applicationf and cmalogie~ Entropy and Its Relation to Work
Middle Georgia College Cochran. Georgia 31014
Reactions between Chiral Molecules: A Handy Analogy
W. S. Richardson Department of Physical Sciences Auburn Universily at Montgomery Montgomery. AL 36193
W. S. Richardson
The rclntionship of entropy to the disorder of a system ran he ex~lninedusine a deck of . davine . - cards. The analor\' can be expanded to i&lude the spontaneity of an event &d its relationship to work.1.2 Begin with a deck of cards wherein each card of a suit is arranged together in descending order from the ace to the two. Drop the ordered cards casually on a tahle so that they fall in a random fashion and then gather them up without paying any attention to their order: an examination of the cards demonstrates the change from order to disorder during this spontaneous event. No work is reauired to disorder the cardssimply drop them. ~ e a r r a n g i i gthem in their original order, however, is not so simple. Indeed, bow many times must one drop the cards to obtain that result? Could such an ordering occur a t all using this method? What is more likely to occur as the cards are continuously dropped? How likely is it that an increase in order will he observed if an unusually large number of cards is used, e.g., 200? If dropping the cards is not likely to order them, then what does ode have to do to reorder them? T h e answer is obvious. They have to be arranged in a given order by working on the deck one card a t a time. Work must he done to accomplish this nonspontaneous event. Thus, a decrease in order is a spontaneous event requiring no work, while an increase in order is a nonsoontaneous event reauirine . work. In fact, work is frequently done during spontaneous events. This can be demonstrated hv releasing (relatively ordered) gas molecules from a tank of &nPressed helium a d allowine.the escapine eas to turn a fan blade. ~ u r i &the discus& of the rrlntivnship hetween entropy. enrhnlpy, and free mrrgy, one should norr that the dewrmination of the spontaneity of a rhemiral reaction must include a consideration of enthalpy as widl as entropy. A spontuneoua reaction may occur and work may be done& the entropy decreases if T A S is small compared to the decrease in enthalpy (AH),according t o the equation AG = AH - T A S . Thus, hydrogen and oxygen form water a t room temperature with n large, negative change in AH and a mnwmitnnt negative change iri AS. Calcium oxide and carhon dioxide form ialcitim carbonate a t room temperature with similar changes in AH and AS. However, a t eievated temperatures the reaction is reversed as the T A S term becomes larger.
In discussing the enantiomeric relationships between chiral (handed)1,2.3molecules, the analogous relationship between the left and right hand, as suggested by the name, encourages students to take an active role in the demonstration. Left and right hands are mirror images (except for the presence of rings, hangnails, or freckles) and, like real enantiomeric molecules, left and right hands are not ideally superimposahle as are two left hands or two right hands. The demonstration makes it clear that, like any two real molecules, no two hands are su~erimoosablein practice, but that a test of superimposahility is, in reality an eiercise of the imagination. This analogy .. is particularly useful and simple when it is extended a oair of ~ ~ -to ~- exnlain ~ -~ - that ~ ~ enantiomers ~ ~ ~ have ~ the same chemical properties except for their rate of reaction with other chiral molecules. This ooint seems more vame to the students than similw cnmpiuisks of physkal pn,p&lies and the effert of lane Dolariled lirhr. One can ask the class u, imnntne that his hands representiptically a ~ t i v e 4(Rand ,~ S) mol&ules of a oarticular acid ( a o ~ r o ~ r i ato t ethe course), and that the hands of another &dent are chiral molecules (Rand S) of a oarticular base. The chemical reaction that takes place between the acid and base is represented by a joining of hands, the faster reaction being the usual handshake of the right hands (Rand R) or the left hands (S and S) of another student and himself. Thus his right hand, the R acid, and the fellow student's right hand. the R base. ioin for the usual handshake. and so do the two ieft hands. y h e slower reactiou is repre: sented by the more unfamiliar and awkward joining of left and right hands (R and S or S and R) as two individuals face each other. This analogy is particularly useful when discussing the complementary fit between molecules such as an amino acid or sugar and the active site of an enzyme found in biological systems. Of course, i t is important to point out that the reaction between R and R or S and S molecules is not necessarily the faster reactiou, and that there is not a general rule t o predict which enantiomer will react faster or slower with an&her chiral molecule. Also note that the clasoed-hand oroducts of the reaction between the R acid, reGesented dy the right hand of one student, with the R a n d S bases, the right
' Spenser,J. N.. Gordon, David, and Schreiber. H. D.. Chemistry,47,
12 11974).
i~etcali,H. Clark, Williams. John E., and Castka. Joseph. R., "Modern Chemistry." Holl, Rinehart. and Winston. New York. 1978,
Department of Physcal Sclences A ~ o ~ ~n r nversry at Montgomery Montgomery. AL 36193
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Tn s feature presents a cdlecuan of descrlptlve appi8catoons and analogies aes gnea to help students unoerstana some of the odlouml concepts freq~enllyencalrntered in Cnemlstry ConIrtbUlwms That w 11 prcdwe a greater appreciation and knowledge of political. religious. economic. historical. and scientificaspects of lifeare encaw@.
Volume 59
Number 8 August 1982
649
