niethylacetaldehyde by the same procedure used in the case of trimetliylacetaldeliyde-1 -d. On removal of the ether, the crude oil containing the neopentyl alcohol-1-d, as well as the normal addition product, was treated with phthalic anhydride in t h e usual manner.7 The crude crystals were recrystallized to give 700 mg. (25% yield based on the ( - ) l-chloro-2-methylbutane-2-d), m.p. 68.5-70.0'. After a second and third recrystallization the m.p. was constant a t 68.5-69.5'. Because of the small amount of material it was difficult t o obt:iin accurate rotations, but within experi-
mental error they were unchanged at ( Y * ~ D-0.07 =t0.02' ( I 1, c 40; acetone). d f t e r the fourth recrystallization a precise value of a Z 7 D -0.072 i.0.010" ( I 1, c 39.9; acetone), [ C Y ] 27D -0.180 iz 0.010" was obtained inaRudolph recording spectropolarimeter. rl deuterium analysis1* revealed 0.56 atom per molecule. The infrared spectrum of this compound was identical with that obtained from the enzymatic reduction of trimethylacetaldehyde-1-d7 with the exception of the differences due to the relative amounts of hydrogen and deuterium in the two samples.
[CONTRIBCTIOS FROM THE r)EPAKT.\IEST O P ClIE\IISTKY, T H E UNIVERSITY O F \vISCOXSIX, h ~ ~ A D I S O 6,S \\.'ISC.]
The Effect of Solvent on Spectra. V. The Low Intensity (n Transition of Cyclic Ketones BY
+ T*)
Electronic
EDWARD 11. K O S O W E R AND ~ ~GUEY-SHUANG WU' RECEIVED SEPTEMBER 6, 1960
A series of cycloalkanones with ring size ( n = 1-10, 13) has been examined in the near ultraviolet region in a series of solvents covering a broad polarity range. The transition energies ( E T )were plotted against the solvent polarity standard, 2 (ref. 4). Cyclobutanone was found to have a II +. r*-transition with rather low sensitivity t o solvent, and brief examination of two other cyclobutanones suggested that such low sensitivity may be characteristic of these rings. The transitions for the ketones with n = 5 through 10 correlate rather well with Z, and the correlation for cyclohexanone is so good t h a t it can be used as a secondary standard of solvent polnrit~-. I t was found t h a t the slope of the correlation for cyclopentadecanone, n = 15, was unexpectedly low. Low solvent sensitivity for the n + r*-transition of the C15-ketonewas rationalized by postulating t h a t the molecule tended toward "folded" conformations in polar solvents, but was "open" or "extended" in non-polar solvents. The CI5-ketoneabsorption band obeyed Beer's law in all solvents.
Kot only are all measures of solvent polarity empirical, but the application of a particular measure to a chemical or physical problem depends upon the congruency of the physical process from which the measure was derived to that under study. The dielectric constant, which averages both the molecular and polymolecular inhomogeneities of a liquid solvent, is not very satisfactory as a measure of solvent polarity on the molecular level. The dielectric constant of the first layer of water molecules around an ion, for example, must be appreciably different from that of the measured (macroscopic) dielectric constant.? For most chemical p r o c e ~ s e s ,and ~ for spectroscopic transitions, the detailed properties of the solvent group (cybotactic region) immediately around the species of concern control the course of events. For the consideration of the effect of solvent polarity on the microscopic level, it is inore desirable to choose model processes which are in their turn subjected to a detailed and searchir?g analysis. For kinetic processes, the rate of solvolysis of tbutyl chloride covers a reasonable range of solvent p ~ l a r i t y ~the , ~ ;parameter based on these rates is called a Y value. A more general parameter became available with the discovery that the chargetransfer absorption hand of 1-ethyl-karbomethoxypyridinium iodide was extremely sensitive to the solvent4; the transition energy corresponding to the absorption band was defined as the Z-value for the solvent in which it was measured. ( l ) ( a ) D e p a r t m e n t of Chemistry, S t a t e Univrrsity of New York, Long Island Center, Oyster R a y , N T. (h) T h e authors are grateful for t h e support of t h e Air Force Research a n d Development Command, Air Force Office of Scientific Research, through Contract AF 49(fi38)282. (2) J. A. Schellman, J . Cherra. P h y s . , 26, 1225 (1957). (3) A. H. Fainberg a n d S. Winstein, J . A m . Chem. Sor., 7 8 , 2770 (1956). ( 4 ) E. M. Kosower, rbid., 80, 3253 (1958). (5) E. Grunwald and S. Winstein, $bid.,70, 848 (1948).
Solvent polarity values defined on the basis of molecular processes can serve a number of useful purposes. Correlation of rates or of spectroscopic maxima can lead to the estimation of rates or maxima in solvents for which the rate or maximum has not been or can not be measured. Response to solvent change, which is an important criterion of chemical mechanism or for establishing the nature of an electronic transition, can be placed upon a semi-quantitativebasis. Last, anomalous behavior with respect to an expected correlation can call attention to chemically interesting situations. The purpose of the present paper is to esamine the effect of solvent on the low intensity (n+n*) electronic transition of cycloalkanones. It will be seen that 2-values are useful in the analysis of the results. Results The ketones used in this investigation were all carefully purified. Our experience indicated that a number of the cyclic ketones were sensitive to photochemically-induced oxidation. One important criterion of purity was therefore the lack of a maximum or shoulder in the ultraviolet absorption curve between 2200 and 2600 A., where CY#unsaturated ketones and other impurities would absorb. The other more usual criteria of purity (melting point, boiling point, refractive index, derivatives, etc.) were also utilized. In the case of cyclobutanone, it proved impractical to remove 2 or 3y0 of two volatile impurities (detected by vapor phase chromatography), but the identical results given by two different samples afford confidence in the supposition that the contaminants are non-light-absorbing in the region of the maximum of c y c l ~ b u t a n o n e . ~ 16) Cf. also E. h i . Kosower, ibid., 80, 3261 (19.58).
