Chapter 13, Problem 4MCP

(a)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ is a primary ketone” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

$\text{2-pentanone}$ or methyl propyl ketone is primary ketone with carbonyl group at the second position.  The structure is,

Hence, the statement $\text{2-pentanone}$ is a primary ketone is true.

(b)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ has a higher melting point than $\text{2-pentanol}$” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is false because $\text{2-pentanone}$ has lower melting point than $\text{2-pentanol}$ due to the presence of weak intermolecular hydrogen bonding in the former.

Explanation of Solution

$\text{2-pentanone}$ is a ketone and $\text{2-pentanol}$ is an alcohol.

Alcohols have strong intermolecular hydrogen bonding.  The $\text{OH}$ group present in the alcohol allows the molecules to engage in hydrogen bonding.  A larger amount of energy is required to break the intermolecular hydrogen bonding in alcohols hence, they have higher boiling and higher melting points.

Ketones have lower boiling points when compared to alcohols, representing the presence of weak intermolecular dipole-dipole forces.  Due to weaker intermolecular hydrogen and weaker dipole-dipole attractions in ketones, they have lower boiling points compared to alcohols.

Hence, the given statement is false because $\text{2-pentanone}$ has lower melting point than $\text{2-pentanol}$ due to the presence of weak intermolecular hydrogen bonding in the former.

(c)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ forms hydrogen bonds with water” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

$\text{2-pentanone}$ can form hydrogen bonds with water.  The hydrogen bonds are formed through their oxygen atoms to the hydrogen atoms of water.  Thus, they are reasonably water-soluble.  Hence, the given statement is true.

(d)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ is nonpolar” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is false because $\text{2-pentanone}$ is polar.

Explanation of Solution

The structure of $\text{2-pentanone}$ is,

The carbonyl group present in $\text{2-pentanone}$ is polar in nature and thereby can participate in dipole-dipole attractions.  Therefore, $\text{2-pentanone}$ is polar in nature.

The statement “$\text{2-pentanone}$ is nonpolar” is false due to the presence of polar carbonyl group in them; $\text{2-pentanone}$ is polar in nature.

(e)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ has a molecular formula of ${\text{C}}_{\text{5}}{\text{H}}_{\text{9}}\text{OH}$” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

The structure of $\text{2-pentanone}$ is,

From the structure of $\text{2-pentanone}$, it can be seen that $\text{2-pentanone}$ comprises of five carbon atoms, ten hydrogen atoms and one oxygen atom.  The molecular formula of $\text{2-pentanone}$ is ${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{O}\text{.}$  It can also be written as ${\text{C}}_{\text{5}}{\text{H}}_{\text{9}}\text{OH}\text{.}$

The statement “$\text{2-pentanone}$ has a molecular formula of ${\text{C}}_{\text{5}}{\text{H}}_{\text{9}}\text{OH}$” is true.

(f)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ has a higher boiling point than hexane” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

The presence of polar carbonyl group and the attractive forces between the carbonyl-containing compounds include London dispersion forces between the hydrocarbon chains and dipole-dipole attractions between the carbonyl groups.  Because the dipole-dipole attractions between the molecules are stronger than London dispersion forces, $\text{2-pentanone}$ has higher boiling point than hexane.  Hence, the given statement is true.

(g)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ is a liquid at room temperature” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

$\text{2-pentanone}$ is clear colorless liquid with the odor of fingernail polish.  It is less dense than water and soluble in water.  It is found in apple and a few other plant sources.  Hence, the statement “$\text{2-pentanone}$ is a liquid at room temperature” is true.

(h)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ is water soluble” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

Ketones cannot form hydrogen bonds with each other, but they form intermolecular hydrogen bonds with water.  As a result, the smaller members of ketones (five or fewer carbon atoms) are soluble in water.  Hence, the statement “$\text{2-pentanone}$ is a water soluble” is true.

(i)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ has a carbonyl functional group” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

Ketone is carbonyl compounds.  The carbonyl group of the ketone is polar and can participate in dipole-dipole attractions.

The structure of $\text{2-pentanone}$ is,

$\text{2-pentanone}$ shows the presence of carbonyl group.  Hence, the statement “$\text{2-pentanone}$ has a carbonyl functional group” is true because it consists of polar carbonyl group.

(j)

Interpretation Introduction

Interpretation:

The statement “$\text{2-pentanone}$ is a structural isomer of pentanal” has to be predicted as true or false.  If the statement is false, the reason for the false statement has to be described.

Answer to Problem 4MCP

The given statement is true.

Explanation of Solution

Structural isomers are those with same molecular formula but differ in arrangement of atoms.

The molecular formula of $\text{2-pentanone}$ is ${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{O}$ and its structure is,

The molecular formula of pentanal is ${\text{C}}_{\text{5}}{\text{H}}_{\text{10}}\text{O}$ and its structure is,

Both these are structural isomers with each other and hence, the statement “$\text{2-pentanone}$ is a structural isomer of pentanal” is true.