Why is direct iodination of alkanes not possible?

The molecular formula of an alkane having a weight of $11 \ g$ and a volume of $5.6 \ L$ at $STP$ is.......

Number of bromo derivatives obtained on treating ethane with excess of $Br_2$ in diffused sunlight is $..........$.

The major product in the following reaction is:

In the bromination of isobutane,the relative reactivity rates of $1^oH, 2^oH,$ and $3^oH$ are $1 : 82 : 1600$ respectively. Calculate the percentage yield of products $(a)$ and $(b)$ in the following reaction:
$CH_3-CH(CH_3)-CH_3 (excess) + Br_2 \xrightarrow{hv} (a) CH_3-C(Br)(CH_3)-CH_3 + (b) CH_3-CH(CH_3)-CH_2Br$

Which reagent is used to convert halides,alcohols,carbonyl compounds,and acids into alkanes?