Many examples are known of differences between comparable P and N compounds, as in the much easier oxidation of phosphines than amines and in the much greater stability of phosphine oxides than amine oxides. Frequently, the paths followed in a reaction with a given reagent can be quite different;thus, with OH , quaternary phosphonium salts undergo cleavage of a C-P bond to form tertiary phosphine oxides ( chapter 4.C ) whereas ammonium salts undergo Hofmann degradation to give tertiary amines. Other differences will be noted here, but there are indeed some useful comparisons that can be made. These are found in the 3-coordinate condition. Phosphines may be primary, secondary, or tertiary and resemble the corresponding amines in stability and basicity. The phosphines are also good nucleophiles, as are amines. In phosphines, the barrier to pyramidal inversion is much higher, typically 35 kcal/mol, and many examples of pyramidal,optically active phosphines are known (e.g., 20; see also chapter 9 ). This is not possible for amines, in which the pyramidal inversion is extremely rapid, as in 21.