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  Reactions of Chromones, Chemistry tutorial

Introduction:

Chromone (or 1,4-benzopyrone) is the derivative of benzopyran having a substituted keto group on the pyran ring. The derivatives of Chromone are collectively termed as Chromones. Most, although not all, Chromones are as well phenylpropanoids.

General Chemical reactions of Chromones:

Chromones vary marginally in their chemistry from Coumarins (benzopyran-2-ones) as the carbonyl group is now conjugated by the oxygen atom through the double bond of the heterocycle. This conjugation doesn't involve the benzene ring (figure shown below).

917_Chromone Conjugation.jpg

Fig: Chromone Conjugation

As a result, the Chromones are instead more basic and strong acids readily protonate the carbonyl oxygen atoms, making crystalline benzopyrilium salts. Once protonated, the molecule must be resistant to further electrophilic attack, however by fuming nitric acid and concentrated sulphuric acid at between 0°C and room temperature, Chromone provides the 6-nitro derivative (figure shown below). Chromone as well undergoes a Mannish-type reaction (figure shown below) with dimethylamine and formaldehyde (methanal) in hydrogen chloride and ethanol; here the product is 3-(N-N-dimethylaminomethyl) Chromone.

1717_Reaction of Chromones.jpg

Fig: Reaction of Chromones

Comparatively hard nucleophiles, like Grignard reagents, might attack at the carbonyl carbon, while softer nucleophiles, example: hydroxide ion, join at C-2 by conjugative addition, and this might then cause ring opening (figure shown below).

2263_Reactions of Chromones with Nucleophiles.jpg

Fig: Reactions of Chromones with Nucleophiles

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