Comparison of chemical reactivity

Bi. Comparison of Chemical Reactivities on the Basis of Relative. Rates of Reaction under Identical Conditions. A comparison is made of the rates of r...
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COMPARISON OF CHEMICAL REACTIVITY HOMER A D ~ N SUNIVER~IIY , OF WISCONSIN, MADISON, WISCONSIN

The relative chemical reactivity of organic compounds may i n general for the present) only be defined i n terms of reference to the experimental method f acquiring the data used i n making the comparison. The various experieental methods for the comparison of the chemical reactivity of two or more ompounds may be classified upon the basis of whether the comparison is made: If the extent of a reversible reaction, of the rates under identical conditions of a eerersible or irreversible reaction which occurs free of side reactions, of the everity of conditions necessary to induce a given type of reaction to occur, Ifthe relatiwe rates of competitive reactions as measured by the ratio of products.

. . . . . .

It is increasingly necessary in teaching and in experimental investiga.ions in organic chemistry to compare the chemical reactivity of the nembers of a group or series of compounds. The phrase "chemical rerctivity" covers such a variety of phenomena that it is lacking in precise lefinition. In fact, it may be defined properly (at least for the present) mly in terms of reference to the particular experimental method of acpiring the data used in making the comparison. A good deal of confusion )f mind and waste of space in the literature has occurred because some ndividuals have attempted to carry over comparisons made by one ex~erimentalmethod into the interpretation of phenomena which were de)endent upon quite a different set of factors.* It therefore seems worthwhile to attempt to differentiate the variousmethods by which conclusions lave been arrived a t with regard to the relativqchemical activity of the nembers of a group of compounds. Comparisons of the relative chemical reactivity of various compounds 911 into two groups, i . e., (A) those that depend upon the concentration a t qnilibrium in a reversible reaction and (B) those that depend upon the 7elative rates of reaction. These two phases have been variously char~cterizedas "affinity and rates," "point of equilibrium and degree o f nobility," "extent and speed." The terminology is relatively unimportant m t the realization of fundamental difference between the two groups of phenomena is of primary significance. The rate phase of the problem of the relationship of constitution to 2hemical reactivity logically is divisible further into three sections because >f the fundamental differences in the types of experimentation which must be resorted to in making the comparisons. In the first type of experimentation (B1) rates are measured under identical conditions for the members of a group of compounds. In the second type of experimentation (B2) the severity of the conditions necessary for bringing about a given transfor-

* Conant, Lapworth, Noiris, Kon, Linstead, and others have been very careful to differentiatebetween comparisons of chemical reactivity made by various methods. 1865

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JOURNAL OF CHEMICAL EDUCATION

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mation are determined for the compounds to be compared. In the third type of experimentation (Bs) the relatine rates of simultaneous or competitive reactions are determined (for each of the compounds to be compared) under a standard set of conditions; actually the amounts of the products obtained from each compound are usually determined and compared with each other or with the amount of the original compound which underwent reaction.

A. Comparison of Chemical Reactivity on the Basis of a Reversible Reaction This method of comparison usually depends upon an experimental determination of the effectiveconcentrations of the reactants at equilibrium (reactions 1-9) or of the potential (reaction 10) at-equilibrium in a system that can be studied electrochemically. Among the systems which have been studied are the following:

- ++ - + -+ -- -

+ +

1. R C a H R'OH RCOzRf H1O (1) 2. RCHO 2R'OH c RCH(OR')$ H20 (2) 3. R L O HC(OEt)s RaC(OEt)* HCOeEt (3) 4. 2RCHaCHO RCHICHOHCH(R)CHO (4) 5. RsC-CRa 2RsC- (5) 6. RC(0)CHsC(O)R' RC(OH)=CHC(O)R' (6) 7. RCHO HCN RCH(OH)(CN) (7) 8. RCH=CHCHR'R" RCHsCH=CR'Rr (8) 9. RCHCH(Br)R RaC(Br)CHpR(9) 10. o=c-c=c-c=c--0 -o+c--c=c-hC4-

+

I

I

I

I

I

I

l

l

l

l

(10)

l

l

Comparisons as to relative chemical reactivity may also be based upon reversible reactions in which there is competition for a reagent as in reaction 11.

