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Advanced Level Organic Chemistry:  Aryl sulfonic acids - preparation, properties, uses

Part 7. The chemistry of AROMATIC COMPOUNDS

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Part 7.7 Electrophilic substitution - ring sulfonation of arenes e.g. benzene and methylbenzene, and properties & uses of alkylbenzenesulfonic acids

Sub-index for this page

7.7.1 Preparation of sulfonic acids - reagents, conditions, equations

7.7.2 The electrophilic substitution mechanism for sulfonation of arenes

7.7.3 The physical properties of sulfonic acids and their salts

7.7.4 The chemical properties and uses of sulfonic acids and their salts


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7.7.1 Preparation of sulfonic acids - reagents, conditions, equations

Benzene is heated with concentrated Sulfuric acid or even better, 'fuming' Sulfuric acid, which has a higher sulfur trioxide content and more efficient at introducing the sulphonic acid group into the benzene ring.

Benzene and methylbenzene are insoluble in sulfuric acid, but the resulting sulfonic acids are.

Therefore the reaction is complete when the hydrocarbon layer disappears.

The sulfur trioxide is the attacking electrophile.

Using fuming sulfuric acid ('oleum', extra SO3 available), at room temperature, benzene reacts in 20-30 minutes, but methylbenzene only takes 1-2 minutes - showing the +I inductive effect of the methyl group in increasing the reactivity of the benzene ring.

Examples of aromatic sulfonation substitution reactions

(a) (c) doc b + H2SO4 ===> (c) doc b + H2O

benzene + sulfuric acid ==> benzenesulfonic acid + water

or (c) doc b + SO3 ===> (c) doc b

benzene + sulfuric acid ===> benzenesulfonic acid + water

or  benzene + sulfur trioxide ===> benzenesulfonic acid

(c) doc b shows the full structure of the sulfonic acid group

(b) (c) doc b + H2SO4 ===> (c) doc b  (c) doc b (c) doc b + H2O

or (c) doc b + SO3 ===> (c) doc b  (c) doc b (c) doc b 

methylbenzene + sulfuric acid ==> 2/3/4-methylbenzenesulfonic acid + water

or  methylbenzene + sulfur trioxide ==> 2/3/4-methylbenzenesulfonic acid

Three structural-positional isomers C7H8SO3 formed in different proportions

A typical yields are 32%, 6% and 62% for substitution at the 2, 3 and 4 positions (from left to right).

 


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7.7.2 The electrophilic substitution mechanism for sulfonation of arenes

Benzene is heated with concentrated Sulfuric acid or even better, 'fuming' Sulfuric acid, which has a higher Sulfur trioxide content and more efficient at introducing the sulphonic acid group into the benzene ring.

diagram of electrophilic substitution reaction mechanisms for sulfonation of benzene ring in aromatic compounds synthesis of benzenesulfonic acids

mechanism 44 - electrophilic substitution by a sulfonic group in the benzene ring

Step (1) Sulfur trioxide is formed (or already present). It is a powerful electrophile, i.e. electron pair acceptor because of the effect of the three very electronegative oxygen atoms bonded to the central Sulfur atom.

Step (2) An electron pair from the delocalised pi electrons of the benzene ring forms a C-S bond with the electron pair accepting Sulfur trioxide forming a second highly unstable carbocation.

It is very unstable because the stable electron arrangement of the benzene ring is partially broken to give a 'saturated' C (top right of ring).

Step 2 has the highest activation energy and is the rate determining step (see mechanism diagram 81F below).

Step (3) A hydrogensulfate ion removes a proton and the complete benzene ring is reformed giving the anion of the aromatic sulphonic acid.

Step (4) is a proton transfer to give the sulphonic acid.

for R = CH3, methylbenzene: C6H5CH3  +  H2SO4  ===>  CH3C6H4SO2OH  +  H2O

and again there is the potential to form three position isomers by substituting in the 2, 3 or 4 position on the ring, the mechanism diagram shows the formation of methyl-2-benzenesulphonic acid.

The overall sulfonation reaction is the substitution of -H by -SO2OH

 

diagram of electrophilic substitution reaction mechanisms for sulfonation of benzene with sulfur trioxide to form benzenesulfonic acid

In conc. sulfuric acid, there is only a little sulfur trioxide from via a self ionisation

2H2SO4    H3O+  +  HSO4-  +  SO3

The sulfur trioxide is a powerful electrophile because the three electronegative oxygens create a relatively positive sulfur atom, and it is a sulfur atom orbital that accept the electron pair from the pi electrons of the benzene ring.

Mechanism diagram 81C: The sulfonation of benzene with sulfur trioxide

A pair of pi electrons are donated to the attacking sulfur trioxide electrophile forming a ring carbon - sulfur bond in the benzene molecule.

Two proton transfers occur to yield the final product of benzenesulfonic acid.

