T H E VELOCITY O F REXCTIOS A S D E S E I I G Y O F .4CTIVATIOS O F HALOGES COLIPOUSDS BY DAVID HENRY P E A C O C K
I n a very large number of cases certain regularities have been observed in the effect of substituents upon the reaction velocity of halogen compounds with other reactants such as water, amines, sodium ally1 oxides, and potassium iodide. The alternating character of these effect,shas been exknsively studied in connection Kith t,he theories of Flurscheim and of Lapworth and Robinson. It has been shown by the present author' that this alternating character is also p-nitrohenzyl chlorides with observed in the energy of activation of 7 ~ and aniline, p-toluidine, o-toluidine and dimethylaniline. .I t,ertiary base was used because with a primary or secondary base there is always th6 possibility of two reactions ( I ) the formation of a salt cf pentavalent nitrogen ( 2 ) direct removal of hydrogen from combination with the nitrogen. The latter possibility may be considered as somewhat remote in the reactions under consideration. It has now been found that methyl-ethyl-aniline with 712- and p-nitrobenzyl chloride and dimethyl-ni- and p-toluidine with benzyl chloride also show this alternating effect. The measurements were carried out in a manner similar to those already described (loc. cit.) and are given in Table I .
TABLE I Energies of Activation :-Tertiary Methyl alcohol solution at 3 j"C and 4j"C. [Base] = o.4hf, [Halogen compound] = 0.111
Bases
k35
Ethyl methyl aniline and meta-nitrobenzyl chloride. j . 0 1 X IO-^ 1, ,, ,> $ 1 para" 3.01 X IO-' Dimethyl-para-toluidine and benzyl chloride 6.j8 X IO-^ f, 9 ) meta>! 91 1.74 X IO-^ ) J
E
8335 8964 13470 148jo
The second case is interesting as showing the alternating effect of a methyl group upon the energy of activation of t,he nitrogen atom in the reaction concerned. Similar cases are being studied. The work of Holleman' on the rates of hydrolysis of substituted benzyl chlorides provides data from which a series of energies of activation can be calculated, Table 11. It is apparent that with exception of the first pair of reactions that reaction which is faster has the smaller energy of activation. Holleman (loc. cit.) drew attention t o this in another way by stating that the differences between the reaction velocities tended to become smaller as the temperature increased. 'Peacock: J Chem. Soc., 127, 2 1 , 7 Rec. Trav. chim., 41, 646 ( 1 9 2 7 ) .
(192j).
DAVID H E S R P PEACOCK
536
TABLEI1 Energies of Activation: H?-drolysis Reaction iHolleman: loc. cit) I,:, 101 Halogen compound k 130") k 'k:j" 2 . 0 4 cals TI -C' H 3 . C ,H 4 . C H 2 C 1 I .04 ' j i j
>>
ill-
0.144
150
2.01
?KI.CEH,.CH?C'l
0.052
184
2 , IO
171-
O.OIj2
242
2.21
~I-B~C~HACH,C'~
0.04ji 0,0147
170
2 . 0 j
227
2.18
5%
7i1-
P-SOz.C'6H;CH?Cl
0.00491 0.0063
11
vi-
234
2.19
222
2.17
From the results given by Hollenian] it is possible to calculate the energies of activation for the reaction between nitro z :4 dichlorobenzene or nitro 2 :j dichlorobenzene and sodium methoside in one case and diethylamine in the other. The results are collected below:(I)
Sodium Alethoside.
S i t r o z:4 dihlorobenzme k25 = 0 . 3 3 0 ks'k26 = 2 . 0 9 E ( j o ! ' ~ j ) = 2.31 X i o 1 Rsj Rso = 3.09 E (8j;jo) = 2 . 2 4 X io4
Sitro
dichlorobenzene 0.0063 1.92
2 :j
2.24
3 ' 24 2.27
X
x
IO?
IO'
i z ) Diethylamine
ksj"
Rile
H85
E (110/85)
0.027
0.0067
3 Ij 1 . 3 6 X io'
3 ' 43 1.34
x
IO'
The faster reaction has the smaller energy of activation in only one pair of these results namely in the reaction between sodium methoside and nitro 2 :5 dichloro benzene at 5o°C and 8 j T . It must be remarked however that in
nitro 2 :4 dichloro benzene both chlorine atoms are labile (cf Holleman: loc. cit. for velocity coefficient with 0- and p-nitrochloro benzene) and this is bound to complicate the results. If the hypothesis of Robinson and Lapworth with regard t o the alternating effect of substituents upon the reactivity of a labile atom be interpreted in the sense that of two compounds that is more reactive which is activated more easily and if this interpretation be applied t o the cases tabulated above it will be seen that where the hypothesis demands that one substance should be more reactive than another then the energy of activation of the more reactive substance is less than that of the less reactive, e.g.: 1
Rec. Trav. chim., 35,
I (191j
l
E S E R G Y O F ACTIT-ATIOS O F H A L O G E S C O M P O C S D S
53 7
lfethyl bsnzyl chloride ( C H 3 . C s H 4 . C H 2 C l ) does not behave in accordance with the hypothesis and the results given by nitro 2 : j dichloro and 2 :j dichloro benzene are contradictory. Bearing in mind the difficulty of obtaining accurate results for the energy of activation the agreement between the order of these values and those predicted by the Robinson and Lapworth hypothesis seems too close to he fortuitous and supports the assumption that the energy of activation of a substance shows alternating effects which can be predicted with the help of the Lapworth-Robinson hypothesis. It is of course understood that the term energy of activation of a substance means energy of activation with a particular reactant.
