Question

(a) Indicate the direction of the net dipole moment for each of the dihaloethenes. (b) Would cis-2,3-dichloro-2-butane have a larger or smaller dipole moment than cis-1,2-dichloroethene? (c) Indicate the direction of the net dipolemoment of cis-1,2 dibromo-1,2-dichloroethene. Will it be larger or smaller than the dipole moment of cis-1,2-dichloroethene? Why?

Short answer

The net dipole moment for dihaloethenes will point towards the Chlorine atom as it is more electronegative. Cis-1,2-dichloroethene has a larger dipole moment than cis-2,3-dichloro-2-butene due to the direct attachment of chlorine atoms to double-bonded carbons. In cis-1,2-dibromo-1,2-dichloroethene, the net dipole moment is directed towards the chlorine atom but has a larger magnitude than cis-1,2-dichloroethene due to the added presence of the bromine atoms.

Step-by-step solution

Understanding dipole moment and electronegativity

A dipole moment is a measure of the molecular polarity and depends on the electronegativity difference between the atoms involved. The greater the electronegativity difference between the atoms, the greater the dipole moment.

(a) Direction of the net dipole moment for dihaloethenes

To determine the direction of the net dipole moment for dihaloethenes, we look at the electronegativity of each halogen: Chlorine (\(\chi\) = 3.16), and Bromine (\(\chi\) = 2.96). The net dipole moment will always point towards the more electronegative atom, which in this case is Chlorine.

(b) Comparing dipole moments of cis-2,3-dichloro-2-butene and cis-1,2-dichloroethene

To compare the dipole moments of these two molecules, we will first examine their structural differences: - In cis-2,3-dichloro-2-butene, the dichloro functional group is attached to carbons 2 and 3, providing a secondary alkene configuration. - In cis-1,2-dichloroethene, both chlorine atoms are directly attached to the double-bonded carbons, imparting higher polarity to the molecule. Consequently, cis-1,2-dichloroethene will have a larger dipole moment than cis-2,3-dichloro-2-butene.

(c) Direction of net dipole moment and comparison for cis-1,2-dibromo-1,2-dichloroethene

In cis-1,2-dibromo-1,2-dichloroethene, the presence of both bromine and chlorine atoms causes the net dipole moment to orient towards the more electronegative atoms; in this case, chlorine. Furthermore, the molecule's dipole moment direction will be more pronounced due to the increased electronegativity of the bromines. Depending on the position of the halogens, the dipole moment of cis-1,2-dibromo-1,2-dichloroethene may vary. However, considering a case where Br is bonded with C1, Cl with C2, the net dipole moment will still point towards the chlorine, but the magnitude is larger than that of cis-1,2-dichloroethene due to the added presence of the bromine atoms.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry that describes the tendency of an atom to attract a pair of electrons. This property is crucial in predicting the behavior of molecules, particularly their polarity.

The differences in electronegativity between atoms in a bond can lead to polar bonds where one atom exerts a stronger pull on the bonded electrons. For example, in dihaloethenes such as chloroethene, chlorine's electronegativity (3.16) is greater than that of bromine (2.96).

This difference causes the dipole moment, a vector indicating the direction of charge separation, to point towards the more electronegative chlorine atom. A molecule's overall dipole moment, combining individual bond dipole moments, determines its polarity.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs when atoms or groups in a molecule can adopt different spatial positions around a double bond or a ring structure. The classic example is 1,2-dichloroethene, where we have two possible arrangements: cis or trans.

In the cis isomer, similar groups are on the same side of the double bond, which can result in a net dipole moment as asymmetrical arrangements generate molecular polarity. For trans isomers, the identical groups are on opposite sides, which usually results in the dipoles canceling out, leading to non-polar molecules.

• The cis isomer often has a higher dipole moment due to lack of symmetry.
• The trans configuration is more symmetrical, often leading to little or no net dipole moment.

Understanding cis and trans isomerism is essential in predicting and explaining the properties and behaviors of different organic molecules.

Molecular Polarity

Molecular polarity arises from the distribution of electrical charges across a molecule and influences many physical properties, such as boiling points and solubility. A molecule's polarity also determines its interactions with other substances, such as solvents.

Polarity depends on two main factors: the difference in electronegativity between bonded atoms and the molecular shape.

• If the shape of the molecule allows for the dipole moments of individual bonds to cancel each other out, the molecule is nonpolar.
• If the dipole moments add up, the molecule is polar, as seen in cis-1,2-dichloroethene where the chlorine atoms are on the same side, leading to a net dipole.

This concept is pivotal in understanding how molecules interact within and between different phases.

Dihaloethenes

Dihaloethenes are a group of organic compounds that consist of two halogen atoms attached to an ethene backbone. These compounds exemplify important concepts in organic chemistry such as electronegativity and molecular polarity.

In dihaloethenes, the positioning of the halogens significantly affects the molecule's properties.

• For instance, the geometric isomerism in cis-1,2-dichloroethene leads to a net dipole moment due to the positioning of chlorine atoms.
• In contrast, if the groups were trans, the molecule might not have a net dipole moment.

Understanding the behavior of dihaloethenes helps in comprehending how molecular structure influences chemical reactivity and interactions, particularly when evaluating the performance and suitability of materials in various applications.