Aromatic Chemistry (5) Phenol: electrophilic substitution reactions

Specification extracts:

OCR

Phenols

6.1.1.  

• (i)  the electrophilic substitution reactions of phenol: 
• (i)  with bromine to form 2,4,6-tribromophenol
  
• (ii)  with dilute nitric acid to form a mixture of 2-nitrophenol and 4-nitrophenol 
• Note that nitration with phenol does not require concentrated HNO3 or the presence of a concentrated H2SO4 catalyst.
• (j)  the relative ease of electrophilic substitution of phenol compared with benzene, in terms of electron pair donation to the π-system from an oxygen p-orbital in phenol (see also 4.1.3 a)
  Explanation is only in terms of susceptibility of ring to 'attack' and not in terms of stability of intermediate. 
• (k)  the 2- and 4-directing effect of electron- donating groups (OH, NH2) and the 3-directing effect of electron-withdrawing groups (NO2) in electrophilic substitution of aromatic compounds
  Learners will not be expected to know further electron-donating or electron-withdrawing groups; relevant additional data will be supplied in examinations. 
• (l)  the prediction of substitution products of aromatic compounds by directing effects and the importance to organic synthesis

Edexcel

6. understand the reaction of phenol with bromine water 

7. understand reasons for the relative ease of bromination of phenol, compared to benzene 

Aromatic Chemistry (5) Phenol: electrophilic substitution reactions

In this fifth post on aromatic chemistry, I’m looking at the electrophilic substitution reactions of phenol with bromine water and nitric acid.

I’m going to discuss the fact that phenol is more easily substituted than benzene and explain this difference illustrating it by looking at the reaction with bromine water and with nitric acid solution.

Phenol with bromine water

Phenol is an aromatic alcohol but it behaves very differently from benzene in electrophilic substitution reactions.

The structure of phenol (C₆H₅OH) means that the OH group attached to the benzene ring can add extra electrons to the ring π system and as result enhance the electron density at certain of the carbon atoms in the ring.  (see below).

The additional electron pair comes from a p orbital in the oxygen of the OH group.  The p orbital overlaps with the ring π system extending the system to include the extra electron pair. 

This addition of electrons to the ring π system is called ring activation.  The carbon atoms at positions 2, 4 and 6 are given enhanced electron density and become activated towards electrophiles such as the bromonium ion Br+ or the nitronium ion NO₂+.

The resultant effect is that much milder conditions are required to brominate phenol i.e. no halogen carrier and simply the addition of bromine water.  The resultant white precipitate is the 2,4,6-tribromophenol. (see the equation below).

The -OH group then is said to be 2,4,6 directing.  

Phenol with nitric acid

In much the same as with bromine water, phenol also reacts under very mild conditions with nitric acid.  Unlike in the case of benzene where concentrated acid was required in the presence of concentrated sulphuric acid, the nitration of phenol takes place in the presence of just dilute nitric acid.  

Two products usually result at room temperature 2-nitrophenol and 4-nitrophenol.  

They can be separated since the molecules have different boiling points.  

Again we see the 2,4,6 directing effect of the OH group in phenol since these positions on the ring are given enhanced electron density and make the ring more susceptible to electrophilic substitution. 

The effect of other groups on the benzene ring is to do the same as phenol so for example the amine group NH₂ is also 2,4,6 directing towards electrophiles as it too adds an electron lone pair from a nitrogen p orbital to the ring π system and makes the ring more susceptible to electrophilic substitution.

Bromination and nitration of phenylamine happen under the same kind of mild conditions as they do in phenol.  (see equation below)

However if the substituent on the benzene ring is not electron pushing but electron withdrawing ( like a nitro group NO2) then the opposite effect on electrophiles is observed.

So for example the nitration of nitrobenzene (at temperature greater than 55^oC ) produces 1,3–dinitrobenzene because the nitro group is 3,5 directing as it deactivates the ring towards electrophilic substitution.

Aromatic Chemistry (4) Phenol

Aromatic Chemistry (4) Phenol

Specification extracts:

OCR

Phenols

6.1.1.(h)  

the weak acidity of phenols shown by the neutralisation reaction with NaOH but absence of reaction with carbonates 

Aromatic Chemistry (4) Phenol as a weak acid

In this fourth post on aromatic chemistry, I’m looking at the acidic properties of phenol.