-
+-
+
CH2 - C1 I
CH&1 I
A+ 1 j +"'-X02
+/)i
-
+SOi
E (aniline)
= 18030 cals. tS(CH3)z
E (aniline) = 17280 cals.
- K(CH3)z I f f"\,-
-
+' - CH3 \/I
-IE (Benzyl chloride) = 13410 cals. f CHZC1 I
E (Benzyl chloride)
= I 48 j o
+ - CHzC1
1
1
'\)4
-
1
- c1
E (water) =
2.10
X
104
cals.
E (water) =
2.21
X
104
cals.
In paPsing it may be remarked that the relative reactivities of the ni- and p-dimethyl toluidines while in accordance with the Lapworth and Rabinson hypothesis do not agree with the Flurscheim hypothesis as ordinarily applied. Toluene shows reactivity in the o and p positions and therefore according to the Flurscheim hypothesis the nitrogen atom in the m position should possess excess of residual affinity and be more reactive than the nitrogen atom in the p position, the opposite is actually the case. Further work is proceeding on this subject.
538
DAVID H E N R Y PEACOCK
The temperature coefficients of simultaneous reactions will naturally affect the relative amounts of the products formed. If the faster reaction has the greater temperature coefficient then the proportion of product formed in the greater amount will increase as the temperature rises. If, on the other hand, the product formed in the greater amount is produced by a reaction with a lower temperature coefficient i. e. a smaller energy of activation, then the relative amounts of the two products will approach one another as the temperature rises and may even change in ratio. Conclusions as to reactivity based on the relative quantities of two products formed at a particular temperature would thus appear to attach undue importance to relative velocities at an arbitrary temperature. In the cases considered the alterations in velocity were produced by the alternating effect of a particular substituent and the difference in velocities is much more marked than the difference in energies of activation. If instead of examining the effect upon velocity of the change in position of a particular substituent we study the effect of substituting one halogen for another then the difference in velocities is often of a much higher order than the difference of temperature coefficients and therefore of energies of activation. Perhaps the earliest case studied is the reaction velocities of allyl halogen compounds with sodium and potassium ethoside osamined by Conrad and Bruckner' who found that the relative rates of reaction for chlorides, bromides and iodides were approximately as I :70:140 while the temperature coefficients of all three reactions were approximatel! the same. Menschutkin*found that the relative rates of reaction for alkyl chlorides, bromides and iodides with amines were approximately as I :IOO:;OO. The temperature coefficients and energy of activation for one particular amine with the halogen derivative of a particular allyl compound in the same solvent do not seem yet to have measured but from the results of Jones and Preston3 the energy of activation for the reaction between allyl iodide and dimethylaniline is found to be 9362 and for tri isoamyl amine 9702 in absoluk alcohol. The velocity constant for the first reaction at z j°C was 0.656. E. It. Thomas4 for the velocity constant of the reaction allyl bromide and dimethyl aniline found the value 1.08at 4ooC in absolute alcohol. The iodide would thus react probably twice as fast at tk.e same temperature. Measurements have been made of the velocity constants of the reaction between diethyl aniline and benzyl chloride and benzyl bromide in methyl alcohol; t,he constants at 3 j ° C are .ooo1i8and ,00473. The energies of activation for the same reactions5 are 1 4 5 5 0 and I j j 6 0 . These results are tentative. Slator and Twiss6 examined ihe rates of reaction of a number of halogen compounds with sodium thiosulphate. Some of their results are given in Table 111. ~~
' Z . physik. Chem., 4, 531 (1889). Z. physik. Chem., 5 , 589 (1890). 3 J. Chem. SOC.,101, 1930 (1912). J. Chem. Soc., 103, 592 (1913). C,H5N ( C Z H ~ = ) ~0.8 ) bl in benzyl chloride reaction and 0.4 II in benzyl bromide reaction. E J. Chem. SOC., 95, 99 (1909).
