Acidity and Basicity in Benzene based compounds

Whilst trawling through your notes you may have noticed a similarity between the explanation why phenol is a little bit acidic and why phenylamine is a little bit basic.

The explanation is effectively the same but the effect it almost the opposite.

Let's first compare the acidity and basicity of their alkyl chained compatriots

• Butanol is neutral, phenol is slightly acidic.

• Butylamine is basic, phenylamine is much less basic

So the presence of a benzene ring as opposed to an alkyl chain makes...

• a hydroxyl functional more acidic 

• an amine functional group less basic 

Before we go any further, remind yourself as to the definitions of an acid and a base.

• Acids are proton donors, so anything that makes a molecule more likely to give away a proton makes it more acidic. 

• Bases are proton acceptors, and to do this they need a lone pair, so anything that makes a molecule less likely to accept a proton would make it less basic.

So. let's answer the questions

• Why is phenol acidic when butanol isn't?

          and

• Why is phenylamine less basic than butylamine?

The answer to both questions comes down to the lone pairs.

If an atom with a lone pair on it is attached to a benzene ring then this lone pair becomes part of benzene's delocalised system. This delocalisation means the negative charge doesn't just sit there on the atom, it moves around the molecule. This fact can be used to explain both the questions above.

So, to explain phenol's slight acidity (compared to butanol's neutrality)...

Phenol is acidic because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form. So Phenol is acidic because the ring draws the lone pair away from the ion

and, to explain phenylamine's reduced alkalinity (compared to butylamine's alkalinity)...

Phenylamine is not that basic because the lone pair is no longer fully available to join to hydrogen ions. Nitrogen is still the most electronegative atom in the molecule so the lone pairs of electrons will still be attracted towards it, but the intensity of charge around the nitrogen is nothing like what it is in butylamine. So phenylamine is a not so good base because the ring draws the lone pair away from the nitrogen.

Notice how the two diagrams are almost identical

Make sense? 

Two effects, one reason.

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.

pH Curves in 60 seconds

pH curves are inherently hard things to get your head around but once you do getting it is very worthwhile, so here is my 1 minute guide to what working them out...

1. Shape - Think of each curve as made up of 2 half curves and there are 4 possible half curves (weak acid, strong acid, strong base, weak base), the shapes will either be almost L shaped (Strong) or almost fallen over S shaped (weak) and either high (base) or low (acid). So, when drawing (or describing) a curve just think, what is in the conical flask before I started titrating and draw the half curve for that first, then think what am I adding to it from the burette and draw that sort of half curve next.
2. Intial pH - is the pH in the conical flask before you added anything
3. Estimate of final pH - this will be very close to the pH of whatever was in the burette but reduced a bit (if its a base) or increased (if its an acid)
4. Equivalence volume - Volume it gets v steep
5. Equivalence pH - Middle of v steep section
6. Getting Ka from graph - At 1/2 equivalence volume the Ka = H+ (or pKa = pH)
7. Where is there a buffering effect? - If you have a weak acid or a weak base, in the middle of the half curve for this solution the curve flattens for a bit, this is the buffering effect, this happens because there is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid)

59, 60. Done