Doc Brown's Advanced level pre-university/college - isomerism - tautomer isomers

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A-Level organic chemistry exam revision notes on isomerism

Constitutional tautomerism

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: isomerism - tautomerism [page RE-EDIT]

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 INDEX of notes on isomerism chemistry

 All Advanced Organic Chemistry Notes

Structural constitutional tautomerism - variations in the connectivity of the atoms to form different functional group molecules which are in a dynamic equilibrium with each other.

The similarities and differences between the physical and chemical properties of the tautomerism  isomers are described and explained.

Abbreviations used: fpt freezing point, mpt melting point, bpt boiling point

Scroll down to study the examples of tautomerism.

14.1.2(d) Structural Isomerism: Tautomerism 

(I don't think you find this term in contemporary UK A level chemistry textbooks?)

diagram explaining tautomerism structural/constitutional isomerism - keto-enol examples described and explained

This is a special case, quite often of functional group isomerism, in which there is a dynamic equilibrium between the two isomers, i.e. they co-exist at the same time in an equilibrium situation.

14.1.2(d) Structural Isomerism - tautomerism: Case study 1d.1

The keto-enol interchange (functional group isomers of C3H6O)

This is a classic example based on functional group isomers involved a dynamic equilibrium.

(1) (c) doc b   (c) doc b (2)

(1) is propanone (a ketone, the keto tautomer) prop-1-en-2-ol and (2) is a bi-functional alkene-alcohol, sometimes referred to as the enol tautomer, which is thermodynamically far less stable and usually difficult to isolate and stabilise.

The equilibrium is very much on the left, but the small % of the 'enol' form is an important intermediate in certain reactions of ketones and if form (2) is a reactive intermediate in a reaction, then (1) is rapidly converted into (2) to attempt to restore the equilibrium (Le Chatelier's Principle in action)

The enol form is an important intermediate in the iodination of propanone (see mechanism).

It's an example of an isomerisation reaction, and because of its 'dynamic' equilibrium nature, its a special case of functional group isomers.

 

There are many examples of this keto-enol tautomerism

R-CO-CH2-R' R-C(OH)=CHR'

R = H, alkyl or aryl for this general equation for keto-enol tautomerism.

14.1.2(d) Structural Isomerism - Tautomerism:

Case study 1d.2 hydroxy-ketone and alkene-diol equilibrium systems

(3) R-CH(OH)-CO-R' (4) HO-CR=CR'-OH

R and R' are alkyl or aryl groups e.g. C6H5 , CH3C6H4 etc.

The hydroxy-ketones with aryl groups tend to form the alkene-diol more readily because of resonance-delocalisation stabilisation.

(3) is a bi-functional alcohol-ketone (hydroxy-ketone) and

(4) is a bi-functional alkene-diol (another sort of 'enol').

Summary of all the types of isomerism you need to know about

tautomerism isomerism diagram showing & explaining all the different types of structural isomerism, constitutional isomerism

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 INDEX of notes on isomerism chemistry

 All Advanced Organic Chemistry Notes

 Index of sets of isomers for a given molecular formula, some include IR and NMR spectroscopy data

 The chemistry of ALKANES and the petrochemical industry

 The chemistry of ALKENES

 The chemistry of organic HALOGEN compound (haloalkanes)

 The chemistry of ALCOHOLS (mention of ethers)

 The chemistry of ALDEHYDES and KETONES

 The chemistry of CARBOXYLIC ACIDS, ESTERS and other derivatives

 The chemistry of ORGANIC-NITROGEN compound e.g. amines

 The chemistry of AROMATIC COMPOUNDS - benzene and derivatives

What you need to know about tautomerism isomerism, tautomerism isomerism is defined, examples of tautomerism isomerism explained, defining what is meant by tautomerism isomerism, similarities and differences between the physical and chemical properties of the tautomerism isomers are described and explained, the structural formula, skeletal formula and IUPAC names are given for the tautomerism isomers

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