I’m going to discuss the fact that phenol is a weak acid and illustrate this looking at its reaction with sodium hydroxide solution and the reaction of carboxylic acids with carbonates.

Phenol as a weak acid

Phenol is an aromatic alcohol but it behaves very differently from aliphatic alcohols like ethanol.

The structure of phenol (C₆H₅OH) means that the OH group attached to the benzene ring can add extra electrons to the ring π system and as result lose the proton from the OH group.  (see below)

The resultant anion (C₆H₅O^–) is stabilised as the negative charge on the oxygen can be distributed around the ring π system. 

The image below shows how the phenoxide anion is stabilised.

As a result phenol is a weak acid.  It has a K_a of 1.6 × 10^—10. and a pK_a of 9.8

As a weak acid it displays some of the conventional properties of acids.  So it will neutralise sodium hydroxide solution forming sodium phenoxide. (see below)

However, if we compare the pKa of phenol (9.8) with those of carboxylic acids such as ethanoic acid (4.76) and with carbonic acid (dissolved CO₂ solution) (pK_a2 10.32) we note that the CO₂ solution has a final second pK_a greater than that of phenol in aqueous solution so addition of phenol to a carbonate does NOT result in the effervescence of CO₂ gas.

However, we can also see that the pK_a of ethanoic acid is less than that of phenol AND dissolved carbon dioxide solution.  Thus addition of carbonates to ethanoic acid and other carboxylic acids will result in the effervescence of CO₂ gas.

Thus we have a simple test to distinguish a carboxyl group from a phenolic group.  Add a few drops of sodium carbonate solution and the carboxyl group will cause the solution to fizz!!.

This is a useful test since many carboxylic acids have a pKa less than that of the carbonate ion as this table of selected acids shows:

Aromatic Chemistry (3) Comparing bromination of benzene and ethene

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(f) and (g) 

—the explanation of the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes.
—the interpretation of unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms.

Aromatic Chemistry (3) Bromination of Benzene and Alkenes compared

In this third post on aromatic chemistry, I’m comparing the reaction mechanism of the electrophilic substitution of benzene with that of the electrophilic addition of bromine to an alkene such as propene (C₃H₆).  

I’m also going to discuss the general mechanism for the electrophilic substitution of benzene and other aromatic compounds such as phenol.

Comparing the mechanisms for the bromination of benzene (C₆H₆) and propene (C₃H₆).

1. The benzene problem

Benzene is a relatively unreactive molecule.  The reason for its unreactivity lies in its distinctive electronic structure.

The 6 carbon ring is bonded in two ways.  There is the end-on overlap of s orbitals to form a series of six sigma bonds between the carbon atoms.  However that does not account for all the electrons in the molecule.  6 electrons remain unbonded in sigma bonds.  There is one delocalised electron per carbon atom.  These in each carbon atom occupy a half filled p orbital.  The genius of the molecule is that these six half filled p orbitals combine in a side-on overlap move to form a ring shaped molecular orbital that resides above and below the plane of the sigma bond ring.

You can see how that forms in the illustration below:

So for a species to access the benzene molecule and add or substitute itself in the molecule it has to at least disrupt this ring shaped molecular orbital and that requires considerable energy around 150kJmol^-1 to be precise.  Hence there is considerable resistance to reaction.

The energy profile for the bromination of benzene reaction is found below:

In an alkene of course there is only one double bond in which the π bond is exposed above and below the plane of the single sigma bond.  

The energy required to break this bond is much less than that required to disrupt benzene’s π system.  Alkenes tend to undergo addition reactions fairly quickly and easily with reagents like bromine and hydrogen bromide.  

The reaction energy profile for the action of hydrogen bromide on an alkene is shown below.

Bromination of benzene on the other hand requires a raised temperature and the use of a halogen carrier to generate the electrophile the bromonium ion Br+.

If the benzene ring is already substituted with an electron rich group such as —OH or —NH₂ then these extra electrons enable the ring to be more susceptible to electrophilic substitution since they become incorporated in the π system.  The result being that the conditions for nitration of phenol are much less severe than they are for benzene as we can see illustrated in the two reactions below.

Aromatic Chemistry (2) Electrophilic Substitution in Benzene

Aromatic Chemistry (2) Electrophilic substitution

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(e) 

the mechanism of electrophilic substitution in arenes for nitration and halogenation
For nitration mechanism, learners should include equations for formation of NO2+.
Halogen carriers include iron, iron halides and aluminium halides. 

