Petroleum ether is added before crystallizing the product. Which of the following best fits the character of petroleum ether?
A Polar Protic
B Polar Aprotic
C Non-Polar Protic
D Non-Polar Aprotic
The correct answer and explanation is:
The correct answer is D) Non-Polar Aprotic.
Petroleum ether is a mixture of hydrocarbons, primarily alkanes, and it is often used in the crystallization process because of its ability to dissolve non-polar substances while not dissolving more polar compounds. In the context of solvent properties, understanding petroleum ether’s characteristics is important for explaining why it is used in crystallization.
- Non-Polar: Petroleum ether is considered non-polar because it lacks significant polarity due to its hydrocarbon composition. It does not have a dipole moment, which is why it is less likely to interact with polar molecules. This non-polar nature makes it an ideal solvent for dissolving non-polar substances or for washing away non-polar impurities during crystallization.
- Aprotic: Aprotic solvents are those that do not have a hydrogen atom attached to an electronegative atom such as oxygen or nitrogen. Since petroleum ether consists of aliphatic hydrocarbons (without any –OH, –NH, or –SH groups), it does not possess hydrogen bonding capabilities. This absence of hydrogen bonding interactions makes it an aprotic solvent. Aprotic solvents do not donate hydrogen ions, which can be an important factor when it comes to the crystallization process or other chemical reactions.
The non-polar aprotic nature of petroleum ether allows it to help crystallize compounds by selectively dissolving certain impurities without participating in hydrogen bonding, which could interfere with the crystallization of the target compound. This characteristic is useful because it does not dissolve polar or hydrogen-bonding compounds that could disrupt the formation of the desired crystals.