Conant has recently reported some very interesting comparisons of the relative acidity of a series of very weak acids (mostly hydrocarbons) based upon competition in a reversible reaction 12. 12. RH

+ R'Na

-

RNa

+ R'H (12)

However, in this case the experimental determinations were not based upon concentrations at equilibrium as in reactions 1-9, nor upon a potential as in reaction 10, nor upon relative amounts of products as in reaction 11, but upon whether in a given case a reaction had taken place. This fact became significant in placing the acids in their proper order because of the choice and number of competitions submitted to test.

VoL. 9, NO. 11 COMPARISON OF CHEMICAL REACTIVITY

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B,. Comparison of Chemical Reactivities on the Basis of Relative Rates of Reaction under Identical Conditions

A comparison is made of the rates of reaction occurring under exactly the same experimental conditions with respect to temperature, concentration, catalyst, solvent, etc., in which only a single set of reaction products is formed. Many such comparisons have been made upon both reversible and irreversible reactions; among the latter may be cited the following:

+

- -+ --

13. RCI KI RI KC1 (13) 14. RX CBHIONH C6Hl0NRHX('14) 15. (C.H.)CHCI ROH (CaHs)L!HOR

16. 17. 18.

+ + RI + CeH&!HaONa ROH + R'COC1RNCS + R'OH

+

HCL (15) CeHsCH20R NaI (16) R1C02R HCI (17) RNC(SH)(OR') (18)

+

+

Be Comparison of the Severity of the Conditions Necessary to Induce a Given Type of Reaction to Occw One of the very common ways of carrying out a comparison of the relative chemical reactivity of two or more compounds is on the basis of the temperature, pressure, concentration, or "activity" of the reagent, catalyst, etc., which are required to bring about a given type of transformation. For example, the concentration of acid required for the absorption of the alkenes varies with the constitution of the hydrocarbon (19). Various substituted malonic acids and triphenylmethyl ethers beginrto decompose to a measurable extent at kmfieratures which depend upon their structure (20).* Alkenes are hydrogenated over nickel a t much lower temperatures than are benzenoid hydrocarbons (22). Certain hydrocarbons are cleaved by sodium-potassium alloy which are stable toward sodium amalgam (21). Oxygen is absorbed rapidly a t 25' by certain hydrocarbons while others are unaffected (11). Di-o-chlorobenzoylethylene, for example, is reduced by a reagent having a lower reduction potential than is effectiveagainst the 9-chloro compound (23). Dialkyl acetoacetic esters may be cleaved by an amount of catalyst (sodium ethoxide) which is but a small fraction of that necessary for the cleavage of monoalkyl acetoacetic esters ( 2 4 ) . Phenols may be halogenated to a greater extent and with a less concentrated reagent than the parent hydrocarbons. Tertiary alcohols may be dehydrated a t a lower temperature than secondary or primary alcohols (25). Aryl chlorides resist hydrolysis under conditions of temperature and reagent which would induce a rapid reaction in the case of alkyl chlorides.

* There was found to be a parallelism between the effect of R upon the temperature of decomposition of the ethers and the effect upon the reactivity of the alcohol in reaction 17.

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Comparison of Chemical Reactivities on the Basis of Relative Rates of Competitive Reactions All of the above methods for the comparison of relative chemical reactivities rest on the assumption that only a single set of reaction products is formed. A more complicated situation is all too often encountered in which a comparison of chemical reactivity must be based on the relative amounts of different sets of products. Under this head come the most numerous comparisons of chemical reactivity. Several illustrations of these competitive reactions which have been more or less carefully studied for a series of compounds are given below: B3.

21. RHgR'

+ HCI

?RH

+ R'HgCL

(27)

( R M ~ )X( R > M ~ . M ~ X ~ ) + ~ gLR-R / * (31) + M~X,

28.

RX

29.

(CH2).(CO&2a

/l(CHz).=CO (39)

~c.,H,. etc.

I

RzCCHzCOpNa HR'COH7R'2RCC(0)R 31.

RR'AOHLR,R~CC(O)R, (.w ./*RH 32. R N C l R'OH (34)

+

LROR,

TOL. 9,

NO. 11 COMPARISON OF CHEMICAL REACTIVITY

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nterpretation of Experimental Results on Comparisons of Chemical Reactivity There are certain inherent difficulties and sources of probable error in tlmost all methods of comparing the chemical reactivity of compounds hat should be kept in mind. There is often the difficulty with reversible eactions if any extensive list of compounds is studied that for many ~f them the reactions will go so far in one direction or the other as to nake the determination of concentrations a t equilibria impossible. In all tudies of the variation in the concentration at equilibrium with variation n the structure of the reactants there is a strong probability that the con,entrations as measured will not be the true concentrations or activities. Chis is true because of the prevalence of molecular association among xganic compounds, especially those containing oxygen and nitrogen, which ue of the most interest. Furthermore, there are few, if any, single equiibria among organic reactions. For example, the reaction of an aldehyde vith an alcohol to form an acetal (reaction 33) certainly involves a t least hree equilihria besides those concerned with the catalyst (38): 3. 14. 16.

