[Mona lizza]

Why we say that grignard reagent is less reactive but more selective as compare to organolithium compound??

[Vidya]

In Grignard Reagents , there is Mg-C bond and in organolithium Compounds, bond is between C-Li.Check the difference in the Electronegativity of C with Mg and with Li.More polar bond is more reactive and less selective because of fast reaction.

[Mona lizza]

I have red that order of reactivity of grignard reagent is RMgI is greater compare to RMgF and grignard reagent become less reactive when length of R increases reason????and why grignard didn't prepared RCaX?

[sjb]

..and why grignard didn't prepared RCaX?

See e.g. http://www.scs.illinois.edu/denmark/wp-content/uploads/gp/2011/gm-2011-3-15.pdf or J. Org. Chem., 1990, 55 (17), pp 5045–5051 (DOI: 10.1021/jo00304a016)

[Vidya]

I have red that order of reactivity of grignard reagent is RMgI is greater compare to RMgF and grignard reagent become less reactive when length of R increases reason????and why grignard didn't prepared RCaX?

Now compare the bond length of R Mg I and RMgF ...check which bond is easy to break?
R is non polar part  and in RMgX ..R- is working like a nucleophile and more bulky nucleophile are less reactive.

[Babcock_Hall]

The reactivity selectivity principle is not always observed to hold empirically.