I began taking Organic Chemistry one week ago, so my question should be very easy for most of you. I am having a lot of problems determining which specie, of a group of given species, is more stable. I can determine it for resonance conformers; for example I know that one conformer with a separation of charges is less stable that one with no separation of charges. I also know that the lone pairs must be in the most electronegative atom...etc. However I am having problems when I am asked to determine which of two or three species (not resonance conformers) is more stable. I read my book and I didn't find any kind of rule for comparing the stability of two different species obeying the octet rule and with no separation of charges. I would reeeaaaallly appreciate if someone could explain me what I must consider. One example of the problems that I cannot do is the following: CH3C(O-)HCH=CH2 --> (the O has formal charge -1, I didn't know how to write it here) VS. CH3C(O-)=CHCH3 The most stable is the second one, but why?