+

-+ -

RCHO R'OH RCH(OH)(OR') KCHO HZO

+

RCH(OH)(OR') R'OH RCH(OR'), RCH(OH)?

+ H,O

12 comparison of rates of reactions of a group of compounds encounters a iomewhat different set of difficulties. This basis of comparison is neces;arily limited to those systems in which a singlg reaction occurs without :ompetition. Impurities so small in amount as to have no appreciable iffect upon concentrations may yet profoundly affect the rate of reaction. rhese impurities may have been present in the original reactants and it nay be practically impossible to avoid them, or they may be produced luring the course of the reaction. One of the normal reaction products nay he far more effective in inhibiting or accelerating the reaction in one :ase than in another. If a catalyst is necessary either in a homogeneous or ~eterogeneoussystem, often the unanswerable question arises as to whether the ratio of catalyst shall be kept constant with respect to the weight of reactant A, to the moles of reactant A or with respect to the weight or moles of reactant B. There is no doubt that the more experience an investigator has had in rate studies the less significance does he attach to any zxcept the larger differencesin rates. Another difficulty arises in that it is usually impractical to find a single jet of conditions of temperature and concentration a t which a large group of compounds react. The comparison then must be made on the basis of severity of the conditions necessary for the reaction. The comparisons made upon this basis are admittedly of considerable practical importance

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and include most of the comparisons that are commonly used in teaching, yet it is diicult to express them in precise quantitative terms. Experience has shown that there is no necessary correlation between the rate at which a reaction proceeds and the extent to which it goes, i. e., there is no necessary connection between the rate of a reaction and its driving force or decrease in free energy (35), (44). For example, furfnral reacts with ethanol several times as rapidly as does acetaldehyde, yet at equilibrium the percentage conversion is much higher for acetaldehyde than for furfural (2). This, of course, is not saying that there is no connection between the rate of a reaction and thermodynamics. It is merely stating that the driving force or decrease in free energy is only one of the factors determining reactions, and that in many reactions it is not a very important one in determining rates of reaction. Space factors, the "a" term in van der Waal's equation, and orienting or activating characteristics, etc., may play the determining I&. The statement made above does mean that it is certainly unsafe to attempt to draw definite conclusions as to electronic structure on the basis of results obtained from studies of rates of reaction. The relative rates of competitive reactions (B3) are so much a function of experimental conditions that there appears to be even less correlation between driving force and rates than for the simpler reactions in the two groups (B,, Bz) previously discussed. Very slight changes in the catalyst, temperature, reagent, or structure of compounds involved oftentimes suffice profoundly to modify the ratio of the products. For example, Ruzicka (32) has shown that the rihgs containing six or more carbon atoms are approximately equally stable, yet there is an enormous variation in the ratio of products obtained in attempting to prepare cyclic ketones from the salts of the dibasic acids as in reaction 29. Similarly, the experimental work of Reynolds on the ratio of nitroparaffins and alkyl nitrites, produced as in reaction 27, showed that the electrical character of the components of RX was not a determiningfactor (30). Despite many such facts more conclusions have been drawn with regard to "electronic strncture" "negativity," and "electronegativity" on the basis of relative rates of competitive reactions than upon the basis of any other experimental procedure. Perhaps this is so because of the superficial resemblance of reactions 19-32, inclusive, with those listed as 11 and 12 under "A," However, in the former case there is no true equilibrium but merely a balance as between the amounts of the products. In such a competitive reaction as studied by Kharasch (27), for example, there is no equilibrium between the products; therefore, the ratio of the amounts of the products is not a measure of the electronegativity of R and R' but is merely an indication of the relative rates of two competitive reactions. It is, of course, possible that the relative electronegativity of the radicals attached to the mercury atom is reaUy the decisive factor in

