Chapter 4, Problem 4.122QA

Interpretation Introduction

Interpretation & Concept Introduction:

the Lewis structure and assign formal charges of each atom in Formamide and methylformate.

Answer to Problem 4.122QA

Solution:

a) Lewis structure:

i. Formamide (HCONH₂)

ii. Methyl formate (HCO₂CH₃)

b) The net formal charge of compound methyl formate is 0.

Explanation of Solution

The electronic configuration of the elements H, C N and O is as follows.

H: 1s¹.

C: 1s² 2s² 2p².

N: 1s² 2s² 2p³.

O: 1s² 2s² 2p⁴.

i. Lewis structure of Formamide (HCONH₂)

Element		Valence electrons
Symbol	# of atoms	In one atom	Total
H	3	1	3
C	1	4	4
N	1	5	5
O	1	6	6
Valence electrons in molecule			18

First we draw a skeletal structure of the molecule.

There are five bonds in the skeletal structure. Therefore 5x2 = 10 electrons are involved in the bond formation. To complete the octet of O and N we will put electron pairs on them.

To complete the octet of C we shift one of the lone pair on O as bond pair between C and O. This completes the octet of C as well as O.

This gives us the Lewis structure of formamide.

ii. Lewis structure of methyl formate (HCO₂CH₃)

Element		Valence electrons
Symbol	# of atoms	In one atom	Total
H	4	1	4
C	2	4	8
O	2	6	12
Valence electrons in molecule			24

First we draw a skeletal structure of the molecule.

There are seven bonds in the skeletal structure. Therefore 7x2 = 14 electrons are involved in the bond formation. To complete the octet of O we will put electron pairs on them.

To complete the octet of C we shift one of the lone pair on O as bond pair between C and O. This completes the octet of C as well as O

This gives us the Lewis structure of methyl formate.

c) Calculating formal charges:

1) Formula:

FC = (no. of valence electrons) – [no. of electrons in lone pairs + ½ (no. of shared electrons]

2) Calculations:

Formal charge on each atom of compound (a) is calculated as:$\mathrm{F}\mathrm{C}=\left(\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\right)-\left[\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{ }\mathrm{p}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{s}+\frac{1}{2}\left(\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{s}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\right)\right]$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{H}=\left(1\right)-[0+\frac{1}{2}\left(2\right)]=0$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{C}=\left(4\right)-[0+\frac{1}{2}\left(8\right)]=0$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{O}=\left(6\right)-[4+\frac{1}{2}\left(4\right)]=0$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{N}=\left(5\right)-[2+\frac{1}{2}\left(6\right)]=0$

The net formal charge on compound formamide is 0.

Formal charge on each atom of compound methyl formate is calculated as:

$\mathrm{F}\mathrm{C}=\left(\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\right)-\left[\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{ }\mathrm{p}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{s}+\frac{1}{2}\left(\mathrm{n}\mathrm{o}.\mathrm{ }\mathrm{o}\mathrm{f}\mathrm{ }\mathrm{s}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{ }\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}\right)\right]$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{H}=\left(1\right)-[0+\frac{1}{2}\left(2\right)]=0$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{C}\mathrm{ }\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}=\left(4\right)-[0+\frac{1}{2}\left(8\right)]=0$

$\mathrm{F}\mathrm{C}\mathrm{ }\mathrm{o}\mathrm{n}\mathrm{ }\mathrm{O}=\left(6\right)-[4+\frac{1}{2}\left(4\right)]=0$

The net formal charge of compound methyl formate is 0.

Conclusion:

By using the Lewis structure, formal charges are calculated for each atom in formamide and methyl formate.