Crystalline Structure, Melting Points

Melting properties of fats depend on the arrangement of the acyl residues in the crystal lattice in addition to other factors attributed solely to the structure of triglycerides. 

Calculations of the energy content of the carbon chain conformation have revealed that at room temperature 75% of the C-C bonds of a saturated fatty acid are present in a fully staggered zigzag or trans conformation and only 25% in the energetically slightly less favorable skew conformation. 

The unsaturated fatty acids, because of their double bonds, are not free to rotate and hence have rigid kinks along the carbon chain skeleton. However, a molecule is less bent by a trans than by 

The corresponding elaidic acid, with a transconfiguration, has a slightly shortened C-chain, but is still similar to the linear form of stearic acid  

The extent of molecular crumpling is also increased by an increase in the number of cis double bonds. Thus, the four cis double bonds in arachi-donic acid increase the deviation from a straight line to 165°  

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This chapter deals with the very important a-amino acids, the building blocks of the proteins that are necessary for the function and structure of living ceils. Enzymes, the highly specific biochemical catalysts are proteins. or-Amino acids are dipolar ions (zwitterions), RCH(N" H,)COO , as is indicated by their crystallinity, high melting point, and solubility in water rather than in nonpolar solvents. The standard (naturally occurring) amino acids are listed in Table 21-1 those marked with an asterisk are essential amino acids that cannot be synthesized in the body and so must be in the diet. They have 1° NHj s except for proline and hydroxyproline (2°). They have different R groups. 

C, 2.60 A to H, 2.32 A). The f (ohexyl)2NLi] amide has been isolated as a colorless crystalline material (melting point 209-211°C), but twinning has so far prevented solution of the structure (157). An unsolvated (AsLi)3 ring trimer [ (Me3Si)2CH]2AsLi 3 has also been reported (158). However, in contrast to the (NLi)3 species (55)-(57), the ring is not planar and deviates from threefold symmetry. One of the... 

Multiple Melting Points A compound may have different crystal structures (i.e., solid phases). For example, carbon tetrachloride has three known solid phases at atmospheric pressure la (face-centered cubic), lb (rhombohedral), and II (monoclinic). Ia and lb melt at temperatures some 5K apart [3]. Multiple melting points have been reported for a large set of compounds, such as many of those listed in the Merck Index [4], Dearden and Rahman improved a structure-melting point correlation for substituted anilines by excluding two outliers on the ground that their Tm values were inadequate, due to different crystalline forms [5]. 

The advantage of sPP over PP is that impact strength and tensile modulus of sPP are significantly higher. While PP has a planar zigzag helical structure, the sPP has a three-dimensional one that leads to lower crystallinity and melting point r (PP) = 165 vs. r ,(sPP) = 133°C. 

At low temperatures, about -78°C, in triisobutylaluminum/ water-initiated polymerization, which is presumed to be a cationic initiation, an amorphous (elastomeric) polymer is obtained from cis-2,3-epoxybutane, and a crystalline polymer, melting point 100°C, is obtained from the trans-isomer. In a copolymerization of the two isomers, the cis-oxide enters the copolymer at about twice the rate of the trans-isomer. Further, the low-temperature, cationic poly(trans-2,3-epoxybutane) with a crystalline melting point of 100°C was found to consist of diad units with a mesodiisotactic structure, while the crystalline polymer formed by coordinate polymerization of the cis-monomer, melting point 162°C, had diad units that were racemic diisotactic. These results make apparent the importance of the monomer coordination step in polymer chain growth in coordinate polymerizations. 

The refractory industry has found chromite useful for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stability of crystalline structure. 

In the case of crystalline polymers such as types E and F the situation is somewhat more complicated. There is some change in modulus around the which decreases with increasing crystallinity and a catastrophic change around the. Furthermore there are many polymers that soften progressively between the Tg and the due to the wide melting range of the crystalline structures, and the value determined for the softening point can depend very considerably on the test method used. 

The two structures appear very similar. Poly( 1,2-propylene adipate) has the same basic structure as poly(ethylene adipate), except for a pendant methyl group. This pendant methyl group on the poly( 1,2-propylene adipate) makes a large difference, however. Poly( 1,2-propylene adipate) has no crystalline melting point. Trappe theorizes that the pendant methyl prevents chain packing and therefore, prevents crystallization [42]. 

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