Carboxylic acid anhydrides

General structural formula of carboxylic acid anhydrides

Carboxylic acid anhydrides are compounds of the general formula R ¹ CO – O – COR ² in which two acyl groups are attached to the same oxygen atom ^[1] . Depending on the nature of the acyl groups, the anhydrides can be “simple” (R ¹ = R ² , for example, acetic anhydride (CH ₃ CO) ₂ O, mixed (various acyl residues) or cyclic (R ¹ and R ² are parts of the same molecule) ^[2] .

Formic anhydride with the formula H − CO − O − CO − H (C ₂ H ₂ O ₃ ) does not exist in free form.

Anhydrides can be formally considered as a condensation product of two −COOH groups:

{\ce {RCOOH + R'COOH -> RC(O)OC(O)R' + H2O}}

{\ displaystyle {\ ce {RCOOH + R'COOH -> RC (O) OC (O) R '+ H2O}}}

{\ displaystyle {\ ce {RCOOH + R'COOH -> RC (O) OC (O) R '+ H2O}}}

Synthesis

Some cyclic anhydrides, such as phthalic anhydride , are formed by heating the corresponding acids. In the case of the synthesis of anhydrides from aliphatic acids, dehydrating agents are used - in particular, phosphoric anhydride in the synthesis of acetic anhydride from acetic acid:

{\ce {2 CH3COOH + P4O10 -> (CH3CO)2P + P4O9(OH)2}}

{\ displaystyle {\ ce {2 CH3COOH + P4O10 -> (CH3CO) 2P + P4O9 (OH) 2}}}

{\ce {2 CH3COOH + P4O10 -> (CH3CO)2P + P4O9(OH)2}}

or carbodiimides , which react with carboxylic acids with the formation of O-acylisoureas, highly reactive compounds capable of acylating carboxylic acids:

{\ce {R-N=C=N-R + R'COOH -> R-NH-C(=NR)OC(O)R'}}

{\ displaystyle {\ ce {RN = C = NR + R'COOH -> R-NH-C (= NR) OC (O) R '}}}

{\ce {R-N=C=N-R + R'COOH -> R-NH-C(=NR)OC(O)R'}}

{\ce {R-NH-C(=NR)OC(O)R' + R'COOH -> (R'CO)2O + R-NH-C(O)NH-R}}

{\ displaystyle {\ ce {R-NH-C (= NR) OC (O) R '+ R'COOH -> (R'CO) 2O + R-NH-C (O) NH-R}}}

{\ce {R-NH-C(=NR)OC(O)R' + R'COOH -> (R'CO)2O + R-NH-C(O)NH-R}}

In most cases, anhydrides are synthesized by acylation of carboxylic acids or their salts. For example, by acylation of sodium formate with formyl fluoride, unstable formic acid anhydride can be obtained ^[3] :

{\ce {HCOONa + HCOF -> (HCO)2OC + NaF}}

{\ displaystyle {\ ce {HCOONa + HCOF -> (HCO) 2OC + NaF}}}

{\ce {HCOONa + HCOF -> (HCO)2OC + NaF}}

When acylating carboxylic acids in laboratory practice, complexes of carboxylic acid halides with pyridine ^[4] or ketenes formally internal carboxylic acid anhydrides are usually used as acylating agents:

{\ce {RCOOH + R'CH=C=O -> RC(O)OC(O)R'}}

{\ displaystyle {\ ce {RCOOH + R'CH = C = O -> RC (O) OC (O) R '}}}

{\ce {RCOOH + R'CH=C=O -> RC(O)OC(O)R'}}

Acetylation of acetic acid with ketene is an industrial method for the synthesis of acetic anhydride.

Reactivity

Anhydrides are acylating agents and react with various nucleophiles:

{\ce {(RCO)2O + NuH -> RCONu + RCOOH}}

{\ displaystyle {\ ce {(RCO) 2O + NuH -> RCONu + RCOOH}}}

forming esters (Nu = OR), amides (Nu = NR ¹ R ² ), hydrazides (Nu = HNNR ¹ R ² ), etc.

Notes

↑ acid anhydrides // IUPAC Gold Book.
↑ cyclic acid anhydrides (cyclic anhydrides) // IUPAC Gold Book.
↑ George A. Olah, Yashwant D. Vankar; Massoud Arvanaghi; Jean Sommer (1979), Formic Anhydride . Angewandte Chemie Int. Ed. Engl., Volume 18, issue 8, page 614. DOI : 10.1002 / anie.197906141 .
↑ ACID ANHYDRIDES (Eng.) // Organic Syntheses : journal. - 1946. - Vol. 26 . - P. 1 . - ISSN 2333-3553 . - DOI : 10.15227 / orgsyn.026.0001 .

[1] ydrides // IUPAC Gold Book.

[2] yclic acid anhydrides (cyclic anhydrides) // IUPAC Gold Book.

[3] George A. Olah, Yashwant D. Vankar; Massoud Arvanaghi; Jean Sommer (1979), Formic Anhydride . Angewandte Chemie Int. Ed. Engl., Volume 18, issue 8, page 614. DOI : 10.1002 / anie.197906141 .

[4] ACID ANHYDRIDES (Eng.) // Organic Syntheses : journal. - 1946. - Vol. 26 . - P. 1 . - ISSN 2333-3553 . - DOI : 10.15227 / orgsyn.026.0001 .

Carboxylic acid anhydrides

Synthesis

Reactivity

See also

Notes

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