Question

Arrange the following in decreasing order of dipole moment: (I) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (II) \(\mathrm{CH}_{2}=\mathrm{O}\) (III) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O}\) \((\mathrm{IV}) \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{O}\) (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (B) \(\mathrm{IV}>\mathrm{III}>\mathrm{II}>\mathrm{I}\) \((\mathrm{C}) \mathrm{II}>\mathrm{III}>\mathrm{IV}>\mathrm{I}\) (D) IV > II > III > I

Short answer

(D) IV > II > III > I

Step-by-step solution

Identify functional groups and geometry

We will identify the functional groups and the geometry of each compound. (I) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) - Ethene - an alkene with planar geometry. (II) \(\mathrm{CH}_{2}=\mathrm{O}\) - Formaldehyde - Carbonyl group with planar geometry. (III) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O}\) - Acetaldehyde - Carbonyl group and an alkyl group with planar geometry. (IV) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{O}\) - Vinylacetaldehyde - Carbonyl group, alkene group, and an alkyl group with a non-planar geometry.

Determine electronegativity differences between atoms

We will look at the electronegativity differences between the atoms in each compound, focusing on the functional groups that contribute to the dipole moment. (I) In ethene, the electronegativity difference between C and H is small, resulting in a negligible dipole moment. (II) In formaldehyde, the electronegativity difference between C and O is significant, resulting in a higher dipole moment. (III) In acetaldehyde, the electronegativity difference between O, C, and H is significant, resulting in an overall dipole moment. (IV) In vinylacetaldehyde, there are several different electronegativity differences due to the presence of multiple functional groups. The overall dipole moment will be influenced by the vector sum of these individual dipole moments.

Compare the overall dipole moment of each compound

We will now analyze the overall dipole moment of each compound based on the information from Steps 1 and 2: (I) Ethene has a negligible dipole moment. (II) Formaldehyde has a high dipole moment due to the C=O bond. (III) Acetaldehyde has an overall dipole moment, but it's smaller than that of formaldehyde, as the CH3 group reduces the effect of the C=O bond. (IV) Vinylacetaldehyde has the highest overall dipole moment because it has the largest number of functional groups with significant electronegativity differences.

Arrange the compounds in decreasing order of dipole moment

Based on our observations, we can arrange the compounds as follows: (IV) Vinylacetaldehyde > (II) Formaldehyde > (III) Acetaldehyde > (I) Ethene The correct answer is (D) IV > II > III > I.

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for most of the chemical reactions of those molecules. They play a crucial role in determining the properties and reactivity of a compound.
Each functional group has a characteristic structure that can predict the compound's characteristics.

• In ethene \((\mathrm{CH}_{2}=\mathrm{CH}_{2})\), the double bond between carbon atoms is a simple alkene group.
• In formaldehyde \((\mathrm{CH}_{2}=\mathrm{O})\), there is a carbonyl group, which is highly polar.
• Acetaldehyde \((\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O})\) also possesses a carbonyl group along with an alkyl chain.
• Vinylacetaldehyde \((\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{O})\) has a mixture of carbonyl and alkene groups.

These functional groups contribute to the molecule's dipole moment due to their inherent polarities, which leads to the attraction of electrons towards more electronegative atoms or groups.

Electronegativity Differences

Electronegativity describes an atom’s ability to attract shared electrons in a chemical bond. The greater the electronegativity, the more strongly it pulls the electron cloud towards itself. This tendency plays a huge role in determining the dipole moment of a molecule.

• In ethene, the difference in electronegativity between carbon and hydrogen is minimal, so the dipole moment is negligible.
• Formaldehyde has a notable electronegativity difference between carbon and oxygen. Oxygen is significantly more electronegative, thus creating a strong dipole moment.
• Acetaldehyde also shows significant electronegativity differences, primarily between oxygen (more electronegative) and the adjacent carbon, affecting its dipole moment.
• Vinylacetaldehyde, with its complex structure, has several notable electronegativity differences. This includes those similar to acetaldehyde, plus differences from additional vinyl and alkene groups, resulting in a complex, overall higher dipole moment.

Understanding these differences is key to predicting molecular properties like solubility and reactivity.

Planar and Non-Planar Geometry

The geometry of a molecule significantly affects its dipole moment. The spatial arrangement of atoms influences how the individual dipole moments combine.

• Planar molecules, like formaldehyde, have all atoms in a single plane, allowing for a clear alignment of any dipole moments.
• Ethene is also planar, which gives a symmetric distribution of charges and leads to minimal net dipole moment.
• Acetaldehyde, despite the presence of a more complex structure, is predominantly planar.
• Vinylacetaldehyde is considered non-planar due to the presence of multiple functional groups and double bond sequences, which introduce three-dimensional complexity into its structure.

In planar molecules, the dipole moments can either cancel each other out or add up straightforwardly. In non-planar molecules, the vectors of individual dipole moments might not align perfectly, resulting in a greater unpredictability and usually a higher net dipole moment.