8.8. Physical Properties

Today, we’re going to discuss the boiling points of carboxylic acids. Can anyone tell me why carboxylic acids boil at higher temperatures than aldehydes or ketones?

Is it because they have larger molecules?

Good thought! But it’s primarily due to hydrogen bonding. Carboxylic acids are able to form hydrogen bonds much more effectively than aldehydes and ketones.

So the stronger the bonds, the higher the boiling point?

Exactly! Think of it this way: the stronger the intermolecular forces, the more heat is required to overcome them. This is why boiling points are higher.

Can you give us some examples?

Sure! For instance, consider acetic acid and propanal—acetic acid has a higher boiling point due to its ability to form stronger hydrogen bonds. Alright, let’s summarize: carboxylic acids have higher boiling points compared to aldehydes and ketones due to hydrogen bonding.

Now let's talk about solubility. How do you think the length of the carbon chain affects the solubility of carboxylic acids in water?

Maybe shorter chains are more soluble because they have less hydrocarbon part?

That’s correct! Short-chain carboxylic acids like butanoic acid form hydrogen bonds with water, leading to higher solubility. However, as the carbon chain gets longer, their hydrophobic properties increase, reducing solubility.

So higher carboxylic acids, like those with ten or more carbons, will be virtually insoluble in water?

Exactly! Let's recall that benzoic acid, despite being a carboxylic acid, is nearly insoluble in cold water. Key takeaway: solubility decreases with increased carbon chain length.

Lastly, let’s discuss the odour of carboxylic acids. Why do you think lower molecular weight acids tend to have stronger scents than higher ones?

Is it because they are more volatile?

Correct! The lower molecular weight carboxylic acids are more volatile and release strong smells, like acetic acid, which has that sharp vinegar-like aroma.

So do higher carboxylic acids smell nice or are they just odourless?

Higher carboxylic acids generally have less noticeable or waxy odours and are often used in positive applications like perfumes and flavorings because of this quality.

So it’s all about balancing volatility and chain length?

Exactly! And that wraps up our discussion on odour properties of carboxylic acids. To conclude, lower carboxylic acids have strong smells due to their volatility, while higher ones tend to be less aromatic.