E S E R G T O F ACTIVATION O F H A L O G E S COMPOUNDS
539
TABLE I11 Rates of Reaction and Temperature Coefficients for Alkyl Halides and Sodium Thiosulphate (kt Io")/kt = 3.051 Iodides CH3I kljO= 0.280 ,, = 2.fj( CHJCOOCzHs klj" = 2 . j j ,l CH3Br klj" = 0 . 2 9 3'1 Bromides 12 2.fj CH9BrCOOC2Hj klj" = 2.36 19 CH3C1 klj" = 0.0066 11 ' Chlorides CH2Cl.COOC2He k?j" = 0.060 2.9
+
1
The final impression produced by all these results is that while the substitution of one halogen for another may make a large effect on the velocity constant the effect on the energy of activation is very small. If the energy of activation is related to the stability of the halogen linkage in the allyl compounds then it is probably also related to the energy required to displace the electrons constituting that bond and to the energy required to break the bond. S o data seem available bearing directly on this point but it is eignificant that the heats of combustion of halogen compounds show no large differences and are usually of the order RCI > RBr > H I . Indirect evidence as to the stability of the H X electronic bond is perhaps afforded by the following figures as to their ionisation potentials given by Mackayl HC1 = 13.7 HBr = 13.3 H1 = 12.7 volts. The hydrogen compounds are not the most suitable to choose in view of the peculiarities of hydrogen but n o other data seems available. Two other points are of interest in this connection. Menschutkin* found that in benzene solution the reaction velocity of m toluidine with allyl bromide was greater than that of p-toluidine and so also wit,h methyl bromide. In acetone solution however both bromides reacted faster with p-toluidine. In the case of allyl bromide it was found that also in broni naphthalin m-toluidine reacted faster than p whereas in acetophenone and propyl alcohol the pcompound reacted faster. The effect of a methyl group in the m or p position thus depended on the solvent. With 2 :4 dimethyl aniline and z : j dimethyl aniline the compound with the methyl group in the p position reacted faster than the m in both benzol and acetone. Thus the anomalous effect of a methyl group in the M or p positions is shown only by m- and p-toluidine in certain solvents. RheinlanderS found that the substitution of iodine for chlorine in z : 4 dinitro chloro benzene produced anomalous results in that the reactionvelocity of dinitro-iodo-benzene with certain bases was lower than that of dinitrochlorbenzene. The effect then of x change in the position of a substituent upon reaction velocity may depend upon the solvent used and the effect of substituting iodine for chlorine depends on the reaction studied. Phys. Rev., ( 2 ) 24, 324 (1924). Z. physik. Chem., 34, I57 (1900) J. Chem. Soc., 123, jog9 (1923).
540
DAVID HENRY PEACOCK
Such considerations raise the question as to th- suitability of velocities of reactions as a measure of reactivity. One obvious disadvantage which has been referred to by Haywood’ is that the order of velocities may depend on the temperatwe chosen for their measurement,s if their temperature coefficients are unequal. Haywood endeavoured t c grt over this difficulty by measurinp the ratio of the velocities under condition of equal entropy change i. e., where Q/T was constant and found that under these conditions the velocity ratios were constant. This result however follow at once from the fact that Q is calculated from the ordinary equation d log k.’clt=Q,/RT?. Haywood’s results have been criticised on other grounds.*
If by the reactivity is meant the ease with which a compound may he brought to the reactive state or activated condition then it is difficult to resist the conclusion that energies of activation or some quantity closely connected with them are the true measure of this property. The theories which have been brought forward to explanation in varying reactivity suffer no distortion if the above meaning is attached to the term hut may perhaps be considered t o acquire a greater degree of precision. Energies of activations are less susceptible of accurate comparison than veloLities of reaction but the latter quantities seem made up of two factors, only one of which can be easily seen t o depend on structure. Several equations have been suggested for expressing the velocity of a. bimolecular reaction in terms of other molecular constants. DushmanS suggests the equation: k = ?i~*d/8?rRT ( I MA I , MB) d e - (&.A QB) / R T or k = e-Q RT
+
+
If we restrict ourselves to reactions such as those between aniline and rn or p nitrobenzyl chloride then h1.k and h 1 will ~ be the same in the two cases and the values of u will probably also be very close. The same remarks apply t o all reactions in which m and p isomers are compared with a fixed reactant. If then the velocitiee for such a pair of reactants be compared at equal temperatures we have: log, k, = log, A, - Qp,!RT and log, k, = log, Am - Qm/RT
If A,
= A, as assumed above, then log, k, - log, k, R T loge kp/km = Qm - Qp.
=
I / R T (Q, - Q,) or
where Qm = sum of the energies of activation for the fixed reactant A , and the m isomer i. e. Qm = Q ’ A Qlm and so with Q,.
