DE2306739A1 - LIQUID CRYSTALLINE ESTER - Google Patents

Description

The invention relates to new compounds of the general formula

COO

wherein R straight-chain alkyl with 4 to 8 carbon atoms or straight-chain alkoxy with 5 to Carbon atoms means processes for their production and their use as a dielectric in electro-optical devices.

The compounds according to the invention are in the liquid-crystalline state

3 098 35/1201

Unii /.- Ί. 1 ι · '(' ■ '

a positive anisotropy of the dielectric constant (£ „> £ ^, where £ _{/ f is} the dielectric constant along the longitudinal axis of the molecule and £, the dielectric constant perpendicular to it).

In an electric field, the nematic liquid crystals according to the invention orientate themselves (because of S ^> E ± ) with the direction of their greatest dielectric constant, ie with their longitudinal axes parallel to the field direction. This effect is used, inter alia, in the interaction between embedded molecules and the liquid-crystalline molecules (guest-host interaction) described by JH Heilmeier and LA Zanoni [Applied Physics Letters 1 ^, 91 (1968)]. Another interesting application of dielectric field orientation is in the rotary cell found by M. Schadt and W. Helfrich [Applied Physics Letters 18, 127 (1971)].

This electro-optical rotary cell is essentially a capacitor with transparent electrodes, whose dielectric is formed by a nematic substance with £ //> 6j_. Molecular longitudinal things of the liquid In the field-free state, crystals are arranged helically between the capacitor plates, the helical structure is determined by the given wall orientation of the molecules. After applying an electrical voltage to the capacitor plates, the molecules position themselves with their longitudinal axes Field-correct (i.e. perpendicular to the plate surface), whereby linearly polarized light in the dielectric is no longer rotated (the liquid crystal becomes perpendicular to the surface of the Uniaxial plates). This effect is reversible and can be used to increase the optical transparency of the capacitor to be controlled electrically.

H ta

In such a light spinning cell, it is desirable to use compounds which have a low melting point

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and have a low viscosity. It has now surprisingly been found that the compounds according to the invention of general formula I have liquid-crystalline properties that meet these requirements. You don't just assign that required large positive anisotropy of the dielectric constant, but are individually or in the form of their mixtures liquid-crystalline with each other or with other nematic or non-nematic substances at a relatively low temperature and show a low viscosity. Operation electro-optical Accordingly, low voltage devices are possible and responsiveness is shorter. An advantage of the invention Compounds compared to the compounds previously used for this purpose is their much greater stability, the allows a much better handling of the substances. Another advantage of the compounds according to the invention is that that they form colorless, milky-white nematic phases.

The compounds of the formula I are preferably used in the form of their mixtures with one another or with other nematic or non-nematic substances are used. Binary or ternary mixtures can be formed. Particularly preferred are mixtures whose composition corresponds to a eutectic.

Mixtures of pn-heptylbenzoic acid-p'-cyanophenyl ester with pn-butylbenzoic acid-p'-cyanophenyl ester or pn-pentylbenzoic acid-p'-cyanophenyl ester in a molar ratio of 2: 1; of pn-octylbenzoic acid-p ^{1 are very particularly preferred} -cyamophenyl ester with pn-butylbenzoic acid p'-cyanophenyl ester or pn-hexylbenzoic acid p'-cyanophenyl ester in a molar ratio of 2: 1 or of pn-hexylbenzoic acid p -eyanophenyl ester with pn-butylbenzoic acid p'-cyanophenyl ester a molar ratio of 2: 1.

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2308739

The compounds of general formula I can be prepared by

a) a compound of the general formula

COX II

wherein R is straight-chain alkyl with 4 to 8 carbon atoms or straight-chain alkoxy with 5 to 8 carbon atoms and X is a leaving group,
reacts with p-hydroxybenzonitrile, or

b) a compound of the general formula

COO // NS CH = MOH III

wherein R has the meaning given above, dehydrated, or

c) a compound of the general formula -

• R —Ρ Λ COO -J / Λ_ Y IV

wherein R is as defined above and Y is halogen or arylsulfonyloxy

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Reacts with copper (I) cyanide or sodium cyanide, or

d) a compound of the general formula

COO

O-HH,

wherein R has the meaning given above, dehydrated, or

e) a compound of the general formula

COO

wherein R has the meaning given above, is diazotized and reacted with copper (I) cyanide.