For the halogenation mechanism, the electrophile can be assumed to be X+. 

Edexcel

18A/5. 

understand the mechanism of the electrophilic substitution reactions of benzene (halogenation, nitration and Friedel-Crafts reactions), including the generation of the electrophile 

AQA

3.3.10.2

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions. 

Students should be able to outline the electrophilic substitution mechanisms of: nitration, including the generation of the nitronium ion and acylation using AlCl3 as a catalyst.
Students could carry out the preparation of methyl 3-nitrobenzoate by nitration of methyl benzoate, purification by recrystallisation and determination of melting point. 

Aromatic Chemistry (2) Electrophilic substitution

In this second blog post on aromatic chemistry, I’m looking simply at the reaction mechanism of the electrophilic substitution of benzene.  

I’m going to discuss a general mechanism for the electrophilic substitution first then focus in on the peculiarities of the different examples quoted in the above A level specifications.  

The mechanism for the electrophilic substitution reactions of benzene

1,. General mechanism

What is an electrophile? 

An electrophile is essentially an electron deficient species be it molecule, ion or atom.  Let’s call it E+

Why should it be that substitution in benzene and other arena/aromatic molecules is facilitated using electrophiles?

Benzene is electron rich.  It has an accessible molecular orbital above and below the plane of the 6 carbon atom ring.  This molecular orbital is often called a π system (it forms in orbital theory from the side on overlap of 6 p orbitals that each contain one electron.)

So above and below the plane of the benzene molecule is this electron rich area open to attack from an electron deficient species E+. 

You can see in stage one of the mechanism above that an electron pair from the delocalised ring system attacks the electrophile.

This attack disrupts the π system and a bond forms between a carbon atom in the ring and the electrophile.  The ring itself now carries a positive charge.

In stage 2 loss of a proton results in the reformation of the π system and the expulsion of a proton from the benzene molecule.  

The overall effect is for the electrophile to take the place off the expelled proton. 

This is generally what happens in electrophilic substitution but let’s now look at some peculiarities of the mechanism in different contexts and we’ll start with the nitration of benzene using concentrated sulphuric and nitric acids.  

2. Mechanism of the nitration of benzene

In nitration the electrophile is the nitronium ion NO₂+

But how does this positive electrophile form?

That can be explained if we look at the role of the concentrated sulphuric acid.

Sulphuric acid is a stronger acid than nitric acid and one in the presence of the other results in sulphuric acid protonating the nitric acid as in the equation below. 

The resultant unstable protonated nitric acid molecule then loses water to form the nitronium ion NO₂+.

The mechanism then follows the general route outlined above now that the electrophile has been formed.

The only other thing to say is that the reaction temperature is kept below 55℃ to prevent the formation of the disubstituted 1,3 dinitrobenzene.

3. Mechanism for the Halogenation of Benzene

Here we need to discuss the role of the halogen carrier.

Iron, iron (III) bromide or chloride and aluminium chloride can act as halogen carriers. They are Lewis acids i.e. they are electron deficient and electron pair acceptors.

Question is how do they function in the mechanism?

For iron the answer is simple, it reacts with the halogen to form the respective higher oxidation number halide.

Iron(III)bromide, iron(III)chloride or aluminium chloride are all three electron deficient Lewis acids.  In the mix with the halogen they form the particular complex as shown below:

The electron deficient aluminium chloride forms the tetrahedral complex AlCl₄^— since it accepts a pair of electrons from the chlorine molecule 

The molecule of chlorine becomes the electrophile (Cl+ )and forms a bond with a carbon atom in the ring.

At this carbon atom, the bonding is no longer planar sp2 but tetrahedral sp3.  The benzene π system has been disrupted.

A second stage follows on from this in which the positive ion loses a proton and reforms the benzene π system.

The aluminium chloride is regenerated — in this sense it is a catalyst in the process.

You observe fumes of hydrogen chloride given off and a temperature rise in the reaction mixture in the lab.

4. Mechanism for the acylation of benzene.

In this example aluminium chloride is used in conjunction with an acid chloride here ethanol chloride to generate the electrophile.

AlCl₃  +  CH₃COCl       →      CH₃CO+          +          AlCl₄^—

The electrophile then can attack the electron rich ring of benzene and form a bond with one of its carbon atoms.

Then the resultant carbocation deprotonates interacting with aluminium chloride complex.

