Feb 5 , 10.58
NEOPENTYL EFFECTIN “PRINCIPAL” ULTRAVIOLET TR.‘INSITION
3 .?9
ORGANIC AND BIOLOGICAL CHELMISTRY [CONTRIBUTION FROM THE
DEPARTMENT O F CHEMISTRY
O F THE UNIVERSITY O F \yASHISGTON]
Solvent and Substituent Effects. The Neopentyl and Other @-BranchedAlkyl Groups and their Effect on the “Principal” Ultraviolet Transition1 BY
\v. hf. SCHUBERT BND JANIS
ROBINS*
RECEIVED JULY 22, 1957 It has been found that in the principal ultraviolet transition of p-alkylnitrobenzenes (gas phase), 4-alkylpyridinium ions and 4-alkyl-1-hydroxypyridiniumions, the neopentyl group is inherently a considerably better electron donor than t-butyl or methyl. The observed excitation energies are in the order: neo-P < i-Bu < Pr < E t < Me. A detailed examination of the solution data indicates that steric hindrance to solvation may be a factor in the apparent effect of these substituents.
IVith regard to steric inhibition of hyperconjugaFor alkyl substituents of the type RCH2, i t has been found that increased branching at the /I- tion, Arnold and Truett,12aand Baddeley and Gorcarbon atom leads to a lowering of the rate constant don, I2b consider hyperconjugation to be a maximum for a number of reactions in solution in which the when the conformation of the molecule is such transition state is electron demanding relative to that the formal contributing valence bond structure the ground state. These reactions include (1) with a double bond between the alkyl group and bromine addition to ethylenes, in which neopentyl- the unsaturated system can make its maximum conis in the ethylene is about ten times less reactive than n- tribution; e.g., when R in structure 1-11 b ~ t y l e t h y l e n e ~(2) ; bromination of alkylben~enes,~plane of the ring. Thus when R is large, as in the and solvolysis of p-alkylbenzhydryl halides,b in neopentyl group, steric interference with cowhich the order of rates is: E t (fastest) > n-Pr > planarity of the R and phenyl groups leads to a isoBu > neoP; and (3) the reaction of RCHBrCH3 lessened contribution by 11. An essentially similar in ethanol containing 1 N sodium ethoxide, for which both substitution and elimination rates are in H ,H HS the order: E t (fastest) > n-Pr > isoBu > neoP.G ;I;;..CC\R I n addition, i t has been found that the retardation of rate accompanying a-deuterium substitution is less for the neopentyl than the ethyl group.’ I I1 Similarly, ratios of solvolysis rates of a-deutero-palkylbenzhydryl chlorides compared to the cor- viewpoint is that maximum hyperconjugative reresponding protium analogs fall off in the order lease by an a-C-H of the alkyl group occurs when CD3 > CH3CD2 > (CH,),CHCD* > (CH3)2CD. the C-H bond lies parallel to the adjacent a-orbital The isotope effects are positive (i.e., k ~ / >k 1) ~ of the unsaturated system13; and that the end methyl groups of the neopentyl substituent hinder and of the order of 6% or less.8 The retarding effect of increased branching a t the approach of the methylene group t o the proper the P-position, observed in the above reactions, has orientation for this.’ On the other hand, JaffC has been attributed to a number of different factors: concluded that hyperconjugation is relatively in(1) second-order hyperconj~gation~;( 2 ) steric sensitive to the conformation of the alkyl group.14 Because of the recent demonstration that inhindrance to bond shorteningg; (3) steric shielding of the reaction centerlo; (4) steric interference with herent alkyl substituent effects may be masked or solvent enhancement (by hydrogen bonding with considerably modified by ~ o l v e n t ,i ~ t was , ~ ~ con~~ the a-hydrogens of the alkyl group) of hyper- sidered desirable to study the effect of branching conjugation*; (5) steric hindrance to solvation of in the /I-position in a gas phase reaction in which a electron deficient sites near the alkyl substituent, large demand for electron release is placed upon coupled with inherent release by alkyl substituents the substituent. As in a previous study of other in the inductive orderllazb,c; and (6) steric inhibition alkyl substituents, the “reaction” chosen for conof h y p e r c o n j u g a t i ~ n . ~ ~ ~ ~ venient study was the gas phase ultraviolet excitation of p-alkylnitrobenzenes that results in the (1) Presented in part a t t h e 132nd S a t i o n a l 3leeting of the American Chemical Society, h-ew York, S . Y.,Sept. 8-13, 1957. so-called principal ultraviolet band in the spec(2) D u P o n t Fellow, 1955-1956. trum.IlC This band has been identified as due to a (3) P. W. Robertson, J. I ; ( c ! W. h l . Schubert, J. Robins and J. I,, Haun, THISJOUE.XAI.. 79, U:30 ( 1 9 3 i ) . (12) (:i) R. T . hrnold and I V . I,. T r u e t t , ibid..73,5508 (1951); (b) (>, T3;rtldelep and h l . Gordon. .I. C h r m . Soc.., 2191 (1952).
