Question

Which member in each pair has the stronger intermolecular dispersion forces? (a) Br_ or \(\mathrm{O}_{2},\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH},(\mathbf{c}) \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Clor}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\)

Short answer

In summary, the molecules with stronger intermolecular dispersion forces for each pair are: (a) Br₂, (b) CH₃CH₂CH₂CH₂CH₂SH, and (c) CH₃CH₂CH₂Cl.

Step-by-step solution

Pair (a): Br₂ and O₂

In this pair, we have Br₂ (Bromine) and O₂ (Oxygen) molecules. Br₂ is a larger molecule with more electrons (70 electrons) compared to the O₂ molecule (16 electrons). As the dispersion forces are dependent on the size of the molecule and the number of electrons, Bromine (Br₂) has stronger intermolecular dispersion forces than Oxygen (O₂).

Pair (b): CH₃CH₂CH₂CH₂SH and CH₃CH₂CH₂CH₂CH₂SH

In this case, we are comparing two organic molecules: 1. CH₃CH₂CH₂CH₂SH (4 carbons and 1 sulfur atom) 2. CH₃CH₂CH₂CH₂CH₂SH (5 carbons and 1 sulfur atom) The first molecule has a total of 34 electrons, while the second molecule has 42 electrons. The second molecule, CH₃CH₂CH₂CH₂CH₂SH, has more electrons and is larger than the first molecule. Therefore, the second molecule has stronger intermolecular dispersion forces.

Pair (c): CH₃CH₂CH₂Cl and (CH₃)₂CHCl

In this pair, we have to compare: 1. CH₃CH₂CH₂Cl (3 carbons and 1 chlorine atom) 2. (CH₃)₂CHCl (3 carbons and 1 chlorine atom) Both molecules have the same number of atoms and electrons (26 electrons each). However, their structure differs. In the first molecule, the chlorine atom is attached to a carbon with two neighboring carbons, while in the second molecule, the chlorine atom is attached to a carbon with two methyl (CH₃) groups attached. The second molecule has a more branched structure, which can lead to weaker dispersion forces due to a reduced surface area. Therefore, CH₃CH₂CH₂Cl has slightly stronger intermolecular dispersion forces than (CH₃)₂CHCl.

Key concepts

Br2 vs O2 Molecules

When comparing bromine ( Br₂) and oxygen ( O₂) molecules, it's crucial to understand the impact of electron count on intermolecular dispersion forces. Dispersion forces, also known as London dispersion forces, are a type of van der Waals force. These are the weakest intermolecular forces but significantly influence how molecules interact and how substances behave. Br₂ and O₂ both exist as diatomic molecules but differ greatly in their electron count. Br₂, with its 70 electrons, is a much larger molecule electron-wise compared to O₂, which has only 32 electrons in total. The greater the number of electrons, the more readily temporary dipoles can form, giving rise to stronger dispersion forces.

• Br₂ - Large size, 70 electrons
• O₂ - Smaller size, 32 electrons

The large number of electrons in Br₂ results in greater moments of instantaneous polarization across the molecule. Consequently, Br₂ exhibits stronger intermolecular dispersion forces than O₂.

Organic Molecule Comparison

When determining which organic molecules possess stronger dispersion forces, we often look at molecular size and the number of electrons. For example, in organic compounds like CH₃CH₂CH₂CH₂SH and CH₃CH₂CH₂CH₂CH₂SH, the difference in carbon chain length and electron count is crucial.

• CH₃CH₂CH₂CH₂SH - 4 carbon atoms, 1 sulfur, 34 electrons
• CH₃CH₂CH₂CH₂CH₂SH - 5 carbon atoms, 1 sulfur, 42 electrons

The presence of more electrons in the longer carbon chain of CH₃CH₂CH₂CH₂CH₂SH enhances the dispersion forces due to greater transient polarizability. Simply put, more electrons allow more significant temporary dipole interactions between molecules, thus leading to stronger dispersion forces. Hence, the molecule with the longer carbon chain, in this case, CH₃CH₂CH₂CH₂CH₂SH, demonstrates stronger dispersion forces.

Molecular Structure and Dispersion Forces

The structure of organic molecules directly affects their ability to interact via dispersion forces. Consider the comparison between CH₃CH₂CH₂Cl and (CH₃)₂CHCl. These molecules possess the same number of electrons but differ structurally, influencing their intermolecular interactions.

• CH₃CH₂CH₂Cl - Straight chain structure
• (CH₃)₂CHCl - Branched structure

The linearity or branching of a molecule affects its surface area and, therefore, the strength of its dispersion forces. Molecules with a straight chain configuration, like CH₃CH₂CH₂Cl, have a greater surface area. This allows for closer contact and interaction between molecules, resulting in stronger dispersion forces. On the other hand, branched molecules like (CH₃)₂CHCl, have a smaller contact area, reducing the efficiency of dispersion interactions. Therefore, despite having the same electron count, CH₃CH₂CH₂Cl exhibits slightly stronger intermolecular dispersion forces due to its more extended structure.