Structure and properties of Carboxylic acids and carboxylic acid derivatives

STRUCTURE AND PROPERTIES

Key Notes

Structure

There are four common carboxylic acid derivaties derived from a parent carboxylic acid – acid chlorides, acid anhydrides, esters, and amides. Car-boxylic acids and their derivatives contain ansp² hybridized carbonyl group linked to a group Y where the atom directly attached to the carbonyl group is a heteroatom (Cl, O or N). The functional groups are planar with bond angles of 120°.

Bonding

The carbonyl group is made up of a strong σ bond and a weaker π bond. The carbonyl group is polarized such that the oxygen acts as a nucleophilic center and the carbon acts as an electrophilic center.

Properties

Carboxylic acids are polar and can take part in hydrogen bonding. They are soluble in water and have high boiling points. Carboxylic acids are weak acids in aqueous solution and form water soluble salts when treated with a base. Primary and secondary amides participate in hydrogen bonding and have higher boiling points than comparable aldehydes or ketones. Acid chlorides, acid anhydrides, esters, and tertiary amides are polar but are not capable of hydrogen bonding. Their boiling points are similar to aldehydes and ketones of similar molecular weight.

Reactions

Carboxylic acids and acid derivatives undergo nucleophilic substitutions.

Spectroscopic analysis

The presence of a carboxylic acid or a carboxylic acid derivative can be demonstrated by spectroscopy. IR spectroscopy shows strong absorptions for carbonyl stretching. The position of the absorption is characteristic of different acid derivatives. Quaternary signals for the carbonyl carbon occur in the ¹³C nmr spectrum and also occur in characteristic regions for each acid derivative. It is important to consider all other lines of evidence when interpreting spectra. This includes elemental analysis, molecular weight and molecular formula, as well as supporting evidence in various spectra.

Structure

Carboxylic acid derivatives are structures derived from a parent carboxylic acid structure.  There  are  four  common  types  of  acid  derivative  –  acid  chlorides, acid  anhydrides, esters, and amides (Fig. 1). These functional groups contain a carbonyl group  (C=O) where both atoms are sp² hybridized (Fig. 2). The carbonyl group along with the two neighboring atoms is planar with bond angles of 120°.

The carbonyl group along with the attached carbon chain is called an acyl group.

Carboxylic acids and carboxylic acid derivatives differ in what is attached to the acyl group (i.e. Y = Cl, OCOR, OR, NR₂, or OH). Note that in all these cases, the atom in Y which is directly attached to the carbonyl group is a heteroatom (Cl, O, or N). This distinguishes carboxylic acids and their derivatives from aldehydes and  ketones  where  the  corresponding  atom  is  hydrogen  or  carbon.  This  is important with respect to the sort of reactions which carboxylic acids and their derivatives undergo. The carboxylic acid group (COOH) is often referred to as a carboxyl group.

Bonding

The bonds in the carbonyl C O group are made up of a strong σ  bond and a weaker π  bond (Fig. 3). Since oxygen is more electronegative than carbon, the carbonyl group is polarized such that the oxygen is slightly negative and the carbon is slightly positive. This means that oxygen can act as a nucleophilic center and carbon can act as an electrophilic center.

Properties

Carboxylic acids and their derivatives are polar molecules due to the polar carbonyl group and the presence of a heteroatom in the group Y. Carboxylic acids can associate with each other as dimers (Fig. 4) through the formation of two intermolecular hydrogen bonds which means that carboxylic acids have higher boiling points than alcohols of comparable molecular weight. It also means that low molecular weight carboxylic acids are soluble in water. However, as the molecular weight of the carboxylic acid increases, the hydrophobic character of the alkyl portion eventually outweighs the polar character of the functional group such that higher molecular weight carboxylic acids are insoluble in water.

Primary amides and secondary amides also have a hydrogen capable of hydro-gen bonding (i.e. RCONHR’, RCONH₂), resulting in higher boiling points for these compounds compared to aldehydes and ketones of similar molecular weight. 

Acid chlorides, acid anhydrides, esters, and tertiary amides are polar, but lack a hydrogen atom capable of participating in hydrogen bonding. As a result, they have lower boiling points than carboxylic acids or alcohols of similar molec-ular weight, and similar boiling points to comparable aldehydes and ketones.

Carboxylic acids are weak acids in aqueous solution, forming an equilibrium between the free acid and the carboxylate ion. In the presence of a base such as sodium hydroxide or sodium hydrogen carbonate, they ionize to form water-soluble salts and this provides a method of separating carboxylic acids from other organic compounds.

