Q36E - Organic Chemistry (Mcmurry)

Question

Assign formal charges to the atoms in each of the following molecules:

a.

b.

c.

Step-by-Step Solution

Answer

a. 

The formal charge on all H in  ${\mathrm{CH}}_3$ is 0

The formal charge on C in  ${\mathrm{CH}}_3$ is 0

The formal charge on O is -1

The formal charge on N is +1

b.

The formal charge on all H in ${\mathrm{CH}}_3$  is 0

The formal charge on C in ${\mathrm{CH}}_3$  is 0

The formal charge on N at left is -1

The formal charge on N in the middle is +1

The formal charge on N at the rightmost is 0.

c.

The formal charge on all H in  ${\mathrm{CH}}_3$ is 0.

The formal charge on C in ${\mathrm{CH}}_3$  is 0.

The formal charge on N at left is 0.

The formal charge on N in the middle is +1.

The formal charge on N at the rightmost is -1.

Step 1: Formal charge

The formal charge is the electric charge present on an atom in a molecule. It indicates magnitude and charge, positive or negative. 

Step 2: Calculation of formal charge

The formal charge on an atom is equal to the number of valence electrons in a neutral and isolated atom minus the number of electrons present in its bonded atom form in a molecule. The number of electrons in a bonded atom is calculated as half the number of bonding electrons plus the nonbonding, that is, lone-pair electrons.

$\begin{array}{c}\text{Formal charge=}\left(\begin{array}{l}\text{Number of}\\ \text{valence electrons}\\ \text{in free atom}\end{array}\right)-\left(\begin{array}{l}\text{Number of}\\ \text{valence electrons}\\ \text{in bonded atom}\end{array}\right)\\ =\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)\end{array}$

Step 3: . Calculation of formal charge on all H in CH 3

Using the above formula:

$\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                        $\begin{array}{c}=1-\left(\frac{2}{2}+0\right)\\ =1-1\\ =0\end{array}$

Step 4: (a) Calculation of formal charge on N

Using the above formula:

 

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                        $\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Step 5: Calculation of formal charge on O

Using the above formula:

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                      $\begin{array}{c}=6-\left(\frac{2}{2}+6\right)\\ =6-7\\ =-1\end{array}$

Step 6: Calculation of formal charge on all C in CH 3

Using the above formula:

   

$\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                        $\begin{array}{c}=4-\left(\frac{8}{2}+0\right)\\ =4-4\\ =0\end{array}$

Step 7: (b) Calculation of formal charge on N bonded with CH 3

Using the above formula:

 

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                        $\begin{array}{c}=5-\left(\frac{4}{2}+4\right)\\ =5-6\\ =-1\end{array}$

Step 8: Calculation of formal charge on N in the middle

Using the above formula:

 

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                      $\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Step 9: Calculation of formal charge on all C in CH 3

Using the above formula:

   $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                          $\begin{array}{c}=4-\left(\frac{8}{2}+0\right)\\ =4-4\\ =0\end{array}$

Step 10: Calculation of formal charge on rightmost N

Using the above formula:

$\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                      $\begin{array}{c}=5-\left(\frac{6}{2}+2\right)\\ =5-5\\ =0\end{array}$

Step 11: (C): Calculation of formal charge on N bonded with CH 3

Using the above formula:

 

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                         $\begin{array}{c}=5-\left(\frac{2}{2}+4\right)\\ =5-5\\ =0\end{array}$

Step 12: Calculation of formal charge on N in the middle

Using the above formula:

 

 $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                          $\begin{array}{c}=5-\left(\frac{8}{2}+0\right)\\ =5-4\\ =1\end{array}$

Step 13: Calculation of formal charge on all C in CH 3

Using the above formula:

   $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                         $\begin{array}{c}=4-\left(\frac{8}{2}+0\right)\\ =4-4\\ =0\end{array}$

Step 6: Calculation of formal charge on the rightmost N

Using the above formula:

   $\text{Formal charge}=\left(\begin{array}{l}\text{Number of}\\ \text{valence }\\ \text{electron}\\ \text{in free atom}\end{array}\right)-\left(\frac{\mathrm{Number}\mathrm{of}\text{ bonding electron}}{2}\text{+Number of nonbonding electron}\right)$

                     $\begin{array}{c}=5-\left(\frac{4}{2}+4\right)\\ =5-6\\ =-1\end{array}$