Q.2-2-35_A - 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 O is +1.

 

The formal charge on B is -1.

 

Step 1: Formal charge

Formal charge means the electric charge present on an atom in a molecule. It indicates the magnitude and whether a charge is 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.

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

Each bond formed has two electrons in it.

 

Step 3: Calculation of formal charge on O:

                                 

Structure showing O linked with two CH₃ and one B.

 

Using the above formula:

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

Step 4: Calculation of formal charge on B:

Using the above formula:

$$\begin{gathered} \text{formal charge=}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{bonded atom}\end{array}\right) \\ \text{ =}\left(\begin{array}{c}\text{number of} \\ v\text{alance electron in}\\ \text{free atom}\end{array}\right)\text{-}\left(\frac{\text{number of bonding electro}n}{2}\text{+ number of non bonding electron}\right) \\ \text{ = 3-}\left(\frac{8}{2}\text{+0}\right) \\ \text{ = 3-4} \\ = -1 \end{gathered}$$