Aldehydes

Aldehydes

Aldehydes have the general formula C_nH_2nO and it contain the oxo (carbonyl) group. In aldehydes, the functional group is –CHO, i.e., one of the available valencies of the carbonyl group is attached to hydrogen, and so the aldehyde group occurs at the end of a chain.

Nomenclature

The lower members are commonly named after the acids that they form on oxidation. The suffix of the names of acids is –ic (the names of the trivial system are used); this suffix is deleted and replaced by aldehyde, e.g.,

        HCHO            (O)⟶     HCO₂H

Formaldehyde                      formic acid

(CH₃)₂CHCHO           (O)⟶          (CH₃)₂CHCO₂H

Isobutyraldehyde                                  isobutyric acid

The positions of side-chains or substituents are indicated by Greek letters, the α carbon atom being the one adjacent to the aldehyde group, e.g.,

According to the I.U.P.A.C. system of nomenclature, aldehydes are designated by the suffix –al, which is added to the name of the hydrocarbon from which they are derived. The longest carbon chain containing the aldehyde group is chosen as the parent hydrocarbon; the positions of side-chains or substituents are indicated by numbers, and the aldehyde group is given the number 1, which may be omitted from the name if it is the only functional group present in the compounds e.g.,

CH₃CHO                                                                      ethanol

CH₃CH₂CHCH(CH₃)CH₂CH₃                       2-ethyl-3-methylpentanal

              ↓

              CHO

 
General methods of preparation of aldehydes

1.   By the oxidation or dehydrogenation of a primary alcohol

                t-Butyl chromate (Prepared by adding chromium trioxide to t-butanol)

                oxidises primary alcohols to aldehydes almost quantitatively

                (Oppenaure et al., 1949).

2.  By heating a mixture of the calcium salts of formic acid and any one of its homologues (yield: variable due to many side reactions):

(RCO₂)₂Ca   +   (HCO₂)Ca    ⟶    2RCHO   +   2CaCO₃

3.  By passing a mixture of the vapors of formic acid and any one of its homologues over manganous oxide as catalyst at 300^oC:

RCO₂H   +   HCO₂H   (MnO) ⟶    RCHO   +   CO₂   +   H₂O

R₂CO and RCHO are obtained as by-products, and the reaction probably proceeds via th manganous salt.

4.  By the reduction of an acid chloride with hydrogen in boiling xylene using a palladium catalyst supported on barium sulphate (Rosenmund’s reduction)

RCOCl   +   H₂    ⟶    RCHO   +   HCl

Aldehydes are more readily reduced than are acid chlorides, and therefore one would expect to obtain the alcohol as the final products. It is the barium sulphate that prevents the aldehyde from being reduced, acting as a poison (to the palladium catalyst) in this reaction. Generally, when the Rosenmund reduction is carried out, a small amount of quinoline and sulphur is added; these are very effective in poisoning the catalyst in the aldehyde reduction.

5.  By means of a Grignard reagent and formic ester.

General properties of aldehydes

Dipole moment measurements of aldehydes has shown that the values are larger than can be accounted for by the inductive effect of the oxygen atom, but can be accounted for if carbonyl compounds are resonance hybrids:

Thus the carbon atom has a positive charge and consequently can be attached by nucleophilic reagents. The carbonyl group also exhibits basic properties; it is readily protonated by strong acids to form oxonium salts, since oxygen is more electronegative than carbon, the second resonating structure will make a larger contribution than the first.

Hence, protonation increases the electrophilic character of the carbonyl group and so it can be expected that nucleophilic additions will be catalysed by acids. It should also be noted that, because of the positive charge on the carbon atom, the CO group has a strong –I effect. Many addition reactions of carbonyl compounds may be represented by the general equation.

Reaction of aldehydes

1. Catalytic hydrogenation readily converts aldehydes into primary and secondary alcohols, respectively. Reduction with dissolving metals also produces alcohols.

This reaction, however may also lead to the formation of a 1,2-glycol.

2.  Aldehydes add on sodium hydrogen sulphite to form bisulphate compounds:

These bisulphate compounds are hydroxysulphonic acid salts, since the sulphur atom is directly attached to the carbon atom. This structure is supported by work with isotope ³⁴S.

Bisulphate compounds are usually crystalline solids, insoluble in sodium hydrogen sulphite solution. Since they regenerated the carbonyl compound when heated with dilute acid or sodium carbonate solution, their formation affords a convenient means of separating carbonyl compounds from non-carbonyl compounds.

3.  Aldehydes add on hydrogen cyanide to form cyanohydrins. The carbonyl compound is treated with sodium cyanide and dilute sulphuric acid:

4 aldehydes add on a molecule of a Grignard reagent, and the complex formed when decomposed with acid, gives a secondary alcohol from an aldehyde (except formaldehyde, which gives a primary alcohol), and a tertiary alcohol from a ketone.