Somuel Soloveichik Yeshiva University New York, New York and Henry Krakauer
Yale University New Haven, Connecticut
Oxidation Stages of Organic Aliphatic Compounds A classification scheme
It is the purpose of this paper to present a classification of organic aliphatic compounds based on the degree of oxidation of the function-bearing carbon atom. The classification is in four broad categories called oxidation stages, and, by further subdivision, into groups. To transfer a carbon atom from one oxidation stage into another requires oxidation or reduc-. tion, while interconversions within a stage are usually accomplished by reactions that are not oxidations or reductions, unless it be of the function itself, e.g., RNOz -+ RNHz. This classification is expected to be of greatest use to the student. It serves to organize both the compounds and, as suggested, their reactions. This point will be illustrated below. There is unfortunately no simple single criterion available for the ordering of compounds into stages and groups. The following are the criteria used, when and as they supply the needed information on the extent of electron deficiencyof the carbon in question: 1. Pauling's electronegativity scale [(I) p. 931. 2. The oxidation number of the parent substance of each homologous series under consideration. 3. Ratio of bond moment to bond length pA (net charge) (#. v..
8). -,.
4. Hammet's sigma function as revised hy T a f t or rather ' (5). Taft's o 5. Chemical reactions of the substance involved in general, and! particularly, hydrolysis, both a s displacement and as additmn, which on the whole, if certain conventions me maintained, does not involve basic changes in oxidation stage
Thus, if a carbon of doubtful oxidation stage appears, after hydrolysis, in a form whose stage is unambiguous, it will be assumed that the original form was also of that stage. Rules for Establishing Stages of Oxidation
Rule I . The oxidation stage of carbon in hydrocarbons of the methane series is zero. Rule II. The oxidation stage of a carbon atom is increased by one for every hond with an H atom or an alkyl group replaced by a bond with a more strongly electron attracting atom or group such as: -NHZ, -SH, -halogen, -COOH, -CCla, -CN, -NOz, etc., or for every removal of a pair of hydrogen atoms from two adjacent carbon atoms (in this case the effect is distributed between the two carbons). When a divalent or trivalent electron attracting (more electron attracting than hydrogen) atom or group of atoms Presented before the Division of Chemical Education a t the 138th Meeting of the American Chemical Society, New York, September, 1960.
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replaces two or three hydrogen atoms or alkyl groups, the oxidation stage of the carbon to which the atom or group is attached increases by two or three. Alkyl chlorides, bromides, and iodides, are in Stage I because these halogens, more electronegative than H or the alkyl groups, replace a H or an alkyl group. Thiols, amines, alkenes, etc., are placed in Stage I because interconversions among them and with the halides take place by addition, solvolysis or displacement, and because their Taft r* value and hond moment to bond length ratio (p/l) indicate that they belong to Stage I in the appropriate groups. When there is both unsaturation and replacement of a H by a more electronegative atom or group of atoms, the carbon to which the more electronegative atom or group of atoms is attached will belong to Stage 11. Rule III. The group of a carbon is defined by its substituent: carbons bearing the same substituent belong to the same group. The groups within a stage are arranged in order of increasing induced electron deficiency of the function bearing carbon. The two criteria that have been found most satisfactory are, again, the Taft a* value and bond moment to bond length ratio (p/l), when both are available (5). The bond moment and Taft r* value of C-H indicate that H tends to withdraw electron density from C so that the degree of oxidation is in the order CH& > R C H X > FLCHX > RsCX (X = any function). This is the order in the groups and is indicated in the tables by (d), (c), (h), (a). Classification of Compound
Note that in this classification, the monovalent group with the most electron attracting ability is the -NO, group. This is in accordance with Taft's sigma value (4). Hence, nitromethane contains the most oxidized carbon of Stage I. I n Table I, nitromethane is the (d) member of Group 13. Note also that in Groups 5, 9, and 11 it is the carbon attached to the carbonyl, carhoxyl, and nitrile group which is of the first oxidation stage. The high oxidation state of the carbon within the function would, in fact, be expected to influence the neighboring atom. Justification for this contention is furnished by Taft's c* values and also by the p/l ratio (net charge). Taft (3) has shown that his r* values are additive to a good approximation, though a saturation effect is likely. This last is more apparent in the case of p/1 ratio. This additivity, nevertheless, can serve as a guide for classifying compounds in Stages 11,111,and IV. The following tables list only some illustrative compounds. The total number of groups, because of the possibility of
Table 1.
