Liberté. Egalité. Acidité.

Ever been confused about the line in the spec...


"...appreciate that carboxylic acids liberate CO2 from carbonates and hydrogen carbonates but phenol does not..."

Appreciate?
Liberate?

In plain English, know that carboxylic acids give off carbon dioxide with carbonates and hydrogen carbonates but phenol doesn't.

That's better,

Lets first establish the reason that carboxylic acids give off CO2 with carbonates and phenols don't is because carboxylic acids are more acidic.

Want to know why?

It comes down to the stability of the thing (anion) formed when the proton has gone. If the anion formed is stable then the proton will happily leave. 

The more stable it is, the more likely the proton is to leave, the more acidic the molecule.

Lets look at the contenders

When carboxylic acids give off a proton they form the carboxylate ion which is stable because the negative charge (lone pairs) forms a miniature delocalised system across the two oxygens, like this...

When phenols give off a proton they form the phenoxide ion which is stable because the negative charge (lone pairs) becomes part of the benzene delocalised system, like this...

The delocalisation is more effective in carboxylic acids then phenol so carboxylic acids are more acidic.

C'est très simple.

Liberté
Egalité
Acidité

My favourite infographic of recent years

Got to love this. A comprehensive infographic detailing the smells of organic compounds.

Download

Sit back

and enjoy telling one of your non-chemistry studying friends that they smell of Octanoic acid

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.

Aspirin giving you a headache?

What do you need to know about aspirin?

Almost nothing. Isn't that what you wanted to here.

Aspirin production was in the old spec but has almost completely gone from the new spec. So you will find questions in the old spec CH4 questions about it.

The only thing you need to know is that making aspirin is one of the uses of acid anhydrides.

So remember that fact, laugh in the face of the aspirin based questions and move on.You will feel much better for it

Special Agent (or why is Manganate sometimes used acidified and sometimes in alkaline solution)

Potassium Manganate (VII) is either used acidified (when oxidising alcohols) or in alkaline solution (when oxidising methyl benzene)

The reason why you have to acidify the manganate VII for the oxidation of alcohols is because the reaction needs H+ to combine to make water from MnO4-'s oxygens. As you saw in CH5 when doing redox half equations for Manganate (VII).

The reason why you need alkaline conditions when oxidising methyl benzene - I have no idea. 

I even googled it and found some degree level notes on it and they said the mechanism was to complicated to explain at degree level. So if I was you, I would assume it is wizardry and move on.

Sounds a Bit Fishy (or Naming N-Substituted Amides - CH4)

N-substituted amides are named in two parts (a bit like esters). So lets look at an example

N-methyl ethanamide

If you look at the molecule, it is an ethanamide with a methyl hanging off the Nitrogen

The ethanamide is on the left (with the amide functional group in the middle) and the methyl is hanging off the N of the amide link (hence N-methyl)

If you remember that the chain that is attached to the N is the one which is the "N-...." at the start of the name then that will tell you which chain goes where.

What about working it out from the reactants (i.e. amine and acyl chlorides)

The amine brings in the N so the amine chain will the N-... chain

and the acyl chloride brings in the rest.

TO be honest this rarely come up in CH4 so I wouldn't worry too much about it!

What's HOT and What's NOT in the world of Chemistry (or Spot the Difference)

Whenever they change specification (like they did 4 years ago) they always take some stuff out and put other new stuff in. 

There is no good reason why, except to confuse pupils and teachers . So, those of you doing the old specification papers will be very confused from time to time when they ask a question that looks like it is written in greek because you don't recognise the stuff at all or you didn't think you needed to know in that level of detail.
So, to help you through, here is a list of all the changes that come to mind. I may have missed something, so please point in out if there is something else that you think is different.

AS
New In!
Hydrogen Emmission Spectrum
Smart Materials
Carbon Nanotubes
Nanotechnology
Green Chemistry

Gone!
Electron Density Distribution

Equilibrium Constants (now in A2)

A2 

New In!

Hydrogen Fuel Cell

Group 3

Chromatography

Gone!

Group 1 and 2

Hydrogen Emission Spectrum (now in AS)

Testing, Testing, 1, 2, 3... (CH4/CH2)

One of the hardest aspects of organic chemistry is remembering all the wet tests that you need to carry out to differentiate between different functional groups that you are likely to come across. 

Not only do you have to remember the compounds that give a positive (and the ones that don't), you need to  remember the observations, the chemicals you need to add to get this thing to work and some seemingly random facts that someone deemed important. 

To help you through this, here is a list of all the tests you are likely to come across and an attempt at the relevant detail. I have also indicated whether these are relevant for CH2 or CH4

Tollen's Reagent (CH4), Fehling's Reagent (CH4), Acidified Dichromate (CH4 and CH2)
These three are collectively known (by me at least) as the oxidation tests, each depends on a molecule being oxidised. So molecules that are easily oxidised give a postive, namely aldehydes, primary alcohols and secondary alcohols. 

The only thing that changes is the observation.

So for Tollen's Reagent you see a silver mirror (or more likely if your test tubes could do with a wash - a grey ppt)

For Fehling's Reagent you see a orangey brown ppt

For Acidified Dichromate you see the orange solution go green

Other facts worth remembering are...

