Question

Arrange the following in decreasing order of boiling point: $$ \mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{CHCl}_{3}, \mathrm{CCl}_{4} $$ (A) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{2} \mathrm{Cl}_{2}>\mathrm{CHCl}_{3}>\mathrm{CCl}_{4}\) (B) \(\mathrm{CH}_{3} \mathrm{Cl}<\mathrm{CH}_{2} \mathrm{Cl}_{2}<\mathrm{CHCl}_{3}<\mathrm{CCl}_{4}\) (C) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}>\mathrm{CHCl}_{3}>\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CCl}_{4}\) (D) \(\mathrm{CHCl}_{3}>\mathrm{CH}_{2} \mathrm{Cl}_{2}>\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CCl}_{4}\)

Short answer

The correct arrangement of the compounds in decreasing order of boiling point is: \(CHCl_3 > CH_2Cl_2 > CH_3Cl > CCl_4\).

Step-by-step solution

Identify the Intermolecular Forces Present in Each Molecule

In halogen-containing organic compounds, certain intermolecular forces can be present, such as Van der Waals forces, dipole-dipole interactions, and hydrogen bonding. However, hydrogen bonding is not present in any of these compounds since none of them has a hydrogen atom directly bonded to a highly electronegative atom (like N, O, or F). As a result, we just have to consider the impact of Van der Waals forces and dipole-dipole interactions on boiling point.

Determine the Polarity of the Molecules and the Strength of Intermolecular Forces

Molecules with a net molecular dipole moment are polar, which usually results in stronger dipole-dipole interactions and consequently higher boiling points. For the given molecules, we can determine their polarity as follows: \(CH_3Cl\): Polar, as the chlorine atom is more electronegative than carbon and hydrogen atoms, leading to a net molecular dipole. \(CH_2Cl_2\): Polar, as the molecule contains two chlorine atoms bonded to a carbon. This creates a net molecular dipole, leading to strong dipole-dipole interactions. \(CHCl_3\): Polar, as the electronegativity difference between carbon and chlorine generates a net molecular dipole. \(CCl_4\): Non-polar, as the molecular geometry is tetrahedral with all four chlorine atoms having the same electronegativity. The dipole vectors cancel each other out, resulting in a non-polar molecule.

Consider the Molecular Weight

Molecular weight also affects boiling point. In this case, the molecular weights are: \(CH_3Cl\): 1 Carbon + 3 Hydrogen + 1 Chlorine = 50 g/mol \(CH_2Cl_2\): 1 Carbon + 2 Hydrogen + 2 Chlorine = 85 g/mol \(CHCl_3\): 1 Carbon + 1 Hydrogen + 3 Chlorine = 119.5 g/mol \(CCl_4\): 1 Carbon + 4 Chlorine = 153.8 g/mol

Rank the Compounds by Boiling Point

Based on the polarity and molecular weights, we can now rank the compounds in decreasing order of boiling point. Polar molecules have stronger intermolecular forces than non-polar molecules with similar molecular weights, so we expect their boiling points to be higher. (B) is incorrect because it ranks boiling points in increasing order. (A) is incorrect because \(CCl_4\) should have a higher boiling point than \(CH_3Cl\) due to the larger molecular size. (C) and (D) are between the polar molecules \(CH_2Cl_2\), \(CHCl_3\), and \(CH_3Cl\). Since \(CHCl_3\) and \(CH_2Cl_2\) have higher molecular weights than \(CH_3Cl\), they also have stronger intermolecular forces, resulting in higher boiling points. Comparing \(CHCl_3\) and \(CH_2Cl_2\), we can see that \(CHCl_3\) has a higher molecular weight and should have a higher boiling point than \(CH_2Cl_2\). Therefore: (D) \(\mathrm{CHCl}_{3}>\mathrm{CH}_{2} \mathrm{Cl}_{2}>\mathrm{CH}_{3}\mathrm{Cl}>\mathrm{CCl}_{4}\) is the correct answer.

Key concepts

Intermolecular Forces

Intermolecular forces are attractions between molecules that determine many properties, such as boiling points. When a substance boils, its intermolecular forces must be overcome. There are several types of intermolecular forces:

• **Van der Waals Forces**: These are weak interactions that are present in all molecules, whether polar or non-polar. They are strongest in substances with larger molecules and higher molecular weights.
• **Dipole-Dipole Interactions**: These occur in polar molecules where a partial positive charge on one end is attracted to a partial negative charge on another molecule. This type of force is stronger than Van der Waals forces and thus increases the boiling point.
• **Hydrogen Bonding**: This strong type of dipole-dipole interaction occurs when hydrogen is bonded to highly electronegative elements like nitrogen, oxygen, or fluorine. It is not relevant for the given organic halogen compounds because they don't have hydrogen bonded to such elements.

Understanding the type and strength of intermolecular forces helps predict the relative boiling points of different compounds.

Polar and Non-polar Molecules

Molecular polarity greatly influences a compound's boiling point. A polar molecule has a net dipole moment due to an uneven distribution of electrons, resulting in sides that carry different charges. These charges lead to stronger intermolecular forces and thus, higher boiling points.

• For example, **CH₃Cl** is polar because the chlorine atom is more electronegative, pulling electrons towards itself and leaving a partial positive charge on the carbon-hydrogen side.
• **CH₂Cl₂** and **CHCl₃** are also polar due to the presence of multiple chlorine atoms creating a net dipole.
• Conversely, **CCl₄** is non-polar as its symmetrical tetrahedral structure allows for an even distribution of charge. The dipole moments cancel each other out, making the molecule non-polar.

Polarity influences boiling points significantly, as polar molecules have stronger dipole-dipole attractions compared to non-polar molecules, which only interact through weaker Van der Waals forces.

Halogenated Compounds

Halogenated compounds are organic molecules that contain one or more halogen atoms, such as chlorine, attached to a carbon backbone. The presence of halogens like chlorine affects several properties of these compounds, including their boiling points.

• **Electronegativity and Polarity**: Chlorine atoms are highly electronegative, contributing to the polar nature of molecules like **CH₃Cl**, **CH₂Cl₂**, and **CHCl₃**. This polarity results in dipole-dipole interactions, raising the boiling points.
• **Molecular Weight**: Halogenated compounds often have higher molecular weights due to the presence of heavy halogen atoms. Higher molecular weight generally means stronger Van der Waals forces, leading to higher boiling points, as seen in **CCl₄** which, despite being non-polar, still has a relatively high boiling point due to its molecular weight.
• **Properties and Uses**: These compounds vary in terms of boiling points and can be used in different industrial applications, from solvents to refrigerants, based on their physical properties, which are affected by the type and number of halogen atoms present.

In summary, the halogenation of organic compounds affects their intermolecular interactions, polarity, and thus their boiling points, playing a crucial role in their chemical behavior and applications.