HLLTON A. SMITH AND C. W. HOLLEY
3714
Vol. SO
The p-bromophenacyl derivative of a-XI11 was obtained Anal. Calcd. for C ~ ~ H ~ ~ N ~C, O S67.89; S: €1, 6.65. as flat, elongated, very small blades after 3 recrystallizations Found: C, 67.91; H, 6.82. from aqueous ethanol; m.p. 157-158'. Remethylation of a-XI with ethereal diazomethane gave a Anal. Calcd. for CgaH23BrOa: C, 63.31; H, 5.09. quantitative yield of a - X , m.p. 75-76'. ( b j By Saponification of p-X.--A 0.500-g. sample of p-1.1 Found: C, 63.28; H, 4.93. (m.p. 127-1%53~was treated n-ith a n excess of anhydrous Coucentration of the filtrates from t h e purification of hydrogen fluoride and the mixture allowed t o stand at rooiii a-XI11 gave 1.39 g. (32YG)of a solid which after 3 recrystemperature for 2 hr. The neutral product, isolated in the tallizations from aqueous ethanol had m.p. 171-172'. usual fashion, consisted of ail oil which did not crl-stallize This material proved t o be a eutectic mixture of a-XI11 and which is presumed t o be 8 - S (cf. below!. Saponificaand &XI11 (cf. below for further details) containing about tion of t h e neutral fraction yielded, after recrystallization 2 parts of a-isomer t o 1 part of p-isomer. On this basis i t of the product, 0.220 g . (50?6) of colorless needles of n1.p. can be estimated t h a t t h e hydrolysis product contained about 173-175' which showeu no depression in 1n.p. when ad8 parts of a-XI11 t o one part of 0-XIII. mixed with a sample of P S I prepared as described above. Treatmeut of 0.1 g. of a - X I I I with ethereal diazomethane p-Isomer of 1,2,3,4,4a,9a,10,10a-0ctahydro-9-ketophenangave 0.08 g. (76%) of t h e starting ester, a - X I I ; m.p. 156thryl-lo-acetic Acid (!3-XI).---LVhen a 8.16-g. sample of P-I-1 156.5". was cyclized with a!uininuni bromide in carbon disulfide The 6-isomer of 6,7,7a,8,9,10,11,11a-octahydro-7-car- as described previously2 a 91%)yield of (%XI1was obtained. boxy-5-keto-5H-dibenzo [ a,c]cycloheptatriene (8-XIII) could In addition, 4c& of an acidic fraction was isolated and not be obtained in pure form. When a 0.60-g. sample of shown t o be @-SI. I t was purified b y recrystallization from @-XI1(m.p. 96-97'j was saponified under the conditions aqueous etliaiiol and obtained as a white ponder, 1n.p. 129described above, 0.55 go (9770) of a n acidic product was 131'; A ~ , " , " ( E ) 249mp(12,100), 290nip(18001. obtained, m.p. 170-172 After 4 recrystallizations from A n a l . Calcd. for C1JlIx07:C 74.39; H , 7.02. Found: aqueous ethanol t h e m.p. mas 171-112.5". T h a t this C, 74.12; H, 6.91. product was actually a mixture of a-XI11 and p-XIII was The methyl ester of $XI ($X'I \\-as obtained by treatment indicated b y retnethrlation experiments. IVhen this material was treated with ethereal diazomethane t h e resulting of the acid described above with diazomethanc. The product was oblaiiied, after evaporative distillation a t 80product could be separated, by fractional crystallization, 90" (0.001 m m . as an oil;".",!:A: (ti 249 m p (lO,900j, 2!10 into 4 8 4 4 % of a-XI1 (m.p. 156-157') and 38-2756 of &XI1 (m.p. 96-97'). On this basis it can be estimated (1800:). A n d . Calcd. for CliH&: C, 74.97;H, 7.40. Found: that the hydrolysis product contained about 2 parts of C, 75.08; H, 7.66. P X I I I t o one part of p-XIII. Ultraviolet Spectra.