Acetonitrile solutions cleave acetals and some ethers. Thioacetals are the sulfur analogues of acetals. These do not form as readily as hemiacetals and acetals. Acetals and ketals are not reduced by LiBHEt. Ytterbium(III) chloride is a powerful catalyst for the formation of acetals using trimethyl orthoformate. Orthoester products are generally favored over ketene acetals. Hemiacetals and acetals are essentially tetrahedral intermediates. Surrogates of cyclopropanone include the acetals. An inherent problem with mCPBA is its inability to oxidize silyl ketene acetals. An example of arylsulfonyl acetals as carbohydrate-protective groups are phenylsulfonylethylidene acetals. With diols, including many sugars, benzaldehyde condenses to form benzylidene acetals. Aluminium hydride can be used in the reduction of acetals to half protected diols. It also rapidly forms an acetals in the presence of alcohols. Acetals and ketals are excellent substrates for additions of allyl- and vinylsilanes. Polyvinyl acetals are prepared by treating PVA with aldehydes.