The expression "leaving group ¹ " denotes halogen, lower alkoxy (preferably methoxy or ethoxy), lower alicanoyloxy (preferably formyloxy or acetoxy), aryl lower s-alkox3 '(preferably benzyloxy), aryl lower alkanoyl oxy (v ^ xza ^ sweise Benzoyloxy), lower alkylaulfonyloxy (pre-. I,>;/; iiiiie mesyloxy) and aryl sulfonyl oxy (preferably tosyloxy).

In one embodiment of the method according to the invention 3098 35/120 1

2306738

a compound of the general formula II with p-hydroxy- benzonitrile implemented. This reaction is useful in one inert organic solvents such as an ether, like diethyl ether or tetrahydrofuran, or dimethylformamide, Benzene, toluene, cyclohexane or carbon tetrachloride carried out.

In the compound of the formula II, --X is preferably for halogen, especially chlorine. In order to bind the hydrogen halide released during the reaction, it is expedient to use an acid binder. Suitable acid binders are tertiary amines, pyridines, etc. Preferably, the acid binder is im large excess is used so that it can serve both as a solvent and as an acid binder at the same time. temperature and pressure are not critical aspects in this reaction and generally atmospheric pressure and temperatures will be used brought between room temperature and the boiling point of the reaction mixture used.

The compound of the formula II in which X is chlorine can be prepared by reacting the corresponding benzoic acid with thionyl chloride getting produced. However, it is by no means necessary for this compound to react with p-hydroxybenzonitrile is isolated from the reaction mixture.

In a further embodiment of the invention In the process, a compound of the general formula III is dehydrated. The dehydration is expediently carried out by means of Acetic anhydride or with anhydrous sodium acetate in acetic acid. The reaction temperature is the reflux temperature of the reaction mixture see. The pressure is not critical and the reaction is advantageously carried out at atmospheric pressure «

The compound of formula III can be prepared by reacting a compound of formula II with p-hydroxybenzaldehyde

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reacts, and the ester obtained reacts with hydroxylamine. The compound of formula III obtained must not necessarily be isolated from the reaction mixture.

In a further embodiment of the invention In the process, a compound of the general formula IV is reacted with copper (I) cyanide or sodium cyanide. This reaction is conveniently in an inert organic solvent such as ethylene glycol, tetrahydrofuran, dimethylformamide, Dimethyl sulfoxide, pyridine or acetonitrile carried out. Temperature and pressure are not critical aspects in this reaction. Conveniently, atmospheric pressure and a Temperature between room temperature and the boiling point of the reaction mixture applied.

In the compound of the formula IV, X is preferably halogen, in particular bromine. This connection can be established, for example by reacting a compound of the general formula II with p-bromophenol and it does not necessarily have to be be isolated from the reaction mixture.

In a further embodiment of the process according to the invention, a compound of the general formula V is dehydrated. Dehydration can be done by boiling with acetic anhydride in glacial acetic acid or by exposure to Pp ⁰ S * SOCl ₂ , etc. This reaction is preferably carried out without a solvent or else in a solvent, for example in benzene, dimethylformamide, pyridine, ethylene dichloride, etc. The reaction temperature is advantageously the reflux temperature of the reaction mixture or the boiling point of the product. The reaction is carried out at atmospheric pressure or reduced pressure.

The compound of the formula V can be produced 309835/1201

by combining a compound of formula II with p-hydroxybenzoic acid converts, the product is converted into the acid chloride and from it with NE, which produces amide »The compound of the formula I does not necessarily have to be isolated from the reaction mixture.