-The spectra of compounds a - S , a-Isomer of 1,2,3,4 ,4a,9aI10,10a-Octahydro-9-ketophe8-X, a - X I I , p-SI1 a n d XIV have been reported prenanthryl-10-acetic Acid (a-XI). (aj By Saponification of o~-X.--X 0.59-g. sample of a-X (m.p. 75-76'] was saponified v i 0 u s l y , ~ ~t h~ e5 d a t a having been obtained on a Beckrna~i under t h e conditions described above t o yield, after one model DU spectrophotometer. T h e spectra of these compounds were redetermined for purposes of a closer comparison recrj-stallization from aqueous ethanol, 0.49 g. (88%) of using, in the present instance, a Cary recording spectroa-XI, m.p. 173-175'. Further recrystallization gave photometer. T h e data agreed very closely with those in the colorless, very small needles, m.p. 175-176.5'; X2": literature, and the differences in extinction coefficient be(4248 mp (11,600), 289 mp (1800j. trveen a - X I I , @-XI1and S I V appear t o be real and reproduAnal. Calcd. for ClsHlsOa: C, 74.39; H, 7.02. Found: cihle. As a further check, t h e areas under the curves for the C, 74.42; H, 6.75. 250 m p bands r%-erecompared (oscillator strengths) and were The S-benzylthiouronium salt of a-XI was obtained as found t o fall in exactly t h e same order as t h e extinction colorless, thick, square blades after several recrystallizations coefficients. ST. LOTJIS.Mo. from ethanol; m.p. 157-157.5".
.
1,
[CONTRIBUTION FROM
THE
DEPaRTMEXT O F CHEMISTRY, THE UNIVERSITY OF
'rENSESSEE]
The Properties of Some Phenylpyridylmethanols in Sulfuric Acid Solution BY HILTONA,SMITHAND C. 1%'. HOLLEV RECEIVED FEBRUARY 27, 1958 The behavior of the following compounds in 1007, sulfuric acid has been studied : p!ien?-I-%-pq-ridplmethanol,phenyl-4pyridylmethanol, diphenyl-2-pyridylmethanol and diphenyl-4-pyridyltnethanol.The first two compounds show no evidence of carbonium ion formation, b u t appear t o give polymers. T h e last two compounds form fairly stable carbonium ions T h e properties of t h e solutions were investigated, utilizing i-factors, ultraviolet and near-visible spectra, titration studies and chemical reaction with methanol and water. T h e influence of the protonation of the pyridine nitrogen on carbonium ion stability of t h e phenylpyridylmethanols is discussed.
Introduction The dissociation of diphenyl- and triphenylmethanols in 100% sulfuric acid to give relatively stable carbonium ions has been the subject of a number of investigati~ns.l-~The influence of (1) H . A. Smith and R . J. Smith, THISJ O U R N A L , 70, 2400 (1948). (2) C. hZ. Welch and H. A. Smith, ibid., 73, 4748 (1950). (3) M . S. Newman and K , C. Deno, ibid., 73, 3644 (1961). (4) V . Gold and F.I-. T y e , J . Chem. Soc., 2172 (1962). (5) H. A . Smith and R. G . Thompson, THISJOURNAL, 77, 1778 (1955). ( 0 ) N. C. Den0 and A. Schriesheim, i b i d . , 1 7 , 3051 (1955). ( 7 ) R. G. Thompson and H. A. Smith, i b i d . , 77, 4432 (1956). (8) H. A . Smith a n d B. B. Stewart, ibid., 79, 3093 (1957). (9) B. B. Stewart a n d H. A. Smith, i b i d . , 79, 5457 (1957).
