$A$. What are silicones? State the uses of silicones.
$B$. What are boranes? Give chemical equation for the preparation of diborane.

(N/A) . Silicones are a group of organosilicon polymers,which have $(R_{2}SiO)$ as a repeating unit. The starting materials for the manufacture of silicones are alkyl or aryl substituted silicon chlorides,$R_{n}SiCl_{(4-n)}$,where $R$ is an alkyl or aryl group.
When methyl chloride reacts with silicon in the presence of copper as a catalyst at a temperature of $570 \ K$,various types of methyl substituted chlorosilanes of formula $MeSiCl_{3}$,$Me_{2}SiCl_{2}$,and $Me_{3}SiCl$ with a small amount of $Me_{4}Si$ are formed.
Hydrolysis of dimethyl dichlorosilane,$(CH_{3})_{2}SiCl_{2}$,followed by condensation polymerization yields straight-chain polymers.
Uses: They are used as sealants,greases,and electrical insulators,and for the waterproofing of fabrics. Being biocompatible,they are also used in surgical and cosmetic implants.
$B$. Boranes are the binary compounds of boron and hydrogen,similar to alkanes. These are covalent hydrides with the formula $B_{2}H_{6}$,called diborane.
Preparation of diborane: It is prepared by treating boron trifluoride with $LiAlH_{4}$ in diethyl ether.
$4BF_{3} + 3LiAlH_{4} \rightarrow 2B_{2}H_{6} + 3LiF + 3AlF_{3}$
$B_{2}H_{6}$ is produced on an industrial scale by the reaction of $BF_{3}$ with $NaH$ (sodium hydride):
$2BF_{3} + 6NaH \xrightarrow{450 \ K} B_{2}H_{6} + 6NaF$