VOL.9. NO.11 COMPARISON OF CHEMICAL REACTIVITY

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determining the ratio of the products. It would be as unwise for the present author to deny the possible validity of the method as it is for Kharasch to maintain it until he has demonstrated that the relative electrouegativity of the radicals is the decisive factor involved. There is no intention on the part of the author of minimizing the importance of studying the relative rates of competitive reactions by insisting that such studies cannot be depended upon to show the relative strength of bonds, relative electronegativity, etc. Quite the contrary view is held, for i t appears to h i that in no inconsiderable degree the progress of organic chemistry lies through such studies. In fact, one of the most important groups of problems in organic chemistry is concerned with the control of the ratio of competitive reactions. It is a matter of hope and of satisfaction to know that in a great many cases the ratio of reaction products is not determined by the laws of thermodynamics, but that i t may be modified more or less in accord with the desire of the chemist. The chemist in this field of experimentation is not like the engineer of a locomotive who must drive where the rails lead, rather is he like a man a t the wheel of a car who has many roads open to him to be followed a t his pleasure. Summary The various experimental methods for the comparison of the chemical reactivity of two or more compounds may be classified upon the basis of whether the comparison is made: (A), qf the extent of a reversible reaction; (BJ, of the rates under identical conditions of a reversible or irreversible reaction which occurs free of side reattions; (Bz),of the severity of conditions necessary to induce a given type of reaction to occur; (Ba), of the relative rates of competitive reactious as measured by the ratio of products.* It has been pointed out that in almost all of these methods of comparison, in so far as they have been developed a t the present time, there are certain difficulties and sources of error which should prevent an investigator or reader from unreserved acceptance of the results. It has been especially emphasized that the method of comparison based upon rates of reaction (B,, Bz, Ba) cannot safely be used for the purpose of drawing conclusions as to the relative electronegativity (in any fundamental sense) of radicals. In many cases, especially with competitive reactions (Bs),

* The phrase, reversible reaction, as used in this paper refers to a readion which proceeds in both directions, in a homogeneous system, to a measurable extent, under a single set of conditions. Perhaps a majority of organic reactions are "reversible" in the sense that under one set of conditions the reaction goes to the "right" and under another set to the "left." However, a "reverse"reaction which is not occurring to a measurable extent, at a given temperat- and other experimental conditions, cannot affect the rate or relative rates of reactions that are being measured under the specified conditions.

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the electrochemical character of the components of the reactants appears to play a rather unimportant r8le in determining the rates or the ratio of the rates of reactions. References to the Literature The references given below were selected largely on the basis of the familiarity of the author with the contents of these particular papers. Many citations might be replaced with others which would be equally significant. The citations have in general been t o the more recent papers containing references to earlier work. (I) (2) (3) (4) (5) (6) (7) (8) (9) (20)

(11) (12) (13)

(14) (15) (16) (17) (18) (19) (23) (21)

(22) (23) (24) (25) (26) (27) (28) (29) (30)

(31) (32) (33) (34)

( 7 (Jri)

MENSCAUTKIN. Ann. ckim. pkys., 30.81 (1883). For references, see J. A m . Chem. Soc., 53, 1853 (1931). ADKINS,CARSWELL, AND PPEIFPER, ibid., 50,235 (1928); 53, 1043 (1931). USHERWOOD. J. C k ~ mSOC., . 123, 1917 (1923). GOMBERG, Ckem. Review$; 1, 104 (1924). KNORR,Ber., 44, 2767 (1911). LAPWORTEAND MANSKE, J. C h a . SOC.,1928,2533; 1930, 1976. KON,LINSTEAD, et al., Annual Reports Chem. Soc., 26, 117 (1929). MICHAEL AND LEUPOLD, Ann., 379, 263 (1911). CONANT AND FIESER,J. A m . Ckem SOL,45,2194 (1923); 46, 1859 (1924). CONANT AND EVANS, ibid., 51, 1925 (1929) CONANT AND WHELAND, ibid., 54,1212 (1932). CONANT, KIRNER,AND HUSSEY.ibid.. 46,232 (1924); 47,476,488 (1925). SEMBAND MCELVAIN, ibid., 53, (90 (1931). N o ~ r u sAND MORTON, ibid., 50, 1795 (1928). HAYWOOD. J. Chem. Soc.. 121,1904 (1922). Nonrus AND CORTESE, J. A m . ChmrSoc., 49,2640 (1927). BRO-E AND DYSON,J. Chem. Soc.. 1931,8285. PLANT AND SIDCWICK.J. SOC. Chem. Ind.. 40, 17 (1921). N o n n ~ AND s YOUNG, J. A m . Chem. Soc. 52,753, 5066 (1930). CONANT AND BLATT.ibid., 50,551 (1928). ZARTMAN AND ADKINS. ibid.. 54, 1668 (1932). CONANT AND LUTZ.ibid., 49,1088 (1927). KUTZAND ADKINS, i b i d . 52,4291 (1931). ADKINSAND PERKINS,ibid., 47, 1163 (1925). LAZIERAND ADKINS.ibid.. 48, 1671 (1926). KnARAscH AND FLENNER, ibid., 54,674 (1932). ADKINSAND COVERT, 3. P ~ Y CS h m . , 35,1684 (1931). BRADLEY AND ROBINSON. J. Ckem. Soc., 129, 2356 (1926); KUTZAND ADK~NS, J . A m . Chem. Soc., 52,4036 (1930). REYNOLDS AND ADKINS,J. A m . Ckem. Sac, 51,279 (1929). JOHNSONAND ADKINS,ibid., 54, 1943 (1932). R u z r a u , Helu. Ckim. Acta, 9, 512 (1926). INCOLD,J. Ckem. Soc., 121, 2676 (1922); IVES,LINSTEAD, AND RnEY, 54, 1093 (1932). HODG~ON AND KEnsnAw, 1930,2784. GETMAN AND DANIELS, "Outlines of Theoretical Chemistry," 5th ed., John Wiley and Sons, Inc., New York City, 1931, p. 332. Annual Rcporfs C k m . Soc., 27, 114 (1930); RACKMANN AND MOSER,3. A m . C h a . Soc.. 54, 1124 (1932).