Table 3.
Stage I
Group 1 Group 2 Group 3 (a) RCNHs R2C==CRn' RaCSR1 (b) RzCHNH, (c) RCHINHl (d) CHaNH2 Group 6 Group 7 Group 5 RGBr (a) CH,COCRn RGI Group 11 Group 10 Group 9 R&CN (a) RGCOOH RaCF Gram 13 (a) R ~ C N O ~ (R' = H, alkyl, acyl, or inorganic radical)
Table 2. Group 1 (a) RzC=NH (b) RCH=NN (c) C H s N H Group 5 ( a ) RzCO Group Q (a) RnCFa
Group 8 RBCCl Group 12 RJCSOZR
Group 1 NH=C(NH& Group 5 CCIGOOH Group 9 CC4
Group 3 Group 4 RnC=CHCl RaCS Group 7 Group 8 RnCClCOOH M(CO0H)a Group 11 Group 12 RnC(SOaR)s RBC(NOd2
combinations of substituents, is too large. Furthermore, too exhaustive a classification, were i t even possible, would not be profitable. I n this system, alkenes have been placed in the first oxidation stage and alkynes in the second. The justification for this inclusion is chemical and physical. Indeed, in reactions such as hydrolysis, which we assume do not involve oxidation or reduction, the alkenes and alkynes respectively yield alcohols and aldehydes as do the other members of their respective stages. Furthermore, unsaturated acids have much higher ionization constants than the saturated ones. In other words, the effect of unsaturation is similar to that of substitution of hydrogens by more electro-negative atoms (inductive effect). Thus, for example, the diisociation constants of isocrotonic acid (double bond) and of 2-butyne-oic acid (CH&kCCOOH) are respectively 4 X 10-5 and 2.5 X 10Wa. The dissociation constant of 2-butyne-oic acid is almost twice as great a s that of a-chlorobutyric acid and 28 times as great as that of P-chlorobutyric acid. When the alkene or alkyne is asymmetric about the double or triple bond, the localization of oxidation (in a formal manner only) may be justified by the application of the reasoning used in interpreting the manner of addition to those bonds (Markovnikov's rule). There is no polarization in the ground state of symmetric alkenes or alkynes. However, during reactions such as hydrolysis, one of the carbons assumes the f d electron deficiency, which was distributed equally between the two carbon atoms before the reaction. Compounds of various groups of Stage I1 exhibit common features and similarities. Thus, alkynes, aldehydes, and ketones are somewhat acidic. The positive charge distribution on the carbon makes it som* what acidic and easily attacked by bases. The reason for the inclusion of CO in Stage I11 is chemical. Indeed, NaOH CO HCOONa is a reaction within a stage. Cathodic reduction of COCL, belonging to Stage IV, yields CO (5). CO++ 2e-
+
-
Group 5 RCIS Group 9 RCFt
G r o u ~2
Group 6 RCBn Group 10 RCN
Table 4.
