Question

The two isomers having the formula ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$ boil at $-{23}^{\circ }\mathrm{C}$ and ${78.5}^{\circ }\mathrm{C}.$ Draw the structure of the isomer that boils at $-{23}^{\circ }\mathrm{C}$ and of the isomer that boils at ${78.5}^{\circ }\mathrm{C}.$

Short answer

The isomer with the boiling point of -23°C is dimethyl ether, having the structure: O || CH3 - - CH3 The isomer with the boiling point of 78.5°C is ethanol, having the structure: CH3 - CH2 - OH

Step-by-step solution

Identify possible isomers with the given formula

With the formula ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$, we can identify the two possible isomers: ethanol (CH3CH2OH) and dimethyl ether (CH3OCH3). The next step is to determine which isomer corresponds to each boiling point.

Analyze the boiling points

Generally, the greater the intermolecular forces, the higher the boiling point of the compound. Ethanol has a hydroxyl group (-OH), which can form hydrogen bonds, a stronger intermolecular force than the dipole-dipole interactions in dimethyl ether. Therefore, we can conclude that the isomer with the higher boiling point (78.5°C) should be ethanol, while the one with the lower boiling point (-23°C) should be dimethyl ether.

Draw the structures

We now know which isomer corresponds to each boiling point, so we can draw their structures: 1. For the isomer that boils at -23°C (dimethyl ether): O || CH3 - - CH3 2. For the isomer that boils at 78.5°C (ethanol): CH3 - CH2 - OH

Key concepts

Isomers

Isomers are fascinating aspects of organic chemistry. They are molecules that have the same molecular formula but different structures or arrangements of atoms. This slight difference in structure leads to different properties, even though they are made of the same elements in the same proportions.
In the exercise provided, we deal with two isomers: ethanol and dimethyl ether. Both have the chemical formula ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$, yet their arrangements differ greatly, and as a result, they exhibit very distinct physical properties, such as boiling points.
Understanding isomers helps in recognizing how structure impacts the functionality and characteristics of molecules, which is a core concept in organic chemistry and essential for applications ranging from drug design to material science.

Boiling Point

Boiling point is an important property in chemistry, indicating the temperature at which a liquid changes into a gas. It is influenced by intermolecular forces; strong forces tend to result in higher boiling points.
In the context of our two isomers, ethanol and dimethyl ether, the boiling points vary significantly despite having the same molecular formula. Ethanol boils at ${78.5}^{\circ }\mathrm{C}$, while dimethyl ether boils at $-{23}^{\circ }\mathrm{C}$.
This stark difference can be attributed to the types of intermolecular forces at play in each compound, highlighting the important relationship between molecular structure and physical properties.

Ethanol

Ethanol is a common alcohol with the chemical structure ${\text{CH}}_3{\text{CH}}_2\text{OH}$. It contains a hydroxyl group ($-\text{OH}$), which has a significant impact on its properties. This group allows ethanol to form hydrogen bonds, a type of strong intermolecular force.
Hydrogen bonds are much stronger than other forces like van der Waals forces or dipole-dipole interactions. As a result, ethanol requires more energy in the form of heat to break these bonds when transitioning from the liquid to gas phase, resulting in its higher boiling point of ${78.5}^{\circ }\mathrm{C}$.
This ability to form hydrogen bonds not only affects its boiling point but also influences ethanol's solubility in water, making it an important solvent in both laboratory and industrial applications.

Dimethyl Ether

Dimethyl ether, with the structure ${\text{CH}}_3{\text{OCH}}_3$, is an isomer of ethanol but with very different properties. It lacks the hydroxyl group that characterizes ethanol, and instead has an ether linkage.
In dimethyl ether, the absence of an $\text{OH}$ group means it cannot form hydrogen bonds. The primary intermolecular forces present are dipole-dipole interactions and van der Waals forces, which are significantly weaker than hydrogen bonds.
Consequently, less energy is required to transition dimethyl ether from liquid to gas phase, resulting in a much lower boiling point of $-{23}^{\circ }\mathrm{C}$. Dimethyl ether's structure and properties make it useful as a substitute for some of the functions traditionally performed by other ether compounds or propellants.

Intermolecular Forces

Intermolecular forces are the forces of attraction that occur between neighboring molecules. They play an essential role in determining a substance's physical properties, like boiling and melting points.
There are several types of intermolecular forces:

• Hydrogen Bonds: Occur in molecules with a hydrogen atom attached to a highly electronegative atom (such as oxygen), resulting in stronger bonds.
• Dipole-Dipole Interactions: Occur between molecules with permanent dipoles as a result of uneven distribution of electrons within the molecule.
• Van der Waals Forces: Weak forces that arise from temporary dipoles that occur when electron distributions become uneven.

The difference in boiling points of ethanol and dimethyl ether can be attributed to the types of intermolecular forces each can form. Ethanol, with its hydrogen bonding capability, has a much higher boiling point compared to dimethyl ether, which relies on weaker dipole-dipole and van der Waals forces.