(13) See, e . ~ . C. , A . Coulsoii, “Valence,” Oxford Press, Oxford, 1952, p. 312. (14) H. H . Jaffi: and J. 1,. Roberts, THIS J O U R N A L , 79, 391 (1957). (13) A. G. Alhrechr and W. T. Simpson, J . f h e m Phys , 23, 1480 ( I Q R t ? ) ; ?‘HIS JOIJRNAI.. 77, 4455 (1957).
w. Lf. SCHUBERT AND JANIS ROBINS
560
on the excitation energy, the position of the principal band of 9-neopentylnitrobenzene also has been measured in a variety of solvents. In addition, solution spectra of certain 4alkylpyridinium perchlorates and 4-alkyl-1-hydroxypyridinium ions were studied.
e
0 . ' , a /
0
s
R
' \,m/ ?,g
I?
Experimental
Vol. 80
Anal. Calcd. for C7H1oNO4Cl: C, 40.79; 1-1,4.85. Found: C, 40.60; H, 4.79. The 4-propylppridiniuin perchlorate melted a t 50-50.5'. Anal. Calcd. for C ~ H I ~ X O ~ C, C I 43.67; : H,5.46. Found: c , 43.45; 11, 5.39. The 4-neopentylpyridinium perchlorate melted a t 120.7121.2'. Anal. Calcd. for C10H1JS04C1: C, 48.10; H , 6.46. Found: C, 48.05; H, 6.45. The 4-(2,2-dimethylbutyl)-pyridiniumperchlorate melted at 79.0-79.3'. Anal. Calcd. for CIIH18SOaC1: C, 50.40; 11, 6.88. Found: C, 50.01; H, 6.79. Pyridiniuni-1-oxides.-These hygroscopic compounds were prepared from the corresponding pyridines by the method of Ochiai.24a Pyridine-l-oxide was fractionally distilled through a 40-plate spinning band column, b.p. 142' (14 m m . ) , then repeatedly sublimed under reduced pressure, I I I . ~ . 66-67" (lit.25m.p. 67"). The ?-picoline oxide was recrystallized three times from ethyl acetate-Inetlianol, 111.1). 165-165.5'. 11 t i n ! . Calcd. for CJI7SO: C, 66.03; 11, 6.47. Found: C , 06.22; H , 6.43. The 4-neopentylpyridine-1-oxide was distilled, b.p. 169' (6 mm.), recrystallized from hexane, and then twice sublimed under 1 mm. pressure, m.p. 97-98'. Anal. Calcd. for CloH16n'O: C, 72.68; H, 9.15. Found: C, 72.82; H, 9.12. Spectral Determinations.-Spectral measurements were made as previously described, using a Beckman DU spectrophotometer equipped with a fused quartz prism and a photomultiplier.11c An air-thermostated cell compartment was used for the gas phase spectral determinatioris rather than the water-bath as previously. Values of vmsx were determined in the graphical manner already describcd.llC
p-Alky1nitrobenzenes.-Neopentylbenzene was prepared by the method of Berliner.I6 The procedure of Selson arid Brown was applied in the nitration of propylbenzene, isobutylbenzene and neopenty1ben~ene.l~Purification of the nitro compounds was achieved by steam distillation, then fractionation through a 40-plate spinning band column foilowed by five low temperature recrystallizations from methanol. The infrared spectra, in the strong out of the plane bending frequencies of aromatic C-H bonds, showed the absence of o- or nz-isomers.'