Question

Which of the following has zero dipole moment? (a) 1-butene (b) Cis-2-butene (c) Trans-2-butene (d) 2 methyl-1-propene

Short answer

Trans-2-butene has a zero dipole moment.

Step-by-step solution

Analyzing 1-butene

1-butene has a simple structure with a double bond between the first and second carbon atoms. The molecule is not symmetrical as the ends have different groups, resulting in a non-zero dipole moment.

Analyzing Cis-2-butene

Cis-2-butene has both methyl groups on the same side of the double bond. This lack of symmetry usually results in a dipole moment as the vector sum of bond dipoles is not zero.

Analyzing Trans-2-butene

Trans-2-butene has its methyl groups on opposite sides of the double bond, creating a symmetrical structure. This symmetry causes the dipole moments of the bonds to cancel each other out, resulting in a zero dipole moment.

Analyzing 2 methyl-1-propene

2 methyl-1-propene is an unsymmetrical molecule due to the presence of the methyl group attached to the first carbon. This results in a non-zero dipole moment.

Key concepts

Dipole Moment

The dipole moment of a molecule is a measure of the extent of charge separation within the molecule. It is particularly important in understanding molecular interactions and properties like polarity. Simply put, a dipole moment occurs when there is an uneven distribution of electrons between atoms of a molecule. This makes one end of the molecule slightly negative and the other end slightly positive.

Dipole moment is measured in Debye units, where one Debye is approximately equal to the dipole moment generated by two charges of one electron charge separated by 0.2082 nanometers.

A dipole moment can result from:

• Differences in electronegativity between atoms in a molecule, which create partial positive and negative charges.
• Geometrical arrangements of atoms that do not allow complete cancellation of these charges.

For example, a molecule like cis-2-butene has an inherent dipole moment because of the placement of identical groups, like methyl, on the same side, leading to an imbalance. Conversely, trans-2-butene has a dipole moment of zero due to the opposing alignment of these groups balancing out the charges.

Symmetry in Molecules

Symmetry in molecules plays an essential role in determining their physical and chemical properties, including dipole moments. Symmetry refers to the balanced arrangement of atoms and groups around a central axis or point within a molecule.

In many symmetrical molecules, dipole moments cancel each other out. This occurs because the molecular geometry ensures that any charge separations are equally opposed by an opposite charge in the structure.

• Molecules like carbon dioxide (CO₂) exhibit linear symmetry, leading to a zero dipole moment.
• Trans-2-butene has similar symmetry around its double bond, resulting in a complete cancellation of dipole moments and a non-polar character.

Understanding symmetry can help predict and explain the stability and reactivity of substances. So, when presented with a molecule, consider the positions and types of atoms or groups to assess symmetry.

Isomers in Organic Chemistry

Isomers are molecules that share the same molecular formula but have different structural arrangements. This fundamentally affects their physical properties and chemical behavior. In organic chemistry, isomers can be classified into several types, with geometrical (cis-trans) isomerism being one of the most common.

Cis-trans isomerism occurs due to the restricted rotation around double bonds or within cyclic structures, leading to distinct spatial arrangements of atoms or groups.

• In cis isomers, similar groups are on the same side of the double bond, often resulting in a compound with a net dipole moment.
• Trans isomers, with groups on opposite sides, generally have less dipole moment due to greater symmetry.

These differences in isomers, like in the compounds cis-2-butene and trans-2-butene, showcase their importance in studying molecular structures. These isomers exhibit significantly different dipole moments due to their structural differences, which directly influence their interactions with other molecules.