(7) T h e observation concerning t h e fact t h a t impurities more volatile t h a n cyclobutanone are present even in purified ketone was also
n - m * TRANSITION OF CYCLIC KETONES
July 20, 1961
3143
I I(
n -m TRANSITIONS.
/
,/'
6 0 % CH,OH-,
8O%CH,OH-/
/
P
60 I-oCt 0
*f: 101
,
,
,
,
l@C
103
104
105
/
I
TFp7
n= 5
i V
I Ll
I;:!
n.6
IO!
P
/
/'
,
,
,
,
95
96
97
98
,
,
99 'I00
,
,
101
102
,
,
,
,
,
,
,
98
99
I00
101
102
103
104
/
~
10:
+
Fig. ET ET (transition energies) wersus 2 (solvent polarity values) for cycloalkanones, n = 4,ET = 0.0819872 96.290; n = 5 , ET = 0.182462 84.252; n = 6, ET = 0.116502 89.234. The arrows on the cyclohexanone ( n = 6 ) line indicate the 2-values derived for these solvents from their experimental ET values ( T F P = 2,2,3,3-tetrafluoropropanc,l!. F o r ?I = 89.890; n = 9, ET = 0.151132 88.404. 7, ET = 0.140382 f 89.371; n = 8,ET = 0.137682
+
+
+
fl-tn'
TRANSITIONS.
80% CHIOH CH,OH
( I C - W JC
,a-,y*i"-d'
'
O
/ -..__ ( HeO 1
ii
0
70:LMe90
P
7
I
o/
0
/
97
s0..1-~t,& /
, '\
102-
-
n 'n TRANSITlOriS VARIATION WITH 91NP SIZE
I
CH3CN
,
Y
103-
,5.. I-PrOH
65
-
105104-
.;i
n=iO
+
0 j
,
,
,
I
95 4
5
6
7
8
9 n
Fig. ET (transition energies) versus 2 (solvent polarity values) for cycloalkanones, n = 10, ET = 0.137272 90.680; n = 15, ET = 0.0734452 95.276.
+
+
The solvents for the ultraviolet measurements were chosen so as to provide the full range of solvent polarity from isooctane (2 60.1) to 2,2,3,3tetrafluoropropanol (TFP) (Z 96.3). The Zvalue for the fluorinated alcohol differs from that previously reported,4and is derived from the linear relationship of transition energies for the n+x*transition of cyclohexanone with 2. All solvents were Spectrograde or of equivalent quality. The maxima were measured carefully in the usual way.4 The maxima for the n+x*-electronic transition of the cyclic ketones I, where n = 4,5,6,7,8,9,
l-----l
(c?Yl.J=o
I 10 and 15, are presented in Table I. I n order to made by R. Arndt, Hs. H. G u n t h a r d and T. GBumann, Hclu. Chim. Acta, 41, 2213 (1958).
1
, ring
0 size.
15
Fig. ET (transition energies) zwsus n, ring size for cycloalkanones in two solvents, water and isooctane, X o t e point for n = 15a t far right.
make the comparisons, the maxima were converted to transition energies in kcal./mole by the relatjon ET (kcal./mole) = 2.859 X lo5 X l / A m a x (in A.). The transition energies were then plotted against 2 ; Fig. 1 (I, n = 4,5,6) and Fig. 2 (I, n = 10,15). Certain quantities can be directly derived from the experimental data. These are the slopes of the least-squares line drawn to relate the points in the figures, and the difference in transition energies for the solvents isooctane and water. Both quantities are related to the degree to which the position of the n+x*-transition responds to the polarity of the solvent, the slope averaging out the changes over a range of solvent polarities. The "solvent sensitivities" derived by difference, ET (HOH) - ET (isooctane), are listed in Table 11, while the slopes and the solvent sensitivities corresponding to these slopes (obtained by multiplying by the 2-value difference between the solvents, AZ)are listed in Table 111.
3144
Vol. s3 TABLE
1
ARSORPTION MAXIMA O P C u c r ~ cKETOSES r
Cyclic ketone, cyc10-
_
~
Water (W.G)= Xrnasb
~
-
3lethanoi (83;ii) a
emnr
hinix
erns.
Butanone Pentanone Hexanone
2706 20 2808 21 2 7 1 21
278-3 20 2870 18 2883 1.7
Ileptanone Octanorie h70nanone Decanone Pentddecanone
-ii,
,,--20 2770 21 2776 25 27 6 > 5 > 4.23 The low solvent sensitivity of cyclobutanone is somewhat puzzling, although in agreement with decreased hydroqen-bonding ability.23 It is furthermore disturbing that a small, relatively rigid ketone does not exhibit an ET versus 2 relationship of g r a t e r linearity; however, the force con(17) Rexiy. (18) (19)
Cf.the bicyclic ketones reported in E. M. Kosower and D. C. T e t r n l i d v o n , 5 , 281 (1959).
N.L. Allinger, J. A m . Chem. SOC.,81, ,5727 (1959). (a) D. C m k , ibid., 80, 49 (1958); (b) S. Diner, Bull. sac. chim. Frarrce, 1025 (19.79). ( 2 0 ) T h e “whole-numbers” are, of course, approximations ( c j . I