 

reaction progress profile diagram of electrophilic substitution reaction mechanisms for sulfonation of benzene ring in aromatic compounds synthesis of benzenesulfonic acids

Mechanism diagram 81F shows the reaction progress profile for the sulfonation of benzene with sulfur trioxide.

The first step here, forming the unstable intermediate carbocation, has the highest activation energy and is the slower rate determining step.

The final step yielding the final product, with the reformed stable benzene ring of benzenesulfonic acid, has the lower activation energy and is much faster.

 

diagram of electrophilic substitution reaction mechanisms for sulfonation of methylbenzene with sulfur trioxide to form methyl-2-benzenesulfonic acid methyl-4-benzenesulfonic acid

Mechanism diagram 81D: The sulfonation of methylbenzene with sulfur trioxide

A pair of pi electrons are donated to the attacking sulfur trioxide electrophile forming a ring carbon - sulfur bond in the methylbenzene molecule.

There are three possible substitution positions, and I've illustrated the formation of 2- and 4-methylbenzenesulfonic acid, because they are the two principal products.

Two proton transfers occur to yield the final product of a methylbenzenesulfonic acid.

 


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7.7.3 The physical properties of sulfonic acids and their salts

Abbreviations used: mpt = melting point oC;  bpt = boiling point oC; sub. = sublimes; dec. = thermally decomposes

Name of aromatic sulfonic acid Structure Mpt/oC Bpt/oC Comments
benzenesulfonic acid structural formula benzenesulfonic acid molecular structure  (c) doc b  C6H5SO2OH 50

anhydrous

? The simplest of the aromatic sulfonic acids.

Soluble in water, crystallises with 1-2 H2O's of crystallisation.

2-methylbenzenesulfonic acid structural formula of methyl-2-benzenesulfonic acid molecular structure of 2-methylbenzenesulfonic acid  structural formula of methyl-3-benzenesulfonic acid molecular structure of 3-methylbenzenesulfonic acid structural formula of methyl-4-benzenesulfonic acid molecular structure of 4-methylbenzenesulfonic acid  67 128 All 3 positional isomers of CH3C6H4SO2OH are soluble in water.
3-methylbenzenesulfonic acid    
4-methylbenzenesulfonic acid 104 140
4-aminobenzenesulfonic acid structural formula of 4-aminobenzenesulfonic acid molecular structure 288 dec. Soluble in hot water.
sodium benzenesulfonate C6H5SO2O-Na+     Very soluble in water
         

Further comments on the data table

Aromatic sulfonic acids are white crystalline solids and very hygroscopic (absorb so much moisture from air, they form a solution).

All sulfonic acids are soluble in water and ethanol (polar solvents), but only slightly soluble in non polar solvents like benzene.

In water the ions will be hydrated, solvation aiding dissolving and in ethanol the sulfonic acid will form hydrogen bonds with the solvent.


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7.7.4 The chemical properties and uses of sulfonic acids and their salts

They are all strong acids - comparable to the 1st ionisation of sulfuric acid ~100% ionised in water.

e.g. for benzenesulfonic acid itself, giving the benzenesulfonate ion and the hydrated hydrogen ion:

  C6H5SO2OH(aq)  +  2H2O(l)  ===>  C6H5SO2O-(aq)  +  H3O+(aq)

The acids can be neutralised to give the soluble metal ion salts.

e.g. the formation of sodium benzenesulfonate on neutralising benzene sulfonic acid with sodium hydroxide solution.

  C6H5SO2OH(aq)  +  NaOH(aq)  ===>  C6H5SO2O-Na+(aq)  +  H2O(l)

 

The salts exhibit surfactant properties and are used as in synthetic detergent formulations.

The salts of sulfonic acids (alkylbenzene sulfonates) are used in numerous personal-care products e.g. soaps, shampoos, toothpaste and household-care products like laundry detergents, dishwashing liquids and spray cleaner cleaners.

The sodium salts of alkylbenzenesulfonates (see below) are particularly useful in 'hard water' containing dissolve calcium salts - the calcium salts of alkylbenzenesulfonates are soluble, so you don't get a precipitate when washing with them i.e. no scum formed!

The diagram below illustrates a 3 step synthesis of a detergent.

synthesis of a detergent sodium alkylbenzenesulfonate LAS surfactant

ALKYLATION: The first step is to react benzene with a long chain linear alkene of typically 10 to 14 carbon atoms i.e. length of R + R' = 8 to 12 in the diagram above. See Part 7.3 Synthesis of arenes including alkylation

SULFONATION: The alkylbenzene product is then sulfonated with fuming oleum (concentrated sulfuric acid with extra sulfur trioxide dissolved in it) yielding a long-chain alkylbenzenesulfonic acid.

NEUTRALISATION: The sulfonic acid is neutralised to make the sodium salt, which acts as a surfactant (lowers surface tension of water) - in this case the sodium alkylbenzenesulfonate salt is an example of a synthetic detergent.

This is a good example of a multi-stage synthesis of a commercially used product.

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