benzene methylbenzene naphthalene Electrophilic substitution ring sulfonation of arenes mechanism properties uses of alkylbenzenesulfonic acids benzenesulfonic acid methylbenzene sulfonic acid advanced A level organic chemistry revision notes doc brown

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

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Part 7.7 The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry sulfonation of benzene and methylbenzene mechanisms physical and chemical properties of sulfonic acids

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All my advanced level aromatic chemistry notes

All my advanced A level organic chemistry notes

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Part 7.7 Electrophilic substitution - ring sulfonation of arenes: benzene and methylbenzene, and properties & uses of alkylbenzenesulfonic acids (brief mention of naphthalene)

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

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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 sulfonic 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) + H₂SO₄ ===> + H₂O

benzene + sulfuric acid ==> benzenesulfonic acid + water

or + SO₃ ===>

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

or benzene + sulfur trioxide ===> benzenesulfonic acid

shows the full structure of the sulfonic acid group

(b) + H₂SO₄ ===> + H₂O

or + SO₃ ===>

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).

If the atom of the original group is directly bonded to the benzene ring does not have any π bonding the ring is usually activated compared to benzene itself. The methyl group tends to increase the electron density of the ring and more so at the 2, 4 and 6 positions, compared to the 3 and 5 positions.

The small electron density shift is sometimes described as a plus inductive shift (+I effect). Therefore the 2 and 4 positions become the 2nd preferred substitution points in the benzene ring so the methyl group promotes 2- and -4 substitution by the sulfur trioxide or sulfuric acid electrophile - you see this is reflected in the % yield of the three isomeric products. (see section 7.14 for more details).

The sulfonation of naphthalene

Naphthalene reacts with hot concentrated sulfuric acid to yield mainly naphthalene-2-sulfonic acid molecular structure structural formula electrophilic substitution mechanism

Naphthalene reacts with hot concentrated sulfuric acid to yield naphthalene-1-sulfonic acid (favoured at a lower temperature and naphthalene-2-sulfonic acid (favoured by higher temperature).

This reaction is in principle the same electrophilic substitution reaction undergone by benzene and methylbenzene.

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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 sulfonic 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. strong 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 highly unstable intermediate.

It is very unstable because the stable electron arrangement of the benzene ring pi electrons 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 of pi electrons is reformed giving the anion of the aromatic sulfonic acid.

Step (4) is a proton transfer to give the sulfonic 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-benzenesulfonic 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.

The stable ring of pi electrons is temporarily destroyed because one of the ring carbons is now saturated.

Two proton transfers occur to yield the final product of benzenesulfonic acid and in doing so the very stable ring of pi electrons is reformed in the benzene ring.

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.

The intermediate is unstable because the pi electrons of the benzene ring no longer form the complete stabilised aromatic ring of either the reactant or product.

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, which now has the reformed stabilising pi electron ring of the benzene ring.

TOP OF PAGE and sub-index

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	C₆H₅SO₂OH	50 anhydrous	?	The simplest of the aromatic sulfonic acids. Soluble in water, crystallises with 1-2 H₂O's of crystallisation.
2-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		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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