Question

Explain why \(\mathrm{CH}_{3} \mathrm{CHO}\) is soluble in water, but \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) has only limited solubility.

Short answer

\(\mathrm{CH}_{3} \mathrm{CHO}\) is soluble in water because it has a polar oxygen-hydrogen bond which can form hydrogen bonds with water. However, even though \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) also has the same polar bond, it also has an additional non-polar hydrocarbon chain, reducing its overall polarity and hence limiting its solubility in water.

Step-by-step solution

Determine the Polarity of \(\mathrm{CH}_{3} \mathrm{CHO}\)

The molecule \(\mathrm{CH}_{3} \mathrm{CHO}\) contains a polarized oxygen-hydrogen bond, which is associated with the polar form of functional group called Aldehyde. Since we are considering the water molecule which is polar, it can form bonds with the polarized oxygen-hydrogen bond present in \(\mathrm{CH}_{3} \mathrm{CHO}\) molecule. This allows \(\mathrm{CH}_{3} \mathrm{CHO}\) to get dissolved in water as 'like dissolves like'.

Determine the Polarity of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\)

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) also contains the same functional group, therefore it has a polarized oxygen-hydrogen bond. But apart from that, it also has an additional \(\mathrm{CH}_{2}\) hydrocarbon chain. This chain is made of carbon-hydrogen bonds, which are non-polar.

Comparison of Solubility

The molecule \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) has the same polar functionality as \(\mathrm{CH}_{3} \mathrm{CHO}\), but with the addition of non-polar Hydrocarbon chain, \(\mathrm{CH}_{2}\). This leads to a decrease in the overall polarity of the molecule, leading to a decrease in its solubility in water. Even though \(\mathrm{CH}_{2}\) is a small chain, the effect on the overall polarity is significant enough to limit the solubility in polar solvents like water.

Key concepts

Polarity of Molecules

Understanding the concept of molecular polarity is like understanding the behavior of magnets. Just as magnets have a north and a south pole, molecules can have areas that are slightly positive and areas that are slightly negative. This happens due to differences in electronegativity between the atoms in a molecule.

Electronegativity is essentially how strongly an atom pulls shared electrons towards itself in a bond. When molecules, such as water (H2O), have a significant difference in electronegativity between the atoms, this creates a dipole, or two poles of charge. The polarity of a molecule affects how it interacts with other molecules. For instance, polar molecules tend to dissolve well in water because water is also polar, creating a 'like dissolves like' situation.

Aldehyde Functional Group

An aldehyde is like a characteristic feature of a building that makes it stand out in a neighborhood. In organic chemistry, an aldehyde is a functional group featuring a carbon atom connected to a hydrogen atom and double-bonded to an oxygen atom (written as -CHO). This arrangement gives the molecule a polar nature, allowing it to interact easily with other polar substances, including water.

In the molecules (CH3CHO) and (CH3CH2CHO), the aldehyde group is the polar 'feature' that initially gives these two molecules the potential to mix with water, much like certain buildings have features that allow them to blend into a specific landscape or city block.

Hydrocarbon Chain

Imagine a hydrocarbon chain as a string of beads where each bead is a carbon atom, and each string is held together with hydrogens. These chains are nonpolar, and they act like oils, which don't mix well with water. The hydrocarbon part of a molecule, like the (CH2) in (CH3CH2CHO), brings nonpolar characteristics to the molecule.

As these chains become longer, they increase the molecule's nonpolar character and decrease its overall solubility in water. This is why even a small change, such as adding one (CH2) unit, can significantly impact the molecule's ability to dissolve in water.

Like Dissolves Like Principle

The 'like dissolves like' principle is a simple way to predict whether a substance will dissolve in another — substances with similar characteristics will generally dissolve in each other.

For example, water is polar, so other polar substances are likely to dissolve in it. Nonpolar substances, on the other hand, do not have a charge dipole and are more likely to dissolve in nonpolar solvents. This principle explains why increasing the length of the nonpolar hydrocarbon chain in a molecule like (CH3CH2CHO) reduces its solubility in polar solvents such as water.