99 Uses for Sodium hydroxide ( or why is it just So-dium useful)

You may have noticed as you travel through the world of organic chemistry with just a tattered A3 map to guide you that many of the paths are marked with sodium hydroxide. So, here is a quick summary of all the uses of NaOH...

1. As a Base
Fairly obviously NaOH is a base, so any time it comes across an acid it neutralises it (as you learnt circa 2008).

The only two acidic functional groups you will come across in CH4 are phenols and carboxylic acids. In both case you will get the salt formed. Either sodium phenoxide or a sodium carboxylate salt (ethanoate, propanoate etc.)

2. As molecular scissors (i.e. base hydrolysis)
Sodium hydroxide is also used in two different places break molecules in half.

Esters and amides (N-substituted or otherwise) are both hydrolysed by NaOH. In both cases you get a carboxylate salt (remember the fact about making carboxylic acids in alkaline conditions - you always get the salt).

This carboxylate salt can then be converted back to the carboxylic acid just by adding a sprinkling of aq H+.

A fact worth remembering here is that the condensation polymers, polyESTERS and polyAMIDES and proteins can be hydrolysed by NaOH like this as they have loads of amide/ester functional groups.

3. For decarboxylation
In all the other reactions so far, the NaOH has been aqueous. In fact NaOH is prety much always aqueous as "conc" NaOH is solid and will absorb the water from the air, turning your solid NaOH into a corrosive mush. For this reaction we need to keep the NaOH solid so we add calcium oxide to it. This prevents it absorbing the water from the air and we call this mixture of NaOH and CaO, soda lime. Strong heat is needed here so don't forget to mention it. You need to know this symbol equation so please remember it.

So, if you are asked for the reagent you write soda lime, If you are doing a symbol equation you write NaOH (or OH-) as the active ingredient in soda lime is NaOH. The CaO is the chemical equivalent of packaging.

Other points to note here are that CO2 gas is not made in the reaction, as even though CO2 is removed from the molecule it goes straight into a molecule of Na2CO3.

Also, worth noting is that the carboxylic acid or carboxylate salt being decarboxylated must be solid. This isn't a problem for the carboxylate salt as all salts are solid (it's that ionic bonding they can do). Carboxylic acids tend not be solid until the chains get really long so you would need add a bit of aq. NaOH first to the carboxylic acid, evaporate off the water to make the solid salt of the carboxylic acid( not the carboxylic acid itself). I wouldn't stress about this point too much though as it doesn't really come up in the exam.


4. To do some nucleophilic substitution (Halogenoalkane to Alcohol)

Finally, this is a straightforward substitution where the halogen is removed to be replaced by an OH. magically transforming this molecule into an alcohol. A bit of heat under reflux is required to get this going other than that there is not much else to say.


If I have missed any reactions please let me know an I will create an addendum to this post.

P.S. There is nothing special about sodium hydroxide in all cases potassium hydroxide, calcium hydroxide etc.would do exactly the same. It's just that in a lab, it is generally sodium hydroxide you have hanging around.