• Tollen's reagent is fairly rubbish as an oxiding agent so it is not strong enough to work with primary alcohols (such as ethanol)
• Fehling's is almost the same Benedicts solution (remember from biology, the test for reducing sugars)

Iodoform Test (CH4)

Iodoform (or triiodomethane or CHI3) is the name of the test but in this case it is not the name of the reagent you add. In fact, Iodoform is the yellow stuff you see at the end that tells you the test has worked. 

The reagent you add is aqueous alkaline iodine (or iodine, mixed with aqueous sodium hydroxide) it also works with Potassium Iodide and Sodium Chlorate (I), You generally don't need to warm it but if its being a bit slow a bit of heat helps.

The observation is a yellow ppt (of Iodoform) that smells of ...erm...Iodoform which is basically an antiseptic smell, which makes sense as iodoform used to be used as antiseptic.

People often come unstuck in what gives a positive. Positives are given by methyl ketones or methyl hydroxyls. That is -OH or C=O groups being on the carbon next to a CH3. For example -  ethanal, ethanol, propan-1-ol, Propanone  all give positives but methanol, propanal and pentan-3-one all give negatives. Think this one through, draw the molecules and try and work it out, if you are still stuck have a look here. 

2,4 DNPH (CH4)

Nice and easy one this one. Positives are given by aldehydes and ketones and it looks like a bright orange ppt. The only other factoid that they want you to know here is that this is an example of an addition-elimination reaction. Don't bother learning the mechanism just remember that fact.

Lucas Test (CH4)

This is technically not in the syllabus so they can't ask a question where this is the only answer and it won't be in the mark scheme as a possible answer but it does work and could get you out of jail if you don't know the actual answer but if you can don't use this as your answer of choice as it won't be in the mark scheme and a half asleep examiner may well mark it wrong if they aren't concentrating.

Anyway (briefly), what is it? Reagent is Conc Hydrochloric Acid and Anhydrous Zinc Chloride. Tertiary Alcohols give a white ppt v quickly, secondary alcohols give a white ppt v slowly. Primary Alcohols don't give a white ppt but neither does anything else so that fact doesn't help you much.

Sodium Carbonate Test (CH2 and CH4)

Simple idea this, acids bubble with metal carbonates. So if you add sodium carbonate solution to any acid (including carboxylic acids) you will get bubbles that turn lime water milky. 

Tests for Phenol (CH4)

Two of these, either... 

•  add Iron (III) Chloride and the solution goes purple

            or

• add bromine water and it decolourises and you get a white ppt

Enough said, lets move on.

Test for Alkenes (CH2 and CH4)

This is an easy one you should all know from GCSE. Alkenes decolourise bromine water. In other words it goes from being a browny-orange colour to being colourless (NOT CLEAR!!!)

Test for Amines (CH4)

This is a spin off from all that di-azo stuff that you do. When you add nitrous acid to an amine it forms a di-azo compound that is really unstable so it immediately breaks down to give off bubbles of nitrogen gas

Test for Amides (CH4)

Again this is a consequence of a conversion reaction. When you sodium hydroxide to an amide and heat it a bit it gives off ammonia. You will know because it stinks but if your nose isn't working then test it with damp red litmus paper and it will go blue. The only possible confusion here is with ammonium salts which also give off ammonia with sodium hydroxide but that will happen in the cold, so it is usually possible to tell the difference.

Test for Halogenoalkanes (CH4 and CH2)

Halogenoalkanes are known for being unreactive so the only way to test for them is to chop a bit off and test that. So, add some sodium hydroxide, this will then release the halide ion. 

The halide ion can then be tested for by adding a mixture of silver nitrate and nitric acid (the acid is just there to neutralise any excess sodium hydroxide from earlier).

The observation will depend on the halide ion - so white ppt means chloride (so it was a chloroalkane) cream ppt means bromide (so it was a bromooalkane) and yellow ppt means iodide (so it was an iodoalkane).

There we have it all the test in one place. If I have missed anything please leave a comment and I will add it. 

Y12 if you are getting stressed by the 4 you have to learn for CH2 spare a thought for Y13 who had to learn all of these others for CH4, as will all of you next year...  

Naming Multifunctional Organic Molecules

If you are naming molecules with multiple functional groups your first priority is to keep the numbers low and then when you write the name you put them in alphabetic order.

So for example this molecule...

...is called,  3-Chloro-1-Iodo butane

This is right because

- it keeps the numbers low (the alternative is "2-chloro-4-Iodo", i.e. 1,3 is lower than 2,4)

- and we have put it in alphabetic order

There is a more complicated rule that groups have different priority based on their Mr but you don't really need to know that,

The other fact it is worth knowing is that if you have a functional group that must be on the end, mainly aldehydes and carboxylic acids, you wouldn't number these because they have to be on the end (obvs) but then when you put other functional groups on you can't start numbering these "end only" functional groups because that would make no sense, it which case you always assume that the "end only" functional group is at 1 and number from there and this overrides the rule about keeping the numbers low.

So for example... 

...is called, 4-Chloro-3-Methyl Butanoic Acid

NOT

1-Chloro-2-Methyl Butan-4-oic Acid

Even though the numbers are kept lower on the alternative name.

The only other thing worth remembering is that of you have more than one of a functional group it becomes, for example, 1,3-dimethyl or 1,3,3-trimethyl and then when you are alphabetising them you go by the "d" or "t" not "m" e.g. it would be "1,3-dimethyl-4-ethyl....." not "4-ethyl-1,3-dimethyl-....."