* The following physical properties were observed: p propylnitrobenzene, b.p. 141.5-142' (10 mm.), 1 2 2 5 ~1.5362, m.p. -14' (lit.Ig b.p. 154' (20 mm.)); p-isobutylnitroben~ 1n.p. -34" zene, b.p. 141-141.5' (8 mm.), n Z 61.5290, p-Keopentylnitro~ (liL20b.p. 125' (3 mm.), n Z 61.529gZ1). Results benzene, a new compound, had m.p. 29". The principal band of the nitrobenzenes was Anal. Calcd. for C I ~ H ~ ~ K C, O ~ 68.38; : I€, 7.53. structureless and quite symmetrical about the Found: C, 68.52; H, 7.85. peak, even in the gas Some remnants Oxidation of a 1-g. sample of p-neopentylnitrobeiizene by hot sodium dichromate in sulfuric acid gave a 70y0yield of the so-called B-band (AI, Bz,) were observed of p-nitrobenzoic acid, m.p. 239', identical with an authentic around 270 mp, but this in no way seemed to insample. Pyridinium Perchlorates.-Pyridine and -,-picoline were fluence the position of the principal band. Also, commercially available (C.P., Baker Chemical). The transitions occurring a t shorter wave lengths did method of Osuch and Levinezz was applied to the preparation n o t interfere. Consequently, the generally :icof the other 4-alkylpyridines. Purification was achieved by cepted practice of using the absorption maxi~nuin fractionation through a 40-plate spinning band column: to indicate the entire band position has been f o l 4-ethylpyridine, b.p. 110' (130 mm.), ?ZZ5~'D1.4997 (lit.23 lowed. The gas phase and solution data are sumb.p. 168' (751 mm.), T Z ~ O D 1.5020); 4-propplpyridine, b.p. ~ (lit.zz b.p. 79-84' (22 mm.)). marized in Tables I and 11. 74' (12 mm.), n 2 5 . 61.4935 The new compound, 4-neopentylpyridine, was obtained in 20% yield, b.p. 134' (101 mm.), n Z s J1.4902. ~ The parent TABLE I peak in the mass spectrum (Consolidated Engineering Co. R A N DOF pmass spectrometer, model no. 21-103) occurred a t mass VALUESOF vNaX I N C x - l FOR THE PRINCIPAL RC8H,X02 I N GASASD I N HEPTANE",^ 149; calcd. mol. wt. 149. Gas p h a s e 4 Heptane Anal. Calcd. for CIUHISI\rT:C , 50.48; 11, 10.13. Found: R Am.,, mrr vmar C, 80.57; H, 9.93. SeoP 253.2 39490 37410 The new compound, 4-(2,2-dimetli~~lbutyl)-pyridine, was obtained in 20% yield, b.p. 156" (103 inin.), ? z * ~i~ 1.1958. i-Bu ?52.5 +110 110 The perchlorate salt was analyzed. 260 200 Pr 251. 6 The perchlorate salt of each pyridine was prepared by the Et0 Yojl.0 350 380 addition of the pyridine t o cold 50y0 perchloric acid. Water Mee 250.2 480 460 was removed under reduced pressure and the rewlting white He 239.1 2330 2'790 precipitate separated by suction filtration and recrystallized several times from methanol-ether. Pyridinium perchloAbsolute values of vnrsX are given for neopentyl; tlie rate melted a t 285-289" uncor. (lit.2* m.p. 287-238'). others are relative to neopentyl. Values of vInnT are The 7-picoline perchlorate melted at 99.8-100.2'. averages of three determinations, duplicatable in gas to Anal. Calcd. for C&h7O4C1: C, 37.50; ET, 4.17. f 3 0 cm.-l; in heptane, to *20 cni.-l. Measurements Found: C, 37.50; H , 4.28. a t 80". Measurements made a t 150' were within the e\perimental error of those at 80". d F o r t-butyl, Y , , , ~ w:is ~ The 4-ethylpyridinium perchlorate melted a t 78-78.5'. 39,760.1lc e Previously reported.llo
-
%IS.