In a further embodiment of the method according to the invention, a compound of the general formula VI is assumed. It is zweckmässigerweiee diazotized and subjected to Sandmeyer reaction ,, The diazotization is carried out in water at a temperature of 0-5 ⁰ C with the addition of hydrochloric acid and sodium nitrate "The resulting diazonium

is then subjected to Sandmeyer reaction subjected This reaction is zweckraässig means of a solution of _ζ β Β "Copper (I) cyanide or a complex salt thereof carried out the reaction is conducted at a temperature between ⁰ ° C-IOO ⁰ Cs, preferably between 70-80 ⁰ C. ■

The pressure is not critical and the reaction ifird with Advantage carried out at atmospheric pressure

The compound of the formula VI can be prepared by reacting a compound of the formula II with IWBensylidene-p-hydroxyaniline and subsequent cleavage of the ship "see base with aqueous Mineral acid are produced.

The physical properties of the invention nematic substances I xierden on the basis of the following Table illustrates

R. A. n-butyl B. n-pentyl C. n-hexyl D. n-heptyl E. n-octyl

Sap ο KIp. *) 67 ⁰ C (Al ₉ 5 ° C) *) 6O _s 5 ^e C (56.5 ° C) 45 ⁰ C 48 ⁰ C 44 ⁰ C 56.5 ⁰ C 46 _P 5 ° C 53.5 ⁰ C -

η-pentyloxy 85-85.5 ° C 2306739 F. n-hexyloxy 71 ° C (76.5-77 ° C) *) G n-heptyloxy 71.5 ° C 82 ° C H n-octyloxy 75-74 ° C 8O ⁰ C I. Clearing points 83 ⁰ C *) monotropic

The compounds according to the invention are preferably used in the form of their mixtures with one another, mixtures, · which correspond to a eutectic are particularly preferred.

Some exemplary compositions are summarized in the following table:

composition

67 % D H A. 67 % D H B. 67 % E H A. 67 % E H C. 67 % C H A. r 33% h 33% η 33% i- 33% η 33%

M.p.

24 ° -26 ° C

30 ° -3O.5 ° C

22 ° -24 ° C,

22 ° -23 ° C

22 ° -23 ° C

50 ⁰ C

56 ⁰ C

48.5 ⁰ C

51 ⁰ C

45 ° -45.5 ° C

The compounds according to the invention can also be in the form their mixtures with other nematic or non-nematic substances, such as Schiff's bases the general formula

CH = N

VII

wherein R ¹ straight-chain alkyl with 2 to 8 carbon atoms, straight-chain alkoxy with k to 7 carbon atoms

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- IG -

—Τ

atoms or straight-chain alkanoyloxy with Means 2 to 8 carbon atoms.

The compounds of the formula VII in which R ^f denotes straight-chain alkyl having 2 to 8 carbon atoms are new and can, for example, by conventional reaction of a compound of the formula

CHO VIII

wherein R 'is straight-chain alkyl having 2 to 8 carbon atoms,
with p-hydroxybenzonitrile.

The preparation of the substances of the formula I according to the invention and the substances of the formula VII is on Hand illustrates the following examples. All temperatures are given in degrees Celsius.

example 1

5> 6 g (28.5 mmol) of pn-butylbenzoic acid chloride are dissolved in 12 ml of abs. Benzene to 3.1 g (26 mmol) of p-hydroxybenzonitrile in 25 ml of abs. Given pyridine. You can overnight at room temperature and stir for 2 hours then at 50 ⁰ C.

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230673S

After cooling, it is extracted with ether and the extract is washed with dilute hydrochloric acid and water. Finally, 9 g of crude pn-butylbenzoic acid ~ p '«cyanophenyl ester are obtained. This crude product is dissolved in toluene and chromatographed on 250 g of silica gel. From the pure fractions in the thin layer, 4 g of product are obtained after recrystallization from hexane. Mp. 67 ⁰ C, clearing point monotropic 41.5 ⁰ C. HMR and IR spectra correspond to the expected structure,

The starting product can be produced as follows:

50 g of pn-butylbenzoic acid are dissolved in 150 ml of thionyl chloride and refluxed for 1 hour. Then first the excess thionyl chloride "at normal pressure, the product is removed and finally distilled under high vacuum. Sdp. 1QO ^east -1Ö5 ° C / 2-5 mm.

