Q.2-2-7P - Organic Chemistry (Mcmurry)

Question

Question: Calculate formal charges for the non-hydrogen atoms in the following molecules:

(a) Diazomethane,H₂C=N=N:

(b) Acetonitrile oxide, H₃C-C$\equiv$N-O

(c) Methyl isocyanides,H₃CN$\equiv$C:

Step-by-Step Solution

Answer

Answer

Total formal charge for the non-hydrogen atom in the molecule is:

(a) 0, +1,-1

(b) 0, 0, +1,-1

(c) 0,-1, +1

Step 1: Formal Charge

The formal charge of an atom in a molecule is the charge that would reside in the atom if all of the bonding electrons were shared equally, regardless of their relative electronegativities.

Step 2: Find the formal charge of an atom in a molecule

1.Draw an electron dot structure of the molecule.

2. Use the formula to determine formal charge for each atom. The periodic table shows the number of valence electrons of the element, and the electron dot structure shows the number of bonding and non-bonding electrons.

 $\text{Formal charge (fc)=}\left[\begin{array}{c}valance\\ electrons\end{array}\right]\text{-}\left[\frac{bonding electrons}{2}\right]\text{-}\left[\begin{array}{c}non bonding\\ electrons\end{array}\right]$

Step 3: Calculate formal charge on non-hydrogen molecules

a.

                       

$$\begin{gathered} \text{For carbon:FC=4-}\frac{8}{2}\text{-0=0} \\ \text{For nitrogen 1:FC=5-}\frac{8}{2}\text{-0=+1} \\ \text{For oxygen 2:FC=5-}\frac{4}{2}\text{-4=-1} \end{gathered}$$

b.

 

               

$$\begin{gathered} \text{For carbon 1:FC=4-}\frac{8}{2}\text{-0=0} \\ \text{For carbon 2:FC=4-}\frac{8}{2}\text{-0=0} \\ \text{For nitrogen:FC=5-}\frac{8}{2}\text{-0=+1} \\ \text{For oxygen:FC=6-}\frac{8}{2}\text{-6=-1} \end{gathered}$$

 c.

                           

$$\begin{gathered} \text{For carbon1:FC=4-}\frac{8}{2}\text{-0=0} \\ \text{For carbon 2 :FC=4-}\frac{6}{2}\text{-2=-1} \\ \text{For nitrogen:FC =5-}\frac{8}{2}\text{-0=+1} \end{gathered}$$