+
(16) E. Berliner, THISJ O U R N A L , 71, 1195 (1949). (17) IC. L. Nelson and H. C. Brown, ibid., 73, 5605 (l9:l). (18) N. B. Colthup, J . 0 3 t . SOC.Apner., 40, 397 (1950). (19) G. Baddeley a n d J. Kenner, J . Chent. Soc., 303 (1936). (20) T. E. Zalesskaya, J . Gen. Chem. ( U . S . S . K . ) , 17, 489 (1947). (21) We are indebted t o Mr, Richard B. NIurphy for preparing these compounds and determining their spectra. (22) C . Osuch a n d R. Levine, THISJ O U R N A L , 78, 1723 (19.56). (23) H. C. Brown and W. A. Murphey, ibid., 73, 3301 (3951). (24) A. I. Popov and R . T. Pflaum, ibid., 79, 570 (1967).
T h e principal band for pyridinium perchlorates appears a t around 210 mp arid has an of about 8,000. The peaks were w r y symmetrical and rather sharp. The results for these compounds arc listed in Table 111. (?In) E. Ochiai, J. Orq. Chem., 18, 534 (195.3). ( 2 : ) 13. P. I,inton, T I I I S J O U R N A L 62, 19-15 (1940)
Feb. 5 , 19%
NEOPENTYL EFFECTI N
"PRINCIPAL"
TABLE I1 VALUESOF vmax
PRINCIPAL BANDOF p R C ~ H ~ NISO SOLVESTS"*~ ~ IN
CM.+ FOR
THE
t-
n-
R
n-C,His
BuOH
BuNHi
HOAc
CHiOH
HCIOiC
NeoP Me H
37410 460 2290
36450 36300 36180 300 320 270 2340 . . 2410
36060 400 2480
32580 440 3140
.
H,O
95% E t O H CHpCN HCOzH
70%
60% HCIOaC
35800 35070 34670 33580 36090 430 390 380 410 360 Me 2920 2480 2660 2770 H 2440 a Absolute values of v are given for neopentyl; the others are relative to neopentyl. Values for nitrobenzene and p nitrotoluene previously reported.llC * Values of vmax are average of three separate determinations; vmaX duplicatable No detectable amount of conjugate acid to A20 cm.-l. present. XeoP
TABLE 111 VALUESOF vmax IS Cw-1 FOR THE PRINCIPAL BANDOF 4ALKYLPYRIDISIUM PERCHLORATES' 4-Group
1% HClOdb
CHiOHC
95% EtOHC
44210 ... ... 44440 44520 44580 ... ... 45320 45680 45660 45680 Me 46080 45980 46020 H 49700 49080 48340 a Average of two determinations; accuracy estimated at k20 cm.-l. b The vms. value for 4-t-butylpyridinium perThese solvents chlorate in this solvent was 45,310 cm.-l. contained 1% HClOd.
2,2-Dimethylbutyl ;\iTeoP Pr Et
The principal band of the conjugate acid of the pyridine-1-oxides was to the red of that of the free base and occurred around 220 mp with an emax of about 5,000. The spectral envelope was symmetrical and did not change in shape upon variation of the 4-substituent. The vmax results for the 1-hydroxypyridinium ions are given in Table IV. TABLE IV VALUESOF vmaX (CM.-~)FOR
THE
ALKYL-1-HYDROXYPYRIDINIUM 4-Group
70% HClOa
9% HClOaC
PRINCIPAL BANDOF 4IONS~.~ Acidic 95% CHiOHd EtOHd
42890 42550 43030 42840 43670 43560 44400 44000 45090 44850 46530 45680 H Average of two runs; estimated accuracy, &20 cm.-l. b The pyridinium oxides were completely in the conjugate acid form in these media. The value for t-butyl in this sold These media contained 10% convent is 43,520 cm.-'. centrated H2SO4by weight. XeoP Me (I
Discussion The Inherent Effect of Neopentyl ahd Other 6Branched Alkyl Groups.-As noted previously, the effect of p-alkyl substitution is to considerably lower the energy of the principal ultraviolet transition of nitrobenzene. This indicates that there is considerably more electron release by the psubstituent in the excited state than in the ground state. I n comparing P-alkyl compounds, i t had been found that in the gas plzase stabilization of the excited state relative to the ground state is in the inductive order: t-Bu > i-Pr > E t > Me >> H.lLC The gas phase data of Table I reveal that
ULTRAVIOLET TRANSITION
561
for p-alkyl groups of the type RCH2a change in R has qualitatively the same effect as when R is directly attached to the aromatic ring; that is, stabilization of the excited state relative to the ground state decreases regularly in the order: neoP > i-Bu > Pr > Et > Me. The results in heptane parallel those in gas (Table I)] except, of course, that the principal band position of the entire series has been shifted to the red (;.e.> lower energy or frequency). A surprising feature of these results is the relatively large activating effect of the neopentyl group. Thus stabilization of the excited state over the ground state is 4SO cm.-' (or 1400 cal.) greater for p-neopentyl than for p-methyl, whereas p-t-butyl lowered the excitation energy by a lesser amount, 210 cm.