Example 2

3.1 g (26 mmol) of p-hydroxybenzonitrile are dissolved in 40 ml of abs. Pyridine dissolved. Then 6 g (28.5 mmol) of pn-pentylbenzoic acid chloride in 20 ml of benzene are added dropwise at room temperature and then stirred overnight. After a short period of heating, it is worked up as in Example 1 and 7.5 g of crude ester are obtained. This is chromatographed on silica gel using toluene / acetone 19: 1. Is obtained from a single fraction after recrystallization from hexane 2.6 g of p-pentylbenzoic-p'-cyanophenyl with sines · melting point of 60.5 ⁰ C and a monotropic clearing point of 56.5 ⁰ C,

The starting product can be produced as follows:

15 g of pn-pentylbenzoic acids are dissolved in 100 ml of thionyl chloride and mzic for 1 hour :? Rilckflaaa cooked. Then the excess thionyl chloride is removed by AMes'Gilli-sren

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and the acid chloride is distilled in a high vacuum. Sdp «104 ° / 2 mm *

Example ? ■ - '"■ ..". -. ■

1.4 g (11.75 fflMol) p-hydroxybene nitrile in g5 ml abs. Pyridine and 2.9 g (13 adlol) p = n-He3cylb © asoesäureö! Uorid in

12 ml abs. Benzene are mixed _? about power b @ i tur. and then geätfilgrt for 2 hours at 50 ⁰ C "- The cooled reaction mixture is extracted on iiro & eser ansgtggoesea and then with ether. It is washed with dilute hydrochloric acid and dilute sodium hydroxide solution and then with water, MS, to give a neutral reaction. After drying and concentration-, "the solution 3.7 g of crude product are obtained which güerat with toluene / acetone 19: 1 chromatographed on silica gel Man _β © rhält schliesslich.1,4 g pn Hexylbengoesäur © -p ^t '-cj © ffl.ophe23yle @ t @ f ⁱ | Smp _e 45 ° 0 »EIp. 48 ⁰ C.

The starting product can be manufactured as follows?

9.4 g of p-n-hexylbeasoic acid and 50 ml of thionyl chloride are mixed and heated to 80® for 1 hour. Daas distilled one first the excess thionyl chloride at -liormaldruck and then the p-n-hexylbeazolic acid chloride am.Hochi§kuU! B · Sdp. 110 ° / 0.05 mm. , ·

■ - Example_4. -

6.6 g (28 mmol) of p ^ n-Heptylbsnzo ^ falis ^ eehlgiiid 'are dissolved in 25 ml of a "bs. Benzene - wiü. Zii 3 g (25 ÄolKp ^ Hyfeöi ^ benzonitr in 50 ml of abs. Pyridine © lug® dropwise _o Reektiöasgsmisch is ütiffiaöltft at 5O ⁰ C and stirred overnight at Zimmertempsratiar ä & wx aoela. 2 ". After the Abkühlen- is EISM @ ser zmg®geb® & isiä DAS

IS3S / 120D,

Product extracted with ether. The extract is washed with dilute hydrochloric acid, dilute sodium hydroxide solution and water and finally 7.6 g of crude pn-heptylbenzoic acid p'-cyanophenyl ester are obtained. This is chromatographed on 300 g of silica gel and eluted with hexane / ether 4: 1. After recrystallization from hexane 6.0 g of product with mp. 44 ⁰ G and ELP are. Received 56.5 ^0C.

The starting product can be produced as follows:

15 g of pn-heptylbenzoic acid and 100 ml of thionyl chloride are refluxed until the evolution of gas has ended. The excess thionyl chloride is then distilled off at normal pressure and finally the pn-heptylbenzoic acid chloride is distilled off in a high vacuum. 13.2 g, bp. L65 ⁰ -l67 / 5 mm ° C.