Substitute the acyl chloride for a chloroalkane under similar conditions gives the mechanism for the formation of an alkyl benzene.

Aromatic Chemistry (1)

Aromatic Chemistry (1)

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(d) 

The electrophilic substitution of aromatic compounds with: 

(i) concentrated nitric acid in the presence of concentrated sulfuric acid
(ii) a halogen in the presence of a halogen carrier
(iii) a haloalkane or acyl chloride in the presence of a halogen carrier (Friedel–Crafts reaction) and its importance to synthesis by formation of a C–C bond to an aromatic ring.

Edexcel

18A/4. understand the reactions of benzene with: 

i  oxygen in air (combustion with a smoky flame)
ii  bromine, in the presence of a catalyst
iii  a mixture of concentrated nitric and sulfuric acids
iv  halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts reaction) 

AQA

3.3.10.2

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions. 

Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines. 

Friedel–Crafts acylation reactions are also important steps in synthesis. 

Aromatic Chemistry (I)

In this first of several blog posts on aromatic chemistry i.e. the chemistry of aromatic molecules, (benzene (C₆H₆)being the simplest), I’m just looking simply at the reactions of benzene.   I’ll present the significant reactions from the different A level specs together with the reaction conditions, reagents and equations.  

But before I go any further I have to say that aromatic chemistry is my kind of chemistry.  As a trained dyestuffs chemist back in the day, it was all aromatic-based all dyes are molecules with aromatic ring structures: sometimes quite simple like some of the indicators you use or sometimes incredibly complex like some deeply coloured vat dyes on cotton.  I show some of the dye structures below at the end of this post.   

Reactions of benzene

1,. Nitration

This is an electrophilic substitution reaction.  

Reagents are concentrated nitric and sulphuric acids.  

Resultant product is nitrobenzene

Typical class practical is the nitration of methyl benzoate or nitrobenzene itself.

Equation:

The product is a mono substituted benzene molecule with the nitro–  group NO₂

Nitrated aromatic molecules form the basis of explosives.  For example nitration of methyl benzene (called toluene) creates the unstable molecule tri nitro toluene TNT).

Nitrated aromatic molecules can be reduced using tin and hydrochloric acid to form amines the precursors of dyestuffs and drugs. 

The reaction scheme below shows the formation of an azo dye from the reduction of TNT.

2. Halogenation

This is also an electrophilic substitution reaction.

Reagents are the halogen in the presence of a halogen carrier such as iron(III) chloride (FeCl₃) or aluminium chloride (AlCl₃).  These two chlorides are often referred to as Halogen Carriers.  Clearly you would use a chloride for the introduction of chlorine and a bromide for the introduction of bromine into the  benzene ring.  Other methods beyond the scope of the A level course are used to introduce iodine and fluorine into the benzene ring.

The product is the halogenobenzene such as chlorobenzene.

Note how this reaction differs from the bromination of an alkene.  Bromine does not added across the π delocalised ring system of 6 electrons in benzene like it does across the double bond in an alkene.  The ring system requires too much energy to be broken into the activation energy for the reception of a bromine atom is too high in the order of 150 kJ per mole.  

Equation:

The product here is bromobenzene.

3. Friedl-Crafts reaction

The Friedl-Crafts reaction takes its name from the two chemists who first carried this reaction.  

The significance of this reaction is that it creates a carbon carbon bond (–C—C–)between the benzene ring and the substituted group. 

There are two types mentioned above in the extracts from the A level specifications.

Type 1 is alkylation:

This where an alkyl group is attached to the benzene ring.

Reagents are a typical haloalkane (e.g. chloroethane C₂H₅Cl ) and aluminium chloride (AlCl₃).

Equation:

The product here would be ethyl benzene:

Type 2 is acylation

This is where an acyl group is attached to the benzene ring.

Reagents are a typical acylhalide (e.g. ethanol chloride CH₃ COCl ) and aluminium chloride (AlCl₃).

Equation:

The product here would be a ketone in this case: phenyl methyl ether.

4. Combustion

Benzene burns in air with a very yellow smoky flame.  

This flame indicates the high carbon content of the molecule.  Benzene is burning incompletely. 

Equation:

C₆H₆  +  4 1/2 O₂  →  2C  + 2CO  +   2CO₂    +   3H₂O

This is not the only possible equation for this incomplete combustion – there are many other.

Some aromatic structures

Methyl orange indicator

Several large vat dye structures