Question

2-Propanol is quite soluble in water, but 2 -decanol is insoluble in water. Draw both structural formulas and explain why one compound is soluble but the other isn't.

Short answer

The structural formula for 2-Propanol is \(CH_{3} - CH(OH) - CH_{3}\), and the structural formula for 2-Decanol is \(CH_{3} - CH(OH) - (CH_{2})_{8} - CH_{3}\). 2-Propanol is polar due to the uneven charge distribution caused by the alcohol group (-OH) in the middle of the molecule and dissolves in water through hydrogen bonding. On the other hand, 2-Decanol is mainly non-polar because of the large hydrocarbon chain that dominates its structure, resulting in it being insoluble in water.

Step-by-step solution

Draw Structural Formulas for both compounds

We need to draw the structural formulas for each compound. The structural formula for 2-Propanol is: \(CH_{3} - CH(OH) - CH_{3}\) The structural formula for 2-Decanol is: \(CH_{3} - CH(OH) - (CH_{2})_{8} - CH_{3}\)

Analyze the Polarity of each compound

Now that we have the structural formulas of both compounds, let's analyze their polarity. A molecule that has polar bonds and an uneven distribution of electron density is considered polar. In these compounds, the carbon and hydrogen bonds are considered non-polar, whereas the oxygen-hydrogen bond (within the alcohol group, OH) is considered polar. 2-Propanol: The polar alcohol group (-OH) is located in the middle of the three carbons, which makes the molecule polar due to the uneven charge distribution. 2-Decanol: The polar alcohol group (-OH) is also located near one end of the long hydrocarbon chain. However, this compound has 10 carbons and is significantly larger in size, which means the effect of the polar OH group is reduced compared to the overall size and non-polar nature of the carbon-hydrogen bonds of the hydrocarbon chain. This molecule, although having a polar OH group, is mainly non-polar due to the presence of a much larger non-polar region.

Determine solubility in water

Solubility in water depends on the "like-dissolves-like" principle. Polar compounds dissolve in polar solvents (such as water), while non-polar compounds dissolve in non-polar solvents. 2-Propanol: This compound has a polar nature, which means it can dissolve in water through hydrogen bonding with the water molecules. 2-Decanol: This compound, despite having a polar OH group, is mainly non-polar due to the large hydrocarbon chain. The non-polar part of the molecule will not interact with water molecules as effectively and will result in it staying as separate entities, meaning it is insoluble in water. In conclusion, 2-Propanol is soluble in water due to its polar nature, while 2-Decanol is insoluble in water due to its mainly non-polar nature.

Key concepts

Polarity and Solubility

Solubility, the ability of a substance to dissolve in a solvent, is deeply influenced by the concept of polarity. Molecules that have an unequal distribution of electrons, and consequently a dipole moment where one end of the molecule is slightly negative and the other end is slightly positive, are deemed polar. Water, famously known for its solvent capabilities, is a polar molecule with a bent shape where the oxygen atom attracts more electrons, making it negatively charged compared to the hydrogen atoms.

Solubility in water is therefore a matter of compatibility between the solute's polarity and water's polarity. If a solute has a similar polar nature, it interacts well with water, creating strong intermolecular bonds, especially hydrogen bonds, and thus it tends to be soluble. Conversely, non-polar solutes, which lack this uneven charge distribution, remain distinct as their molecular attractions are very different from those of water. The concept of polarity is key to understanding the divergent behavior of compounds like 2-Propanol, which is polar and thus soluble in water, and 2-Decanol, which is largely non-polar due to its long carbon chain and is insoluble in water.

Structural Formulas

The structural formula of a compound is a visual representation showing how atoms are arranged and bonded within the molecule. This detailed depiction can provide insights into the chemical properties and reactivity of the compound, including solubility. For example, the structural formula of 2-Propanol ((CH_{3} - CH(OH) - CH_{3})) reveals a short chain with an alcohol group (-OH) centrally located, indicating a polar molecule that, as previously mentioned, is soluble in water.

On the other hand, the structural formula of 2-Decanol ((CH_{3} - CH(OH) - (CH_{2})_{8} - CH_{3})) displays a much longer hydrocarbon chain that diminishes the impact of the single polar alcohol group. The structural formula is a powerful tool that, upon examination, allows chemists to predict the solubility of a compound by analyzing the balance between polar and non-polar parts of the molecule.

Alcohol Solubility in Water

Alcohols are organic compounds characterized by the presence of an -OH (hydroxyl) group. Their solubility in water varies depending on the balance between the polar hydroxyl group and the non-polar carbon chain. Short-chain alcohols like methanol and ethanol have a strong affinity for water due to the dominance of their polar nature, making them highly soluble. This is exemplified in alcohols such as 2-Propanol, where the polar alcohol group provides sufficient polarity for the molecule to mix with water effectively.

However, as the carbon chain lengthens, the non-polar nature begins to overpower the polar hydroxyl group's influence. In longer-chain alcohols like 2-Decanol, the extensive non-polar region impedes interactions with water, rendering the alcohol less soluble or insoluble. It's a delicate balance where the chain's length determines the alcohol's compatibility with water.

Like-Dissolves-Like Principle

The 'like-dissolves-like' principle is a fundamental rule in chemistry describing how substances with similar polarities tend to dissolve each other. According to this principle, polar solvents, such as water, are effective at dissolving polar solutes due to the ability to form strong intermolecular interactions, like hydrogen bonds. This interaction is essential for substances such as 2-Propanol, whose polar nature aligns well with water, facilitating its dissolution.

In contrast, non-polar solutes, such as oils and fats, do not mix well with polar solvents but will readily dissolve in non-polar solvents like hexane. The principle illustrates why 2-Decanol, with its predominantly non-polar hydrocarbon chain, does not dissolve in water. The large non-polar region cannot establish the necessary interactions with water molecules to become soluble. The 'like-dissolves-like' principle serves as a guiding rule to predict and understand solubility behaviors in various compounds.