letters Organic Chemistry:
An Experimental Approach
To the Editor: There are several statements in the review of "Organic Chemistry: An Experimental Approach" (see THIS JOURNAL, 47, A596 (1970)) that may lead a reader to erroneous conclusions. The implication that the placement of the KBr technique in the Appendix is erroneous or leads to a skimpy treatment is misleading. Since ir is not needed for spectra of solids until later chapters, its inclusion in an earlier chapter is not necessary or perhaps even desirable. Since there are adequate references to the Appendix in most chapters that describe synthesis of solids and references in the index, and since the section on KBr pellets includes 11 pages and 13 illustrations, the material is not likely to be overlooked. The statement: "In thin layer chromatography, no mention is made of visualization procedures" is incorrect. Perhaps what led to the reviewer's statement
was that the introduction to t,lc has no discussion of visualization procedures because this introduction involves the separation of plant pigments where obviously no visualization procedure is needed. In later tlc experiments more than adequate laboratory instruction and discussion of tlc visualization procedures are given where they are needed. The reference to no solubility classificat,iondiscussion before "Chapter 32 though there are several chapters devoted t o . . .qualitative analysis before this" is incorrect. The beginnings of solubility classification tests are in Chapters 14,23, and 28. Perhaps the objection was that solubility classification was not given more emphasis. I t is admittedly arranged in such a way that t,he solubility classification can be easily omitted or included at the instructor's discretion. The comment t,hat there are "some essent,ial additions" required apparently because the sect,ions on spectra interpretation are brief is misleading. Like all laboratory manuals, "Organie Chemistry: An
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Experimental Approach" is intended for use with a senarate text for lecture. The theoret,ical,internret,ive. a i d experimental material in this manual com&emeni and supplement rather than duplicate the theoretical and intirpretive sections of most modern lecture texts. Since these texts contain adequate sections on spectra interpretation, most instructors will find no additional material is essential for the first-year organic course. The comment on the omission of discussion of nmr techniques is correct. When this material was prepared there were very few instruments being used in introductory organic laboratories. Now that there are more, material is being prepared on this subject and will be included in the revision of the hook ant~cipated for 1971.
Redox Titralon Calculations
To the Editor: I n a recent textbook errors column, Heyn [J. CHEM. EDUC.47,240 (1970)l has shown that in the calculation of the equivalence point potential for redox titrations the usual textbook equation
(where Eol and Eoz are the standard half-reaction potentials, and a and b are the number of electrons transferred per mole in each half reaction) does not apply to reactions which involve hydrogen ions. However, even with his correction for hydrogen ion activity, the above equation is limited to titrations in which, for each half-reaction, one mole of reactant produces one mole of product. This ismot true for several common analytical reagents. To illustrate this point, consider the iron-dichromate titration' 6Fee+
-
+ Crr0,2- + 14HC
6Fes+
+ 2Cra+ + 7H.O
(2)
At the equivalence point, for each CaOlz- ion which has not reacted, there remain six unreacted Fez+ions. For each Crz07Z-ion which has react.ed, there are produced two Cra+ ions and six Fe3+ ions. The equilibrium concentrations of these ions may then be represented as: [Crz07Z-]= x; [Cr3+]= C; [Fez+]= 6x; [Fea+]= 3c. If it is assumed that at the equivalence point [H+] = 1 (Heyn has discussed the correction used when this is not true), then the half-cell reduction potentials are given by E
=
E'o,
+ 0.0591 log c*
E
=
E"s.
3c + 0.0591 log 62
(4)
Summing 6X eqn. (3) and eqn. (4) in order to have the same denominator
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Here, unlike the examples usually-~resented, the log . term does not go to zero: E
=
6E0c.
+ ED=._ 0-591log
7
2C
7
At the concentrations normally present in titrations the additional term will be small and, as Heyn ha! pointed out, the assumptions made in using the Nerns equation (e.g., that concentrations equal activities lead to calculated values which may in any case, deviatt widely from those measured. Nevertheless, when r derivation is presented in generalized form, it is un sound pedagogical practice to include unstated as sumptions, limiting its general applicability. A studen1 who tries to apply the usual textbook procedure to thf dichromate and similar systems is likely to be confusec when the concentration terms do not cancel out as thej are "supposed" to do. DENISQUANE EASTTEXAS STATEUNIVERSITY COMMERCE 75428
' A similar treatment of the endpoint potential in a ferrous. dichromate titration is presented in J. R. Wnzm "Quantitstivt Chemistry," W. A. Benjamin, New Yark, 1964 p. 32&321, ant for the ferrous-bromine titration in H. A. LAITINEN "Chemi~a Analysis," McGrilw-Hill, New York, 1960,O. 329.
Carbonium Ion-Carbanion ~omanciature
To the Editor: I wish to add my voice to the many others who have pled that the chemical world adopt consistent nomenclature systems and stick to them. One of the most abused examples is the carbonium ion-carbanion nomenclature. In the present state of affairs,if one does not include a sketch of the structure of the ion in question with any usage of this system the reader is not likely to know what is meant with any certainty. In addition to the hoary misuseage of "tertbutyl carhonium ion" when what is really meant is trimethyl carbonium ion, or perhaps more explicitly, tertbutyl cation, a misuseage which appears not only in the most widely used organic text but also in the November issue of THIS JOURNAL on page 732, there are many other examples. Recently the writer has noted "cyclopropyl carbonium ion" for cyclopropyl cation, and "isopropyl carbonium ion" for isopropyl cation. There appears to be a rampant misunderstanding about this nomenclature system such that many writers use carbonium ion as synonymous for cation and carbanion as synonymous to anion, with the understanding that the anion or cation is at carbon. In view of the widespread misuseage of the carbonium ion-carbanion nomenclature, I suggest that it be dropped and that all anions and cations henceforth he named as such. Tertbutyl cation, methyl cation and ethyl anion seem to be instantly intelligible whereas the carbonium ion and carbanion equivalents would be cloaked in ambiguity.
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