substituents on the stability of such ions has been studied by means of freezing point lowering, reaction with water and alcohols, ultraviolet absorption spectra and minimum sulfuric acid concentration necessary to produce a visible color. It has been demonstrated that the substitution of electronreleasing groups in the phenyl rings of diphenylmethanol increases the stability of the carbonium ion formed in sulfuric acid solution while electronwithdrawing groups decrease the thermodynamic stability and also promote the tendency for polymerization. Compounds such as tri-p-aminophenylmethanol and tri-p-dimethylaminophenylmethanol when dissolved in sulfuric acid form stable
PHENYLPYRIDYLMETHANOLS I N SULFURIC ACID
July 20, 1958
3715
place. The lack of color and other evidence indicates that carbonium ions are not formed. Reactions of Sulfuric Acid Solutions with Methanol and Water.-When solutions of the pyridyldiphenylmethanols in sulfuric acid were poured into methanol and then water and the products isolated, the corresponding methyl ethers were obtained. These were identified by elemental analy((CHa)2NC6H4)3COH+ 4H2SO4 --f HaO+ 4HSOasis. When the same solutions were poured into (( CH3)2rjHCeH4)&k~H&( CH3)2 (i = 6.0) water, the original carbinols were regenerated. If one of the benzene rings in diphenyl- or tri- These are the expected reactions if diphenylpyridylphenylmethanol is replaced by a pyridine ring, carbonium ions are present in the solution. When sulfuric acid solutions of the phenylprotonation of the nitrogen atom would be expected to decrease the stability of the carbonium ion pro- pyridylmethanols were poured into methanol and duced in sulfuric acid. Four such methanols were then water or into water directly, elastic materials prepared and their solutions in sulfuric acid were of variable composition were isolated which slowly crystallized on standing. These charred and sinstudied. tered a t temperatures from 220-260' and appeared Results to be polymeric in nature. Titrations to Form Carbonium Ions.-When dii-Factors.-Table I gives the i-factors for the phenylpyridylmethanols studied. For all of these phenyl-2-pyridylmethanol is dissolved in sulfuric compounds solution was relatively rapid, and the acid, an intense wine color is produced and with initial freezing point measurement was recorded the corresponding 4-pyridyl compound a dark within the first 15-20 minutes. Formation of a reddish-brown color is obtained. Titrations of the carbonium ion together with protonation of the diphenylpyridylmethanols dissolved in an inert nitrogen in the pyridine ring should give an i-factor solvent in contact with water were carried out with lOOyosulfuric acid until the color of the carbonium of 5.0 as shown, for example, by the equation ion was barely visible. The minimum weight per (CeHs)2(C&H4X)COH 3HzSOi + cent. of sulfuric acid necessary to produce the ( C B H ~ ) ~ ( C ~ H ~ & H )H30i C+ 3HSOa- (i = 5 ) characteristic color of the diphenyl-2-pyridylcarThis represents the initial behavior of the diphenyl- bonium ion was 66.0, while for the corresponding pyridylmethanols. The increase in the freezing diphenyl-4-pyridylcarbonium ion the corresponding point depression as the sulfuric acid solutions are value was 56.5. For triphenylmethanol the value is allowed to stand is probably caused by sulfonation 49.3 and for diphenylmethanol it is 72.0. No such of the benzene rings. Each sulfonic acid group titrations were performed with the phenyl-2which is introduced in the carbonium ion will cause pyridylmethanol which gives no color with lOOyo an increase of 2 in the value of i. acid or with phenyl-4-pyridylmethanol which forms a very light yellow color.
