Question

The molecules that will have dipole moment (1) 2,2 -dimethylpropane (2) trans-1-pent-2-ene (3) cis-hex-3-ene (4) \(2,2,3,3\)-tetramethylbutane (a) 2,3 (b) 1,3 (c) \(1,2,3\) (d) all of these

Short answer

Only cis-hex-3-ene has a dipole moment. The correct answer is (a) 2,3.

Step-by-step solution

Understanding Molecular Dipoles

A dipole moment occurs in a molecule when there is a separation of charge due to differences in electronegativity between bonded atoms. This results in one end of the molecule having a partial negative charge and the other a partial positive charge.

Analyzing 2,2-Dimethylpropane

2,2-Dimethylpropane is a symmetrical alkane with no difference in electronegativity between its carbon and hydrogen bonds. This molecule is symmetrical, resulting in no net dipole moment.

Analyzing trans-1-pent-2-ene

In trans-1-pent-2-ene, the trans configuration across the double bond creates a symmetrical arrangement, where the slight polarities cancel each other out, leading to no net dipole moment.

Analyzing cis-hex-3-ene

In cis-hex-3-ene, the cis configuration leads to asymmetry because the substituents on the double bonded carbon atoms are on the same side. This does not cancel out potential dipole moments, resulting in a net dipole moment.

Analyzing 2,2,3,3-Tetramethylbutane

2,2,3,3-Tetramethylbutane is also symmetrical. The symmetry of the structure ensures that any polarities cancel each other out, resulting in no net dipole moment.

Identifying Molecules with Dipole Moment

Based on the analysis, only the molecule cis-hex-3-ene (step 4) has a dipole moment due to its asymmetry. Therefore, the correct choices for molecules with a dipole moment are molecules in option (a) 2,3.

Key concepts

Molecular Symmetry

At its core, molecular symmetry refers to a situation in which a molecule has a balance of shape and charge. Symmetry affects whether a molecule will have a dipole moment, which results when charges in a molecule are unbalanced. In chemistry, molecules with perfect symmetry often do not have a dipole moment because the electric charges are evenly distributed. For example, in symmetrical molecules like 2,2-Dimethylpropane and 2,2,3,3-Tetramethylbutane, any polarities arising from electronegativity differences are canceled out.

Symmetry includes various elements such as rotational axes or planes of symmetry. When these elements feature prominently, the molecule’s physical and electronic structure is such that it can be divided into identical halves. This means each half reflects or rotates to match the other perfectly. As a result, the molecule tends to be non-polar, and any possible dipole moments are negated. Even if some bonds are polar, the symmetric geometry ensures that the polar effects are neutralized, leading to no net dipole moment.

Electronegativity

Electronegativity is a fundamental concept that helps predict the kind of bond that forms between two atoms. It represents an atom's ability to attract and bind with electrons. In molecules, differences in electronegativity between bonded atoms can lead to polar bonds and, when not canceled out by symmetry, to a dipole moment. For instance, the carbon-hydrogen bonds in 2,2-Dimethylpropane exhibit no significant electronegativity difference, maintaining a non-polar character.

Elements with high electronegativity tend to attract electrons strongly. Fluorine, oxygen, and nitrogen are elements known for having high electronegativities. Checking the Periodic Table gives a clear picture, where electronegativity increases across a period from left to right and decreases down a group.

• When atoms of differing electronegativities bond, the shared electrons tend to associate more with one atom.
• This results in one atom having a partial negative charge and the other a partial positive charge, creating a polar bond.

Understanding electronegativity is crucial when analyzing dipole moments in molecules, as it directly affects the charge distribution.

Alkene Structures

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The presence of this double bond directly influences the geometry and symmetry of the molecule, which, in turn, can affect the dipole moment. For instance, in alkenes like trans-1-pent-2-ene and cis-hex-3-ene, the arrangement of hydrogen and carbon groups around the double bond determines whether the molecule is symmetrical or asymmetrical.

In the case of trans-alkenes, such as trans-1-pent-2-ene, substituents on opposite sides of the double bond typically create symmetry. This can lead to a cancelation of dipole effects, resulting in no net dipole moment. Conversely, for cis-alkenes like cis-hex-3-ene, substituents are on the same side of the double bond. This asymmetrical placement does not allow for cancellation of the dipole moments and often results in a net dipole moment.

• The presence of a double bond restricts rotation, giving these alkenes a fixed geometry.
• Such a geometry influences how molecular forces like polarity and dipole moments occur.

Understanding these structures helps in predicting and rationalizing the behavior of alkenes in chemical reactions and their properties.