Ketone halogenation: Difference between revisions
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In [[organic chemistry]] '''ketone halogenation''' is a special type of [[halogenation]]. |
In [[organic chemistry]] '''ketone halogenation''' is a special type of [[halogenation]]. |
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The reaction may be carried out under either acidic or basic conditions in an aqueous medium with the corresponding elemental halogen. In this way, chloride, bromide, and iodide (but notably not fluoride) functionality can be installed selectively in the [[Alpha and beta carbon|alpha position]] of a ketone. |
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The ketonic reaction may therefore be either in basic medium or in acidic medium. We just make solution of ketone in water and halogen is also added. Therefore ketonic halogenation accurs. Reference = Engineer Rana Farhan |
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The position alpha |
The position alpha to the [[carbonyl]] group in a [[ketone]] is easily halogenated, due to the ability to form an [[enolate anion|enolate]] in [[basic (chemistry)|basic]] solution, or an [[enol]] in [[acidic]] solution. An example is the [[bromination]] of [[acetone]] in basic solution: |
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:<big>CH<sub>3</sub>–CO–CH<sub>3</sub> + |
:<big>CH<sub>3</sub>–CO–CH<sub>3</sub> + NaOH<sup>−</sup> → CH<sub>3</sub>-CO–CH<sub>2</sub>Na + H<sub>2</sub>O</big> |
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:<big>CH<sub>3</sub>–CO–CH<sub>2</sub> |
:<big>CH<sub>3</sub>–CO–CH<sub>2</sub>Na + Br<sub>2</sub> → CH<sub>3</sub>-CO–CH<sub>2</sub>Br + NaBr</big> |
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In acidic solution, usually only one alpha hydrogen is replaced by a halogen, because each successive halogenation is slower than the first. The halogen decreases the basicity of the carbonyl oxygen, thus making protonation less |
In acidic solution, usually only one alpha hydrogen is replaced by a halogen, because each successive halogenation is slower than the first. The halogen decreases the basicity of the carbonyl oxygen, thus making protonation less favorable. However, in basic solution successive halogenations are more rapid due to inductive electron withdrawal by the halogen, making the remaining hydrogens more acidic. In the case of methyl [[ketone]]s, this reaction often occurs a third time to form a ketone trihalide, which can undergo rapid substitution with water to form a carboxylate in what is known as the [[haloform reaction]].<ref>"Organic Chemistry" Fifth Edition, by Paula Yurkanis Bruice. Pearson Prentice Hall, Upper Saddle River, NJ, 2007</ref> |
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==References== |
==References== |
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Revision as of 15:59, 8 January 2014
In organic chemistry ketone halogenation is a special type of halogenation. The reaction may be carried out under either acidic or basic conditions in an aqueous medium with the corresponding elemental halogen. In this way, chloride, bromide, and iodide (but notably not fluoride) functionality can be installed selectively in the alpha position of a ketone.
The position alpha to the carbonyl group in a ketone is easily halogenated, due to the ability to form an enolate in basic solution, or an enol in acidic solution. An example is the bromination of acetone in basic solution:
- CH3–CO–CH3 + NaOH− → CH3-CO–CH2Na + H2O
- CH3–CO–CH2Na + Br2 → CH3-CO–CH2Br + NaBr
In acidic solution, usually only one alpha hydrogen is replaced by a halogen, because each successive halogenation is slower than the first. The halogen decreases the basicity of the carbonyl oxygen, thus making protonation less favorable. However, in basic solution successive halogenations are more rapid due to inductive electron withdrawal by the halogen, making the remaining hydrogens more acidic. In the case of methyl ketones, this reaction often occurs a third time to form a ketone trihalide, which can undergo rapid substitution with water to form a carboxylate in what is known as the haloform reaction.[1]
References
- ^ "Organic Chemistry" Fifth Edition, by Paula Yurkanis Bruice. Pearson Prentice Hall, Upper Saddle River, NJ, 2007
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