Question

Which have zero dipole moment? (1) 1,1 dichloroethane (2) cis-1,2-dichloroethane (3) trans- 1,2 -dichloroethane (4) none

Short answer

trans-1,2-dichloroethane

Step-by-step solution

Understand Molecular Dipole Moment

A molecule has a dipole moment if there is an uneven distribution of electron density. This can happen due to differences in electronegativity and the geometry of the molecule. Determine if the individual dipoles cancel each other out.

Analyze 1,1 dichloroethane

In 1,1 dichloroethane, both chlorine atoms are bonded to the same carbon atom, resulting in two dipoles pointing towards the chlorines on the same side. The dipoles do not cancel out, leading to a non-zero dipole moment.

Analyze cis-1,2-dichloroethane

In cis-1,2-dichloroethane, the chlorine atoms are on the same side of the double bond. The dipole moments do not cancel out because they are pointing in the same general direction, resulting in a non-zero dipole moment.

Analyze trans-1,2-dichloroethane

In trans-1,2-dichloroethane, the chlorine atoms are on opposite sides of the double bond. The dipole moments are equal in magnitude but opposite in direction, leading to their cancellation. Thus, this molecule has a zero dipole moment.

Conclusion

Based on the analysis, trans-1,2-dichloroethane has a zero dipole moment. Therefore, the answer is trans-1,2-dichloroethane.

Key concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. This arrangement plays a crucial role in determining the molecule's properties, such as polarity and dipole moment. For instance, in molecules like 1,1-dichloroethane, the geometry involves a central carbon atom bonded to two chlorine atoms on the same side. This results in an uneven distribution of electron density and a non-zero dipole moment.
By contrast, in trans-1,2-dichloroethane, the chlorine atoms are opposite each other around a carbon-carbon double bond, making the molecule symmetrical. This symmetry cancels out the individual dipoles, leading to a zero dipole moment. Understanding molecular geometry helps in predicting how dipoles interact within the molecule.

Electronegativity

Electronegativity is the ability of an atom to attract electrons towards itself. It varies across the periodic table, with fluorine being the most electronegative element. When atoms with different electronegativities form bonds, the electrons are not shared equally, creating a dipole.
For example, in dichloroethane molecules, chlorine is more electronegative than carbon, creating dipoles pointing towards the chlorines. In 1,1-dichloroethane and cis-1,2-dichloroethane, these dipoles do not cancel out, leading to a net molecular dipole moment.
On the other hand, in trans-1,2-dichloroethane, the dipoles created by the two chlorine atoms are opposite and equal, resulting in no net dipole moment for the molecule.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism where molecules with the same molecular formula differ in the spatial arrangement of their atoms. This is common in molecules with double bonds or rings.
In cis-1,2-dichloroethane, the chlorine atoms are on the same side of the double bond, resulting in a non-zero dipole moment due to the additive effects of the dipoles in the same direction.
In contrast, trans-1,2-dichloroethane has chlorine atoms on opposite sides of the double bond. This symmetrical arrangement cancels out the dipoles, resulting in a zero dipole moment. Understanding cis and trans isomerism is essential for predicting molecular polarity.

Organic Chemistry Principles

Organic chemistry principles, such as those involving dipole moments, stereochemistry, and electronegativity, are foundational for understanding molecular behavior. These principles help explain why different isomers of the same molecule (like the dichloroethanes) can have vastly different properties.
For instance, knowing that the arrangement of atoms in space (molecular geometry) and the differences in electronegativity between atoms can lead to polar bonds is crucial. This polarity affects the molecule's interactions with other substances, influencing properties like solubility and reactivity.
Overall, mastering these principles allows students to predict and explain the outcomes of various organic reactions and the physical properties of compounds.