Q.2-2-35_B - Organic Chemistry (Mcmurry)

Question

Question: Calculate the formal charges on the atoms shown in red.

                             

Step-by-Step Solution

Answer

Answer

 

The formal charge on C is -1

The formal charge on N bonded with C is +1

The formal charge on N bonded with N is 0

Step 1: Formal charge

Formal charge is the electric charge present on an atom in a molecule. It shows the magnitude and if a charge is positive or negative. 

Step 2: Calculation of formal charge

Formal charge on an atom is calculated as 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 lone-pair electrons.

$$\begin{gathered} \text{formal charge=}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{bonded atom}\end{array}\right) \\ \text{ =}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\frac{\text{number of bonding electro}n}{2}\text{+ number of non bonding electron}\right) \end{gathered}$$

Step 3: Calculation of formal charge on C

                                           

                                                    Structure showing bond electrons

 

Using the above formula:

$$\begin{gathered} \text{formal charge= }\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\frac{\text{number of bonding electro}n}{2}\text{+ number of non bonding electron}\right) \\ \text{ =4-}\left(\frac{6}{2}\text{+2}\right) \\ \text{ =4-5} \\ =-1 \end{gathered}$$

       

Step 4: Calculation of formal charge on N bonded with C:

Using the above formula:

$$\begin{gathered} \text{formal charge = }\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\frac{\text{number of bonding electro}n}{2}\text{+ number of non bonding electron}\right) \\ \text{ =5-}\left(\frac{8}{2}\text{+2}\right) \\ \text{ =5-4} \\ =1 \end{gathered}$$

Step 5: Calculation of formal charge on N bonded with N:

Using the above formula:

$$\begin{gathered} \text{formal charge =}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\frac{\text{number of bonding electro}n}{2}\text{+ number of non bonding electron}\right) \\ \text{ =5-}\left(\frac{6}{2}\text{+2}\right) \\ \text{ =5-5} \\ =0 \end{gathered}$$