-I (or 600 cal.), relative to p-methyl. I n addition] the spreads in excitation energy in the series E t to neoP are somewhat larger than found in the series Me to t - B u . ' I C The neopentyl compound retains its position of lowest excitation energy in all the solvents studied, although the V N ~ ~ P - V M spread ~ is modified (Table 11). For the 4-alkylpyridinium ions in solution (Table 111) the effect of changing the alkyl group is qualitatively the same as for the P-alkylnitrobenzenes in gas and heptane; Le., the excitation energies are in the order: neoP (lowest excitation) < Pr < E t < Me; also, t-Bu < E t < Me. I n addition] the 2,2-dimethylbutyl group has a somewhat greater activating effect than neopentyl in the 4-alkylpyridinium ion seriesz6 Fewer alkyl groups are compared in the solution spectra of 4-alkyl-lhydroxypyridinium ions. but the general trends are the same as for the 4-alkylpyridinium ions (Table IV). It is to be noted that the spreads between excitation energies are greater for the 4-alkylpyridinium ions and 4-alkyl-1-hydroxypyridinium ions than for the p-alkylnitrobenzenes. Presumably this reflects a greater demand for electron release placed upon the alkyl group in the excited state (relative to the ground state) of the ions. It is concluded from the above results that the neopentyl group is inherently a more efficient electron-releasing group than either methyl or t-butyl, a t least in these spectral excitations. Furthermore, the order of electron release by RCHz groups is: 2,2-dimethylbutyl (greatest release) > neoP > i-Bu > P r > Et > Me. Therefore, if steric inhibition of hyperconjugation is a factor in determining the electronic effects of these groups, i t is not a sufficiently important factor to reduce their inherent electron release tendencies below that of methyl. If inherent substituent effects in spectral excitation reactions are no different in principle from those in ordinary chemical reactions, then one must look elsewhere for an explanation of the apparent relative retarding effect of @-branched groups such as neopentyl in the chemical reactions cited in the Introduction. In a previous publication i t has been suggested that substituent effects may be qualitatively "understood" and perhaps quantitatively correlated by assuming that electron release or accept(26) T h e spectra of the free pyridines were n o t measured because the wave lecgthof the principal band is so low t h a t accurate measurem e n t of its position is difficult.
,562
\\-. \ I
SCFIVRFRT ixn J ITIS ROBINS
1'01. 80
ance by a substituent is merely a function of the ment of C-H hyperconjugation (for a more comelectronegativity difference I)etm.een it and the plete discussion see reference 1IC).3n .Iplot of v n e o p - v ~ tis, Y H shows a scatter similar to group to which it is xttaclied (in ground or excited states) and the "~~olariz~il~ilities" o f the two por- that found iii the correspondirig plot for j - t tions.?' In these terms. if oiie :mxuiies the electro- t)utyliiitrobenzene.llc A s in the case of the t-butyl iiegativity difference bt.!mwi alkyl groups to be cwnpound, the scatter can be interpreted as indirelatively small, then thc "cxtra" activating effect cating disproportionate solvation effects betvieell of the neopentyl substituent (and to a lesser extent, the neoperityl compound and nitrobenzene and is of isobutyl and propyl) relative to say ethyl would consistent with the operation of steric hindrance to in the main be attributed to its greater "polariz- ring solvation. 3 1 The contrasting fact that p-neopentplnitrobenability." The "extra" polarizability of the neopentyl substituent conceivably may be due to the zene has a lower excitation energy than p-riitroclose proximity of the terminal methyl groups to toluene in solution whereas in certain reactions the aromatic ringsg5 Thus, on excitation of the (Introduction') the rate constant for the neopentyl molecule, the terminal portion of the substituent compound is less than for the corresponding may be polarized directly across space, by the ring methyl compound, may be attributable, in part a t as well as via the bond to the substituent; i.e., the least, to the operation of the Franck--Condon printerminal portion of the substituent