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p-CYANO SUBSTITUTED CINNAMIC ACID ESTERS
F. Jones, Jr, J. Ratto
To cite this version:
F. Jones, Jr, J. Ratto. p-CYANO SUBSTITUTED CINNAMIC ACID ESTERS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-413-C1-418. �10.1051/jphyscol:1975168�. �jpa-00216247�
JOURNAL DE PHYSIQUE Colloque C1, suppliment au no 3, Tome 36, Mars 1975, page C1-413
Classification Physics Abstracts
7.130
p-CYAN0 SUBSTITUTED CINNAMIC ACID ESTERS
F. B. JONES, Jr. and J. J. RATTO
Science Center, Rockwell International, Thousand Oaks, California 91360, USA
RBsnrnB. - L2 synthke d'un nombre d'esters d'acide cinnamique contenant un cyano-groupe terminal a etB r6alis6e. Certains exemples de ces coinposes sont caracterisks par des tempkratures de transition mksomorphe relativement basses et des domaines de temperature mhomorphe inter- mediaires. Plusieurs m5langes de ces esters cinnamzte ont kt6 prBparBs, montrant de larges domaines de tempbraturs nkmztique. Des mbthodes pour la prkparation de ces composes, la classification de leurs temperatures m6somorphes et les domaines de melanges appropries sont discutks.
Abstract. - A numbsr of cinnamic acid esters containing a terminal cyano group have been synthesized. Some examples of these compounds are characterized by relatively low mesomorphic transition temperatures and intermediate mesomorphic temperature ranges. Several mixtures of these cinnamate esters have been prepared which show broad nematic temperature ranges. Methods for the preparation of these compounds, the tabulation of their mesomorphic temperatures and the ranges of appropriate mixtures are discussed.
1. Introduction. - Most of the current research in liquid crystalline materials is being generated because of the need in display applications for the development of broad temperature range nematogens with increased stability toward atmospheric contaminants [I-41.
Moreover, it is quite desirable that these materials possess large dielectric anisotropies [5]. In an earlier publication we reported on the liquid crystalline pro- perties of some dialkyl, alkyl-alkoxy and alkyl-acyloxy trans-cinnamic acid esters [6]. Surprisingly, several of these compounds were found to possess positive dielectric anisotropies. The dielectric anisotropies (BE = E,, - E ~ ) of -f 4.0 and + 0.1, respectively, for 4-butyl, and 4-butylcarbonyloxy, 4'-butylcinnamates indicate the overriding effect of dipolar forces operating along the major axis of the molecules.
A high positive value of the dielectric anisotropy is a necessary requirement for low voltage threshold for twisted nematic device applications [7]. Therefore, we reasoned that placing a
CN group, with a group moment of 4.05 D, at R or R'
positions of formulae (1) would have the effect of substantially increasing the dielectric anisotropy. We therefore chose to investigate a number of 4-cyano- phenyl, 4'-n-alkyl-cinnamates and 4-n-alkylphenyl, 4'-cyanocinnamates synthesized by the scheme given below :
Scheme
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975168
C1-414 F. B. JONES, JR. AND J. J. RATTO 2. Experimental. - All melting points are correct-
ed. Infra-red spectra were obtained on a Perkin-Elmer 421 spectrophotometer. The number of phase transi- tions were analyzed on a calibrated Fisher 300 QDTA.
Transition temperatures were obtained using a Wild polarizing microscope in conjunction with a Mettler (FP 5 and FP 52) hot and cold stage apparatus. The enthalpies of transition (AH) were determined by a Perkin-Elmer DSC- 1 B.
2.1 PREPARATION OF 4-SUBSTITUTED PHENOLS. -
Typical synthetic procedures for the preparation of phenols are given below.
4-Hydroxyheptanophenone. The procedure is similar to that followed by Close, Tiffany and Spielman [8].
Aluminum chloride (26.7 g, 0.2' mole) was added slowly to a stirred and cooled (0 OC) solution of phenol (9.4 g, 0.1 mole) in 100 ml of carbon disulfide.
The mixture was stirred for thirty minutes and then heptanoyl chloride (15 g, 0.1 mole) was added drop- wise at a slow rate. Hydrogen chloride gas was evolved and the mixture was allowed to come to room tempe- rature while stirring was continued overnight. The mixture was poured into ice and concentrated hydro- chloric acid, the layers separated, and the water layer extracted with diethyl ether. The organic layers were combined, washed with water and dried over magne- sium sulfate. Removal of the solvent and distillation under vacuum gave 20 g of the desired 4-ketophenol.
