Problem 96 - Chemistry The Molecular Science

Question

Organic compounds with structures based on benzene, \(\mathrm{C}_{6} \mathrm{H}_{6},\) can be formed by substituting an atom or a group of atoms for one of the hydrogens. Such substituted benzenes have their own properties, different from benzene and from each other. Explain the order of experimental boiling points for these four compounds. (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\left(80{ }^{\circ} \mathrm{C}\right)\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\left(131{ }^{\circ} \mathrm{C}\right)\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\left(156^{\circ} \mathrm{C}\right)\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\left(182{ }^{\circ} \mathrm{C}\right)\)

Step-by-Step Solution

Answer

The boiling points increase with stronger intermolecular forces: Benzene < Chlorobenzene < Bromobenzene < Phenol.

Step 1: Understanding Boiling Points

Boiling point is the temperature at which a substance changes from liquid to gas. It depends on intermolecular forces: stronger forces result in higher boiling points.

Step 2: Analyzing Benzene ( C_{6}H_{6} )

Benzene has the simplest structure among the given compounds, with only C-H bonds. It has weak London dispersion forces, resulting in a relatively low boiling point of 80°C.

Step 3: Chlorobenzene ( C_{6}H_{5}Cl )

Chlorobenzene has a chlorine atom attached to the benzene ring. The C-Cl bond is polar, which introduces dipole-dipole forces in addition to London dispersion forces. This strengthens intermolecular attraction, leading to a higher boiling point of 131°C.

Step 4: Bromobenzene ( C_{6}H_{5}Br )

Bromobenzene is similar to chlorobenzene but with a bromine atom. Bromine is larger and more polarizable than chlorine, resulting in stronger London dispersion forces and a further increased boiling point of 156°C.

Step 5: Phenol ( C_{6}H_{5}OH )

Phenol has an -OH group which can form hydrogen bonds, the strongest type of intermolecular force. This significantly raises the boiling point to 182°C, as hydrogen bonding adds strong, additional intermolecular attraction.

Key Concepts

Intermolecular Forces

Intermolecular forces are the weak forces of attraction between molecules. They are crucial in determining the physical properties of substances, like boiling and melting points. There are several types of intermolecular forces, each with varying strengths:

• London Dispersion Forces: These are the weakest forces, arising from temporary dipoles created when electrons move around a molecule.
• Dipole-Dipole Forces: These occur between molecules with permanent dipoles, where positive and negative ends of different molecules attract each other.
• Hydrogen Bonding: This is a special, stronger type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

A stronger force means that more energy is needed to separate the molecules, leading to higher boiling points. When analyzing substituted benzene compounds, these intermolecular forces play a pivotal role in their boiling point determination.

Organic Chemistry

Organic chemistry is a branch of chemistry focused on carbon-based compounds. These include a diverse array of structures and compounds, many of which are found in living organisms. Carbon's unique ability to bond with itself and other elements forms the basis of organic chemical reactions and structures.
In organic chemistry, the bohring point of compounds can be influenced by different factors:

• Functional Groups: These are specific groups of atoms within molecules that are responsible for characteristic chemical reactions. For example, the (-OH) group in alcohols can participate in hydrogen bonding.
• Substituents: The atoms or groups of atoms that replace hydrogen in organic molecules, like the chlorine or bromine in chlorobenzene or bromobenzene, can influence a compound's properties, including boiling point.

Understanding the structural diversity and reactive nature of organic compounds is key to understanding how compounds like substituted benzenes behave differently.

Substituted Benzene Compounds

Substituted benzene compounds are derived by replacing one or more hydrogen atoms in benzene with other atoms or groups. This substitution can significantly alter the chemical and physical properties of the compound, including its boiling point.
In the context of the given problem:

• Benzene ( (C_{6}H_{6}) ): This is the parent hydrocarbon with a relatively low boiling point due to weak dispersion forces.
• Chlorobenzene ( (C_{6}H_{5}Cl) ): Introducing a chlorine atom creates a dipole-dipole force, raising the boiling point compared to benzene.
• Bromobenzene ( (C_{6}H_{5}Br) ): Replacing chlorine with the larger bromine increases polarizability, further increasing boiling point.
• Phenol ( (C_{6}H_{5}OH) ): The presence of an (-OH) group allows hydrogen bonding, resulting in the highest boiling point among these substances.

The type of substituent and its ability to alter intermolecular forces are key to understanding why these benzene derivatives have dissilar boiling points.