Red ant venom. Vinegar. And old rancid butter. What can they have in common? They each contain the same type of acid: organic acids.
In order to see how the molecules of these acids are structured, we'll start with something simpler. In organic chemistry, hydrocarbons -- or alkanes -- form the basis of many compounds. If you replace one of the hydrogens with a hydroxy group - OH - you form an alcohol. This alcohol is formed from ethane. It's called ethan-ol.
We're going to make one more change to this molecule. Let's replace the other two hydrogens - with an oxygen. Wait, now the carbon is only bonded to three atoms. Carbon atoms want to have four bonds! Yes, but look at the bond between the carbon and the oxygen.
It's a double bond. Instead of just sharing one electron pair… ... this bond consists of two pairs of electrons. We started with ethane which became ethan-ol. And now it is ethanoic acid.
We can turn any alkane into an organic acid by replacing the three hydrogens at the end… ... with one OH-group and a double bonded oxygen. Methane becomes methanoic acid. Propane becomes propanoic acid. Then there is butanoic acid, pentanoic acid, and so on.
This group, the C... the O... and the OH... have a special name. It's called a carboxyl group.
The acids are also called carboxylic acids. When we write their chemical formula, we show that the two oxygens are different. One is attached to the carbon, and the other belongs to the hydroxy group. That's why we don't write the chemical formula of the carboxylic group with "O-two", but as C, O... O, H.
The chemical names tell us how many carbon atoms the molecules have. Some of the acids also have older names, that are still used. Ant venom contains methanoic acid, also known as formic acid. Vinegar contains ethanoic acid, also known as acetic acid. When butter is left at room temperature for too long, it starts forming foul-smelling butanoic or buteric acid.
Why are these molecules acidic? An acid is something that can release hydrogen ions. And in the carboxyl group, COOH, the hydrogen is only loosely attached. When the molecule loses a hydrogen, it becomes a negative ion. The ions have names that begin in the same way, but now their names end in "o-ate": methanoate, ethanoate, propanoate and so on.
Organic acids are weak acids. That means that most of the time, the hydrogen stays with the molecule. The longer the carbon chain, the weaker the acid is. To summarize: organic acids are common in nature. Organic acids are also known as carboxylic acids, from C, O, O, H - group of atoms - the carboxyl group.
Their names show which hydrocarbon they are made from. Methanoic Ethanoic Propanoic Butanoic And so on. The hydrogen in the carboxyl group can leave the molecule. That's what makes these compounds acidic. When they lose a hydrogen, what's left is a negative ion, with a name ending in "-oate": Methanoate, ethanoate, and so on.
The shorter the carbon chain is, the stronger the acid gets. So which of these has the strongest acid?