8-34b - Organic Chemistry (Mcmurry)

Question

Iodine azide, adds to alkenes by an electrophilic mechanism similar to that of bromine. If a monosubstituted alkene such as 1-butene is used, only one product results:

b) Calculate formal charges for the atoms in both resonance structures you drew for ,in part (a).

Step-by-Step Solution

Answer

Formal charge for both the resonance structures are:

Formal Charge

       I          N(i)     N(ii)         N(iii)

A) 0          0          +1          -1

B)  0           -1          +1          0

Formal charge

Formal charge is an imaginary charge. In a polyatomic molecule the charge is not distributed on any single atom whereas the charge is distributed in a molecule as a whole. 

Mathematical formula of formal charge is:

$F=v-\frac{s}{2}-u$

Where, 

V = Number of valance electrons

S = Total shared electrons

U = Number of unshared electrons

Example

Formal charge of carbon dioxide molecule

$\mathrm{O}=\mathrm{C}=\mathrm{O}$

Formal charge of one oxygen atom = 6-2-4 = 0

Formal charge for other oxygen atom will be same i.e. 0.

Formal charge of carbon atom = 4-4-0 = 0

Calculation of formal charge of resonance structure

As we know that, Formula for calculating formal charge is:

$F=v-\frac{s}{2}-u$

By substituting the values in above formula we will calculate formal charge for both the resonance structures.

Formal charge for the atoms of resonance structure A is: 

Formal charge for I atom = $7-\frac{2}{2}-6=7-1-6=0$

Formal charge for N(i) atom = $5-\frac{6}{2}-2=5-3-2=0$

Formal charge for N(ii) atom = $5-\frac{8}{2}-0=5-4-0=+1$

Formal charge for N(iii) atom = $5-\frac{4}{2}-4=5-2-4=-1$

Formal charge for the atoms of resonance structure B is:

Formal charge for I atom = $7-\frac{2}{2}-6=7-1-6=0$

Formal charge for N(i) atom = $5-\frac{4}{2}-4=5-2-4=-1$

Formal charge for N(ii) atom = $5-\frac{8}{2}-0=5-4-0=+1$

Formal charge for N(iii) atom = $5-\frac{6}{2}-2=5-3-2=0$