Stage II
Group 2 RC5CH Group 6 RC(Ch)R Group 10 R2C(CN)s
Group 1
Group 4 R,CORf
+
-
Group 2 CO(NH& Group 6 OC(OH)* Group 10 C(NOdr
Stage Ill Group 3
Group 7 RCC1,
Group 4
Group 8 RCOCl
Stage IV Group 3 COOH-COOK Group 7 C0a
Group 4 CN-CN Group 8 COCh
CO (in the electrolysis of a solution of A1CL in phosgene). Also, the oxidation number of C in CO is the same as that of C in HCOOH. The reason for its inclusion in Group 3 is that it is the pseudoanhydride of HCOOH, just as ketene is the pseudoanhydride of CH&OOH. The carbons of diierent groups in Stage I11 exhibit common features. Thus, the carbon in HCN, which according to the rules is the most oxidized carbon of Group 10, behaves like an electronegative atom and the hydrogen attached to it forms hydrogen bonds with the nitrogen of another HCN molecule. Similarly, when molecules of chloroform and acetone are brought together, the hydrogen linked to the carbon in chloroform (which is the most oxidized carbon of Group 7) forms a hydrogen bond with the oxygen of acetone. Moreover, compounds containing carbon-carbon double bonds, usually, act as Lewis bases and form pi complexes with Lewis acids such as boron tduoride, ALBr8, silver ion, etc. Yet, when the hydrogens in ethylene are substituted by strongly electronegative groups, the pi base is converted into a pi acid. Thus vinylidene cyanide CHFC(CN)% which contains a carbon of oxidation Stage I11 is already a pi acid. Tetra-cyanoethylene which contains carbons of oxidation Stage I11 is an even stronger pi acid. It should be emphasized that compounds containing carbons in Stage I11 are mostly acids. Thus in Group 4 we have the organic acids with HCOOH containing the most oxidized carbon of the group. The hydrogen in chloroform is acidic, and HCN which is a weak acid in comparison to most inorganic acids is the strongest acid known in which the acidic hydrogen is attached to carbon. Even the amidines of Group 1were called carbazylic acids by Franklin (6) because the carbon is oxidized and therefore deficient in electrons, and acts as a Lewis acid. Hydrolysis of the amidines yields the corresponding amide and then the carboxylic acid, just as the hydrolysis of nitriles does. In oxalic acid and cyanogen one carbon belongs to oxidation Stage I11 and one to oxidation Stage IV. A problem of classification arises when two polysubstituted carbons are attached to each other. Which shall be considered the function and which the bearer of the function? In symmetric compounds such as oxalic acid or cyanogen, no a priori distinction between the two Volume 43, Number 10, October 1966
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carbons can be made. Polarization takes place upon 1,eaction. Thus, hydrolysis of (CN)2yields HCN (111) and HOCN (IV). However, there may be some justification for making a distinction in the case of CC1, COOH. The more electronegative carhon which, in this case, is the carbon bonded to three chlorine atoms, will be considered of Stage 111, while the Stage IV is assigned to the carhon of the carboxyl gronp. Indeed, CCl&OOH CHCla(II1) and CO1(IV). A general rule is that when two carbons, each of oxidation Stage I1 or higher are attached to each other, only the one of higher electronegativity will affectthe other, and not the reverse. A simple extension of these principles will he sufficient to cover compounds such as the very important Grignard reagents. Since magnesium is more electropositive than hydrogen and alkyl groups, the carbon attached to Mg will be considered to be in oxidation stage minus one; displacement of h4g by H will thenimply an oxidation.
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Interpretation of Reactions
The proposed classification immediately exhibits a large number of relationships between compounds. It may he expected that compounds containing carbons of like oxidation stage, being similar, wonld undergo similar reactions (subject of course to the qualifications arising from specific and local effects), and that interconversion within one family is easier than derivation of a compound from another belonging to a different oxidation stage. The derivation would necessarily involve an oxidation or reduction. Compounds belonging t o the same stage undergo similar reactions: Hydrolysis of a compound of the first, second, third, and fourth oxidation stage yields respectively an alcohol, an aldehyde or ketone, an acid, and carbonic acid. Anzmonolyszs of a compound of the first, second, third, and fourth oxidation stage yields respectively an amine, an imino compound, an amide or nitrile, and urea or guanidine. Alcoholyszs of a compound of the first, second, third, and fourth oxidation stage yields respectively an ether, an acetal, an ester, and an ester of carbonic acid or orthocarbonic acid. Acidolysis of a compound of the first, second, and third oxidation stage yields respectively an ester, a