A-level Chemistry/WJEC/Module 4/Carboxylic acids

A carboxylic acid is characterised by the presence of the carboxyl group -COOH. The chemical reactivity of carboxylic acids is dominated by the very positive carbon, and the resonance stabilisation that is possible should the group lose a proton. These two factors contribute both to acidity and to the group's dominant chemical reaction: nucleophilic substitution.

Preparation

1) from primary alcohols

RCH₂OH + 2 [O] → 3 RCOOH + H₂O

3 RCH₂OH + 2 K₂Cr₂O₇ + 8 H₂SO₄ → 3 RCOOH + 2 Cr₂(SO₄)₃ + 2 K₂SO₄ + 11 H₂O

Aliphatic carboxylic acids are formed from primary alcohols or aldehydes by reflux with potassium dichromate(VI) acidified with sulfuric acid.

2) from MethylBenzene ("toluene") etc.

C₆H₅CH₃ + 2 KMnO₄ → C₆H₅COOH + 2 MnO₂ + 2 KOH

Alkyl benzenes (MethylBenzene, EthylBenzene, etc) all react with potassium manganate(VII) to form benzoic acid. All alkyl benzenes give the same product, because all but one alkyl carbon is lost.

No acidification is needed. The reaction is refluxed and generates KOH. The benzoic acid is worked up by adding a proton source (i.e. an acid, such as HCl).

3) hydrolysis of nitriles and amides

The hydrolysis of an amide or a nitrile by a dilute acid leads to a carboxylic acid. Hydrolysis using sodium hydroxide produces the sodium salt of the acid, plus ammonia.

Examples:

ethanamide + hydrochloric acid → ethanoic acid + ammonium chloride

CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl

propanenitrile + sodium hydroxide → sodium propanoate + ammonia

CH₃CH₂CN + H₂O + NaOH → CH₃CH₂COONa + NH₃

4) hydrolysis of esters and acid chlorides

Acid chlorides rapidly hydrolyse if water is present:

ethanoyl chloride + water → ethanoic acid + hydrochloric acid

CH₃COCl + H₂O → CH₃COOH + HCl

Esters also hydrolyse but much more slowly. An acid or alkali catalyst is usually required. If an alkali catalyst is used then the product is the salt of the carboxylic acid:

methyl propanoate + water → propanoic acid + methanol

CH₃CH₂COOCH₃ + H₂O → CH₃CH₂COOH + CH₃OH

methyl octadecanoate + sodium hydroxide → sodium octadecanoate + methanol

CH₃(CH₂)₁₆COOCH₃ + NaOH → CH₃(CH₂)₁₆COONa + CH₃OH

Sodium salts of long-chain fatty acids (e.g. sodium octadecanoate) are soaps, and the alkaline hydrolysis of long-chain fatty acid esters is called "saponification".

Properties

Nomenclature

The systematic IUPAC nomenclature for carboxylic acids requires the longest carbon chain of the molecule to be identified and the -e of alkane name to be replaced with -oic acid.

The traditional names of many carboxylic acids are still in common use.

Nomenclature of carboxylic acids

formula	IUPAC name	traditional name
HCOOH	methanoic acid	formic acid
CH3COOH	ethanoic acid	acetic acid
CH3CH2COOH	propanoic acid	propionic acid
CH2=CH-COOH	propenoic acid	acrylic acid
HOOCCOOH or (COOH)2	ethanedioic acid	oxalic acid

The systematic approach for naming dicarboxylic acids (alkanes with carboxylic acids on either end) is the same as for carboxylic acids, except that the suffix is -dioic acid.

Acidity

Most carboxylic acids are weak acids. To quantify the acidities we need to know the pK_a values: The pH above which the acids start showing mostly acidic behaviour: Ethanoic acid: 4.8 Phenol: 10.0 Ethanol: 15.9 Water: 15.7

Acidity of carboxylic acids in water

acid	formula	pKa
methanoic acid	H-COOH	3.75
ethanoic acid	CH3-COOH	4.75
propanoic acid	CH3CH2-COOH	4.87
propenoic acid	CH2=CH-COOH	4.25
benzoic acid	C6H5-COOH	4.19
trifluoroethanoic acid	CF3-COOH	0.3
phenol	C6H5-OH	10.0
ethanol	CH3CH2-OH	15.9
water	H2O	15.7

Data from CRC Handbook of Chemistry & Physics, 64th edition, 1984 D-167-8 Except http://en.wikipedia.org/wiki/Trifluoroacetic_acid

Clearly, the carboxylic acids are remarkably acidic for organic molecules. Somehow, the release of the H⁺ ion is favoured by the structure. Two arguments: The O-H bond is polarised by the removal of electrons to the carbonyl oxygen. The ion is stabilised by resonance: the carbonyl oxygen can accept the charge from the other oxygen. The acid strength of carboxylic acid are strongly modulated by the moiety attached to the carboxyl. Electron-donor moiety decrease the acid strength, whereas strong electron-withdrawing groups increase it.

