For each pair of compounds, predict the one with a higher boiling point. Which compounds have zero dipole moments?
a. cis-1,2-dichloroethene or cis-1,2-dibromoethene

Verified step by step guidance

1. Step 1: Understand that boiling point is influenced by molecular weight and intermolecular forces. Compounds with stronger intermolecular forces or higher molecular weights generally have higher boiling points.
2. Step 2: Compare the molecular weights of cis-1,2-dichloroethene and cis-1,2-dibromoethene. Bromine is heavier than chlorine, so cis-1,2-dibromoethene has a higher molecular weight.
3. Step 3: Consider the types of intermolecular forces present. Both compounds have dipole-dipole interactions due to the polar C-Cl and C-Br bonds, but the heavier bromine atoms in cis-1,2-dibromoethene contribute to stronger London dispersion forces.
4. Step 4: Predict that cis-1,2-dibromoethene will have a higher boiling point due to its higher molecular weight and stronger London dispersion forces.
5. Step 5: Determine the dipole moment. Both compounds are cis isomers, which means they have a net dipole moment due to the asymmetrical arrangement of the polar bonds. Therefore, neither compound has a zero dipole moment.

Key Concepts

Boiling Point and Intermolecular Forces

The boiling point of a compound is influenced by the strength of its intermolecular forces. Stronger forces, such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces, result in higher boiling points. In comparing similar compounds, the size and polarizability of the molecules can also affect the boiling point, with larger, more polarizable molecules typically having higher boiling points.

Dipole Moment

A dipole moment occurs in a molecule when there is an uneven distribution of electron density, resulting in a partial positive and negative charge. Molecules with symmetrical structures, like trans isomers, often have zero dipole moments because the individual bond dipoles cancel each other out. In contrast, asymmetrical molecules, such as cis isomers, usually have a net dipole moment due to the non-cancellation of bond dipoles.

Cis and Trans Isomerism

Cis and trans isomerism is a type of stereoisomerism where molecules have the same formula but differ in the spatial arrangement of atoms around a double bond. In cis isomers, similar groups are on the same side of the double bond, often resulting in a net dipole moment. Trans isomers have similar groups on opposite sides, which can lead to a cancellation of dipole moments, affecting properties like boiling point and solubility.