Carbonyl carbon-14 kinetic isotope effects and substituent effects on the relative reactivity and on the cis-trans product ratio were determined in the Wittig reaction of XCBH4CH0 with Ph3P= CHCsH4Y in THF at 0" C. The isotope effect and the Hammett p value were positive under both Li salt-free (12k/14k= 1.060 i 0.003 and px= 2.77 i 0.15) and Li salt-present (12k/" k= 1.015 i 0.004 and px= 1.38 i 0.12) conditions, although they were much larger in the ...