Carboxylic acid halides

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From top to bottom: General formula of a carboxylic acid halide (X = F, Cl, Br or I; R = H or organyl radical), formic acid fluoride, benzoic acid chloride. The halocarbonyl residue is marked in blue .

In chemistry, carboxylic acid halides , also known as alkanoyl halides , are compounds of a group of substances that are derived from carboxylic acids and can be referred to as reactive carboxylic acid derivatives . In them, the hydroxyl group of the carboxylic acid has been replaced by a halogen atom, so that a halocarbonyl group results. The most frequently used carboxylic acid halides are the carboxylic acid chlorides .

nomenclature

It can be named either as an acid halide, based on the acyl radical , or as a halocarbonyl. Examples:

  • Acetic acid fluoride or acetyl fluoride or fluorocarbonyl methane
  • Benzoic acid chloride or benzoyl chloride or chlorocarbonylbenzene
  • Formic acid chloride or formyl chloride or chlorocarbonyl is a substance that only exists hypothetically or under high pressure and very low temperatures. Under normal conditions it breaks down into HCl and CO. Formyl fluoride H-COF is much more stable.

presentation

Methods for the preparation of the carboxylic acid chlorides from carboxylic acids are described under the heading carboxylic acid chlorides .

The synthesis reactions shown there also work for the corresponding fluorine and bromo analogues . Carboxylic acid fluorides are also obtained from carboxylic acid chlorides and potassium hydrogen difluoride with elimination of potassium chloride and hydrogen chloride:

Acyl halide synthesis1

Carboxylic acid iodides are prepared from carboxylic acid chlorides by reaction with hydrogen iodide and with elimination of hydrogen chloride :

Acyl halide synthesis

properties

Most low molecular weight carboxylic acid halides are colorless, pungent-smelling liquids that smoke in moist air due to hydrolysis to carboxylic acids and hydrohalic acids . Compared to the respective carboxylic acid, the melting and boiling points are lower, since no hydrogen bonds can form. All carboxylic acid halides are flammable. Due to the –I effect of the halogen atom, the carbon atom of the carbonyl group is more positively charged and therefore significantly more reactive than that of the corresponding carboxylic acid. All carboxylic acid halides are among the particularly reactive carboxylic acid derivatives.

Reactions

hydrolysis

Carboxylic acid halides react with water with a stormy, strongly exothermic reaction to form the respective carboxylic acid and the corresponding hydrogen halide :

X = F, Cl, Br, or I.

For the exact mechanism, see also: addition-elimination mechanism .

Amide formation

The corresponding carboxamides can be produced by reaction with ammonia :

X = F, Cl, Br, or I.

A hydrogen halide is also split off.

Ester formation

The corresponding carboxylic acid esters can be prepared by reacting with alcohols :

X = F, Cl, Br, or I.

In contrast to that of carboxylic acids and alcohols, this reaction is irreversible.

Friedel-Crafts acylation

Aromatic ketones are formed through a reaction with aromatics , here benzene , through Friedel-Crafts acylation :

X = F, Cl, Br, or I.

The carboxylic acid halides must first be activated with a Lewis acid, here aluminum trichloride .

For further reactions see: Carboxylic acid chlorides .

use

The carboxylic acid halides are the carbonyl compounds with the greatest reactivity. This is why many reactions that, starting from the pure carboxylic acid, are only possible under special conditions, proceed much more easily. Examples of the use are the preparation of the carboxylic acid esters or Friedel-Crafts acylations .

safety instructions

Since the carboxylic acid halides hydrolyze very easily and with development of heat , they must be stored as dry as possible. Escaping hydrohalic acids irritate the mucous membranes, eyes and skin. The reaction with lower alcohols usually proceeds with a similar stormy course, with the formation of the ester and the hydrohalic acid. All carbonyl halides are also flammable.

Individual evidence

  1. ^ Hans Beyer and Wolfgang Walter : Organic Chemistry , S. Hirzel Verlag, Stuttgart, 1984, pp. 238-240, ISBN 3-7776-0406-2 .
  2. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 1236.
  3. ^ A b Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 414, ISBN 3-342-00280-8 .
  4. ^ Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 319, ISBN 3-342-00280-8 .
  5. ^ Hans Beyer and Wolfgang Walter : Organic Chemistry , S. Hirzel Verlag, Stuttgart, 1984, p. 245, ISBN 3-7776-0406-2 .
  6. ^ Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, pp. 381-384, ISBN 978-3-527-32292-3 .