which over- ciple in spectra. Thus, in the spectral excitations, hangs the ring may be acting in much the same way solvent stabilization of the excited state depends as would an adhering hydrocarbon solvent niole- in part on the position of solvent molecules asjiised by the ground state; whereas the transition states of cule. ,Inother factor leading to increased activation ordinary chemical reactions are maximally solby the branched alkyl groups may be the relief of vated. T h ~ i ssteric hindrance to solvation would strain on spectral excitation. It has been proposed be expected to be a n even more important factor previously, in connection with the spectra of a,p- in "ionic" reactions in solution. The shifts in the position of the principal band unsaturated carbonyl compounds, that the excitation energy will be lowered when the electron dis- brought about by changing solvent are small for placement in the spectral excitation is away from the pyridinium ions and 1-hydroxypyridinium a site of angle strain, even though no movement. of ions (Tables I1 and III), in contrast to the large the atomic nuclei is allowed in the short time of the shifts in the nitrobenzene series. This is not surelectronic excitation (Franck-Condon Principle) . 2 9 prising since the excitation of the ions, crudely reIf increased branching of the alkyl groups does lead presented by equation 2, involves a redistribution to any angle strain at or near the substituent (this of a positive charge. Thus some sites, e . g . , the is not evident from inolecular models) then the protonated group, become less positive in the exexcitation energy should be lowered since the elec- cited state than in the ground state; other sites, tron displacement on excitation is away from this e . g . , the ring and the substituent, become more positive in the excited state. Solvent stabilization region. Solvent Effects.-For the neopentyl substituent, at sites becoming less positive on excitation (e.g., the solvent effect on the Y R - Y I I ~ spread in the nitro- by hydrogen-bonding a t the N-H hydrogen) will benzene series is qualitatively not as dramatic in be less in the excited than in the ground state. its appearance as for the series Me, Et, i-Pr, f-Hu Approximately balancing this will be greuter solvent in which the effect of more basic solvents was to stabilization in the excited state than in the ground tend to invert the inductive order of excitation state (e.g., of the ring and possibly the substituentj energies found in the gas phase, giving a jumbled :it thy sites becorning more positive on excitation. order of excitation energies tending toward a con-plete Baker-Nathan order.1ic On the other hnlld, the neopentyl compound retains its position ( i f lowest excitation energy in all the solvents studied. (2) However, there is an experimentally significarit reduction in Vneop-YMe in the basic solvents (Table H €I 11) that is comparable to the reduction in ~ 1 - 1 3 ~ v l t e . The YneoP-Vhre spread is lowest in n-butylamine The red shift for the unsubstituted pyridiriium while the Y ~ B ~ - Y R spread . ~ ~ is lowest in water.'!' ion in proceeding from 15,; aqueous perchloric acid The decrease in spread is qualitatively consistent to ly0 perchloric acid in ethanol indicates that, with the operation of either steric hindrance to ring (30) I n t h e absence o i steric hindrance t o solvation effects it would solvation or steric hindrance to solvent enhanc: ii i o l v e n t rei1 shift in t h e spectrum t o be ac( 2 7 ) W. AT. Schubert, J. A I . Crai,en, I-I. Ovg. Chena., 2 2 , 1286 (1057).
(28) I n the molecular model of p-netil)entylnitrobenzene the siilistituent is constrained in such a mannrr t h a t 3 termin'tl port!, closely overhang; one side of t h e aromatic ring in the neighbor1 t h e $-position. ThiT is true t o n lesser Pxtent in the model5 isobutyl a n d propyl compounds. T h e fact t h a t p-neopentyitiitrohenzene and 4-neopentylpyridinium perchlorate have a noticeably highcr melting point t h a n t h e corresponding propyl or isobutyl curnp~, (see Experimental) m a y be a reflection of the zdditiorlal conforinat restraint placed upon the neoprntyl grrnii>. (20) \V, \T, Schllhert :lUd b 'y A . S&v?nc'v, '?I115 [ o I J R K . \ T 77, '?2')7 (1~15.5).
