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Alkanoic (Carboxylic) Acids

Key Concepts

  • Alkanoic acids belong to a class of organic (carbon) compounds known as carboxylic acids.

  • Carboxylic acids contain the carboxyl (-COOH) functional group:

      O
    ||
     
    -C-OH

  • General formula of alkanoic acids1 : CnH2n+1COOH or R-COOH

  • Naming alkanoic acids2:
  1. Number the longest carbon chain starting with the carbon atom of the COOH functional group.

  2. Name the carbon chain as the alkane.

  3. Drop the "e" from the name of the alkane.

  4. Add the suffix -oic acid
  • Properties of alkanoic acids:
  1. Alkanoic acids are polar molecules so are expected to be more soluble in polar solvents like water, and to have higher melting and boiling points than the corresponding alkane.

  2. Alkanoic acids are weak acids so are expected to display similar physical and chemical properties to other acids.
  • Reactions of alkanoic acids:
  1. react with bases in neutralisation reactions3

  2. react with carbonates to produce salt, carbon dioxide gas and water

  3. react with active metals to produce hydrogen gas

  4. react with alkanols (alcohols) in esterification reactions

Naming Alkanoic Acids

  1. Number the longest carbon chain starting with the carbon atom of the COOH functional group.

      H
    |
      H
    |
      H
    |
      O
    ||
    H-C4-C3-C2-C1
      |
    H
      |
    H
      |
    H
      |
      OH

  2. Name the carbon chain as the alkane.

    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

    alkane with 4 carbon atoms in the chain is named butane

  3. Drop the "e" from the name of the alkane.

    butane becomes butan

  4. Add the suffix -oic acid

    butan becomes butanoic acid
Examples of Some Alkanoic Acids
formula longest
carbon chain
C-C
single bonds
functional
group
Preferred IUPAC Name4
(alternative name)
occurrence
HCOOH C1
meth
 
an
-COOH (carboxyl)
oic acid
formic acid
(methanoic acid)
ant sting
CH3COOH C2
eth
 
an
-COOH (carboxyl)
oic acid
acetic acid
(ethanoic acid)
vinegar
C2H5COOH C3
prop
 
an
-COOH (carboxyl)
oic acid
propanoic acid
(propionic acid)
dairy
products
C3H7COOH C4
but
 
an
-COOH (carboxyl)
oic acid
butanoic acid
(butyric acid)
rancid butter
C4H9COOH C5
pent
 
an
-COOH (carboxyl)
oic acid
pentanoic acid
(valeric acid)
valerian root

Physical Properties of Alkanoic Acids

Alkanoic acids are weak acids, and as such are expected to have the properties of acids:
  • taste sour
  • conduct electricity in solution
  • turn blue litmus red

Melting Point and Boiling Point of Alkanoic Acids

Alkanoic acids are polar molecules.

2 hydrogen bonds (shown in dotted red lines) can form between them.

For this reason, alkanoic acids have higher melting and boiling points than other compounds with the same number of carbon atoms, as shown in the table below:

compound formula Number of atoms melting point
(oC)
boiling point
(oC)
C atomsO atoms
acetic acid
(ethanoic acid)
CH3COOH 2 2 more O 17higher M.P. 118higher B.P.
ethanol CH3CH2OH 2 1 -114 78
acetaldehyde
(ethanal)
CH3CHO 2 1 -123 20
ethane CH3CH3 2 0 less O -183 lower M.P. -89 lower B.P.

The melting point and the boiling points of alkanoic acids increase with increasing number of carbon atoms in the carbon chain.
The boiling points of alkanoic acids increase by about 20oC for each carbon that is added to the chain, as shown below:

name formula No. C atoms molar mass
(g/mol)
Trend boiling point (oC)
formic acid
(methanoic acid)
HCOOH 1 46 lower 100
acetic acid
(ethanoic acid)
CH3COOH 2 60 118
propanoic acid C2H5COOH 3 74 141
butanoic acid C3H7COOH 4 88 164
pentanoic acid C4H9COOH 5 102 186
hexanoic acid C5H11COOH 6 116 205
heptanoic acid C6H13COOH 7 130 higher 223
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As the carbon chain becomes longer, the intermolecular interactions between these carbon chains becomes greater, so the weaker Van der Waals forces (dispersion forces or london forces) 5 become more significant, and the amount of energy required to separate the molecules so that the liquid will boil becomes greater.

Solubility of Alkanoic Acids in Water

Short chain alkanoic acids, formic acid (methanoic acid) to butanoic acid, are soluble (miscible) in water as a result of hydrogen bonding between the OH group of the carboxyl group and water molecules.

name formula No. C
atoms
molar mass
(g/mol)
Trend solubility
(g/100g water)
formic acid
(methanoic acid)
HCOOH 1 46 miscible
acetic acid
(ethanoic acid)
CH3COOH 2 60 miscible
propanoic acid C2H5COOH 3 74 miscible
butanoic acid C3H7COOH 4 88 miscible
pentanoic acid C4H9COOH 5 102 greater 3.4
hexanoic acid C5H11COOH 6 116 1.0
heptanoic acid C6H13COOH 7 130 least 0.2
However, after butanoic acid, as the number of carbon atoms in the carbon chain increases, the solubility of the alkanoic acid in water decreases.
The weak intermolecular forces of attraction between the hydrocarbon chains (Dispersion forces or London forces) become more significant as the chains become longer, so the alkanoic acids are more attracted to each other than they are to the polar water molecules.

