edited by: JOHNJ. ALEXANDER University of Cincinnati Cincinnati, Ohio 45221
Invert Sugar (and a Honey of a Problem] Ray Wendland Carroll College. Waukesha, WI 53186
During many years of teaching organic chemistry, I have assigned this problem to second-semester organic students. Even though i t looks easy, its solution eludes at least 50%of those who otherwise have good records in a standard one-year course. The problem tests cognitive skills at the applications level. Question (a) Given a 1.00 M aqueous solution of sucrose, calculate the initial optical rotation of the solution as observed through a 2.00 decimeter polarimeter tube. (b) Assume that afew drops of sulfuric acid or sucrose enzyme are added to the sugar solution. The sucrose then undergoes gradual hydrolysis t o its component sugars, dextrose and lev"lose. At a certain moment, the observed optical rotation will have dropped ta 0.0'. Calculate the concentrations of dextrose, levulose, and residual sucrose when the rotation is 0.0'. Express these concentrations in moles I-' and grams lF1. Essential data fur the problem are
= 0.00
deg.
The equation is simplified by factoring out "1" and "1000"; whereupon we find that X = 0.762 males for dextrose; likewise for levulose. Sucrose = (1 - X)= 0.238 males. Obviously, concentration of sucrose a t zero rotation in grams per liter = 0.238 moles 1-' X 342 g malecl = 81.4 g I-'. Approach 11. The second approach is to calculate the total change in rotation from 100% sucrose a t the start, and zero sucrose after eomolete conversion to 1.00 mole each of dextrose and levulose, intermixed in 1.00 1 of solution. The change to zero rotation will be a fraction of the total. Initial rotation of sucrose in 1.0 M solutivn in 2.00 dcm tube is +45.5O. Rotation far dextrose, 1.00 M
Rotation for levulose, 1.00 M
Sum, R D
+ RL = -14.14O
Total rotational change, 100%conversion.
[a]$for sucrose = +66.5" for dextrose = +52.1° far levulose = -92.0° R = rotation = [a]C - 1 where C is in g ml-'
.
Change, initial to zero rotation = 45.5' Fractional conversion af sucrose = 45.5159.65 = 0.762.
Acceptable Solution (a) Initlal rotation of sucrose in 1.00M solution (assume a 2.00-dcm polarimeter tube)
(h) Two approaches are apparent, based on the following chemical equation:
sucrose
+ R(dextrose) +R(levulose)
R(sucrose)
+ H20 +
dextrose
I t is obvious that the two methods give identical end results for the amount of sucrose converted. The problem as stated specified a 1.00 M solution of sucrose a t the beginning, and this was done merely for convenience. It is of interest to find whether it makes any difference in the relative amount of sucrose converted, if the original solution were 1.5 M, 0.8M, etc.
+ levulose
"invert sugar"
A Novel Exam Format for Advanced Courses Accordingly, the three concentrations in grams per milliliter must be (1 - X)moles 1-I X 342 g mole-' sucrose 1000 ml l-I dextrose
X moles 1-'
levulose
X 180 g molec11000 ml
l-I
X mules 1-' X 180 g moleC1 1000 ml1-I
The algebraic sum of the three rotations must he zero:
+92.0
deg xX(180)gml-1~1dcm=0.0deg g ml-I dcm 1000
John C. Cochran Colgate Un~vers~ty, Ham~lton,NY 13346
Advanced undergraduate, or senior level, courses in chemistry usually take on the character of special topics courses, and the contents depend on the special interest of the instructors. Depending on who teaches the course, the range of topics under a single course title (i.e., Advanced Organic Chemistry) can vary within a single department. Certainly there are broad variations from school to school. The breadth of content carries over to the methods and their level of sophistication attained after four years of training in chemistry. Accordingly, I use the following format for the final exam in my course in Advanced Organic Chemistry. The topics covered in the course are typical of a physical organic course with some emphasis on organometallic chemistry. Volume 59
Number 3
March 1982
217
In essence, the exercise asks the student to write a miniresearch wrowosal. The format allows the student to use the
Acceptable Solution An appropriate answer will be given for the paper used most recently (3). This communication concerns sulfur participa-
"
integration of pertinent components into a coherent package. References to papers used in previous years appear below ( I , 2 , 3 ) . An alternative to a paper from the literature would he a problem from one's own research. The exam is a take-home, and the students are given one week to prepare the answer.
Question Attached is a recent article reporting neighboring group participation by sulfur. Suppose you wished to become the "World's Authority" on this reaction. Describe how you would proceed. What experiments would you carry out and what information would you gather? You may use any source available at the university with the exception of another person.
218
Journal of Chemical Education
of the problem with special attention to solvolytic processes, anchimeric assistance and, in this case, the Thorpe-Ingold Effect. Next, the student should propose a plan of attack which would lead to a comprehensive investigation of the problem. This is really the heart of the exam. In this case, electronic and steric effects seem to be most pertinent, and the students are expected to anticipate a variety of results and then interpret the significance of each. Finally, the student is expected to outline reasonable synthetic routes to all compounds to he used and propose methods for carrying out rate studies. Since this paper involves extensive use of the literature, a complete bibliography is expected. Literature Cited (11 Eliel,E.L., et al., Tet. M I . 21,331 (19ROl. (21 Clsesrun. A.and Sahiberg, C., Tei. Lett ,5049 110781. Chem.. 40, C69 (19751 (31 Shsrma, R. K. and Felleri, N.H , I I Or~onom~toi.
