a t first complete bydrolyzation to take place. Since [NH4+] = [CN-1, the amounts of OHand H + used up thereby will be equal. Let also the remaining H + and OH- recombine to form HzO. Xow dissociation takes place, uiz., a moles HCK
--
6 moles NH8.H8O
H'
+C W
NH,+
+ OH-
(4) (5)
in such manner as to satisfy (1) and (2) above. Equation (1) independently would yield [H+] = [CN-] = 40.1 X 7 X 10-'0 = 8.4 X 10-6 moles and (2) independently would yield [OH-] = [NH4+] = moles, so that the 1.8 X 10-5 = 1.3 X combination of (1) and (2) would yield [H+J[OH-I
= 1.1
X 10-8
> lo-''
a
z
w
\
NHlt
y
NHa.H80 C -b
b
The amount of H + and OH- formed by (4) and (5) and recombining to form HnO is w moles. From the diagram it becomes evident that the amount of H + removed from HCN -+ H + CN- in order to form water is equal to the amount of OHremoved from NH3.Hz0 + NH4+ OH- in order to form water, i. e.,
+
+
w = a - z = b - y
m
(6)
This is the fourth relation sought above. It may also he stated as [CN-]
-
[Ht]
=
ISHI+]
-
[OH-]
(7)
Assumption a, b
