NATURAL WAXES 329 



acetate, C27H45O • OCCH3, is readily prepared by the interaction of choles- 

 terol and acetic anhj^dride; it crystallizes in plates or needles from an 

 alcohol-ether solution and melts at 115°C. Cholesterol palmitate, C27H45O • 

 OC(CH2)i4CH;j, can be synthesized by heating one mole of cholesterol with 

 two moles of palmitic acid for 3 hours at 200 °C. This ester is present in 

 blood serum, skin, adrenal glands, and wool fat. The stearic acid ester, 

 C27H450-OC(CH2)ir,CH3, is not found in blood semm, but it does occur in 

 the adrenal glands. Cholesteryl stearate can be prepared in a manner 

 analogous to that used for the palmitate. It can be crystallized from an 

 ether-alcohol mixture in the form of white platelets. Cholesteryl oleate, 

 C27H450-OC(CH2)7CH:CH(CH2)7CH:i, is another common ester. It is 

 present in blood serum, wool fat, and the adrenal glands. The synthesis is 

 identical with that of the palmitate and stearate. The oleate ester sepa- 

 rates from solution in the form of long thin needles which are readily soluble 

 in ether, chloroform, and hot acetone, but only difficultly soluble in hot 

 alcohol. 



On heating, the cholesteryl esters exhibit the unusual property of chang- 

 ing to a turbid liquid state before melting to a clear liquid at a higher 

 temperature. Reinitzer'-^ observed this phenomenon in 1888 in the case 

 of cholesteryl benzoate; it has since been shown to be a general property of 

 the natural cholesteryl esters, as well as of the synthetic ones. The turbid 

 liquid state has properties actually intermediate between a solid and a 

 liquid phase. Lehmann^^** referred to this as the "liquid crystalline" state, 

 which is a misnomer, in that there is actually no crystalline phase. The 

 more appropriate name, mesomorphic state, was proposed by Friedel^^^ 

 and is in current usage. 



Two types of the mesomorphic phase are now recognized, i.e., smectic and 

 ncmatic. In the first case, the liquid does not exhibit a normal flow, but 

 rather gliding movements in one plane. In the nematic tj^pe of meso- 

 morphism, the liquid flows readih^ l)ut it has a thread-like stmcture and a 

 comparatively low viscosity. A third type of mesomorphism is likewise 

 demonstrable in some cholesteryl esters; this is kno^vn as the cholesteric 

 phase. This is quite similar to the nematic phase, but differs in having a 

 definite layering, and in showing distinct color effects in polarized light. 

 The molecular layers appear to be thicker than in the smectic phase. 



The cholesteryl esters from the formate to the myristate can assume the 

 cholesteric form, although the esters from the caprylate to the myristate 

 usually appear first in the smectic modification, and then change to the 

 cholesteric phase on further heating, before melting to the clear liquid 

 state. These changes are re\'ersible and occur in the opposite order on 

 cooling. The cholesterjd esters having fatty acid chains longer than those 



"8 O. Lehmann, Z. phijsik. Chem., 4, 462-472 (1889). 

 1" G. Friedel, Ann. phijs. [9], IS, 273-474 (1922). 