carbonium ions in spite of the strong electron-withdrawing groups present in the form of protonated amino substituents. However, the measured ifactors for these compounds are only 6.0 instead of the predicted value of 7.0. Apparently protons can exist on only two of the amino groups of the carbonium
+
+
+
+
+
TABLE I
VAN'T HOFF FACTORS OF PHENYLPYRIDYLMETHANOLS IN l O O ~ SULFURIC , ACID i Methanol
At zero time (extrapolated)
At 24 hours
Phenyl-2-pyridylDiphenyl-2-pyridylPhenyl-4-pyridylDiphenyl-4-pyridyl-
3.85 5.00 3,65 4,90
3 . 9 8 (constant) 7.30 (constant) 3 , 7 0 (constant) 5.66 (increasing slowly)
Simple solution of the methanol in sulfuric acid together with protonation of the nitrogen would give an i-factor of only two
+
C K H ~ ( C ~ H ~ N ) C H OHzSO4 H --f CeH5(CbH4kH)CHOH
+ HSOa- (i = 2)
while polymerization together with protonation of the nitrogen would give values of 2.5-3.0. If the carbinol group of the polymer reacted with the acid to form a carbonium ion, the values of i would vary from 4 ( n = 2) to 3 (n a) --f
+ +
nCoHb(C5HJV)CHOH HsSOd ---+ (C5H4~H)CHoHCeHaCHCeH4(CsHai'fH),,-zn-1 (3n-2)HSOrCHCGH~(CSH~~H ) H30+ (i = 4 for n = 2 to 3 for n +
+
m)
The values of i for the phenylpyridylmethanols do not correspond to simple solution or to solution and polymerization unless some sulfonation takes
Experimental
i-Factors.-The apparatus, technique and solvent for t h e cryoscopic measurements have been described previously .2,10 The temperature was maintained below 15' during an entire run. Preliminary experiments with triphenylmethanol as the solute were used to check the reproducibility of the apparatus. Reaction of the Phenylpyridylmethanol Solutions with Water.-A 0.25-g. sample of each diphenylpyridylmethanol was added with constant stirring to 18 g. (10 ml.) of 100% sulfuric acid. Solution was completed in approximately one minute. Each solution was poured onto 100 g. of cracked ice, neutralized with sodium hydroxide and extracted with ether. The extracts yielded 60 to 80% of the original methanols on evaporation and these showed no depression of the melting point when corresponding reactants and products were mixed. Sulfuric acid solutions of phenyl-2-pyridylmethanol and phenyl4pyridylmethanol were allowed to stand for 24 hours and then poured over cracked ice. After neutralization with sodium hydroxide, the solutions were extracted with ether which yielded no residue on evaporation. The solutions were then extracted with pyridine, evaporation of which gave solid polymeric materials which charred and sintered a t about 220' for the 2-pyridyl derivative and 230260" for the 2-pyridyl compound. Reaction of the Phenylpyridylmethanol Solutions with Methanol.-A 0.25-g. sample of each diphenylpyridylmethanol was added with constant stirring to 18 g. (10 ml.) of 100% sulfuric acid and this solution added cautiously with vigorous stirring to 100 ml. of cold absolute methanol. The mixture was then poured into 100 g. of cracked ice, neu(10) L. P. Hammett and A. 1. Deyrup, THISJOURNAL, 55, 1900 (1933); H.P.Treffers and L. P.Hammett, i b i d . , 59, 1708 (1937).
HILTONA. SMITH AND C.LIT. HOLLEY
cedure given by \Vilds" was followed. The melting poiiit of the product was 121-122". The literatureI2 value is 123-1250. Phenyl-2-pyridylmethanol Iyas obtained in 86c/;.yield b>-aluminum isopropoxide reduction of 2-benzoylpyridine nhich vas purchased from Reilly T a r and Chemical C O . The product melted a t 75.,5-76" which compares \Tit11 :L literature value of 76-78O.13 Diphenyl-4-pyridylmethanolw c i b obtxiiied coinnierci:tll~ f r i m Reill>- Tar and Chemical Co. Diphenyl-2-pyridylmethanolwas prepared in 73 '1, yields by the Grignard reaction of benzoyl-2-pyridine with phenylmagnesium broiiiide. The product melted u t 104.6--10R.(io which coniprtres to the literature value of 105".'4
4.0
3.0
-2
i-01. ti0
iy
2.0 1.0
800
,730 400 450 IVave length, nip
500
550
