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UNIT-1-P BLOCK ELEMENTS
Q:-Define Boranes& its Generic formula?
Ans: –In chemistry, boranes comprise a large group of compounds with the generic formula of BxHy. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The class is named after the parent chemical called “borane” itself, chemical formula BH3. This compound is only known to exist as a transient intermediate since it dimerises to form diborane, B2H6. The larger boranes all consist of boron clusters that are polyhedral. In addition to the charge-neutral boranes, a large number of anionic boron hydrides are known. The most important boranes are diborane B2H6 and two of its pyrolysis products, pentaborane B5H9 and decaborane B10H14. The development of the chemistry of boron hydrides led to new experimental techniques and theoretical concepts. Boron hydrides have been studied as potential fuels, for rockets and for automotive uses, but the only commercial applications involve derivatives of borane.
Generic formulaof boranes
The four series of single-cluster boranes have the following general formulae, where “n” is the number of boron atoms:
|closo-||BnH2−n||No neutral BnHn+2boranes are known|
|hypho-||BnHn+8||only adducts established|
Nomenclature:Boranes are named as follows:
a) The Latin prefixes mono-, di-, tri-, etc. are used before “borane” to indicate the number of boron atoms in the compound.
b) Immediately following the “e” in “borane” the number of hydrogen atoms is placed in parentheses using Arabic numerals.
Example: B5H11 is pentaborane (11).
Boranes fall into 6 structure categories, the 3 most important of which are:
a)closo – BnHn2-
b)nido – BnHn+4
c)arachno – BnHn+6
These are prefixes that can be given to the borane designation above. However, they are not necessary because each structure type has only one general formula, thus the formula gives the structure.
The boron atoms will form a shape by occupying vertices on a polyhedron (e.g. for 4 B atoms the polyhedron can be a tetrahedron), whether or not the boron atoms are actually bound to each other. This arrangement is frequently called a “cage.” Closo- refers to the most symmetrical possible arrangement, a closed polyhedron (e.g. 6 B atoms form an octahedron, see figure below). Nido- comes from the next higher closo- polyhedron with the most highly connected boron vertex removed (e.g. for 5 B atoms, an octahedron less any one vertex to yield a square pyramid). Arachno- comes form the next higher nido- structure with the most highly connected boron vertex on the open face removed (e.g. for 4 B atoms, a square pyramid less any basal vertex to yield a butterfly structure). The words closo-, nido-, and arachno- are derived from words meaning “closed” (Greek), “nest” (Latin), and “spider’s web” (Greek). A large number of examples are shown on p. 799 of Huheey, Keiter, and Keiter.
Examples: B4H42- is closo and the 4 B’s lie on the 4 vertices of a tetrahedron.
B4H8 is nido and the 4 B’s lie on 2 equatorial and 2 axial positions of a trigonalbipyramid.
B4H10 is arachno and the 4 B’s lie on an octahedron with adjacent vertices removed (one edge).