How is overall molecular polarity affected by molecular symmetry (specifically shown for alkenes in book)? And what are the consequences for melting and boiling points (in cases of high molecular symmetry versus low molecular symmetry)?
To determine the overall molecular polarity of a molecule, we need to analyze the polarity of its individual bonds and the molecular geometry. Molecular symmetry plays a significant role in this analysis.
For alkenes, which contain carbon-carbon double bonds, molecular symmetry can affect the overall polarity. To understand this, we first need to consider the concept of bond polarity. A bond between two atoms is polar if the electronegativity (ability to attract electrons) of the two atoms differs. In general, carbon and hydrogen have similar electronegativities, so a carbon-hydrogen bond is considered nonpolar.
Next, let's examine the molecular geometry of alkenes. The double bond in alkenes restricts the rotation around the carbon-carbon axis, leading to a planar or nearly planar structure. This planar geometry often results in molecular symmetry.
In high molecular symmetry alkenes, such as ethene (C2H4), the double bond is centrally located between two carbon atoms, with two hydrogen atoms on each carbon. Since carbon and hydrogen have similar electronegativities, the carbon-hydrogen bonds are nonpolar. Due to the symmetric arrangement of the bonds and the molecule's planar geometry, the polarities of the individual bonds cancel each other out, resulting in an overall nonpolar molecule.
In contrast, low molecular symmetry alkenes, such as propene (C3H6), have a slightly different arrangement. In propene, the double bond is located between two carbon atoms, but there is an additional single bond to another carbon atom. This asymmetric arrangement creates an imbalance in the distribution of electron density, resulting in a polar molecule.
Now let's explore the consequences of molecular symmetry on melting and boiling points. Generally, substances with higher molecular symmetry tend to have lower melting and boiling points compared to substances with lower molecular symmetry. This is because molecules with high symmetry pack together more efficiently, allowing for weaker intermolecular forces (forces between molecules). Weaker intermolecular forces require less energy to overcome, thus resulting in lower melting and boiling points.
In the context of alkenes, alkenes with high molecular symmetry, such as ethene (C2H4), have weaker intermolecular forces and, therefore, lower melting and boiling points compared to alkenes with lower molecular symmetry, such as propene (C3H6).
To summarize, molecular symmetry can affect the overall molecular polarity of alkenes. High molecular symmetry generally leads to nonpolar molecules, while low molecular symmetry often results in polar molecules. The consequences for melting and boiling points are that higher molecular symmetry typically correlates with lower melting and boiling points due to weaker intermolecular forces.