Fig 1.--;lbsorption spectra of sulfuric acid solutions of phenylpyridylmethanols: ,phenyl-4-pyridyl; - - - -, phenyl-2-ppridyl; - . - . -, diphenyl-4-pyridyl; . . , diphenyl-2-pyridyl. tralized with sodium hydroxide and evtracted mith ether. Evaporation of the solution gave a 30G>yield for the methyl ether of diphenyl-2-p~-ridylmethanolwhich melted a t 53.054.2". Anal. Calcd. for C1$HliSO: C , 82.37; H, 6.22. Found: C, 82.14; H, 6.00. Fur the methyl ether of diphenyl-4-pyridylmethanol the yield was 35y0 of a compound melting a t 93.4-94.3'. A m l . Calcd. for CISHli?;O: C, 52.87; H, 6.22. Found: C, 82.71; H, 5.97. Sulfuric acid solutions of the pheiiy1-2-pyrid!-l- and phenyl-4-pyridylmetlianols were allowed to stand and then poured into methanol and then over ice. The solutions were extracted with ether. An elastic material was observed between the ether and aqueous layers for preparations from both of the phenylpyridylinethanols. These materials were collected and dried under vacuum. They became liard and brittle, and their weights mere approximately the same as for the starting pyridylphenylniethanols. S o residue was obtained from the ether extracts. The product from the 2-pyridyl compound charred and sintered a t 720230" while t h a t from the 4-pyridy1 compound started to sinter a t 240". Titrations to Form Carbonium Ions.-Small samples (0.0002 mole) of each of the pyridyldiphenylmethanols were dissolved in 10 ml. of carbon tetrachloride in a 125-mI. erlenriieyer flask. T o this was added 25 ml. of water and with vigorous shaking the two-phase mixture was titrated with 100';; sulfuric acid until a faint but permanent color was observed. The temperature of the solution was maintained a t 22 f 2' throughout the titration by means of an ice-bath. From the volume of acid required, the percentage of acid necessary for visible carbonium ion formation in the aqueous layer was determined. Spectra.-The ultraviolet and near-visible spectra were obtained with a Beckman DU spectrophotometer and quartz cells 1 cm. thick. h hydrogen discharge lamp was used for the range 25@-350 mp; a tungsten filament lamp was tired for the range 330-600 nip. The spectra were taken as quickly as possible after solution of the pyridylphenylmethanols in sulfuric acid. The spectra are shown in Fig. 1. Preparation of Compounds. Phenyl-4-pyridylmethano1..!-Benzoylpyridine obtained commercially from Reilly Tar mid Chemical Co. was reduced with aluminum isopropoxide t o give the desired product in 927, yield. T h e general pro-
Discussion The results of this research support the expectation that substitution of a pyridine ring for a benzene ring in a phenyl-substituted methanol decreases the ease of formation of carbonium ions in sulfuric acid solution. For the two diphenylpyridylniethanols, the initial i-factor of approximately 3, the characteristic spectra shown in Fig. I, and the reaction of the sulfuric acid solution with water and methanol to form the original compound and the ether all indicate stable carbonium ion formation. The titration experiments show that the stabilities of these ions are less than for the triphenylcarbonium ion but greater than for the diphenylcarbonium ion. They also show that the 2pyridyldiphenlrlcarboniurn ion is less stable than the -2-I'yridyldiphenylcarboniuni ion. This might be expected since the protonated nitrogen atom in the former compound is closer t o the central carbon than in the 4-pyridyl derivative. The increase in the i-factor with time is apparently due to slow sulfonation of the benzene ring. The spectra of the sulfuric acid solutions of the two pheny-lpyridylmethanols give little if any indication of carbonium ion formation, for there is no major absorption band it1 the neighborhood of 1,50-300 nip. If simple solution and nitrogen protonation had occurred, an i-factor of 2 would ha\-e been anticipated. The values of 3.7-4 indicate that something more than simple solution and nitrogen protonation did occur, and the chemical experiments give evidence t h a t a process of poly merization took place. The spectra suggest that the polymer in solution was a methanol derivative rather than a carbonium ion. The constancy of the i-factors indicates that the polymers were formed rapidly, and that slow sulfonation of the benzene rings did not occur. KSOXVILLE, TESSES~;EE (11) .I. I,. \Vilds in "Urznnic Reactions," Vol. I, John Wiley and Sons, Inc., New York, 1;.Y . , 1942, p. 178. (1'2) X I . Kharasch, U. Schwartz, 31. Zimmerman and W. A-udeIibarg, J . Org. CIze?n., 1 8 , 1051 (19.53); M. R. Keselman and E. V. Brown, THISJ O U R N A L , 75, 46-19 (19.53). (13) H. Gilman and S . Spatz, J . O r g . Chent., 16, 1485 (1951). (14) A. E. Tschitschibahin and S. TV. Benewolenskaja, Be?'., 61, 551 (1928).
