Increased acidic properties depend on what?? I've given a question to arrange 3 compounds, the structures are drawn out. One is alkyne, the 2nd is a carboxylic acid and 3rd one is an alcohol. So how do u know which compound is more acidic than other? Like for example something random i'm think would an alkane be more acidic than an alkene due to a greater number of hydrogen present?? could someone plz help clear up my misconception thx.

Question

anonymous · Answer

Acidity is caused by a losing $H^+$ ions. How acidic a substance is depends on how easily it will split with the hydrogen ions in water. For example, in hydrochloric acid (HCl), the hydrogen bond is polar with the hydrogen being strongly partially positive, so this a strong acid. In your question, carboxylic acids are the most acidic because they have two oxygen atoms on the carbon to which the carboxyl group is attached, whereas the alcohol only has one and the alkyne has none, like I said earlier, this makes the hydrogen ion partially positive and therefore easier to lose.

anonymous · Answer

So Acidity does partially depend on how easily the hydrogen is lost, but it also has to do with the stability of the conjugate base. So when you are looking at an aklyne, a carboxylic acid, and an alcohol look at the conjugate base and see which one is more stable. In this case I urge you to look at the resonance structures of the conjugate base. 

A carboxylic acid has two resonance structures because of the two oxygens that can share the negative charge. The deprotonated alcohol however, does not have any resonance structures so it the conjugate base is less stable than the carboxylic acid conjugate base. 

When you compare the alcohol to the alkyne conjugate bases neither one has resonance so you look at what the negative charge is on. The alcohol puts the negative on the oxygen which is electronegative whereas the alkyne puts it on a carbon which is not so electronegative. Thus the alcohol is more stable than the alkyne. and is a stronger acid. 

Also, if you know the pKa of the compounds then you can determine the order of acidity because the lower the pKa the more acidic.

anonymous · Answer

thanks allot guys ben the answer you gave was what i was actually looking for :D I asked my professor today and they pretty much told me the same. Thanks again guys!

anonymous · Answer

Glad to be helpful.