Draw the best Lewis structure for #CH_3^-1#. What is the formal charge on the #C#?

1 Answer
Feb 18, 2017

There is a formal negative charge on the carbon nucleus.........


Consider the electronic distribution of the parent #"methane"#. Methane is a gaseous, neutral molecule. The carbon contributes 6 electrons (why), and the 4 hydrogen atoms contribute 1 electron each: 10 electrons in total. In the carbon nucleus, and in the FOUR hydrogen nuclei, there are 10 nuclear protons altogether. since there are as many positive charges as negative charges, the element must be neutral.

Now consider the electronic structure of methyl carbanion:

#""^(-):CH_3#. There are #3xxC-H# bonds, each composed of 2 electrons (as is standard for a covalent bond). When we formally break these, one electron is claimed by hydrogen, and the other bonding electron devolves to carbon:

#""^(-):CH_3rarr3xxH* + C^(-)#

Around carbon, therefore, there are 2 inner core electrons (i.e. the #1s^2# pair. The #2xx2s# and #3xx2p# electrons are presumed to be the valence electrons. There are SEVEN electrons in total associated with carbon: 2 inner core; 3 from the #C-H# bonds; and 2 from the lone pair of electrons. And thus the carbon centre is a FORMAL carbanion, as the 6 protonic or nuclear (+ve) charges are countered by 7 electronic charges. And the difference is the negative charge that we assign to the carbon.

In solution, the methyl carbanion is presumed to oligomeric, i.e. a tetramer or dimer or something.