Question

Why is it necessary to react the sodium hydroxide in a greater concentration then the concentration of crystal violet?

Answer 1

R-Cl     + NaOH ---------------------> R -OH + NaCl

(violet)    

mechanism : it is first order reaction infact.

rate determianing step is carbocation formation

R -Cl ------------------------> R+   + Cl-

The reaction order with respect to R⁺ will be determined first. The concentration of sodium hydroxide in the reaction mixture will be much greater than that of the crystal violet. It is so much larger, that even after all of the crystal violet has reacted, the sodium hydroxide concentration has not changed very much.

[OH-] >>>[R+]

so rate depends on [R+]

rate = k [R+]