Reactions of Carboxylic Acids

Reduction

RCOOH + LiAlH₄(ether) → RCH₂OH

N.B. NaBH₄ reduces aldehydes and ketones, but it does not reduce carboxylic acids.

Acid Chloride Formation

Carboxylic acids are converted to acid chlorides by a range of reagents: SOCl₂, PCl₅ or PCl₃ are the usual reagents. Other products are HCl & SO₂, HCl & POCl₃ and H₃PO₃ respectively. The conditions must be dry, as water will hydrolyse the acid chloride in a vigorous reaction. Hydrolysis forms the original carboxylic acid.

CH₃COOH + SOCl₂ → CH₃COCl + HCl + SO₂

C₆H₅COOH + PCl₅ → C₆H₅COCl + HCl + POCl₃

3 CH₃CH₂COOH + PCl₃ → 3 CH₃CH₂COCl + H₃PO₃

Esterification

Alcohols will react with acid chlorides or carboxylic acids to form esters. This reaction is catalysed by acidic or basic conditions.

C₆H₅COCl + CH₃CH₂OH → C₆H₅COOCH₂CH₃ + HCl

With carboxylic acids, the condensation reaction is an unfavourable equilibrium, promoted by using non-aqueous solvent (if any) and a dehydrating agent such as sulfuric acid (non-nucleophilic), catalysing the reaction.

CH₃COOH + CH₃CH₂CH₂OH ⇌ CH₃COOCH₂CH₂CH₃ + H₂O

Ethanoic Acid reacts with Propan-1-ol to form Propyl Ethanoate

Reversing the reaction is simply a matter of refluxing the ester with plenty of aqueous acid. This hydrolysis produces the carboxylic acid and the alcohol.

C₆H₅COOCH₃ + H₂O → C₆H₅COOH + CH₃OH

Alternatively, the reflux is done with aqueous alkali, in which case the salt of the carboxylic acid is produced.

Amide Formation

Conceptually, an amide is formed by reacting an acid with ammonia or an amine compound, releasing water.

RCOOH + H₂NR' → RCONHR' + H₂O

However, the acid-base reaction is much faster:

RCOOH + H₂NR' → RCOO^- + H₃NR'⁺

Heating the ammonium salt is required to produce the amide:

RCOO^- + H₃NR'⁺ → RCONHR' + H₂O

Alternatively, an acyl chloride can be used instead of the carboxylic acid:

RCOCl + H₂NR' → RCONHR' + HCl

Decarboxylation

When carboxylic acids are heated with soda lime, their -COOH groups are replaced with a simple -H atom. Soda lime is a mixture of NaOH and CaO. In equations, it is generally written as simply "NaOH", and the inorganic product is Na₂CO₃. Usually, the sodium salt of the carboxylic acid is chosen to react with soda lime.

RCH₂COONa + NaOH + heat → RCH₃ + Na₂CO₃

C₆H₅COOH + 2 NaOH + heat → C₆H₆ + Na₂CO₃ + H₂O

CH₃CH₂COOH + 2 NaOH → CH₃CH₃ + Na₂CO₃ + H₂O

Note how a carbon is lost from the main chain. The product of the reaction may be easier to identify than the original acid, helping us to find the structure.

Polyester Formation

Polyester can be made by reacting a diol (ethane-1,2-diol) with a dicarboxylic acid (benzene-1,4-dicarboxylic acid).

n HO-CH₂CH₂-OH + n HOOC-C₆H₄-COOH → (-O-CH₂CH₂-O-OC-C₆H₄-CO-)_n + (n-1) H₂O

Polyester makes reasonable fibres, it is quite inflexible so it does not crease easily; but for clothing it is usually combined with cotton for comfort. The plastic is not light-sensitive, so it is often used for net curtains. Film, bottles and other moulded products are made from polyester.

Distinguishing carboxylic acids from phenols

Although carboxylic acids are acidic, they can be distinguished from phenol because:

Only carboxylic acids will react with carbonates and hydrogencarbonates to form CO₂.

2 CH₃COOH + Na₂CO₃ → 2 CH₃COONa + H₂O + CO₂

C₆H₅COOH + NaHCO₃ → C₆H₅COONa + H₂O + CO₂

Some phenols react with FeCl₃ solution, giving a characteristic purple colour.

Derivatives of Carboxylic Acids

The carboxyl group (abbreviated -CO₂H or -COOH) is one of the most widely occurring functional groups in chemistry as well as biochemistry. The carboxyl group of a large family of related compounds called Acyl compounds or Carboxylic Acid Derivatives.

