Question

"Ethylamine" and "diethylamine" are soluble in water. Explain why "triethylamine" is only slightly soluble in water.

Short answer

Triethylamine is only slightly soluble in water because its three ethyl groups make it more hydrophobic, reducing water interaction.

Step-by-step solution

Understand Solubility

Solubility in water is greatly influenced by the ability of a compound to form hydrogen bonds with water molecules. Compounds with groups that can donate or accept hydrogen typically have higher solubility.

Analyze Ethylamine and Diethylamine

Both ethylamine and diethylamine are amines, compounds containing nitrogen with lone pair electrons that can form hydrogen bonds with water. Additionally, they have smaller alkyl groups, which don't significantly hinder solubility.

Introduction to Triethylamine

Triethylamine is also an amine and can theoretically form hydrogen bonds with water using the lone pair on the nitrogen atom. However, it has three larger ethyl groups attached to nitrogen.

Influence of Alkyl Groups

In triethylamine, the presence of three ethyl groups increases the hydrophobic character of the molecule. These bulky alkyl groups reduce its ability to interact with water molecules, overriding the hydrogen bonding potential.

Conclusion on Solubility

Due to the increased hydrophobicity from the three ethyl groups, triethylamine becomes only slightly soluble in water, as the hydrophobic interactions counteract the hydrogen bonding capacity.

Key concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in determining a compound's solubility in water. Water molecules are polar, meaning they have partial positive and negative charges. The oxygen atom in water carries a partial negative charge, while the hydrogen atoms have a partial positive charge. This charge difference allows water molecules to form hydrogen bonds easily. For a solute to dissolve in water, it must form these hydrogen bonds with the water molecules.

Amines, like ethylamine and diethylamine, are particularly adept at forming hydrogen bonds due to the nitrogen atom within them. The nitrogen atom has a lone pair of electrons, which can attract the hydrogen in water molecules. This attraction facilitates a strong enough intermolecular force to dissolve these amines effectively in water. Triethylamine, despite being an amine, has less effective hydrogen bonding due to the structure of the molecule being more blocked by large alkyl groups.

Amines

Amines are organic compounds characterized by one or more alkyl or aromatic groups attached to a nitrogen atom. They can be primary (one carbon group), secondary (two carbon groups), or tertiary (three carbon groups). This distinction is vital because it influences their chemical behavior and interactions with solvents like water.

Ethylamine and diethylamine, as primary and secondary amines, respectively, have less steric hindrance around the nitrogen atom, allowing them to participate effectively in hydrogen bonding. This enables them to dissolve readily in water. In contrast, triethylamine is a tertiary amine, which means the nitrogen is connected to three carbon groups. While still capable of forming hydrogen bonds through the lone pair on nitrogen, the steric bulk from the three ethyl chains surrounding the nitrogen atom inhibits these interactions. This results in a decreased solubility in water.

Alkyl Groups

Alkyl groups are simple chains of carbon and hydrogen atoms, such as methyl, ethyl, propyl, etc. These groups are nonpolar and contribute to the hydrophobic nature of organic molecules. The size and number of alkyl groups have a significant impact on a compound's solubility in water.

In compounds like ethylamine and diethylamine, fewer or smaller alkyl groups mean there is less hindrance to hydrogen bonding with water. These molecules remain more soluble. Triethylamine, on the other hand, is burdened by three bulky ethyl groups. These groups increase the hydrophobic (water-fearing) character of the molecule by preventing effective interaction between water molecules and the nitrogen atom. The presence of larger alkyl chains in triethylamine means more of the molecule is repellent to water, reducing its overall solubility.