Has anyone an idea how acidic the surface of normal phase silica is? I ask this question because we have noticed some skeletal isomerization of compounds (alkenes) when pumped (at room temperature) through a silica column. We also suspect that acetals may be formed from the dehidration of aldehydes when adsorbed (at room temperature) onto silica, or must we look somewhere else for the formation of these acetals?

Dear Anonymus;

The silica surface can be acidic or basic depending on the way it was prepared. In most cases the Si-OH group behave like weak acids and have a pKa of about 4.5. However, there are diffrent types of silanols present, and if I remember well some materials such as Zorbax are acidic (pH 3), and others such as Hypersil can be basic (pH 8).

Long ago I worked on some studies on the isomerization of terpenes. It was relatively easy to do it, but the reaction conditions were not those typically found in LC operations.

I hope this information helps;

Benjamin

alkenes very unlikely. aldehydes possible if very reactive. The catalyst literature is where you will find more on topic

Benjamin, thanks for the valuable information. You are a star !!! :-)
Bill, i will look into your suggestion...thanks