All the reactions and compounds covered in this section will yield Carboxylic Acids on hydrolysis, and thus are known as Carboxylic Acid Derivatives. Hydrolysis is one example of Nucleophilic Acyl Substitution, which is a very important two step mechanism that is common in all reactions that will be covered here.

Structure

This group of compounds also contains a carbonyl group, but now there is an electronegative atom (oxygen, nitrogen, or a halogen) attached to the carbonyl carbon. This difference in structure leads to a major change in reactivity.

Nomenclature

The systematic IUPAC nomenclature for carboxylic acid derivatives is different for the various compounds which are in this vast category, but each is based upon the name of the carboxylic acid closest to the derivative in structure. Each type is discussed individually below.

Acyl Groups

Acyl groups are named by stripping the -ic acid of the corresponding carboxylic acid and replacing it with -yl.

CH₃COOH = ethanoic acid

CH₃CO-R = ethanoyl-R

Acyl Halides

Simply add the name of the attached halide to the end of the acyl group.

CH₃COOH = ethanoic acid

CH₃COBr = ethanoyl bromide

Esters

Esters are created when the hydrogen on a carboxylic acid is replaced by an alkyl group. Esters are known for their pleseant, fruity smell and taste, and they are often found in both natural and artificial flavors. Esters (RCOOR¹) are named as alkyl alkanoates. The alkyl group directly attached to the oxygen is named first, followed by the acyl group, with -ate replacing -yl of the acyl group.

CH₃COOH = ethanoic acid

CH₃CH₂CH₂CH₂OH = butan-1-ol

CH₃COOCH₂CH₂CH₂CH₃ = butyl ethanoate

Amides

Amides which have an amino group (-NH₂) attached to a carbonyl group (RC=O) are named by replacing the -oic acid or -ic acid of the corresponding carboxylic acid with -amide.

CH₃COOH = ethanoic acid

CH₃CONH₂ = ethanamide

Nitriles

Nitriles (RCN) can be viewed a nitrogen analogue of a carbonyl and are known for their strong electron withdrawing nature and toxicity. Nitriles are named by adding the suffix -nitrile to the longest hydrocarbon chain (including the carbon of the cyano group).

CH₃CH₂CN = propanenitrile

Structure and Reactivity

Stability and reactivity have an inverse relationship, which means that the more stable a compound, generally the less reactive - and vice versa. Since acyl halides are the least stable group listed above, it makes sense that they can be chemically changed to the other types. Since the amides are the most stable type listed above, it should logically follow that they cannot easily changed into the other molecule types, and this is indeed the case.

The stability of any type of carboxylic acid derivative is generally determined by the ability of its functional group to donate electrons to the rest of the molecule. In essence, the more electronegative the atom or group attached to carbonyl group, the less stable the molecule. This readily explains the fact that the acyl halides are the most reactive, because halides are generally quite electronegative. It also explains why acid anhydrides are unstable; with two carbonyl groups so close together the oxygen in between them cannot stabilize both by resonance - it can't loan electrons to both carbonyls.

The following derivative types are ordered in decreasing reactivity (the first is the most reactive):

Acyl Halides (CO-X) > Acyl Anhydrides (-CO-O-OCR) > Acyl Thioester (-CO-SR) > Acyl Esters (-CO-OR) > Acyl Amides (-CO-NR₂)

As mentioned before, any substance in the preceding list can be readily transformed into a substance to its right; that is, the more reactive derivative types (acyl halides) can be directly transformed into less reactive derivative types (esters and amides). Every type can be made directly from carboxylic acid (hence the name of this subsection) but carboxylic acid can also be made from any of these types.

Reactions of Carboxylic Acid Derivatives

Acyl Chlorides

Conversion to acids

R-COCl + H₂O ----> R-COOH + HCl

Conversion to esters

R-COCl + R'-OH --- pyridine ---> R-COOR' + Cl^- + pyr-H⁺

Conversion to amides

R-COCl + R'NHR" (excess) ---> R-CONR'R" + R'NH₂R"Cl

R' and/or R" may be H

Esters

Hydrolysis

R-COOR' + H₂O <--- HA ---> R-COOH + R'-OH

R-COOR' + OH^- ----> RCOO^- + R'-OH

Amides

Hydrolysis

R-CON(R'R") + H₃O⁺ --- H₂O ---> R-COOH + R'-N⁺H₂R"

R-CON(R'R") + OH^- --- H₂O ---> R-COO^- + R'-NHR"

R,R' and/or R" may be H.

Dehydration (conversion to nitriles)

R-CONH₂ --- P₄O₁₀, heat ---> R-CN + H₂O

Nitriles

Hydrolysis

R-CN + H⁺ + 2 H₂O + heat ---> RCOOH + NH₃⁺

R-CN + OH^- + H₂O + heat ---> RCOO^- + NH₃

Reduction to amine

R-CN + LiAlH₄ ---> R-CH₂NH₂