Harvard University Library (HUL)

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Harvard University Library (HUL)
It was founded in 1638. It has stock nearly 17 million volumes.

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What Is Melting Points

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Most organic compounds have melting points below 200 ºC. Some decompose before melting, a few sublime, but a majority undergo repeated melting and crystallization without any change in molecular structure. When a pure crystalline compound is heated, or a liquid cooled, the change in sample temperature with time is roughly uniform. However, if the solid melts, or the liquid freezes, a discontinuity occurs and the temperature of the sample remains constant until the phase change is complete. This behavior is shown in the diagram on the right, with the green segment representing the solid phase, light blue the liquid, and red the temperature invariant liquid/solid equilibrium. For a given compound, this temperature represents its melting point (or freezing point), and is a reproducible constant as long as the external pressure does not change. The length of the horizontal portion depends on the size of the sample, since a quantity of heat proportional to the heat of fusion must be...
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What Is Atomic And Molecular Orbitals

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A more detailed model of covalent bonding requires a consideration of valence shell atomic orbitals. For second period elements such as carbon, nitrogen and oxygen, these orbitals have been designated 2 s , 2p x , 2p y   & 2p z . The spatial distribution of electrons occupying each of these orbitals is shown in the diagram below.   Very nice displays of orbitals may be found at the following sites: J. Gutow, Univ. Wisconsin Oshkosh R. Spinney, Ohio State  M. Winter, Sheffield University The valence shell electron configuration of carbon is 2 s 2 , 2p x 1 , 2p y 1  & 2p z 0 . If this were the configuration used in covalent bonding, carbon would only be able to form two bonds. In this case, the valence shell would have six electrons- two shy of an octet. However, the tetrahedral structures of methane and carbon tetrachloride demonstrate that carbon can form four equivalent bonds, leading to the desired octet. In order to explain this covalent bonding,...
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Uses OF Alkynes In Nature And Medicine

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Alkynes in nature and medicine According to Ferdinand Bohlmann, the first naturally occurring acetylenic compound, dehydromatricaria ester, was isolated from an Artemisia  species in 1826 . Polyynes, a subset of this class of natural products, have been isolated from a wide variety of plant species, cultures of higher fungi, bacteria, marine sponges, and corals.  Some acids like  tariric acid  contains an alkyne group. Diynes and triynes, species with the linkage RC ≡ C-C ≡ CR' and RC ≡ C-C ≡ C-C ≡ CR' respectively, occur in certain plants ( Ichthyothere , Chrysanthemum ,  Cicuta ,  Oenanthe  and other members of the  Asteraceae  and  Apiaceae  families).  Some examples are  cicutoxin , oenanthotoxin ,  falcarinol  and  carotatoxin . These compounds are highly bioactive, e.g. as  nematocides .  1-Phenylhepta-1,3,5-triyne is illustrative of a naturally occurring triyne. Alkynes occ...
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