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Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion (HCO−2) or its derivatives such as ester of formic acid. The salts and esters are generally colorless.^[1]

When dissolved in water, formic acid converts to formate:

HCO₂H → HCO−2 + H⁺

Formate is a planar anion. The two oxygen atoms are equivalent and bear a partial negative charge. The remaining C-H bond is not acidic.

Formate is a common C-1 source in living systems. It is formed from many precursors including choline, serine, and sarcosine. It provides a C-1 source in the biosynthesis of some nucleic acids. Formate (or formic acid) is invoked as a leaving group in the demethylation of some sterols.^[2] These conversions are catalyzed by aromatase enzymes using O₂ as the oxidant. Specific conversions include testosterone to estradiol and androstenedione to estrone.^[3]

Formate is reversibly oxidized by the enzyme formate dehydrogenase from Desulfovibrio gigas:^[4]

HCO−2 → CO₂ + H⁺ + 2 e⁻

Formate esters have the formula HCOOR (alternative way of writing formula ROC(O)H or RO₂CH). Many form spontaneously when alcohols dissolve in formic acid; contrariwise, they hydrolyze easily in base.^{[5]: 674, 682} Some formate esters arise by the addition of formic acid to alkenes.^[6]

An important formate ester is methyl formate, which is produced as an intermediate en route to formic acid. Methanol and carbon monoxide react in the presence of a strong base, such as sodium methoxide:^[1]

CH₃OH + CO → HCOOCH₃

Hydrolysis of methyl formate gives formic acid and regenerates methanol:

HCOOCH₃ → HCOOH + CH₃OH

In laboratory, formate esters can be used to produce pure carbon monoxide.^[7]

Reaction with phosphorus pentachloride does not chlorolyze the ester bond, instead forming an (unusual) dichloromethyl ether.^[5]: 684

Formate esters often are fragrant or have distinctive odors. Compared to the more common acetate esters, formate esters are less commonly used commercially because they are less stable.^[8] Ethyl formate is found in some confectionaries.^[1]

Formate salts have the formula M(O₂CH)(H₂O)_x. Such salts are prone to decarboxylation. For example, hydrated nickel formate decarboxylates at about 200 °C with reduction of the Ni²⁺ to finely powdered nickel metal:

Ni(HCO₂)₂(H₂O)₂ → Ni + 2 CO₂ + 2 H₂O + H₂

Such fine powders are useful as hydrogenation catalysts.^[1]

• ethyl formate, CH₃CH₂(HCOO)
• sodium formate, Na(HCOO)
• potassium formate, K(HCOO)
• caesium formate, Cs(HCOO); see Caesium: Petroleum exploration
• methyl formate, CH₃(HCOO)
• methyl chloroformate, CH₃OCOCl
• triethyl orthoformate
• trimethyl orthoformate, C₄H₁₀O₃
• phenyl formate HCOOC₆H₅
• amyl formate