Question

. Even if glass is oven-dried, the SiO2 still has surface OH groups. Is this a concern in our Grignard reaction? Given that the surface density of hydroxyl groups of glass is about 4 OH groups/nm2, how much would your yield of Grignard reagent be reduced by surface OH groups, presuming complete reaction with these groups? Show your calculation. Assume that the glass surface area in contact with your solution is 5 cm2.

Answer 1

5 cm2 has 5e+14nm2 . So 5 cm2 will have 4 x 5e+14 = 20e+14 OH- groups which will react with 1:1 ratio with grignard reagent. So number of grignard molecule 20e+14 .

The use of other solid materials as catalyst supports is also common in Ziegler-Natta catalysis. Generally these materials include simple binary metal oxides, but more complex mixtures or materials have been used. Because of the inherent hydrolytic and oxidative instability of the polymerization sites, solid supports must be adequately pretreated to remove surface functional groups that might poison catalytic active sites. Grignard reagents have been used to chemically clean or modify the hydroxylic surfaces of a variety of supports, such as magnesia, silica, or alumina. Typically, the reactions are carried out in an ether solvent, as illustrated for the surface hydroxyl group of silica gel. SiO2{OH}+RMgCl - SiO2{OMgCl}+RH The chemically dehydroxylated silica is then heated to drive off the ether residues. The reaction medium may be altered to allow the deposition of an excess of the Grignard reagent beyond the stoichiometry required for dehydroxylation; the excess reacting with titanium compounds in the fashion described in the previous section. An additional scheme allows the pacification of the residues of the Grignard reagent with the various halo compounds described earlier, to remove the remaining magnesium alkyl groups and prevent overreduction of the transition metal center. The addition of organic reactants containing protic or other functional groups has also been used to moderate the reducing power of

Moles of grignard = 20 x 1014 6.022 x 1023 = 3.21 x 10-9