The product was shown by proton nuclear magnetic resonance and infra-red spectroscopy to be the desired compound.
The physical data for three of the phenols are given in Table I.
Physical data for the compounds
Yield M. P. B. P .
- R (%) (OC) (OC, mm)
- - -
0
nC,H, ,--C- II 96 61 167 (1.0 mm)
nC6H13-C- II 92 90 171 (1.0 mm)
nC,H, ,-C- II 90 59 183 (1.0 mm) 4-n-heptylphenol. Following a procedure similar to that adopted by Read and Wood [9], a mixture of heptanoylphenol (20.6 g, 0.1 mole), 7.02 g of amal- gamated mossy zinc, 60 ml of water, 60 ml of concen- trated hydrochloric acid, and 25 ml of 95 % ethanol were stirred vigorously at reflux temperature until reduction was complete (12-18 hours). The reduction
was followed by infra-red spectroscopy by observing the disappearance of the carbonyl absorption at 1675 cm-l. Toluene (100 ml) was added to the reaction mixture, and stirring was continued an addi- tional 15 min. The toluene layer was separated, washed with water and dried over magnesium sulfate. Distil- lation under reduced pressure afforded 16.7 g of the desired product. The results for the three phenols are given in Table 11.
Physical data for the compounds
Yield M.P. B. P.
- R (%I - ("C) - (OC, mm) -
C 6 H ~ 3 - 83 112 (1.0mm)
C7H15- 87 123 (1 .O mm)
csH17- 80 42 134 (1 .O mm)
2.2 GENERAL METHODS FOR THE PREPARATION OF 4-ALKYLBENZALDEHYDES. - Acylation of alkyl- benzenes. The method used is that of Mowry, Reno11 and Huber [lo]. To a suspension of aluminum chloride (1.2 mole) in 400 ml of carbon disulfide cooled in an ice bath, acetyl chloride (I .0 mole) was added drop- wise over 5-10 min. To this mixture, the alkylbenzene was added dropwise over a period of one hour while maintaining the temperature below 5 OC. The mixture was stirred over night at room temperature and poured into ice and concentrated hydrochloric acid. The orga- nic layer was removed and the water layer extracted with diethyl ether. The organic layers were combined, washed with water, and dried over magnesium sulfate.
Removal of solvent and vacuum distillation gave the 4-alkylacetophenones listed in Table 111.
TABLE 111
Physical data for 4-substituted acetophenones
R ' Yield (%) B. P . (OC, mm)
- - -
n-C,H,
,-
85 92- 95 (0.3 mm)n-C6H13- 83 128-130 (0.5 mm)
n-C7H,,-- 89 144-147 (0.5 mm)
n-C8HI7- 73 160-1 62 (0.5 mm)
Preparation of 4-alkylbenzoic acids. Using the pro- cedure of Johnson, Gutsche and Offenhauer [ll], a solution of sodium hypobromite was prepared by dissolving sodium hydroxide (0.8 mole) and bromine (0.4 mole) in 400 ml of water while maintaining the temperature below 10 OC. The sodium hypobromite solution was added to a stirred mixture of the
p-CYAN0 SUBSTITUTED CINNAMIC ACID ESTERS C1-415 4-alkylacetophenone(0.1 mole) in 300 ml of p-dioxane.
During the addition process the reaction temperature is allowed to slowly rise to 35-40 OC. After stirring for 30 min, the suspension of sodium salt was treated with enough sodium bisulfite to destroy excess hypo- bromite. Two liters of water were added, and 200 ml of liquid was boiled off. Cooling, acidification and filtering gave the 4-alkylbenzoic acids. The acids (see Table IV) were recrystallized several times from a 75/25 mixture of ethanollwater.
Physical data for 4-alkylbenzoic acids
R ' Yield ( %) M. P . (OC) -
n-C5H,,- 9 1 88
n-C6H!,- 89 97
n-C7Hi5- 88 101.5
n-C8H17- 85 100
2.3 PREPARATION OF ACID CHLORIDE AND REDUCTION T o THE ALDEHYDE. - Acid chloride. The acid chlorides were prepared by the standard technique described in Vogel [12]. Thionyl chloride (0.15 mole) and the 4-alkylbenzoic acids (0.1 mole) were combined with 75 ml of benzene. The solution was heated until the evolution of hydrogen chloride and sulfur dioxide ceased. The excess thionyl chloride was distilled and the alkylbenzoyl chlorides were distilled at reduced pres- sure. The yieIds in most cases were quantitative.
Rosenmund reduction. The procedure is similar to that given in Vogel [13]. This is the Rosenmund reduction using a palladium-barium sulfate catalyst.
