[Keiko]

1.Exactly what weight of dry ice is needed to react with 2 mmol of phenyl magnesium bromide?
2.In the synthesis of benzoic acid, benzene is often detected as an impurity.  How does this come about?
 
 3 . The benzoic acid could have been extracted from the ether layer using sodium bicaarbonate solution.  What equation shoes how this might be done and how the benzoic acid would be regenerated. Where could I get this. What practical reason makes this extraction method less desirable than sodium hydroxide extraction?

Please Help Anybodyy

[barcrphd]

1 mole of carbondioxide weighs 44 gms. you need 2 mmols. so the weight of dry ice needed can be calculated as 1/44 (multiplied by) 2 x 10^-3.

got it?

see the method for the synthesis of benzoic acid, then you will know from where the benzene impurity had come.

[netscape]

1.Exactly what weight of dry ice is needed to react with 2 mmol of phenyl magnesium bromide?
2.In the synthesis of benzoic acid, benzene is often detected as an impurity.  How does this come about?
 
 3 . The benzoic acid could have been extracted from the ether layer using sodium bicaarbonate solution.  What equation shoes how this might be done and how the benzoic acid would be regenerated. Where could I get this. What practical reason makes this extraction method less desirable than sodium hydroxide extraction?

Please Help Anybodyy

2.  Think about what you can add to phenyl magnesium bromide to convert it to benzene... (hint: that's the reason why Grignard reactions are usually run with dry glassware under dry atmosphere)

3.  Remember sodium bicarbonate is a base..  in the presence of a base, benzoic acid will be deprotonated:

     C6H5CO2H    +   NaHCO3    -------->      C6H5CO2- Na+    +    CO2 (gas)   +    H2O
   (benzoic acid)                                    (sodium benzoate)

     As a salt, sodium benzoate is now soluble in water and will go into the aqueous layer.

      I will let you think about how to regenerate benzoic acid from sodium benzoate.   

      Yes, sodium hydroxide can accomplish the same thing, since it's also a base:

        C6H5CO2H    +   NaOH    -------->      C6H5CO2- Na+     +    H2O
   (benzoic acid)                                    (sodium benzoate)

     Now compare the two equations and you will see why sodium hydroxide is more "desirable, " at least for
     practical reasons:  after sodium bicarbonate gets protonated, it decomposes rapidly into CO2 gas and water. 
     This gas evolution can sometimes be troublesome because you can build up a lot of pressure inside your   
     separatory funnel, especially in large scale (that's why you should always remember to vent you separatory 
     funnel  frequently!!)  In the case of sodium hydroxide, the only by-product is water.