im.,

im.,

Vo!.. 9, No. 11

COMPARISON OF CHEMICAL REACTIVITY

1Xi:i

NOKRISAND YOUNG,J. Am. Chem. Soc., 52, 753 (1930). ibid., 50, 178 (1928). ADXINSAND BRODERICK, GELLET,Trans. Irish Acad., 25, 371 (1875); NEKNST,"Theoretical Chemistry," Macmillan and Co., New York City. 1923, p. 540. AYRES,Ind. Eng. C h m . . 31,899 (1929). AND PORTER, J. Am. Chem. Soc., 54, 1199 (1932). This reaction and YABROPP reference are given merely as illustrative of the many competitive reactions that have been studied in connection with the problem of orientation in the benzene series. AND DINSMORE, ibid., 54,1025 (1932). NOLLKR For references see KHARASCHAND REINMUTH, J. CHEM.EDUC.,8, 1725 (1931). The "Chandler Lecture" for 1932 was an illuminating discussion entitled "Equilibria and Rates of Some Organic Reactions," by PROPESSOR CONANT, I d Eng. Chem., 24, 466 (1932).

GROUP RESEARCH The viewpoint of science is changing with the rapid extension of our knowledge. There was once a time whe? an Edison could know all there was to know about the electric light and the phonograph, or when a Bell could know all there wm to know about the telephone. Each little phase of these fields has now become afield of its own in which intense specialization is needed for a competent knowledge of the subject. Technical workers frequently isolate themselves in order to focus their attention more effectively on the one narrow field of their scientific endeavor. Industry has recomized these changes and has neated large research and develoo-

of the field. B Y bringing together it& a single picture the knowledgewhich each of these possesses, industry can carry forward the type of scientific development required to meet present-day competition. Let us take an example in the field of materials of construction. To select the proper material for a piece of work, it was once only necess a r y t o know the tensile or compressive strengfh. With our growing knowledge of the suhject, physical properties of materials now represent a field in itself; but the present problem of selecting materials ha? become more complicated than that. It requires also the expert advice of a metallurgist, of a man who knows behavior of materials under corrosion, and possiblv the advice of a paint exmrt on protective coatinzs. Up t o this point, we have recognized the importance of intense specialization as an aid t o industrial research, hut these research organizations have another problem. namely, th: comhnation of knowl~dpv. There ir a nrerl for a scienti.1 wh.) l ~ a. w d e knuwled~euf his ticlrl, r h o cdn iktmnine what tools and mcncurcmenrs will he nrcci-ary in the solution of his problem. He must possess the imagination to consider possibilities outside the range of the immediate experimental program and he must have the wisdom to select those new trends which appear mast promising. Such a man does not develop in isolation, but finds his training field in the association with a wide variety of scientists and business men. Real leaders in research and development are rare and there is a tendency in industry t o recognize that even the hest of these are not alone competent to render decisions of policy which affect the major phases of an industry. I n such cases T the decisions are delegated to groups of such men acting as a committee.-LIN~~~N WonK via The Cheniml Digest