Question

Write the resonance structure that would result from moving the electrons as the curved arrows indicate. 

Answer 1

Hint: Keto and enol are two tautomeric forms i.e. the interconversion of both involves movement of alpha hydrogen atoms. Deprotonation of enol gives enolate anion which is a strong nucleophile. This shows that the anion present at the adjacent position of the carbonyl group is in resonance with the oxygen atom.

Complete step by step answer:
-Resonance structures are two or more Lewis structures which describe the delocalization of electrons in a molecule or polyatomic ions. Resonance in chemistry describes the bonding in particular molecules or ions by merging many contributing forms, referred as canonical structures or resonance structures as per the theory of valence bonding into a hybrid resonance or structures.
-The delocalization of electrons takes place by fractional bonds and formal charges in a resonance hybrid. Not all resonance structures are equivalent and we need to determine which one is best describing the actual bonding. To predict which resonance structures are favoured, we use formal charges.
-Let us look at the anion given to us. In aqueous solution, the carbonyl compounds which have an alpha hydrogen can lose it to water and give hydronium ion and the conjugate base of the carbonyl compound, referred to as an enolate. The reason for its acidic behaviour is the strong resonance stabilization of the enolate, which has both carbanion and alkoxide character.
-Enolates have two resonance forms on the basis of position of negative charge i.e. we can put the negative charge either on the oxygen or the carbon. Both resonance structures are comparably stable and this tells us that the resonance stabilization is large. As per the arrow moving from double bond to oxygen atom, it clears that the double bond of oxygen will split into two ions, positive and negative. The negative ion will reside on oxygen whereas the positive charge on carbon of carbonyl will form a pi-bond with the negative charge already present on the near-by carbon atom. This is how we get the resonance structure of given anion.

Note:
Out of the two resonance structures of the enolate ion that we have drawn above, the one which places the negative charge on the oxygen is the most stable and less reactive. This is because the negative charge will be stabilized more by the greater electronegativity of the oxygen atom.