Example 5

To 3 g (25 mmol) of p-hydroxybenzonitrile in 50 ml of abs. Pyridine are 7 g (ί-7.5 mmol) of pn-octylbenzoic acid chlorine id in 25 ml of abs. Benzene is added dropwise, then the mixture is ^{stirred at room temperature and at 50 °} C. for 2 hours overnight. The reaction mixture is poured onto ice water and the product is isolated as in Example 4. 8.7 g of crude ester are chromatographed on silica gel with benzene. After recrystallization from hexane 46.5 ⁰ C and cl.p. obtained 5.2 g of pure p-octyl benzoic acid-p'-cyanophenyl with mp.. 53.5 ⁰ C.

The starting product can be produced as follows:

20 g of p-n-octylbenzoic acid and 100 ml of thionyl chloride are refluxed for 1 hour. The excess thionyl chloride is distilled off at normal pressure, and the residue is added one with abs. Toluene and concentrated in vacuo. The raw one

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p-n-Octylbenzoic acid chloride can be used without further purification will.

Example 6

4.5 g (37 mmol) of p-hydroxybenzaldehyde are dissolved in 50 ml of pyridine. Are added dropwise at room temperature, 8 g (40.5 mmol) of p-Butylbenzoesäurechlorid in 25 ml of benzene, the mixture is stirred overnight at room temperature and liber ansohliessend further 2 stood at 50 ⁰ C. The mixture is then poured onto ice water and extracted with ether. The extract is washed with water, dilute hydrochloric acid and water and finally, after drying and concentrating the solution, 10.2 g of crude product and, after recrystallization from ether / hexane, 8.2 g of crystalline ester are obtained. 8 g (27 »5 mmol) of this ester are refluxed for 16 hours with 2.85 g (^ 1 mmol) of hydroxylamine hydrochloride and 4.5 g (55 mmol) of anhydrous sodium acetate in 100 ml of glacial acetic acid. Butylbenzoic acid p'-oximinophenyl ester is formed and this is then dehydrated. It is allowed to cool, most of the glacial acetic acid is distilled off in vacuo, water is added and the product is extracted with ether. 7 * 0 g of crude product are obtained from the extracts washed with water and dried with sodium sulfate. This is chromatographed on silica gel with benzene. After recrystallization from hexane, pn-butylbenzoic acid p'-cyanophenyl ester, which corresponds completely to that obtained in Example 1, is obtained from uniform fractions.

Example 7

3 * 7 ß (9 * 5 mmol) of pn-octylbenzoic acid-p'-bromophenyl ester are refluxed with 1.1 g (12.5 mmol) of CU ¹ CN in 5 ml of dimethylformamide. Then the reaction mixture

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poured onto 2 ml of ethylenediamine and 6 ml of water while Shaken vigorously for 5 minutes and then extracted twice with benzene. The organic extracts are used again Ethylenediamine in water and then washed several times with water. 1.9 S crystallized crude product still contain little starting material, which by chromatography with toluene is removed on silica gel. The p-n-octylbenzoic acid p'-cyanophenyl ester obtained by recrystallization with Melting point 46.5 ° C and KIp, 54 ° C is identical to that in Example 5 obtained product.

The starting material can be produced as follows!

Ί, 5 6 (26 mmol) p-Broniphe.nyl are abs in 50 ml. Pyridine dissolved and with 7 g (28 mmol) of pn-octylbenzoic acid chloride in 25 ml of abs. Benzene added. The mixture is then stirred overnight at room temperature, then poured into ice water and extracted with ether. The organic phase is washed with dilute hydrochloric acid, dilute sodium hydroxide solution and water, dried with sodium sulfate and concentrated. 8.4 g of practically pure pn-octylbenzoic acid p ¹ -bromophenyl ester are obtained in a thin layer,

Example 8

2 g (16.5 mmol) of p-hydroxybenzonitrile are dissolved in 5 ml of abs. Pyridine dissolved. A solution of 4 g (16.5 mmol) of pn-hexyloxybenzoic acid chloride in 2 ml of abs is added at room temperature. Benzene. The mixture is initially left to stir overnight at room temperature, then taken up in ether and washed successively with dilute hydrochloric acid , dilute sodium hydroxide solution and distilled water. After drying and concentrating, 9 g of crude product are recrystallized twice from ethanol. 4.3 g of pn-hexyloxybeneic acid ~ p ^s -oyanophenyl ester are obtained

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M.p. 71 ° and Klp. 82 °.