Reactions

Carboxylic acids and carboxylic acid derivatives commonly react with nucleophiles in a reaction known as nucleophilic substitution (Fig. 5). The reaction involves replacement of one nucleophile with another. Nucleophilic substitution is possible because the displaced nucleophile contains an electronegative heteroatom (Cl, O, or N) which is capable of stabilizing a negative charge.

Spectroscopic analysis

The presence of a carboxylic acid can be demonstrated by spectroscopy. The IR spectrum of an aliphatic carboxylic acid shows a very broad O–H stretching absorption covering the range 3200–2500 cm⁻¹, as well as a strong absorption for carbonyl stretching in the range 1725–1700 cm⁻¹. Less obvious absorptions may be observed in the fingerprint region for O–H bending and C–O stretching (1320–1220 cm⁻¹). If the carboxylic acid is conjugated to a double bond or an aromatic ring, the carbonyl absorption occurs at relatively lower wavenumbers (in the ranges 1715–1690 and 1700–1680 cm⁻¹ respectively). The ¹H nmr spectrum of a carboxylic acid will contain a singlet for the acidic proton at a high chemical shift (9–15 ppm) which is D₂O exchangeable, while the ¹³C nmr spectrum shows a quaternary signal for the carbonyl carbon (166–181 ppm). In the mass spectrum, fragmentation ions may be observed due to loss of OH (M-17) as well as loss of CO₂H (M-45). A fragmentation ion due to [CO₂H]⁺ may be present at m/e 45.

Distinguishing a carboxylic acid derivative from a carboxylic acid is easy since the ¹H nmr spectrum of the former will lack a signal for the acidic proton. More- over, the characteristic broad O–H stretching absorption will be absent from the IR spectrum. Distinguishing between different carboxylic acid derivatives is less straightforward, but it is still feasible. All the acid derivatives show a carbonyl stretching absorption in the IR spectrum. The position of this absorption can indicate the group present (e.g. acid chloride 1815–1790 cm⁻¹; acid anhydride 1850–1710 cm⁻¹; ester 1750–1735 cm⁻¹; amide 1700–1630 cm⁻¹). Unfortunately, there is an overlap between the regions, which can sometimes make the assignment difficult. This problem is compounded if the functional group is conjugated to an aromatic ring or double bond since this lowers the possible frequency range and increases possible overlap. For example, the carbonyl absorption for an aliphatic ester is in the range 1750–1735 cm⁻¹, whereas the absorption range for an α,β-unsaturated ester and an aromatic ester are both within the range 1730–1715 cm⁻¹. For that reason, it is important to look at supporting evidence before assuming the presence of a particular functional group. For example, there are other characteristic absorptions in the IR spectrum that can help to distinguish one acid derivative from another. Acid anhydrides are distinctive in having two car-bonyl absorptions and may also have two visible C–O stretching absorptions in the range 1300–1050 cm⁻¹. Esters may also have two visible C–O stretching absorp-tions in the same region but will only have one carbonyl absorption. Primary and secondary amides have N–H stretching absorptions (3500–3400 cm⁻¹ and 3460–3400 cm⁻¹ respectively) as well as N–H bending absorptions (1640– 1600 cm⁻¹ and 1570–1510 cm⁻¹ respectively). Tertiary amides naturally lack these absorptions.

The ¹³C nmr spectra for acid derivatives will contain a quaternary signal for the carbonyl carbon. For an aliphatic ester this appears at 169–176 ppm while conju-gated esters have a signal at lower chemical shift (164–169 ppm). The carbonyl signal for acid anhydrides occurs in the range 163–175 ppm. For amides it occurs at 162–179 ppm, and for acid chlorides it is present at 167–172 ppm.

If a mass spectrum and elemental analysis have been taken, the molecular weight and molecular formula will be known. This can establish whether a particular acid derivative is present or not. For example, the lack of chlorine or nitrogen in the formula clearly rules out the possibility of an acid chloride or amide; the presence of only one oxygen rules out the possibility of an ester or acid anhydride. This may seem obvious but it is surprising how often this information is ignored when students attempt to interpret IR spectra.

The chemical shifts of certain groups can give indirect evidence of particular acid derivatives, both in the ¹H and ¹³C nmr spectra. For example, the methyl group of a methyl alkanoate appears at 3.7 ppm in the ¹H spectrum and at 51–52 ppm in the carbon spectrum. In contrast, the N-methyl group of an amide occurs at 2.9 ppm (¹H nmr) and 31–39 ppm (¹³C nmr).