Rate and equilibrium constants are reported for the reactions of benzenethiol, p-nitrobenzenethiol, and thioacetic acid with acetaldehyde to form the corresponding hemithioacetals. The hydroxide ion catalyzed breakdown of these hemithioacetals occurs with rate constants near 1010 M-1 sec-1 and is largely or entirely limited by the rate of diffusion-controlled encounter of the substrate and catalyst. The energy of activation is 2-3 kcal/ mole and the solvent deuterium isotope effect koA-/koD-is 1.25. In the synthesis reaction, formation of the carbon-sulfur bond and proton transfer (eq 11) occur rapidly and separation of hydroxide ion from the hemithioacetal (eq 10) is the rate-determining step. Weaker bases catalyze the breakdown reaction at a slower rate with a Brpnsted slope p of 0.8 =t 0.03. This indicates that carbon-sulfur bond formation or cleavage and proton transfer are in some sense concerted. Equilibrium constants for hemithioacetal formation are independent of the acidity of the thiol over a range of acidity of lo7.he addition to acetaldehyde of the weakly acidic (3) R.

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Praseodymium and ytterbium chlorides were used as nuclear magnetic resonance shift reagents to resolve the inner and outer choline methyl resonances of single-walled dimyristoylphosphatidylcholine bilayer vesicles. The gel to liquid-crystalline phase transition of these vesicles was monitored by observing the proton and carbon-13 nuclear magnetic resonance line widths of the choline methyl group nuclei. In the absence of ions the transition occurred at 21.5 degrees C in both halves of the bilayer. With Pr3+ or Yb3+ added to the outside of the vesicles, the phase transition temperature of the outer half of the bilayer was raised several degrees, while the transition temperature of the inner half was unchanged. In vesicles containing 20 mol % cholesterol the phase transition of the outer monolayer was considerably broadened, while the inner half still melted sharply at 21.6 degrees C. By use of dipalmitoylphosphatidylcholine vesicles with UO2(2)+ added to the outside, phase transitions at 41.5 and 44 degrees C were detected by electron spin resonance with the spin-label 2,2,6,6-tetramethylpiperidinyl-l-oxy. These results imply that the two halves of the bilayer in phospholipid vesicles are so weakly coupled that they can undergo the gel to liquid-crystalline phase transition independently.