Question

Rationalize the trend in water solubility for the following simple alcohols: $$\begin{array}{lc} \text { Alcohol } & \begin{array}{c} \text { Solubility } \\ \left(\mathrm{g} / 100 \mathrm{g} \mathrm{H}_{2} \mathrm{O} \text { at } 20^{\circ} \mathrm{C}\right) \end{array} \\ \hline \text { Methanol, } \mathrm{CH}_{3} \mathrm{OH} & \text { Soluble in all proportions } \\ \text { Ethanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} & \text { Soluble in all proportions } \\ \text { Propanol, } \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & \text { Soluble in all proportions } \\ \text { Butanol, } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{2} \mathrm{CH}_{2} \mathrm{OH} & 8.14 \\ \text { Pentanol, } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CH}_{2} \mathrm{OH} & 2.64 \\ \text { Hexanol, } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CH}_{2} \mathrm{OH} & 0.59 \\ \text { Heptanol, } \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{5} \mathrm{CH}_{2} \mathrm{OH} & 0.09 \\ \hline \end{array}$$

Short answer

In summary, the trend in water solubility for the simple alcohols decreases as the size of the hydrocarbon chain increases, because the non-polar character of the alcohol increases, reducing its ability to interact favorably with the polar solvent (water). The polar hydroxyl groups can participate in hydrogen bonding with water, but as the proportion of the hydrocarbon chain increases, the overall solubility in water decreases.

Step-by-step solution

Understanding molecular structure and solubility

In order to rationalize the trend in water solubility for the given simple alcohols, it's necessary to first understand how molecular structure can influence solubility. In general, solubility depends on the ability of a substance to form favorable interactions with the solvent. In this case, the solvent is water, which is a highly polar molecule due to the presence of polar O-H bonds. Thus, substances that are also polar and can participate in hydrogen bonding will generally be more soluble in water.

Analyze the structures of the given alcohols

The alcohols given in the exercise have the following general structure: $$\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)^{n}\mathrm{CH}_{2}\mathrm{OH}$$ where n is the number of methylene (-CH2-) groups in the molecule. As the number n increases, the size of the hydrocarbon chain increases as well. Notice that each alcohol has a polar hydroxyl (O-H) group at the end of the molecule, which can participate in hydrogen bonding with water molecules.

Relate molecular structure to solubility

As n increases in the general structure of the alcohols, the hydrocarbon chain (the non-polar part of the molecule) becomes larger, while the polar hydroxyl group remains the same. This means that as the alcohols become larger, the proportion of the molecule that can effectively interact with the polar water molecules decreases. Consequently, the balance between the polar and non-polar portions of the molecule shifts towards the non-polar portion, causing the overall solubility in water to decrease.

Consider the solubility values

The given solubilities are as follows: - Methanol, Ethanol, and Propanol: Soluble in all proportions - Butanol: 8.14 g/100g H2O - Pentanol: 2.64 g/100g H2O - Hexanol: 0.59 g/100g H2O - Heptanol: 0.09 g/100g H2O It is evident that as the size of the alcohol molecules increases (from methanol to heptanol), the solubility in water decreases. This trend supports our reasoning that as the hydrocarbon chain becomes larger, the molecule becomes less polar, resulting in a decrease in solubility in water. In summary, the trend in water solubility for the simple alcohols can be rationalized by considering the balance between the polar hydroxyl groups and the non-polar hydrocarbon chains in the molecules. As the size of the hydrocarbon chain increases, so does the non-polar character of the alcohol, causing a decrease in solubility in the polar solvent (water).

Key concepts

Alcohols

Alcohols are organic compounds consisting of one or more hydroxyl (-OH) groups attached to a carbon atom. These compounds are known for their versatility and are commonly used in various applications, from household products to biofuels. The hydroxyl group (-OH) is what classifies a compound as an alcohol, and it plays a significant role in their chemical behavior.

• Alcohols can be primary, secondary, or tertiary, based on the carbon atom that the hydroxyl group is attached to.
• Methanol, ethanol, and propanol are examples of simple alcohols, where the hydroxyl group is attached to a carbon atom at the end of a hydrocarbon chain.
• The presence of the hydroxyl group makes alcohols polar, which influences their interactions with water.

Understanding alcohols in the context of their solubility requires exploring their molecular interactions, particularly how they interact with water through hydrogen bonding.

Molecular Structure

The molecular structure of an alcohol significantly impacts its solubility in water. Alcohols have a general structure of \[ \text{CH}_3(\text{CH}_2)^n\text{CH}_2\text{OH} \] where the hydroxyl group is polar and can form hydrogen bonds, while the hydrocarbon chain is non-polar.
As the length of the hydrocarbon chain increases, the non-polar character of the molecule dominates, reducing its ability to dissolve in water.

• Short-chain alcohols like methanol and ethanol are more soluble due to the dominance of the polar hydroxyl group over the hydrocarbon chain.
• As the chain lengthens, the influence of the non-polar region increases, thus affecting the alcohol's overall polarity.
• The molecular weight also plays a role, as larger molecules have more extensive electron distributions that make them less soluble in polar solvents like water.

This balance between polar and non-polar regions is essential in determining the solubility pattern of alcohols in water.

Hydrogen Bonding

Hydrogen bonding plays a vital role in the solubility of alcohols in water. Water, being a polar molecule, has the ability to form hydrogen bonds with other polar entities, such as the hydroxyl group of alcohols. This interaction is critical for solubility.

• The hydroxyl group (\(-\text{OH}\)) in alcohols serves as both a hydrogen bond donor and acceptor, interacting with water molecules effectively.
• In smaller alcohols, hydrogen bonding with water helps to overcome the non-polar character of the hydrocarbon chain, allowing them to dissolve more readily.
• As the carbon chain increases, these hydrogen bonds become insufficient to overcome the non-polar nature, leading to decreased solubility.

Thus, the capacity for hydrogen bonding explains why smaller alcohols tend to be miscible in water, while larger ones are not.

Polar and Non-Polar Interactions

Understanding the interplay between polar and non-polar interactions is crucial when discussing the solubility of alcohols. These interactions are fundamental to the dissolution process.
Alcohols have both polar (hydroxyl group) and non-polar (hydrocarbon chain) sections, influencing how they interact with water.

• Polar interactions are primarily caused by the hydroxyl group, allowing the alcohol to mix with water, a polar solvent.
• Non-polar interactions arise from the hydrocarbon chain, which tends to resist mixing with water. The longer this chain, the less the alcohol can blend with water.
• The balance or imbalance of these interactions determines how soluble an alcohol is in water.

In essence, high water solubility in small alcohols is due to strong polar interactions outweighing non-polar influences, whereas the reverse is true for larger alcohols.