VOLUME 33, NO. 3, MARCH. 1956
GEORGE K. ESTOK Texas Technological College, Lubbock, Texas
INTHE early development of a student's background in chemistry it seems desirable to encourage an integrated understanding of the types of chemical particles and the formal ways in which they may react. Chemical reactions may vary in complexity from simple one-step processes to those involving a number of steps. Two very simple schemes for indicating the important formal possibilities for one-step reactions involving one or two particles are presented here. The several steps involved in moat "over-all" reactions may be indicated by a simple notation based on the one-step reaction schemes. The following involves a consideration of only the simplest essentials: (1) the fundamental particles in chemical reactions, (2) the concept of the transition state, (3) the different formal ways particles may react, and (4) the representation of "over-all" reactions as a sequence of one-step reactions. Revision of a paper presented before the Division of Chemicd Education a t the 8th Southwest Regional Meeting of the American Chemical Society, Little Rock, Arkansea, December, 1952.
' The Fundamental Particles. Every chemical reaction involves particles of matter. The term particle is quite useful since it can include all the possibilities; i. e., free radicals (or free atoms), ions, and molecules. Nuclear reactions are not considered in this paper. Free radicals (or free atoms) are neutral but generally very reactive particles with a free unpaired electron available for bond formation. Examples: Na ., H ., H3C ., Cl., etc. The dot represents the free unpaired electron. Ions (charged particles) are particles of varying reactivity having one (or more) full electronic formal charge(s), either positive or negative. For certain electrolytic reactions the anode and cathode may be considered as positive and negative particles, respectively. Examples of charged particles (ions): Ag+, C1-, H&+, H-, (anode)+, (cathode)-, etc. Molecules are neutral particles with valence require ments already satisfied but nevertheless of varying reactivity depending on their nature. Examples: CzH4, CzHs, Br,, NHa, BFa.
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The Transition State. If two particles collide properly, i. e., with sufficientenergy and proper orientation of their reactive centers, they may be considered to form a momentary coalescence unit, or "transition state." This transition state may then further change to yield the product(s) or it may return to the reactants. I n the following reaction schemes the letter T is used to represent this state. Generalized One-Step Reaction Schemes. The following two schemes constitute patterns for the important ways in which one or two particles may react.
SCHEME A
'/?
Scheme A: Molecular Transition State (I-IV) Comhination of free radicals. HIC . . CHI CHJCH, (1-111) Disproportionation. CHsCHn . CHsCHl. + CHZCHI CHFCH~ (11-1) Kolbe electrolysis. (anode)' RC02. (anode)+ RCO1(II-IV) Combination of ions. (CH&Ct CI(CH,),C-C1 (11-111) Nucleophilic displacement. OHR-OH RaS R-SRlt (III-IV) Coordination. BFs H8N-BF, H8N: (111-111) A o n ~ s t e pesterificsti~n.~ ROH RCOZR RCOZH RCOICOR (IV-IV) An isomerization. cis-isomer trans-isomer
+
+
+
+
Scheme B: Free Radical, or Ionic, Transition State Free radical: (11)' Steps in chlorination of methane. CL . CH* HCI . CHa HJC. CIS CHsCl CI. (1-2)' Free redieel attack on an olefin. R . R2C=CHs RLLCHBR Calion:
Acid catalysis of an olefin. H + (CH&C=CH2 (CH8)Gi Carbonium ion rearrangement. (CH&CHCH2+ (CH3)>Ct
(2-2)+
4
Anion: ( 1 1
(
Scheme A involves a molecular transition state; scheme B covers those cases where the transition state is a free radical or ion. An unfilled circle represents a molecular particle; a circle with charge sign stands for an ionic particle; a circle with a dot is used to show a free radical (or free atom). I n using scheme B the appropriate charge sign, or free radical designation, is chosen and maintained for any given application. Various one-step reactions are readily represented by appropriate pairs of Roman or Arabic numerals. Thus (I-IV) indicates the reaction of two free radicals to yield a molecule; (II-IV), the reaction of cation and anion to form a molecule; (1-2) +, the reaction of cation and molecule t o yield another cation. When both numerals in a pair are identical, the reaction does not change the type of particles involved. Thus (IV-IV) indicates a molecular isomerization; (1-1) ., the reaction of a free radical and molecule to yield some other free radical and molecule. Some examples are now offered t o illustrate the applicability of the schemes to certain specific cases.
+ + + + + - + + + + + -
+
+
(1-2)+
SCHEME B
-
+
1
-
Nucleophilic displacement. ClC&Br CHaCl BrCoordination. AICI, AICI,C1-
+ +
+
"Over-All" Reaction Sequences. Most over-all chemical reactions involve several steps before the final product is attained. As an example, the following overall, acid-catalyzed esterification reaction is postulated R-C02H
+ HO-R
H
+
R-COnR
+ HzO
t o occur in four one-step reactions as follows:
The two generalized schemes, A and B, serve to outline in a definite and easily comprehensible way the various important possibilities which may be postulated in formulating the various steps involved in overall reactions. Rarer cases involving polyvalent ions or multiparticle collisions could be handled by an adaptation of the principles already indicated in the schemes. 'It should not be inferred that all the examples given have been proved authentic; some are well substantiated, others represent reasonable postulations.
