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    Hi,
    I was doing some chemistry revision, and in specific was looking at the formation of ethanol from ethene and the mechanism for the reaction. I quickly realized that the whole idea of charges on atoms changing in mechanisms confused me very much. The first instant of this is the formation of the carbocation after the carbon-carbon double bond breaks, and one of the carbons forms a covalent bond with the H+ ion. How does the other carbon become C+ when it loses two electrons i.e. the one it donates to the electron pair and the one it gets from the other carbon? Secondly, when the steam molecule's lone pair forms a dative covalent bond with the carbocation, why does it become a neutral carbon atom, instead of a 1- carbon atom. If it gains two electrons, due to it being a dative covalent bond, why does its charge only drop by 1?

    I would really appreciate if someone could help me out here , thanks in advance.
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    (Original post by z4_)
    Hi,
    I was doing some chemistry revision, and in specific was looking at the formation of ethanol from ethene and the mechanism for the reaction. I quickly realized that the whole idea of charges on atoms changing in mechanisms confused me very much. The first instant of this is the formation of the carbocation after the carbon-carbon double bond breaks, and one of the carbons forms a covalent bond with the H+ ion. How does the other carbon become C+ when it loses two electrons i.e. the one it donates to the electron pair and the one it gets from the other carbon? Secondly, when the steam molecule's lone pair forms a dative covalent bond with the carbocation, why does it become a neutral carbon atom, instead of a 1- carbon atom. If it gains two electrons, due to it being a dative covalent bond, why does its charge only drop by 1?

    I would really appreciate if someone could help me out here , thanks in advance.
    Think of it like this.

    You can agree that there are two bonds between the carbon atoms in ethene: the sigma and pi bond.
    Since the electrons in the pi bond are loosely bound to the carbon atoms, they will be accepted by an electrophile.

    When you add the proton, the electrons in the pi bond form a bond with the proton. But the sigma bond remains. So one of the carbon atoms is now making three bonds with hydrogen atoms and a sigma bond (four in total).

    The other carbon atom has lost the pi bond so now it's making three bonds only (two bonds with hydrogen and the sigma bond).

    Thus there are three electrons in its orbitals; since carbon originally had four, it becomes positively charged (just like Na becoming Na+ when it donates an electron)


    When the water molecule forms a coordinate bond, it doesn't mean it's losing both electrons to a species. What it's doing is just forming a sigma bond with the carbon atom, i.e. sharing the electrons between the carbon and oxygen.

    Thus carbon is getting one electron from the pair.
    But because oxygen has to give up one electron, there's a positive charge on the oxygen atom. That's why we draw the positive charge on the oxygen.
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    (Original post by RMNDK)
    Think of it like this.

    You can agree that there are two bonds between the carbon atoms in ethene: the sigma and pi bond.
    Since the electrons in the pi bond are loosely bound to the carbon atoms, they will be accepted by an electrophile.

    When you add the proton, the electrons in the pi bond form a bond with the proton. But the sigma bond remains. So one of the carbon atoms is now making three bonds with hydrogen atoms and a sigma bond (four in total).

    The other carbon atom has lost the pi bond so now it's making three bonds only (two bonds with hydrogen and the sigma bond).

    Thus there are three electrons in its orbitals; since carbon originally had four, it becomes positively charged (just like Na becoming Na+ when it donates an electron)


    When the water molecule forms a coordinate bond, it doesn't mean it's losing both electrons to a species. What it's doing is just forming a sigma bond with the carbon atom, i.e. sharing the electrons between the carbon and oxygen.

    Thus carbon is getting one electron from the pair.
    But because oxygen has to give up one electron, there's a positive charge on the oxygen atom. That's why we draw the positive charge on the oxygen.
    Ahhh, I understand now. I wasn't thinking of it in terms of each atom getting one electron in the pair. I can finally get the right charges by thinking of it this way. Thank you very much, it's really appreciated
 
 
 
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