Question

question 3: the enolate ion, known as a formula of c2h3o, has two possible lewis dot diagrams.
(a)
h - c = c - h
:o:
(b)
h - c - c - h
:o:
:o:

a) using formal charge, determine which structure is the most likely correct structure, a or b? why?

b) for carbon atom x in structure a and b:
what is the hybridization around carbon atom x?
how many sigma and pi bonds has carbon atom x formed?

c) how many sigma bonds and pi bonds are in the enolate ion for structure a?

d) a hydrogen ion attaches itself to the enolate ion in structure a, creating c2h4o. draw the lewis dot structure of the new molecule:

Explanation:

Step1: Calculate formal charge for structure A

Formal - charge = Valence electrons - Non - bonding electrons - $\frac{1}{2}$Bonding electrons. For oxygen in structure A, valence electrons of O = 6, non - bonding electrons = 6, bonding electrons = 2. Formal charge of O in A = $6 - 6-\frac{2}{2}=- 1$. For the carbon atom marked 'X' in A, valence electrons of C = 4, non - bonding electrons = 0, bonding electrons = 8. Formal charge of C in A = $4 - 0-\frac{8}{2}=0$.

Step2: Calculate formal charge for structure B

For oxygen in structure B, valence electrons of O = 6, non - bonding electrons = 4, bonding electrons = 4. Formal charge of O in B = $6 - 4-\frac{4}{2}=0$. For the carbon atom marked 'X' in B, valence electrons of C = 4, non - bonding electrons = 0, bonding electrons = 8. Formal charge of C in B = $4 - 0-\frac{8}{2}=0$. Since the enolate ion has a negative charge, structure A is more likely as it has a negative formal charge on the more electronegative oxygen atom.

Step3: Determine hybridization of carbon 'X' in structure A

The carbon atom 'X' in structure A has 3 electron - groups (a double - bond counts as one group). Using the VSEPR theory, the hybridization of this carbon atom is $sp^{2}$. There are 3 regions of electron density around the carbon atom 'X' in A.

Step4: Count sigma and pi bonds for carbon 'X' in structure A

A double - bond consists of 1 sigma and 1 pi bond, and single bonds are sigma bonds. The carbon atom 'X' in structure A has 3 sigma bonds (2 C - H single bonds and 1 C - C single bond) and 1 pi bond (from the C = O double - bond).

Step5: Analyze new Lewis structure after hydrogen attachment

When the hydrogen ion attaches to the enolate ion in structure A, the negative charge on the oxygen will react with the hydrogen ion. The new Lewis structure of $C_{2}H_{4}O$ will have a carbonyl group (C = O) and a methyl group (CH₃). The structure will be $CH_{3}CHO$ (acetaldehyde).

Answer:

a) Structure A is more likely because it has a negative formal charge on the more electronegative oxygen atom.
b) The hybridization of carbon 'X' in structure A is $sp^{2}$, and there are 3 regions of electron density around it. The carbon atom 'X' in structure A has 3 sigma bonds and 1 pi bond.
c) When the hydrogen ion attaches to the enolate ion in structure A, the new Lewis structure of $C_{2}H_{4}O$ is $CH_{3}CHO$ (acetaldehyde).