Replacement of hydrogen by other elements

Replacement of hydrogen by other elements and functional groups and introduction of unsaturation... 

REPLACEMENT OF HYDROGEN BY OTHER ELEMENTS AND FUNCTIONAL GROUPS, AND INTRODUCTION OF UNSATURATION... 

In Section 23.1, you learned that hydrogen atoms bonded to carhon atoms in hydrocarbons can be replaced by halogen atoms. Many other kinds of atoms or groups of atoms also can bond to carbon in the place of hydrogen atoms. In addition to structural variations, replacement of hydrogen by other elements is a reason that such a wide variety of organic compounds is possible. 

The experiences which found expression in Dumas substitution theory led to a second type of names. The sensational observation of hydrogen replaced by other elements without fundamental change of type was immediately visualized by Dumas (19) as having a bearing on nomenclature. Lavoisier s binary nomenclature was now no longer sufficient. Dumas writes ... 

All three elements are susceptible to ipso replacement by electrophiles - such reactions have been studied extensively for arylsilanes and arylstannanes, where they occur via an electrophilic addition/silicon elimination mechanism analogous to other aromatic substitutions, but at a much faster rate than the corresponding replacement of hydrogen." Ipso substitutions also take place on heterocycles and, in the case of electron-rich systems, probably via the same type of mechanism. 

Up to now you have probably (and rightly) assumed that isotopes of an element are chemically identical. They differ only in the number of neutrons in their nuclei chemistry generally depends on charge, orbitals, and electrons. It may come as a surprise to find that this is not quite true. Isotopes may differ chemically, because some chemical properties do depend on atomic mass. However, this difference is only significant for hydrogen—no other element has one isotope twice as massive as another Kinetic isotope effects are the changes in rate observed when a ( H) hydrogen atom is replaced by a ( H) deuterium atom in the same reaction. For any reaction, the kinetic isotope effect (KIE) is defined as... 

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