The acid chloride required for the reaction is made from p-hexyloxybenzoic acid by boiling with thionyl chloride and Distillation obtained at 130 ° / 0.1 mm.

Example 9

A mixture of 5 »» 9 g (0.05 mol) of p-aminobenzonitrile and 6.7 g (0.05 mol) of p-ethylbenzaldehyde is gassed with nitrogen in 100 ml of benzene after addition of 150 mg of p-toluenesulfonic acid heated under reflux for 1 hour (bath temperature 120 °). The resulting water is separated off with a water separator. For a further hour, the benzene condensing in the reflux condenser is then returned to the reaction vessel through a layer of 50 g of aluminum oxide (Act. I). After cooling, 2 g of solid potassium carbonate are added, the mixture is filtered and the solvent is removed from the piltrate in vacuo at a bath temperature of 50 °. There remain 11 * 5 g of yellow oil, which crystallizes on cooling. For purification, recrystallization is carried out several times from i-propanol until the melting point is constant and the by-products in the gas chromatogram have disappeared. The pure p - [(p-ethylbenzylidene) amino] benzonitrile melts at 76.2-77 * 0 ° and is liquid crystalline on cooling from 63.0-59.7 °.
UV (EtOH): £ ₂₇₇ = 25800 (shoulder at 316 nm). NMR, MS, IR and microanalysis correspond to the substance.

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Claims (1)