Chemical Properties of Alkanoic Acids

Alkanoic acids are weak acids, the acid dissociation constant, Ka, is small.

Soluble alkanoic acids dissociate slightly in water :

general reaction: R-COOH R-COO-(aq) + H+(aq)
relative amount of species
present in aqueous solution:
almost 100%small amount small amount

The acidity of carboxylic acids increases with the substitution of highly electronegative atoms, such as chlorine, in the molecule:

name formula Number of
chlorine atoms
Acid Dissociation Constant
(Ka)
acidity
ethanoic acid CH3COOH 0 10-4.8 lower
chloroethanoic acid ClCH2COOH 1 10-2.9
dicholorethanoic acid Cl2CHCOOH 2 10-1.3
trichloroethanoic acid Cl3CCOOH 3 10-0.7 higher

Neutralisation Reactions

neutralisation reaction: acid + strong basesalt + water
general word equation: carboxylic acid
(alkanoic acid)
+ strong base salt
(metal alkanoate)
+ water
general chemical equation: RCOOH + MOH RCOO-M+ + H2O
chemical equation example: CH3COOH + NaOH CH3COO-Na+ + H2O
word equation example: acetic acid
(ethanoic acid)
+ sodium hydroxide sodium acetate
(sodium ethanoate)
+ water

Soluble salts of long-chain carboxylic acids (fatty acids) are soaps

neutralisation reaction: acid + strong basesalt + water
general word equation: long-chain carboxylic acid
(fatty acid)
+ strong base soap
(soluble metal alkanoate)
+ water
general chemical equation: RCOOH + MOH RCOO-M+ + H2O
chemical equation example: C17H35COOH + NaOH C17H35COO-Na+ + H2O
word equation example: stearic acid + sodium hydroxide sodium stearate + water

Reaction with Carbonates

general reaction: acid + carbonatesalt + carbon dioxide gas + water
general word
equation:
alkanoic acid+ metal carbonate metal alkanoate + carbon dioxide + water
general chemical
equation:
RCOOH +M2CO32RCOO-M+ + CO2(g) + H2O
chemical equation
example:
2CH3COOH + Na2CO32CH3COO-Na+ + CO2(g) + H2O
word equation
example:
acetic acid
(ethanoic acid)
+ sodium carbonate sodium acetate
(sodium ethanoate)
+ carbon dioxide + water
chemical equation
example:
CH3COOH + NaHCO3CH3COO-Na+ + CO2(g) + H2O
word equation
example:
acetic acid
(ethanoic acid)
+ sodium hydrogen carbonate6
(sodium bicarbonate)
sodium acetate
(sodium ethanoate)
+ carbon dioxide + water

Reaction with Active Metals

general reaction: acid + active metalsalt + hydrogen gas
general word equation: alkanoic acid + active metalmetal alkanoate + hydrogen gas
general chemical equation: 2RCOOH + 2M(s)2RCOO-M+ + H2(g)
chemical equation example: 2CH3COOH + 2Na(s)2CH3COO-Na+ + H2(g)
word reaction example: acetic acid
(ethanoic acid)
+ sodiumsodium acetate
(sodium ethanoate)
+ hydrogen

Esterification Reactions

Esters are produced in a condensation reaction between an alkanoic acid (carboxylic acid) and an alkanol (alcohol).

This is known as an esterification reaction.

general word equation: carboxylic acid
(alkanoic acid)
+ alcohol
(alkanol)
ester + water
general chemical equation: RCOOH + R'OH RCOOR' + H2O
chemical equation example: CH3COOH + CH3OH CH3COOCH3 + H2O
word reaction example: acetic acid
(ethanoic acid)
+ methanol methyl acetate
(methyl ethanoate)
+ water


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1Alkanoic acids are those carboxylic acids in which an oxygen atom (=O) has been substituted for two of the hydrogen atoms in the corresponding alkane, and, an OH functional group has substituted for another H atom on the same carbon atom.

2This refers to the naming of carboxylic acids in which there is only one functional group, the COOH functional group, and that occurs on a terminal carbon atom of an alkane chain.

3The term neutralisation reactions is not the best description of the reaction between a weak acid, such as a carboxylic acid, and a base, because the resulting solution at the equivalence point is not neutral. Instead, they are better described as proton transfer reactions.

4The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

5Some Chemists refer to all intermolecular forces as Van der Waal's forces, others use the term Van der Waal's forces synonymously with London forces or dispersion forces. It is probably best to avoid using the term Van der Waal's forces at all and use one of the other, unambiguous, terms instead.

6If you are an organic chemist, you name the HO-CO-O- ion as the hydrogen carbonate ion (two words separated by a space, hydrogen carbonate).
If you are an inorganic chemist, you name the same ion as the hydrogencarbonate ion (one word, hydrogencarbonate).

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