Suggest a reason for why the phosphorus-containing side product from the horner-emmons wittig reaction is more water-soluble than the triphenylphosphine oxide side product obtained from the standard wittig reaction.

Co-product (rather than side product) of Horner-Emmons Wittig reaction is alpha-hydroxy phosphonate which by definition is hydrophilic as opposed to triphenylphosphine oxide. In addition, the side groups in Horner-Wittig reagents are typically Me or Et which are less bulky than phenyls are more compatible with hydrophilicity.