+
In Table 1; are collected values for R T log,k,/k, and of Q,-Q, for several reactions; the hydrolysis of substituted benzyl chlorides was studied by Holleman (loc. cit.) the other results are by the present author. 1 2
8
J. Chem. Soc., 121, 1904 (1922). Cf. Goldsworthy: J. Chem. Soc., 1926, J. Am. Chem. Soc., 43, 397 (1921).
I 102.
E S E R G Y O F ACTIVATIOS O F HALOGES COMPOTSDS
541
TABLE IV l‘elocity Constants and Energies of Activation for m and p Substituents. (a) Hydrolysis Reaction T = 303’ React ants log (kp k m ) RTln i k p k m ) Qm - Qp m and p - CH3.CsH4.CHXl o 8586 I 506 -300 m - and p- Cl.CsH,.CH?CI 0 5342 i45 IIO0 0 4926 688 I100 nz- and p - Br.CsH4.CH2C1 -0 1082 - I49 -2 0 0 m - and p - KO1 CsH4.CHzC1 (b) ni- and p-nitrobenzyl chloride T = 308’ Aniline -0.1088 -154 -0.168; -239 p-Toluidine o-Toluidine -0.1698 - 240 -0.2203 -310 Dimethyl aniline -0.2212 -312 Ethyl methyl aniline
M-
(c) Benzyl chloride T = 308’ and p-dimethyl toluidine 0 . I424
201
-j j o - 2620 -3470 - 2840 -629
1380
The lack of agreement between the values of RTln k, k, and Qm - Q, may indicate either experimental error in the determinations or lack of equality between Am and Certain interesting relationships appear however when the results are examined and it is proposed to study these more fully. The ratio k,,,, k,, for o-toluidine is practically equal t o that for p toluidine while that for dimethyl aniline is practically equal to that for ethyl methyl aniline. This indicates that for each reaction A, = -1,and also that Q,,, = Q’a Q’,,l. Therefore, Q,,, - (1, = Q’,,, - Q’Djconstant as long as the and p isomers react with substancies of similar constitution. X strict comparison between chlorine, bromine, and iodine compounds cannot he made because all the necessary data are not known but the results of Conrad and Bruckner (loc. cit.) show that with alkyl oxides the temperature coefficients of the reaction velocities are approximately equal while the actual velocities vary very greatly; thus th$re is a greater discrepancy be-
+
tween the values of E, E? and-
log k, than would be expected from the varialog k,
tion in u and molecular weight in passing from one halogen compound to another. A reference to the discussion of the reaction velocities of chlorine, bromine and iodine compounds with amines and with sodium thiosulphate shows that here also the velocity constant show much greater variation in value than is shown by the energies of activation. Energies of activation like heats of combustion and ionisation potentials show only small changes in passing from chloro to bromo and iodo compounds. I t therefore follow that, the equation for a bimolecular react’ion is of the type: k = kl e - B RT where k1 is a constant much more dependent on the
542
DAVID H E N R Y P E A C O C K
constitution of the compounds concerned than the expression involving molecular diameters and molecular xeights. Perrin and Job suggest that this term represents the sensibility or ease of activation of the molecules. I t is also possible that kl represents some function oi the stability of the activated molecule. The high rates of reaction frequently observed n-ith iotlo compounds may then he due mainly to the longer life of the activated molecules of the iodo compound as compared with those of a bromo or chloro compound. Pirnilarly the very much greater reactivity of n- or p-nitro h o m o benzene as compared with m nitro brom benzene may lie ascribed not entirely to an alternating effect on the energy of activation but also to an effect on the stability of the activated state. I t is also possible that the effect of the solvent on reaction velocity may he partly due to the same cause. X comparison of reaction velocities or what is the same thing of the quantities of isomers formed in a particular reaction thus involves two quantities at least, the energy of activation and another quantity which may be the scnaibility to activation, the stability of the activated molecule or a combination of both. Summary -Attention is drawn to the alternating effect of substituents upon the (I) energy of activation for the reactions studied. The effect of energy of activation in altering the ratios of velocities (2) and therefore the ratios of the products formed, at different temperatures is pointed out. ( 3 ) I t is suggested that the alteration in activation energy is a more truthful expression of the effect of substituents upon reactivity than is the velocity of the reaction. (4) A comparison is made of the effect of substituting one halogen for another upon the velocity constant and energy of activation for a number of reactions. The order of energies of activation i s compared with the order of ionisation potentials for the halogen acids. ( 5 ) Velocities of reaction are affected not only by alterations in activation energy but also by variations in the sensibility of the molecule (Perrin. Job) and the Ftability of he activated state. (6) The applicability of Dushman’s equation to the cases concerned is examined. I’niz’ersify College I-nirersity of Rangoon S e p l . PO, 1026.
