Toward the Consistent Use of Regiochemical and Stereochemical Terms in Introductory Organic Chemistry David L. A d a m Babson College, Babson Park, M A 02157 A recent text in introductory organic chemistry defmes the term regiospecific as a reaction exhibiting "a preference for one of the two possible products" (I).Another defines "a reaction in which two isomeric products could be obtained, but one predominates" as a regioselective reaction (2). Although both internally consistent, these two texts use different terms to describe the same kind of reaction. Examination of additional organic texts reveals that these two terms are defined and used in several different ways. For instance, texts by Fessenden and Fessenden (21, Loudon (3)and Solomons (4)use the term reeioselectiue to designate a reaction in which one orientatik of addition oredominates. McMum (5) and Raber and Raber (1) . use ;he term regiospecific & describe this same reaction characteristic. Lowry and Richardson (6)use both terms with different meanings. Streitweiser and Heathcock (7) and Wade (8)use neither of these terms. Finally, Orchin et al. (9) define both terms synonymously and IUF'AC (10) defines regioselective but not regiospecific. This same type of inconsistency exists among organic texts for the terms stereoselective and stereospecific. Texts by Loudon and Streitweiser and Heathcock define stereoselective as a reaction in which one of several possible stereoisomers predominates. McMurry, on the other hand, uses stereospecific to describe this same reaction characteristic. Wade and Raber and Raber use the term stereos~ecific.but do not mention stereoselective. to describe a' reaction in which a specific reactant steredisomer vields a specific oroduct stereoisomer. Solomons. Fessenden and Fessenden, and Lowry and Richardson use both terms with diffcrcnt defiitions! Why are there such inconsistencies among texts in the use of these basic organic chemistry terms? Why do these inconsistencies continue? This inconsistent use, as documented above, stems from a series of factors, including personal preferences of instructors and authors, the fact that reference works do not unequivocally define these terms (9,10), and the lack of a dialog to resolve this issue. At some ooint instructors and authors of oreanic chemistrv should b;? able to agree on and use one set i f terms. ~ e g i i nine students in oreanic chemistrv are sufficientlv intimidatid by its large n;mber of termsand definitionswithout us exacerbating the situation by using vague and inconsistent definitions. The perpetuation of the multiple definitions ascribed to each of these terms in todav's texts is undesirable. A lack of cll'ectivc communication among instructors and authors contnhutes to this problem. The intent here is to propose consistency and ilarity in the use of definitibns for regioselective, stereoselective, and stereospecific in introductory organic chemistry. The goal is to stimulate dialog among instructors and authors in order that we may agree on and use one set of consistent regiochemical and stereochemical terms. ~
Proposals Aregioselective reaction is one in which one direction of bond making or bond breaking occurs preferentially over
all other possible directions (11).Examples include the addition of hydrogen chloride to 2-methylpropene to yield 2chloro-2-methylpropane, the addition of HBr to styrene to yield 1-bromoethylbenzene,and the E2 elimination of 2bromobutane to yield a preponderance of 1-or 2-butene depending on reaction conditions. Regiospecific is not recommended for Further use for reasons detailed below. A sterwselective reaction is one in which one sterwisomer (or enantiomeric pair of stereoisomers) is formed or destroyed preferentially over all others that may be formed or destroyed (12).Examples are the addition of one mole of bromine to propyne to yield trans-l,2dibromopropene and the reactions of cis- a n d tmns-2phenylcyclohexyl tosylate to give 1-phenylcyclohexene under elimination conditions. A stereospecific reaction is one in which different stereoisomeric s&rting materials yield different stereoisomeric products (9). Examples are the addition of bromine to cisand trans-2-butene to give an enantiomeric pair of stereoisomers of 2,3-dibromobutane and meso-2,3-dibromobutane.. res~ectivelv.and the Sw2 .. nucleoohilic disolacement reaction occurring at a chiral carbon atom where inversion of relative configuration occurs.
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Rationale and Explanation A major driving force in the acceptance of defmitions is their internal consisteucv. The oro~oseddefinitions of regioselective and stereoselective are consistent with each other. As a suffix "selective" denotes that one isomer forms preferentially to another, with the prefix indicating the type of isomer. As a suffix "specific" denotes a definite relationship between the isomers of the reactant and product with the prefix indicating the type of isomer. A related term, regiospecific, sometimes defined as a reaction which yields one isomer exclusively over other possible isomers, should be eliminated from use for two reasons. First, it defines a condition which has no basis in realitv. Virtuallv no organic chemical reactions yield a singleproduct to the exclusion of all others. Such a term implies analytical criteria that are unwarranted. Second, the use of this term is not consistent with the meanine of the suflix "s~ecific"as defined. The proposed use of t h e t e r m regioselective differs slightly from that recommended by IUPAC (10). IUF'AC uses the term "completely" regioselective for exclusivity and the term "partially regioselective" for predominance. This distinction, for the reasons mentioned above, is unnecessarv and not recommended. Orchin et al. (9) defmes regioselective and regiospecific synonymously. Redundancy in terms and definitions invariably leads to unnecessary confusion and, therefore, should be removed. In this case, regiospecific should be eliminated for the reasons explained. The proposed use of the terms stereoselective and stereos~ecificis that recommended bv IUPAC (10). and as contained in Orchin et al. (9).
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All stereospecific reactions are, by definition, stereoselective since a s~ecificstereoisomer is s refer entially formed. The converse, however, is never true. In a stereospecific reaction a relationship exists between the stereochemical configurations of the startimg material and product, but no such relationship exists in stereoselective reactions. Reactions in which starting materials or products lack stereoisomers can never be stereospecific.Thus, the reaction of propyne with equimolar quantities of brois stereoselective, mine to yield trans-12-dibromopropene . . but not stereospecific. The term stereospecificis useful pedagogically, because it describes an important characteristic of many organic reactions; namely, the stereochemical relationship between reactant and product. This characteristic is sufficiently noteworthy to identify explicitly, because it represents the essence of stereochemical transformations.
scription of organic reactions in introductory organic chemistry. As defined, these terms are consistent, descriptive, and pedagogically useful. The Fessenden and Fessenden (2)and Solomons (4) texts provide a model for the use of these terms in the proposed manner. Literature Cited 1. &her D.J.; Raber,N. K Organic Chemistry; West:St. Paul,MN,1988. 2. Fessenden,R.:Fesse"den, J. OrgonicChem&tv.4Uled:B~~~ka~oIe: Pacifiehve, C h 1990. 3, la"don,G. M. OrganieCkm&f'y,2nd ed: B~Riamin-Cvmmi"g8:R~dddCit~ cq .an0 A " " " .
Solornons,T.W. C. Organic Chemisby, 4th ed; Why,NewYork, 1988. 5. MeMuny, J. O w n i c Chernistry,Znd ed:Bmoks-Cole: P ~ h vEe , CA, 1990. 6. L o w , T. H.: Richardson, K S. Mechanism and Theow in Organic Chemistry, 3rd ed: Harper and Row:New York, 1987. 7. Streitweiser,A,: Hesthme*,C. H. Org.nicCkm&tv,%rded;MaemiUsn:NewYorL, 4.
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8. Wade. L. C Organic Chemistry. 2nd ad; Benjamiami4ummk.p: Redwad City, CA, 1991.
Summary
The proposed definitions of regioselective, stereoselective, and stereospecific are recommended for use in the de-
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11. Ha8sner.A J. Olg. Chern. 1888.83.2684. 12. Zirnrneman, E.; Singe5 L.; Thyagagadan, B. S. J.Am. Chem. Sac. 1859,81,108.