The alkylbenzoyl chloride (0.1 mole), 2 g of palladium catalyst, 0.2 ml of quinoline-sulfur poison, and 100 ml of sodium dried p-xylene were combined. The air was displaced with hydrogen gas, and the mixture was heated at 140-150 OC while stirring vigorously. The course of the reaction was followed by hydrogen chloride evolution monitored with damp pH paper at
Physical data for 4-alkylbenzaldehydes
R' Yield ( %) B. P. (OC, mm)
- - -
n-pentyl 88 107-112 (0.5 mm)
n-hexyl 78 110-1 15 (0.3 mm)
n-heptyl 74 120-127 (0.4 mm)
n-octyl 84.5 1 1 5-120 (0.4 mm)
the exit port. At the end of the gas evolution, the mixture was cooled, filtered and distilled under vacuum to give the desired 4-alkyl-benzaldehydes which are listed in Table V.
In each case the precursor aldehydes, and phenols, whose syntheses are described above, were utilized to prepare cyanocinnamate 1 according to procedures given in a previous publication [6].
3. Results and discussion. - The melting points and mesomorphic transition temperatures for the various 4,4'-disubstituted cinnamates are given in Table VI.
Compounds 1-7 have melting points that lie between 40 and 75 OC, while compounds 8-11 have somewhat higher melting points. All compounds have very broad nematic temperature ranges and clearing points over
100 OC.
Transition temperatures (7 for
trans-4-cyanocinnamic acid esters
(") C = crystal ; N = nematic ; I = isotropic liquid.
(b) Taken from reference [6].
There are two factors evident when comparing the transition temperatures of these compounds : (1) Lower N -t I transition temperatures for cinnamates 1-5 (Series 1) exist as opposed to higher N -+ I transition temperatures for cinnamates 8-11 (Series 2) and (2) the N -+ I transition temperatures increase as the number of electronegative atoms are increased (ex. 4, 6 and 7).
In the first case, it seems likely that the higher N + I transition temperatures for cinnamates 8-11 as opposed to 1-5 are due to increases in the anisotropy of polariza- bility along the major axis of the molecules. Thus when R' represents the CN group (1-5), there is some CN n-overlap with the phenolate z-system but consi- derably less than when CN becomes part of the extended n-system of the styrenecarbonyl moiety (8-11).
C1-416 F. B. JONES, JR. AND J. J. RATTO This effect is further exemplified when comparing the
N -+ I transition temperature of cinnamate 12 with that of 1 or 8. With cinnamate 12 there is a further reduction in the amount of n-overlap at the para position, hence a reduction in molecular polarization.
For an alkyl group, as opposed to CN, this leads to a decrease in the anisotropy of polarizability along the major axis, resulting in a lower value of induced polari- zation and concomitantly a lower N -, I transition.
In the second case, the increase in the N -, I transi- tion temperatures with increasing number of electro- negative atoms, most likely can be traced to a diffe- rence in polarization forces and dipole moments between alkyl, alkoxy and acyloxy groups. Conse- quently, the polarizabilities of the three groups may be ordered as follows : C,H15C0, > C7H150 > C7H15.
Hence the N + I transition temperatures increase as their group polarizabilities. Moreover, an addi- tional increase is caused by contributions due to ordered differences in dipole moments ( p ) of alkyl, alkoxy and acyloxy groups. For example, the dipole moments ( p ) of the three groups and their respective angles, (8) subtended from the CAr bond, are methyl, 0.37 D, 00 ; methoxy, 1.28 D, 720 ; acetoxy, 1.83 D, 1000 [14]. Hence, the dipole moments increase as the number of oxygen atoms increase. This difference is reflected in the corresponding increase in N -, I tran- sition temperatures of cinnamates 4, 6 and 7.
Figure 1 shows a plot of the transition temperatures against the number of carbon atoms in the alkyl chain for the series of 4'-cyanophenyl-4-n-alkylcinnamates 1-5. The N -+ I transition temperatures for the alkyl
N W l l E R O F CARBON ATOMS I N ~ H p n + , ~ C H = C H C 0 2 - @ C N
FIG. 1. - Plot of transition temperatures vs. the No. of C atoms for 4'-cyanophenyl, 4-n-alkylcinnamates.
compounds show an alternating effect with those for the even homologues lying below the odd homologues.
Figure 2 shows a plot of transition temperatures against
FIG. 2. - Plot of transition temperatures vs. the No. of C atoms for 4'-n-alkyl-phenyl, 4-cyanocinnamates.
the number of carbon atoms in the alkyl chain for the series of 4'-n-alkylphenyl-4-cyanocinnamates (8-11).
