Spectroscopic analysis:
The presence of a carboxylic acid can be illustrated through spectroscopy. The IR spectrum of an aliphatic carboxylic acid depicts an extremely broad O-H stretching absorption covering the range 3200-2500 cm-1, also a strong absorption for carbonyl stretching in the range between 1725-1700 cm-1. Less clear absorptions might be observed in the fingerprint region for O-H bending and C-O stretching (1320-1220 cm-1). If carboxylic acid is conjugated to a double bond or an aromatic ring, the carbonyl absorption takes place at relatively very lower wave numbers (in the ranges between 1715-1690 and 1700-1680 cm-1 correspondingly). The 1H nmr spectrum of a carboxylic acid will consist of a singlet for the acidic proton at a high chemical shift (9-15 ppm) that is D2O exchangeable, whereas the 13C nmr spectrum depicts a quaternary signal for the carbonyl carbon (166-181 ppm). In the mass spectrum, fragmentation ions might be observed because of loss of OH (M-17) also loss of CO2H (M-45). A fragmentation ion because of [CO2H] + may be available at m/e 45. Distinctive a carboxylic acid derivative from a carboxylic acid is simple because the 1H nmr spectrum of the former will lack a signal for the acidic proton. Furthermore, the characteristic broad O-H stretching absorption will be not present from the IR spectrum. Distinguishing among different carboxylic acid derivatives is less simple, but it is still feasible. All the acid derivatives depict a carbonyl stretching absorption in the IR spectrum. The position of this absorption can point out the group present (for example 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 among the regions that can occasionally make the assignment hard. This difficulty is compounded if the functional group is conjugated to an aromatic ring or double bond because this lowers the probable frequency range and increases possible overlap. For instance, the carbonyl absorption for an aliphatic ester is in the range between 1750-1735 cm-1, while the absorption range for an α, β-unsaturated ester and an aromatic ester are both in the range 1730- 1715 cm-1. For this reason, it is significant to look at supporting proof before assuming the existence of a specific functional group. For instance, there are other characteristic absorptions in the IR spectrum which can assist to distinguish one acid derivative from other. Acid anhydrides are distinctive in comprising two carbonyl absorptions and might as well have two visible C-O stretching absorptions in the range between 1300-1050 cm-1. Esters might also have two visible C-O stretching absorptions in similar region but will only have carbonyl absorption. Primary and secondary amides comprise N-H stretching absorptions (3500-3400 cm-1 and 3460-3400 cm-1 correspondingly) as well as N-H bending absorptions (1640- 1600 cm-1 and 1570-1510 cm-1 correspondingly). Tertiary amides naturally lack these types of absorptions.