one example of this is the cheletropic reactions of 1,3-dienes with sulfur dioxide. chemists have carefully looked at the kinetics of this reaction. in 1976, isaacs and laila measured the kenetic factors for the addition of sulfur dioxide to butadiene derivatives. rates of addition were monitored in benzene at 30 °c with an initial twentyfold excess of diene. they used a spectrophotometer to study light at 320 nm to measure the disappearance of so2. the reaction showed "pseudo first-order kinetics". the chemists found that electron-withdrawing groups on the diene decreased the rate of reaction. also, the reaction rate was affected considerably by steric effects of 2-substituents, with more bulky groups increasing the rate of reaction. (in other words, the larger the group of atoms hanging from the second carbon atom, the quicker the reaction ran.) the authors attribute this to the tendency of bulky groups to favor the cisoid conformation of the diene which is essential to the reaction (see table below). in addition, the rates at four temperatures were measured for seven of the dienes. from these measurements, chemists used the arrhenius equation to calculate the enthalpy of activation (δh‡) and entropy of activation (δs‡) for each reaction. this was one of the first important efforts to study the kenetics of a cheletropic reaction.