Claims
1. Process for the production of compounds of the general common formula COO where R is straight-chain alkyl with 4 to θ carbon atoms or straight-chain alkoxy with 5 to carbon atoms,
characterized in that one a) a compound of the general formula COX II wherein R has the meaning given above and X is a leaving group, reacts with p-hydroxybenzonitrile, or b) a compound of the general formula r \ COO CH = NOH III wherein R has the meaning given above, dehydrated, or c) a compound of the general formula 309835/1201 COO wherein R has the meaning given above and Y is halogen or arylsulfonyloxy,
with copper (I) cyanide or sodium cyanide, or d) a compound of the general formula COO CO-NH, wherein R has the meaning given above, dehydrated, or e) a compound of the general formula R_ COO wherein R has the meaning given above, diazotized and reacted with copper (I) cyanide. 2. The method-according to claim la, Ib, ld or le, thereby characterized in that a compound of the formula I in which R is straight-chain alkyl having 4 to 8 carbon atoms, n-hexyloxy or is n-heptyloxy. 309835/1201 3. Process according to Claim 2, characterized in that p-n-butylbenzoic acid p'-cyanophenyl ester is prepared. 4. The method according to claim j3, characterized in that that one p-n-butylbenzoic acid chloride with p-hydroxybenzonitrile converts or p-n-butylbenzoic acid p'-oximinophenyl ester dehydrated. 5. The method according to claim 2, characterized in that p-n-pentylbenzoic acid p'-cyanophenyl ester is prepared. 6. The method according to claim 5 *, characterized in that that one p-n-pentylbenzoic acid chloride with p-hydroxybenzonitrile implements. 7. The method according to claim 2, characterized in that p-n-hexylbenzoic acid p-cyanophenyl ester is prepared. 8. The method according to claim 7 »characterized in that p-n-hexylbenzoic acid chloride with p-hydroxybenzonitrile implements. 9. The method according to claim 2, characterized in that that p-n-heptylbenzoic acid p'-cyanophenyl ester is prepared. 10. The method according to claim 9 » characterized in that pn-heptylbenzoic acid chloride is reacted with p-hydroxybenzonitrile. 11. The method according to claim 2, characterized in that p-n-octylbenzoic acid p'-cyanophenyl ester is prepared. 12. The method according to claim 11, characterized in that p-n-octylbenzoic acid chloride with p-hydroxybenzonitrile 309835/1201 or p-n-octy! benzoic acid -ρ'-bromophenyl ester with Copper (I) cyanide converts. 13. The method according to claim 1, characterized in that pn-pentyloxybenzoic acid p ¹ -cyanophenyl ester is prepared, 1 ^. Method according to claim 13 * characterized in that that p-n-pentyioxybenzoic acid chloride with p-hydroxy * - benzonitrll implements. 309835/1201 15. Nematic mixture for electro-optical purposes, characterized in that it consists of a mixture of compounds of the formula I according to claim 1 with one another and / or with other nematic substances and / or with non-nematic substances. 16. Nematic mixture for electro-optical purposes, characterized in that it consists of a mixture of compounds of the formula I in which R is straight-chain alkyl having 4 to 4 carbon atoms, n-hexyloxy or n-heptyloxy with one another and / or with other nematic substances and / or with non-nematic substances. 17 · Nematic mixture according to claim 16, characterized in that it consists of p-n-heptylbenzoic acid p'-cyanophenyl ester and p-n-butylbenzoic acid p'-cyanophenyl ester. 18. Nematic mixture according to claim 16, characterized in that it consists of p-n-heptylbenzoic acid p'-cyanophenyl ester and p-n-pentylbenzoic acid p'-cyanophenyl ester. 19. Nematic mixture according to claim 16, characterized in that that it is made from p-n-octylbenzoic acid p'-cyanophenyl ester and p-n-butylbenzoic acid p'-cyanophenyl ester. 20. Nematic mixture according to claim 16, characterized in that that it is made from p-n-octylbenzoic acid p'-cyanophenyl ester and p-n-hexylbenzoic acid p'-oyanophenyl ester. 21. Nematic mixture according to claim 16, characterized in that it consists of p-n-hexylbenzoic acid p'-cyanophenyl ester and p-n-butylbenzoic acid p'-cyanopheny! ester. 309835/1201 22. Use of a according to one of claims 1, 13 and 14 produced connection in a dielectric for electro-optical purposes, 23. Use of one according to one of claims 2 to 12 produced connection in a dielectric for electro-optical Purposes. 24. Use of a according to claim 1, 1 "5 and 14 produced Connection in a rotating cell. 25. Use of a compound produced according to one of claims 2 to 12 in a rotary cell. 26. Use of a mixture according to claim 15 as a dielectric for electro-optical purposes. 27. Use of a mixture according to one of claims 16 to 21 as a dielectric for electro-optical purposes. 28. Use of a mixture according to claim 15 in a rotating cell. 29. Use of a mixture according to one of claims 16 to 21 in a rotating cell. 309835/1201 30. Compound of the general formula COO wherein R is straight-chain alkyl of 4 to 8 carbon atoms or straight-chain alkoxy with 5 to carbon atoms. 31. Compound of the general formula COO where R is straight-chain alkyl having 4 to 8 carbon atoms, n-hexyloxy or n-heptyloxy, 32. p-n-Butylbenzoic acid p ¹ -cyanophenyl ester. 33. pn-Pentylbenzoic acid p ¹ -cyanophenyl ester. 34. p-n-hexylbenzoic acid p ¹ -cyanophenyl ester. 35. pn-heptylbenzoic acid p ^f -cyanophenyl ester. 36. p-n-Octylbenzoic acid p'-cyanophenyl ester. 37 _# pn-pentyloxybenzoic acid p ¹ -cyanophenyl ester. 38. pn-Hexy1oxybenzoic acid p'-cyanophenyl ester. 309835/1201 39. pn-heptyloxybenzoic acid p ⁵ -eyanophenyl ester. 40. pn-octyloxybenzoic acid p ^s -oyanophenyl ester. 309835/120