Question

Give an explanation for the following observations: a. Methanol is soluble in water, but ethane is not. b. 2-Propanol is soluble in water, but 1 -butanol is only slightly soluble. c. 1 -Propanol is soluble in water, but ethyl methyl ether only slightly soluble.

Short answer

Methanol is soluble due to hydrogen bonding; ethane is not. 2-Propanol is more soluble than 1-butanol due to a shorter hydrophobic chain. 1-Propanol is more soluble than ethyl methyl ether due to better hydrogen bonding capability.

Step-by-step solution

- Understand Hydrogen Bonding

Methanol (CH3OH) can form hydrogen bonds with water due to its -OH group which has a highly electronegative oxygen atom capable of hydrogen bonding. Ethane (C2H6), on the other hand, is a nonpolar hydrocarbon without the capability to hydrogen bond with water, making it insoluble in water.

- Compare Alcohol Solubility

2-Propanol (isopropanol) has a -OH group and a relatively small hydrocarbon chain making it very soluble in water due to the hydrogen bonding with water molecules. 1-Butanol has a longer hydrocarbon chain which makes it more hydrophobic (water repelling) and slightly reduces its solubility in water despite having a -OH group for hydrogen bonding.

- Analyze Solubility of Ethers and Alcohols

1-Propanol has a hydroxyl group (-OH) that can form hydrogen bonds with water, making it soluble. Ethyl methyl ether (CH3OCH2CH3) has an oxygen atom that can form hydrogen bonds, but the ether's larger nonpolar parts (ethyl and methyl groups) reduce its ability to mix with water comparatively, making it only slightly soluble.

Key concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in determining the solubility of different compounds. These bonds occur when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This interaction is stronger than Van der Waals forces but weaker than ionic or covalent bonds.

Solubility is significantly influenced by hydrogen bonding because it enhances the interaction between solute and solvent molecules.

• Methanol (CH3OH) can form hydrogen bonds with water due to its -OH group.
• Hydrogen bonding allows alcohols to dissolve in water more effectively compared to nonpolar compounds like ethane (C2H6), which cannot form such bonds.

Understanding hydrogen bonding helps explain why some organic compounds dissolve in water while others do not. Without this interaction, many polar compounds would not mix well with water.

Alcohol Solubility

Alcohols are organic compounds with one or more hydroxyl (-OH) groups attached to a carbon chain. The solubility of alcohols in water varies depending on the size of the hydrocarbon chain and the presence of the hydroxyl group.

• Smaller alcohols like methanol and 2-propanol are highly soluble in water because their short carbon chains allow them to form hydrogen bonds with water easily.
• As the hydrocarbon chain length increases, the solubility decreases. This is because longer chains increase the hydrophobic (water-repelling) nature of the alcohol. For example, 1-butanol is only slightly soluble in water.

Thus, the balance between the hydrophilic hydroxyl group and the hydrophobic carbon chain determines alcohol solubility.

Polar vs Nonpolar Compounds

The solubility of a compound is greatly influenced by its polarity. Polar and nonpolar compounds have different interactions with solvents like water.

• Polar compounds, which have a distribution of electric charge leading to regions of partial positive and negative charges, are generally soluble in polar solvents like water. Methanol and 2-propanol, which are both polar, are good examples.
• Nonpolar compounds, like ethane, lack this charge distribution and do not form hydrogen bonds with water, making them insoluble in polar solvents.

The principle 'like dissolves like' is often used to predict and explain solubility. Polar solvents dissolve polar solutes well, while nonpolar solvents dissolve nonpolar solutes better.

Ether Solubility

Ethers are compounds where an oxygen atom is bonded to two alkyl or aryl groups. They exhibit differing solubility patterns compared to alcohols.

While ethers contain an oxygen atom that can form hydrogen bonds, the extent of this bonding is limited compared to alcohols due to the lack of a hydrogen directly attached to the oxygen.

As a result:

• Ethers like ethyl methyl ether (CH3OCH2CH3) are generally less soluble in water compared to alcohols of similar molecular weight.
• Large nonpolar groups attached to the ether decrease its overall solubility in water, despite its polar oxygen atom.

Therefore, while ethers can exhibit some solubility in water, they are generally not as soluble as comparable alcohols.