What is the O.N of carbonyl carbon in Ethyl 4-methylpentanoate ???
The oxidation number of the carbonyl carbon is +3.
As you know, oxidation numbers are assigned by treating all bonds an atom has as ionic.
The bond-line notation for ethyl-4-methylpentanoate looks like this
A carbonyl group is simply a carbon atom double bonded to an oxygen atom. In the case of ethyl-4-methylpentanoate, the carbon that's part of the carbonyl group is also bonded to another carbon atom and to another oxygen atom, th oxygen from the ether group..
When a carbon atom is bonded to another carbon atom, the difference in electronegativity between these two atoms is equal to zero.
So the carbonyl carbon will neither take, nor lose electrons from its bond to the other carbon atom. Remember that the "taking" or "losing" of electrons is hypothetical.
Since oxygen is more electronegative than carbon, it will take the electrons carbon is contributing to the bond. In the case of the double bond, oxygen will take all four bonding electrons, i.e. both electrons carbon shares to the bond.
This will leave carbon deprived of two electrons, giving it a +2 oxidation state.
The ether oxygen that is single bonded to the carbonyl atom will take both bonding electrons, i.e. the electron carbon shares to the bond.
This means that the carbonyl carbon has "lost" an additional 1 electron, bringing the total number of electrons it "lost" to 3. As a result, the carbonyl carbon's oxidation state is +3.