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.

Azo-Dyes

What is there to say on this self contained little topic.

If you have a benzene interconversion map you should have it all on there.

Here is a little summary of the topic just to make sure you know what is going on.

I am not going to go through the pathway from benzene to benzenediazonium chloride.. .oh, OK then I will

1. Benzene with Conc H2SO4 and Conc HNO3 (at about 50C) makes Nitrobenzene
2. Nitrobenzene with Tin and Conc HCl makes Phenylamine
3. Phenylamine with Sodium Nitrite and Conc HCl - Nitrous Acid (below 5C) makes Benzenediazonium chloride

Happy now?

This is benzenediazonium chloride

Benzenediazonium chloride and nitrous acid are unstable above 5C so you have to keep it cool when this stuff is around.

If you let things get a bit hot you get phenol and some bubbles of nitrogen gas given off (that sounds like a test to me).

Benzenediazonium chloride can be added to loads of molecules with benzene rings to make dyes. The three examples you need to know are with...

1. Phenol

2. Napthalen-2-ol

3. Phenylamine

Pretty repetitive really.

The only relatively interesting thing to point out her is why the O- on napthalen-2-ol and phenol (as opposed to -OH. This is explained by the condition needed which is in NaOH (i.e. alkaline conditions). All acids lose their proton in alkaline conditions and as both of these phenolic groups are alkaline then  - protons begone!

These two benzene rings linked by a N=N are a chromophore so all of these molecules are dyes. 

You don't need to be able to name them as they have several possible names, just remember the formula.

OK, enough chemistry for me for one evening it's back to the mince pies and TV.

Reaction Types - Fool Proof Guide

Ever wondered how to work out which reaction are electrophilic, nucleophilic, substitutions, additions?

There is a fairly fool proof way to work it out.

Electrophilic - This needs electrons to be attacked. So will happen to benzene (cloud of them), alkenes (double bond full of them)

Nucleophilic - This needs a very electronegative element to pull the electrons away and create a delta positive carbon - so alcohols, aldehydes/ketones, halogenoalkanes are the main culprits.

Addition - If you are going to add on an extra atom you need some space to add some extra bonds, (i.e a double bond) so that means alkenes with almost anything (electrophilic) and aldehydes and ketones with HCN (nucleophilic)

Substitution - There are two reasons why you would get substitution reactions...

1. The molecule is stable as it is and adding anything would disrupt that stability (i.e. benzene)
2. The carbon being attacked doesn't have space for extra atoms so if something is coming in,
    something else has to leave  - like a one in one out policy (i.e. alcohols, halogenoalkanes).

There we have it. Four different types of reactions. A (relatively) fool proof way to tell them apart.

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.