Functional groups as substituents

In methods C through G, oxadiazoles are formed from unstable N-acylamidoximes but the starting materials from which the unisolable intermediates are produced, are not amidoximes. Except for method C which is general, methods D, E, F, G lead to particular compoimds containing a functional group as substituent. 

Aliphatic compounds comprise hydrocarbons and their derivatives in which the molecular skeletons consist of tetrahedral carbon atoms connected by C-C single bonds. These tetrahedral carbon atoms can be arranged as chains, rings, or finite frameworks, and often with an array of functional groups as substituents on various sites. The alkanes CMH2n+2 and their derivatives are typical examples of aliphatic compounds. 

As a rule functional groups as substituents on the carbon atoms of trithiadi- and trithiatriazepines exhibit their typical reactivity, but it should be noted that these rings are unstable towards strong bases. Thus, demethylation of methyl 1,3,5,2,4,6-trithiatriazepine (17) was carried out by treatment with boron tribromide in dichloromethane or hydrochloric acid and gave the desired acid (95). 

Polymers with different structures and physical-mechanical properties have been manufactured in these ways. Using a wide range of monomers bearing alkyl, aryl or various functional groups as substituents, the physical-mechanical properties of the polymers could be varied for desired practical applications. Numerous other examples of polymerization reactions of various substituted and unsubstituted polycyclic olefins are found in the open and patent literature [164-169]. 

With the 4-, 6-" and 15- ° keto steroids the degree of epimerization at C-5 and C-14 depends on the presence of other functional groups and substituents. Exchange of the 16- " and 20- keto pregnanes yields the 17/5-epimer as the main product. 

In certain polyfunctional compounds, an aldehyde or ketone group can also be named as a substituent on a molecule with another functional group as its root. The aldehyde carbonyl is given the prefix formyl-, and the ketone group is named 0x0- with a number to show its position in the molecule. Compounds with both an aldehyde and ketone are named as aldehydes, because aldehydes have functional group priority over ketones. A ketone containing a benzene ring is named as a -phenone. 

Most commonly, correlations are made to chemical constants that are defined by the effect of substituents on a reference reaction. They are usually designated o and are applied to QSARs in the form of Hammett s equation or its various extensions. Alternatively, a descriptor can be a property of a substrate molecule that is available, is readily measurable, and/or can be calculated by independent means, such as octanol/water partition coefficients. Despite their numerous successful applications in QSAR studies, experimentally determined o constants also have some disadvantages. They are available only for a limited set of substituents and are not of very good quality for uncommon functional groups. As an alternative, the use of quantum-chemical parameters may be a solution. 

A ketone or aldehyde group can also be named as a substituent on a molecule with a higher priority functional group as its root. A ketone or aldehyde carbonyl is named by the prefix oxo- if it is included as part of the longest chain in the root name. When an aldehyde —CHO group is a substituent and not part of the longest chain, it is named by the prefix formyl. Carboxylic acids frequently contain ketone or aldehyde groups named as substituents. 

If more than one type of functional group is present, the one highest in the list is treated as the principal functional group (Table 3.5). Some groups can never be functional groups, only substituents, e.g., chloro-, nitro- (distinct from the very early literature where nitro, for example, was treated as a functional group). 

In all this, we must remember that what makes macromolccules special is, of course, their great size. This great size permits a certain complexity of structure, not just on the molecular level, but on a secondary level that involves the disposition of molecules with respect to each other. Are the molecules stretched out neatly alongside one another, or coiled up independently What forces act between different molecules What happens to a collection of giant molecules when it is heated, or cooled, or stretched As we shall see, the answers to questions like these are found ultimately in structure as we have known it the nature of functional groups and substituents, their sequence in the molecule, and their arrangement in space. 

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