Alkanoic acids

Alkanoic acids ( alkanoates, alkanates or alkanecarboxylic acids ) are carboxylic acids that are built up from an alkyl radical and a carboxy group (-COOH). Alkanoic acids have the general empirical formula C _n H _{2 n +1} COOH for (i.e. n = 0, 1, 2, 3, ...). By definition, an alkyl radical has neither aliphatic nor aromatic double bonds nor other heteroatoms . Compounds with linear alkyl radicals are also known as saturated carboxylic acids. ${\ displaystyle n \ in \ mathbb {N} _ {0}}$

Alkanoic acids with a carboxy group belong to the monocarboxylic acids . Alkanoic acids with two carboxy groups are referred to as alkanedioic acids; they are dicarboxylic acids . The simplest alkanoic acid is methanoic acid, which is known under the common name formic acid . Many alkanoic acids from butanoic acid (butyric acid) are among the saturated fatty acids .

Sodium alkanoates

Alkanoic acid salts (for example: rubidium butanoate / rubidium butyrate or rubidium propanoate / rubidium propionate ) and alkanoic acid esters (for example: methyl butanoate / methyl butyrate ) are referred to as alkanoates or alkanates.

Structure of alkanoic acids

Alkanoic acids consist of the carboxy group and the aliphatic alkyl radical:

Ethanoic acid ( acetic acid )	n -hexanoic acid ( caproic acid )

properties

The shorter-chain alkanoic acids, especially butanoic acid , are strongly malodorous liquids and are acidic in water.

Longer chain alkanoic acids are white solids at room temperature. Due to the polar character of the carboxy group, the alkanoic acids are able to form hydrogen bonds , which is why they have an elevated boiling point. The acid strength and the ability to form hydrogen bonds decrease with increasing chain length, since the influence of the carboxy group on the entire molecule then decreases.

Reactions

dissociation

Short- chain alkanoic acids dissociate in water (here: acetic acid):

{\ displaystyle \ mathrm {H_ {3} C {-} COOH + H_ {2} O \ \ rightleftharpoons \ H_ {3} C {-} COO ^ {-} + H_ {3} O ^ {+}}}

.

The resulting acid residue ion is named after the alkane and -oate involved in the structure of the acid (sometimes there is also the ending -at ), for example:

Ethane + -oate → ethanoate ion (also: acetate ion)

However, alkanoic acids do not dissociate completely, i.e. only some of the alkanoic acid molecules give off a hydrogen ion. As a rule, alkanoic acids are weak acids, which is expressed in the acid strength .

Esterification

If a mixture of an alkanoic acid and an alcohol is heated in a water bath, esterification takes place. This takes place in the presence of an inorganic acid as a catalyst , usually concentrated sulfuric acid . The OH group of the alcohol reacts with the COOH group of the alkanoic acid and an ester is formed with elimination of water :

{\ displaystyle \ mathrm {H_ {3} C {-} COOH + HO {-} H_ {2} C {-} CH_ {3} \ longrightarrow H_ {3} C {-} COO {-} H_ {2} C {-} CH_ {3} + H_ {2} O}}

.

Ethanoic acid ( acetic acid ) and ethanol react to form ethyl ethanoate ( ethyl acetate ).

Complete oxidation

Alkanoic acids are oxidized to carbon dioxide and water during combustion :

{\ displaystyle \ mathrm {H_ {3} C {-} COOH + 2 \, O_ {2} \ longrightarrow 2 \, CO_ {2} +2 \, H_ {2} O}}

.

Ethanoic acid is oxidized to carbon dioxide and water by oxygen.

If the oxygen supply in the air is insufficient, carbon monoxide or carbon (in the form of soot) are produced instead of carbon dioxide .

Names and formulas of alkanoic acids

Common name	Chemical name	C atoms	Molecular formula	Melting point in ° C	Boiling point in ° C	p K _s value
Formic acid	Methanoic acid	1	CH ₂ O ₂	−08.3	101	3.75 (25 ° C)
acetic acid	Ethanoic acid	2	C ₂ H ₄ O ₂	-16.64	117.9	4.756 (25 ° C)
Propionic acid	Propanoic acid	3	C ₃ H ₆ O ₂	−20.5	141.15	4.87 (25 ° C)
Butyric acid	Butanoic acid	4th	C ₄ H ₈ O ₂	0−5.1	163.75	4.83 (25 ° C)
Valeric acid	Pentanoic acid	5	C ₅ H ₁₀ O ₂	−33.6	186.1	4.83 (20 ° C)
Caproic acid	Hexanoic acid	6th	C ₆ H ₁₂ O ₂	0−4	205.2	4.85 (25 ° C)
Enanthic acid	Heptanoic acid	7th	C ₇ H ₁₄ O ₂	0−7.2-7.5	222.2	4.89 (25 ° C)
Caprylic acid	Octanoic acid	8th	C ₈ H ₁₆ O ₂	-16.5	239	4.89 (25 ° C)
Pelargonic acid	Nonanoic acid	9	C ₉ H ₁₈ O ₂	-12.4	254.5	4.96 (25 ° C)
Capric acid	Decanoic acid	10	C ₁₀ H ₂₀ O ₂	-31.4	268.7

Other fatty acids are palmitic acid (hexadecanoic acid, C ₁₅ H ₃₁ COOH) and stearic acid (octadecanoic acid, C ₁₇ H ₃₅ COOH).

swell

Reiner Altmann : Chemical-technical material values. Publisher Harri Deutsch , Thun u. Frankfurt a. M. 1987 ( German publishing house for basic industry , Leipzig 1987).

Individual evidence

↑ ^a ^b David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-1 - 3-523.
↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Dissociation Constants of Organic Acids and Bases, pp. 8-42-8-51.

Web links

Commons : Alkanoic Acids - Collection of pictures, videos and audio files

[CRC90_3_1_523-1] David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-1 - 3-523.

[CRC90_8_42_51-2] David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Dissociation Constants of Organic Acids and Bases, pp. 8-42-8-51.