Problem 4 - Chemistry A Guided Inquiry

Question

Recall that the electronegativity of \(\mathrm{C}\) and \(\mathrm{H}\) are roughly the same, but that \(\mathrm{O}\) has a significantly higher electronegativity. For each type of compound (alkane, ketone, alcohol) predict whether or not the compound is expected to be polar or nonpolar.

Step-by-Step Solution

Answer

Alkanes are nonpolar, whereas ketones and alcohols are both polar.

Step 1: Identify Polarity in Alkane

Alkanes consist of carbon and hydrogen atoms. Since the electronegativity of carbon and hydrogen is roughly the same, the bonding electrons are equally shared between them, causing the bond to be nonpolar. Moreover, alkanes have a symmetric molecular structure which doesn't allow a buildup of charges on any side, affirming that alkanes are nonpolar.

Step 2: Identify Polarity in Ketone

Ketones have a carbonyl functional group which involves a carbon-oxygen double bond. Oxygen is significantly more electronegative than carbon, so the electrons in these bonds are not equally shared - they are pulled more towards the oxygen atom, making the bond polar. Overall, the presence of the carbonyl group makes the entire molecule polar.

Step3: Identify Polarity in Alcohol

Alcohols have a hydroxy functional group. This is a bond between an oxygen and a hydrogen atom - there is a big difference in electronegativity between these two, leading to a polar bond. Therefore, alcohols are polar molecules.

Key Concepts

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. Each element has its own electronegativity value, which helps us understand how atoms will interact with each other in molecules.
Carbon ( C ) and hydrogen ( H ) have similar electronegativity values, which means they share their electrons almost equally when they bond. This equal sharing typically leads to nonpolar covalent bonds. On the other hand, oxygen ( O ) has a much higher electronegativity than both carbon and hydrogen. When oxygen is involved in a bond, it tends to pull electrons closer to itself, causing polar covalent bonds.

• Carbon and Hydrogen: Similar electronegativity, nonpolar bonds.
• Oxygen: Higher electronegativity, forms polar bonds.

Understanding electronegativity is crucial because it influences the polarity of molecules, which affects properties like solubility and boiling points.

Alkanes

Alkanes are hydrocarbons consisting solely of carbon and hydrogen atoms. The carbon-to-hydrogen bonds within alkanes are generally nonpolar, as mentioned earlier, due to their similar electronegativity values. These molecules tend to be quite symmetrical, which means that there is a uniform distribution of electron charge.
This symmetry is key in confirming that alkanes are nonpolar. Nonpolar substances like alkanes typically do not mix well with polar substances like water. Instead, they are more soluble in other nonpolar solvents. This quality is important for understanding how alkanes behave in different chemical reactions and environments.

Ketones

Ketones are characterized by the presence of a carbonyl group, which is a carbon atom double-bonded to an oxygen atom. This carbonyl group is what makes ketones unique, as it introduces polarity into the molecule.
Since oxygen is much more electronegative than carbon, it attracts the shared electrons towards itself, creating a polar bond. This polar bond within the carbonyl group leads to an overall dipole moment in the molecule, allowing it to interact more readily with polar substances. As a result, ketones are generally polar molecules, making them more soluble in water and affecting their boiling point compared to similar nonpolar compounds.

Alcohols

Alcohols are organic compounds that contain one or more hydroxyl ( OH ) groups. The O-H bond within the hydroxyl group is highly polar because of the significant difference in electronegativity between oxygen and hydrogen.
This polar bond results in strong intermolecular forces such as hydrogen bonding, significantly influencing the physical properties of alcohols. Because of these polar characteristics, alcohols are often able to dissolve well in water, contrasting with nonpolar alkanes. The polarity of alcohols is also a key factor in their relatively high boiling points for organic compounds of similar size.