i n u , , , , i ~ - u n e , pnrticularl? 111 hydrtigeii ther biords. hydrogen bond solvation of t h r nitro group would be stronger for the neopentyl compound t h a n f u r the methyl in t h e ground state (greater inherent electron release by neopentyl). I n view of the 1;ranck-Condon principle, this would contribute t o an even greater solvent itatdization of t h e excited state o l the nropentyl compi,unri relative t o the methyl compound, acting to increasr the spread in excitation energies. Witness the increasrd ob\erved r,>ic-z+1 :iircad accompanying solvent red 5hifts. (~,'31)I n the Fisher-l'aylor-Hirscllfelder model of p-neopentylnitrobenzene part < i f onr siilc of thc ring is eliectively 5hielded hy terminal ~ i n e t h v l Lroii{>. , s f t l i c ~ ~ i ! l ~ ~ t i t u v tr hi tr o t l ~ r r.ii! p-I > p-C1> p-Br, whereas the m-halogen substituents all have negative Au-values. These observations are discussed in terms of transition-state resonance and by comparison with similar systems. The unusual rate-enhancing influence of iodine has been further ascertained by solvolysis of the two iodobenzyl tosylates in dioxane-water and acetone-water mixtures of varying composition,
Recent work in this Laboratory showed that the solvolysis of m- and p-substituted benzyl tosylates gives a limited correlation with the Hammett equation.2p3 Donor substituents such as methoxyl and methyl in the p-position provide exceptionally large driving forces above those to be expected from their normal c-constants. T h a t these deviations are due to important contributions of transition-state resonance is shown by the relatively normal behavior of m-methoxy- and m-methylbenzyl tosylates. This is reminiscent of the deviations of acceptor substituents such as p-nitro in the reactions of substituted phenols and anilines. These are reactions which involve the freeing of an electron-pair whereas in the solvolysis of benzyl tosylates the vacating of an orbital is important in stabilizing the transition state. The sensitivity of the solvolysis of benzyl tosylates to substituent effects as indicated by the large negative p-values2s3has prompted an investigation of the behavior of the m- and p-halogenated compounds. There has been substantial disagreement in the literature as to the relative effects of halogen substituents. It was felt that the solvolysis of halobenzyl tosylates would make a good case for the evaluation of the polar effects of these substituents. The deviation from the Hainmett crconstant can be taken as a measure of the ability (1) Presented in p a r t before t h e 130th Meeting of t h e American Chemical Society, Atlantic City, h’. J., September, 1956. (2) J. K . Kochi a n d G. S. Hammond, THISJ O U R N A L , 76, 3445 (1953). (3) G.S. Hammond. C. E. Reeder, F.T. F a n g nnd J K. Kochi, ibid., 80,BG8 (1958).
of a substituent to accommodate a positive charge in the transition state regardless of the origin of the stabilizing influence. Since the completion of our work the results of a similar study of halogen substituent effects on the solvolysis of phenyldimethylcarbinyl chlorides has been published. Previous work in this Laboratory included the solvolysis of p-fluoro- and p - ~ h l o r o - m, ~ and pbromobenzyl tosylates2 in various solvent systems. In order to ensure a fair correlation, all eight mand p-halobenzyl tosylates have been prepared and their rates of solvolysis have been determined in the same mixed solvent, namely, 55.6% aqueous acetone. In addition, the rates of solvolysis of the two iodobenzyl tosylates have been measured in five other dioxane-water and acetone-water mixtures of low dielectric constants and varying composition.
Experimentalj Preparation and Purification of Materials. Benzyl Alcohol.-The commercial alcohol (Matheson, Coleman and Bell, chlorine-free) was distilled under reduced pressure through a glass helices packed column, b.p. 91.9-92.4’ (11.5 mm.). Halobenzyl Alcohols.-All except m-fluorobenzyl alcohol were prepared from the corresponding benzoic acids by reduction with lithium aluminum hydride.6 Most of the commercial acids were purified by recrystallization from ethanol followed by vacuum sublimation. This procedure was found to increase the ease of purification of the benzyl alcohols. The liquid alcohols were purified by distillation under H.C.Brown, Y.Okamotu and G. Ham,ibid., 79, 1906 (1957). (5) All melting a n d boiling points a r e uncorrected. (6) W.G. Brown, in R. Adams, “Organic Reactions,” Vol. VI, John Wiley and Soua, Inc., New York. N. Y., 1951, p. 491. (4)
