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c2h6n and c3h5br2

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    is the reason why they cant exist is the fact they form unstable compounds which are negatively charged?

    not sure of your question. what are c2h6n and c3h5br2 ?

    can you give them names as the molecular formulae you have given are ambiguous?

    im not sure how either would exist, structural forumla pls?

    C2H6N (I'm going to assume as NITROGEN) - Cannot exist, as there would be too many hydrogens if all the electrons formed a covalent bond with another atom. You would think that you can make a atom like [CH3-CH=NH2](1+) where 1+ is the overall charge on the molecule (One of the hydrogens form a dative covalent bond with the lone pair on the nitrogen), but it is more likely that the C=N double bond will break, as it is more unstable. Also, in order for a ion to be made, high pressures will be needed, so (assuming you're in room temperature and pressure) it won't exist. Woo. essay, haha.

    C3H5Br2 - Is unlikely to exist, because a carbocation (a positive carbon in a organic molecule) would exist. So it'll probably exist for a fraction of a second before a nucleophile (electron pair donator) will attack it and for a dative covalent bond with it, changing the structure.

    I think it's right, but I'm only in A2 Chemistry, so hope this helped! =]

    These molecules would be charged but you haven't specified what the charges are.

    C2H6N+ wouldn't exist as you'd never be able to remove H- from dimethylamine. You could remove H+ though with a very strong base like butyl lithium. So C2H6N- (negative charge on nitrogen) can exist. You can even buy this from sigma aldrich as as the lithium salt - LiN(CH3)2. This molecule will be very reactive though as it really really wants to pick up a proton so it will exist as long as there isn't any other molecule around with even a slightly acidic proton.

    C3H5Br2+ is too unstable. Maybe as a very short lived SN1 intermediate but you'd never isolate it. As for C3H5Br2- (assuming it's CH3C(-)HCHBr2), the free electron pair on carbon would spontaneously kick into the adjacent C-C bond to eliminate a bromide and give CH3CH=CHBr.
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