diester, and an anhydride. The student with a reasonably good understanding of the concept of oxidationstage and therulesof classification can anticipate and therefore understand the majority of preparations and reactions presented to him since they generally involve processes wherein no change in oxidation stage occurs. Reactions such as the hydrolysis of an alkyl halide, esterification, ester hydrolysis, passage from keto or aldo form to the en01 form, the production of alcohol from an alkene, or ether from alcohol, are either reactionswithin a stage, or reactions in which the sum of oxidation stages is conserved. Similarly, the large number of additions to aldehydes and ketones, e.g., of HCN, NaHS03, ROH, etc., no longer appears so arbitrary. Neither is RCHO PXs = RCHX2 POXa, in principle, unexpected. These reactions are all interconveisions within a stage. The following somewhat more complex reactions, overloolc-
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Journal of Chemical Education
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ing problems of mechanism, also become more straightforward:
+
CIsCCHO Hs0 ClaCCOOH RCOCC18 NaOH CO NaOH COCIS ROH
+ + +
----
+
Cl&HCOOH HC1 CHClr COI RCOONa CHC1, HCOONa ClCOOR HCI
+
+ +
The hydrolysis of chloral wonld seem, a t first glance, to he an oxidation-reduction reaction inasmuch as i t involves the conversion of an aldehydic carbon into a carhoxyl carbon. Actually, it is not an oxidation-reduction reaction, hut a reaction within the stages. Indeed, the two carbons in chloral belong to Stage 111. The carbon attached to the three chlorine atoms belongs to Stage I11 because it has 3 atoms of hydrogen replaced by 3 more electronegative atoms. The aldehydic carbon is of Stage 111 because it has an oxygen in place of two hydrogen atoms and is the bearer of the extremely electronegative function -CCl,. The two carbons in the product, dichloroacetic acid, also belong to the same Stage 111. I n this compound the more electronegative function is the carboxyl group; hence, the carhon to which the two chlorine atoms are attached is of Stage I11 because of the two chlorine atoms and because it is the hearer of the COOH group. A student with this classification will be able to distinguish between reactions in the same stage and between reactions which involve internal oxidationreduction. For instance, in the reaction CHaCOONa 1
3
+ NaOH
-
CH, 0
+ NanCOi 4
or in the thermal decomposition of acetaldehyde
the student can see that while there is oxidation-reduction, the sum of the stages is conserved on both sides of the equations. The classification of carbons according to stages and groups within a stage correlates many chemical phenomena. Thus, the addition of HBr to vinyl bromide takes place as follows: CHFCHB~
+ HBr + CHnCIIBr.
I n our classification, the carbon to which the bromine is attached is of Stage 11. A carbon of Stage I1has a high positive charge and will repel the proton but will atr tract the bromide ion (just as the carhonyl carbon of aldehydes and ketones belonging to the same stage attracts bases such as the CN- ion). Similar reasoning about the much higher positive charge on the carhon of the nitriles belonging to Stage I11 explains why the hydrolysis reaction proceeds: RCN HzO RCONH2 and not in any other way (RCN H20 + ROH HCN). The classification scheme also may suggest preparatory pathways. It should be possible, in principle, to convert any compound in a given oxidation stage to any other in the same stage by displacement, addition, elimination, and redox attack on the substituent. Also, because compounds of the same oxidation stage are expected to undergo similar reactions, the use of analogy may prove fruitful. To illustrate, acetylene reacts with acids to form a diester (ethylidene diacetate if the acid is acetic acid).
+
+
+
-
I t would be logically correct to expect substances containing carbons of similar oxidation stages to yield like products on acidolysis. [CH&HCL, 2CH3COOH+ CHsCH(0-COCHa)2 2HC11. This classification focuses the attention of the student on the topic, "The Chemistry of Oxidized Carbons" as t,he chemistry of stages, of groups within the stages, and of members within a group. We wish to acknowledge a valuable suggestion by Dr. Gerald Rrakower, Squibb Research.
+
+
Cornell University Press, Ithaea, N. Y., 1940, pp. 160-169; 3rd. ed. pp. 221-231. (2) Dm RE, G., "Electronic Aspects of Biochemistry," (B. Pullman, ed.), Academic Press, Inc., New York, N. Y., 1964, pp. 221-237. (3) TAFT,R. W. JR., "Steric Effects in Organic Chemistry," Chap. 13, (M. S. Newman, ed.), John Wiley & Sons, Ine., New York, N. Y., 1956, pp. 556-675. Tables, pp. 617. 619. [See also TAFT.R. W. JR.. J . Am. C h m . Sac. 74.2729 (16521: ihid.. 75.'4231. 11958)l
Literature Cited (1) PAULING, L., "Nature of the Chemical Bond," 2nd ed.,
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