Iodination (chemistry)

In organic chemistry, iodination is the introduction of iodine with the help of iodinating agents in the form of an addition or substitution . It is a special case of halogenation .

Iodinating agents

The following substances or groups of substances are used as iodination agents :

• elemental iodine or the equimolar aqueous solution of iodine and potassium iodide in water (formation of I ₃ ^- ions)
• Hydriodic acid
• Iodine monochloride
• N -iodoacetamide
• N -iodosuccinimide
• 1,3-diiodo-5,5-dimethylhydantoin
• Bis sym -collidino-iodoniumperchlorat
• Sodium iodide or potassium iodide
• Phosphorus triiodide
• Acid iodides

substitution

A common method of production, even on an industrial scale, is the reaction of alcohols with hydriodic acid. The hydroxyl group is protonated by the acid and the carbon atom can then either be easily attacked by the iodide ion (S _N 2 mechanism) or water is split off, forming a carbocation which is then attacked by the iodide ion can be (S _N 1 mechanism). Aromatic hydroxyl groups ( phenols ) cannot be iodinated in this way.

addition

Elementary iodine, like bromine, adds electrophilically to alkenes. However, the yields are generally low due to the higher reactivity of the vicinal diiodides. A special case, however, is the iodolactonization of allyl esters, which form iodolactone in good yields when reacted with elemental iodine. The highly reactive geminal diiodide is not formed here because the iodonium ion is opened nucleophilically and intramolecularly directly by the ester carbonyl oxygen atom.

Finkelstein reaction

By the Finkelstein reaction , the blank halogen -atoms of alkyl chlorides or - bromides in an acetone solution by means of sodium iodide into the corresponding alkyl iodide transfer ( "halogen exchange reaction"). The equilibrium of this reaction is shifted to the side of the alkyl iodide by the formation of bromides or chlorides which are insoluble in acetone.

Iodination of aromatics

Many aromatic hydrocarbons cannot readily be electrophilically iodized directly because iodine is not a good electrophile. Aromatic heterocycles , on the other hand, can often be iodinated very easily and very regiospecifically.

However, the direct iodination from the unsubstituted aromatic succeeds in the presence of oleum , nitric acid or mercury (II) oxide , which oxidize elemental iodine to the stronger electrophile I ₂ ⁺ :

${\displaystyle \mathrm {2\,I_{2}+2\,SO_{3}+H_{2}SO_{4}\rightarrow 2\,I_{2}^{+}+SO_{2}+2\,HSO_{4}^{-}} }$

The subsequent iodination takes place as follows:

The synthesis of iodinated aromatics can also be carried out by a Sandmeyer-like reaction, e.g. B. can iodobenzene by diazotization of aniline a iodide solution are prepared and then adding.

See also

• Iodized salt

Individual evidence

1. ↑ ^{a b} Author collective: Organikum . 22nd edition. Wiley-VCH, 2004, ISBN 978-3527311484 .
2. ↑ FB Dain, RQ Brewster: Iodobenzene In: Organic Synthesis . 9, 1929, p. 46, doi : 10.15227 / orgsyn.009.0046 ; Coll. Vol. 1, 1941, p. 323 ( PDF ).
3. ↑ Paula Y. Bruice, Organic Chemistry, Pearson Education, 5th ed., P 755, ISBN 978-3-8273-7190-4 .
4. ↑ HJ Lucas, ER Kennedy: Iodobenzene In: Organic Syntheses . 19, 1939, p. 55, doi : 10.15227 / orgsyn.019.0055 ; Coll. Vol. 2, 1943, p. 351 ( PDF ).

swell

• H. Beyer, W. Walter: Textbook of organic chemistry . 20th ed., Hirzel, Stuttgart, 1984. p. 446.
• Entry to iodination. In: Römpp Online . Georg Thieme Verlag, accessed on January 2, 2015.