Título del gráfico
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10. COMPROBACIÓN DE LA HIPÓTESIS
81
instruments.
Because of the overlapping melting ranges of � · 2 and � · 1 for OPP , OPS and OSP (Fig.
3-23 ) ,
it was impossible t o det ermine i f a single phase was present init ially and t rans formed during the course of the the rmogram or i f a mixture o f phases was produced by the t empering procedures.Howev er , estimat es of the heat of fusion of � · 2 of OPP and OPS were made on t he assumpt ion that only the � ·2 phase was present at the start of the thermogram (Table
3. 1 1 ) .
Since the 29.8°C form of OSP was well separate d from � · 2 and� · 1 ,
it c ould b e examined at high heat ing rat es without the complet e loss of resolut ion which occurs for overlapping forms. At a0 0
8
heat ing rate of
1 6
C/min , the 29.8 C endotherm accounted for ""07�
o f thet ot al heat of fusion of the phase ( s ) obtained by transformat ion of a
( 5 min) or crystallisat ion of the melt ( 1 5 min ) at the a melt ing point .
Thus , a reliable est imat e of the heat of fusion of this form could be
obtained (Table 3.1 1 ).
The heat s of fusion and melt inG points of the � · 1 forms obtained by t ransformat ion were significantly lower than those of the solvent crystallised forms. The differences were typically 5 - 1 5% for the heat of
0
fusi on and 1.5-3.5 C for the melt ing point . This disparity and the
correspondinG spectral variat i ons previously noted v1ere presumably due t o lncomplet e equilibrat ion o f the phases obtained by transformat ion.
A schemati c sumr.1ary of the polymorphism of OSS , OSP , OPS and OPP
is present ed in Fis. 3-24. The figure Gives phase assignments , melt ing
or t ransition points and transformat ion relat ionships. Correspondence
with the phases report ed in the literature is also indicat ed.
3- 3
3. 3 . 1
of Enant i omeric
of the of Enant i omeric and
Racemic
Three enanti omeric TGs were prepared, �-SSB , SSO and PPO , and the polymorphism of these glycerides was invest igat ed in relat i on to that of their racemate s.
(a )
Solvent FormsThe IR spectra of the solvent crystallise d forms of equivalent
enant iomeric and racemic TGs were essent ially ident ical (Fig. 3-25 ).
-1
All spectra sh01·1e d a symmet rical doublet near 720cm and the forms were
therefore assigned as � · phases. X-ray diffract i on patterns of the solvent crystallised forms are sho\vn in Fig.
3-26
and the c orresponding short spacings are list ed in Table 3.12. In contrast to the spectroscopicresults , t here were obvious di fferences between the X-ray pat t erns of
the racemat es and thei r antipodes . \Vhile the short spacings of the racemic TGs were dist inct , the short spacing patt erns of the enant i omers were diffuse and relat i vely fe\v spac ine;s v1ere resolved. Similar pat t erns were obtained for the stable forms prepared by t ransformation. Howe v er ,
0
all patt erns shO\ved st rong spac ings near 3.1:! and 4.2A, conf irming the
� · assignments made by IR spectroscopy.
( b )
ThermalRepresentat ive t herrnograms for sn- and �-SSB , SSO and PPO are present e d in the Appendix (?igs. 30 , 31 ; 40, 41 ; and 47 , L�8 respect ively).
The corresponding melt ing points und heats of fusion are summarised in Table 3 . 1 3 . In general , equivalent enant iomeric and ro.c emic TGs exhibited
similar polymorphic b ehaviour except that the a forms of the ant ipodes transform e d more rapidly than the a forms of the racemates. Thus , whi l e
the a form o f rac-SSB melted without transformation t o a higher form a t a
0 .
heating rat e of 4 C/mln , the a form of �-SSB t ransformed readily t o � · 2
and � · 1 under the same c ondit ions (Fig. 3-27). Similarly , at a heat ing rat e of 8°C/min the a form of rac-SSO melt e d without t ransformat ion to a higher form , but at the same rat e t he a. form o f sn-SSO t ransformed without melt ing (Fig. 3-27 ). Although the correspondinG thermogr��s for PPO are not present e d in Fig. 3-27 , the a f orms of rue- and �-PPO shO\ved anulor;ous
b ehaviour t o those of SSO exc ept that the t ransformat ion of a sn-PPO was even more ro.pid ( Appendix , Fig. 47).
\hth the except ion of the f3 •2 form of rac-PPO, for every polyr.10rph
of a rac emic TG there vms an e quivalent polymorph for the c orresponding
antipode. I·;elt ing point s and heut s of fusion of the equivalent poly
morphs were very similo.r although not ident ical (To.ble 3.1 3). For
exmnple , the melt ing point and heat of fusion of the solvent crystallised , ()
form of l:'ac-SSB were 51.b C and 131.5kJ/mol, while the correspondin�
values for the solvent crystullise d form o f sn-SSB were 51.8°C and
129.4kJ/mol respecti ve ly.
Because of the ro.pid transformat i on o f the enant i omeric glycerides, they \vere not invest igat ed by vo.riuble t emperature IR spectroscopy.