Chapter 13, Problem 24E

Interpretation Introduction

(a)

Interpretation:

The electron-pair geometry for each carbon atom in ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is to be explained and the Lewis diagram for the molecule is to be drawn.

Concept introduction:

The electron pairs in Lewis diagrams repel each other in real molecule and thus they distribute themselves in positions around the central atoms which are far away from one another. This arrangement of electron pairs is called electron-pair geometry. The electron pairs may be shared in covalent bond, or they may be lone pairs.

Answer to Problem 24E

The Lewis diagram for the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown below.

The electron pair geometry for the carbon atoms $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ are tetrahedral and for the carbon $-\text{COOH}$ is trigonal planar.

Explanation of Solution

To write the Lewis diagram for a compound, first the number of valence electrons is to be calculated. In the molecule, ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$, each carbon has four valence electrons, each hydrogen has one valence electron, and each oxygen has six valence electrons. The total number of valence electron for the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3\times 4+6\times 1+6\times 2\right)e^{-}\\ & =& 30e^{-}\end{array}$

The atom which is least electronegative and lesser in number is the central atom. In ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$, carbon is least electronegative element and is lesser in number. Therefore, carbon is the central atom. Carbon is bonded with hydrogen atoms and oxygen atom. In Lewis diagram, each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown below.

Figure 1

The electron-pair geometry depends on the number of electron pairs around the central atoms. In the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$, there are four electron-pairs around the each carbon atom shown as $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$, and there are three electron-pairs for the carbon atom in $-\text{COOH}$ due to the presence of $\left(\text{C}=\text{O}\right)$ double bond. Therefore, the electron pair geometry for the carbon atoms $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ are tetrahedral and for the carbon $-\text{COOH}$ is trigonal planar.

Conclusion

The Lewis diagram for the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown in the Figure 1. The electron pair geometry for the carbons $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ are tetrahedral and for the carbon $-\text{COOH}$ is trigonal planar.

Interpretation Introduction

(b)

Interpretation:

The molecular geometry predicted by the valence shell electron-pair repulsion theory for each carbon atom in the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is to be explained and the Lewis diagram for the molecule is to be drawn.

Concept introduction:

Molecular geometry is the precise term that is used to describe the shape of molecules and arrangement of atoms around the central atom. The molecular geometry of a molecule is predicted by valence shell electron-pair repulsion theory or in short VSEPR theory. VSEPR theory applies to substances in which a second period element is bonded to two, three, four, or other atoms.

Answer to Problem 24E

The Lewis diagrams for ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown below.

The molecular geometry is tetrahedral for the carbons $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ and trigonal planar for the carbon $-\text{COOH}$.

Explanation of Solution

To write the Lewis diagram for a compound first the number of valence electrons is to be calculated. In the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$, each carbon has four valence electrons, each hydrogen has one valence electron, and each oxygen has six valence electrons. The total number of valence electron for the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3\times 4+6\times 1+6\times 2\right)e^{-}\\ & =& 30e^{-}\end{array}$

The atom which is least electronegative and lesser in number is the central atom. In ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ carbon is least electronegative element and is lesser in number. Therefore, carbon is the central atom. Carbon is bonded with hydrogen atoms and oxygen atom. In Lewis diagram each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown below.

Figure 1

The molecular geometry depends on the number of electron pairs as well as number of unpaired electron on the central atoms. In the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$, there are four electron-pairs around the each carbon atom shown as $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$; and there are three electron-pairs for the carbon atom in $-\text{COOH}$ due to the presence of $\left(\text{C}=\text{O}\right)$ double bond and no lone pair present on the central atoms. Therefore, the molecular geometry would be same as electron-pair geometry that is tetrahedral for the carbon atoms $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ and trigonal planar for the carbon $-\text{COOH}$.

Conclusion

The Lewis diagram for the molecule ${\text{CH}}_{\text{3}}{\text{CH}}_2\text{COOH}$ is shown in the Figure 1. The molecular geometry is tetrahedral for the carbons $-{\text{CH}}_{\text{3}}$, and $-{\text{CH}}_{\text{2}}$ and trigonal planar for the carbon $-\text{COOH}$.