The N + I transition temperatures for these com- pounds also show an alternating effect but one that is less pronounced than for compounds 1-5. The general decrease in the nematic thermal stability of (1-5) can be attributed to a decrease in axial polarizability as each successive methylene unit is added. The anisotropy of polarizability may be used to explain the alternation of N -+ I transition temperatures, since each successive methylene unit makes a different contribution to the axial polarizability of the molecule, which is dependent upon an odd-even extension of the carbon chain [ S ] . The higher average nematic stability for series 2 should be expected because the CN group appended to the phenylester moiety will lead to a significant change in the polarizability of the molecule, as discussed earlier.
The melting points and the N + I transition tempe- ratures of series 1 are in every case lower than that of the corresponding compounds of series 2. In both cases there is a general decrease in melting points with an increase in the homologous series.
In Table VII results are shown for the enthalpies of melting where the A H values are given for the most stable transitions. These values were determined from data collected on the differential scanning calorimeter (Perkin Elmer-1 B). The enthalpies for the C -+ N transitions range from 5-6 kcal mol-I with the excep- tion of cinnamates 5 and 8 which respectively have higher and lower enthalpies of melting.
p-CYAN0 SUBSTITUTED CINNAMIC ACID ESTERS C1-417
TABLE VII Enthalpies of melting for
R R' A H (kcal mol- l)
- - -
I
1 n-C4H9 CN 5.0
2 n-C5Hl, CN 5.5
3 n-C6H13 CN 5.3
4 n-C7HiS CN 6.1
5 n-C,H1, CN 7.7
6 n-C7H150 CN 6.2
predict the compositions of eutectic mixtures chosen from the same family. The expression (1)
relates the mole fractions (xi) of pure components to the melting point (T) of the mixture where AHi is the heat of fusion from the crystalline solid to the nematic phase. Temperature (Ti) is the melting point tempera- tures of pure components and R is the gas constant.
Simultaneous solution of' N equations using expres- sion (2)
N
7 n-C7H1,C02 CN 5.9
8 CN n-C4Ng 4.2 permits the calculation of x i and Tcorresponding to the
9 CN n - C ~ H 1 ~ 5.1 eutectic mixture. The N 4 I transition temperature
10 CN n - C 6 H ~ 3 5.2 TCN-,) of the mixture was determined by utilizing
11 CN n - w - b ~ 5.1 expression (3), where T(,-,), are the N + I transition temperatures of each component in the mixture
As is evident from Table VI none of the compounds N
listed are room temperature nematogens. However, T(N+I) = C ~i T ( N - + ~ ) ~ .
i (3)
these compounds can be utilized as additives to
expand the useful nematic temperature range of multi- These three expressions provided us with a method to component systems. Toward such an end we have predict the eutectic composition of several multi- employed the Schroder-van Laar equations [15] to component systems.
TABLE VIII
Nematic temperature ranges for mixtures of several cinnamic acid ester nitriles
Mixture
No. Components
- -
Predicted Actual
Predicted Nematic Temperature
mole % Range (NTR) OC
- - -
(@)Shows polymorphism after storage at -20 OC for several days.
C1-418 F. B. JONES, JR. AND J. J. RATTO Data for the mole fractions, predicted nematic tem-
perature ranges and experimental mixtures prepared according to the predicted compositions are given in Table VIII. Several of these mixtures exhibited poly- morphism which eventually produced higher melting points when kept at temperatures considerably below the melting point. Where polymorphism occurs, the figures in parentheses refer to the most stable form.
The electrical properties of several of these mixtures were measured. The low frequency dielectric constants at 25 OC for mixture C (Table VIII) are clI = 13.8, zl = 6.3, A& = 7.5 and mixture D are c,, = 15.8, zl = 7.5, A c = 8.3. The resistivity for both samples
was typically 10'' Q-cm at 100 Hz. The unusually low values of B e for both samples was surprising when compared to that of dibutylcinnamate 12, since it was expected that the CN-to-C4H, substitution would substantially enhance the anisotropy of polarizability along the major axis, at least to the extent of that found for cyano-biphenyls [5], -Schiff bases [7] and -benzoyl- oxybenzoates U]. Apparently the CN appended at the para position of cinnamic acid esters makes only a small contribution in affecting resultant polarization and dipolar forces within these compounds.
A 12 pm layer of mixture D which responded to the twisted nematic electro-optical effect had a threshold voltage of 2 Vrms and a turn-on time of 0.4 s.
References
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