Aldehydes and ketones both contain the carbonyl group – a group in which a carbon atom has a double bond to oxygen.
The carbonyl group in aldehydes is bonded to at least one hydrogen atom, and in ketones it is bonded to two carbon atoms. The general formula for an aldehyde is
And the general formula for a ketone is
Aldehydes and ketones have a trigonal planar arrangement of groups around the carbonyl carbon atom. The carbon atom is sp2 hybridized,
On the other hand, the acidic compounds are the carboxylic acids and the phenols. The carboxylic acids have the general formula of
The functional group,
is called the carboxyl group (carbonyl + hydroxyl).
Further, phenols are commonly known carbolic acids.
The presence of the hydroxyl groups in the molecules of phenols means that phenols are like alcohols in being able to form strong intermolecular hydrogen bonds. This hydrogen bonding causes phenols to be associated and, therefore, to have higher boiling points than hydrocarbons of the same molecular weight. Thus, phenols have a modest solubility in water.
Although phenols are structurally similar to alcohols, they are much stronger acids, however they are weaker acids compared to carboxylic acids. In the experiment, the following samples of aldehydes are used, 1) Benzaldehyde
It is the simplest aromatic aldehyde. It is employed to confer almond flavor. It is used chiefly as a precursor to other organic compounds, ranging from pharmaceuticals to plastic derivatives.
It is the simplest form of aldehyde, also called methanal. It is used in low concentrations for the process C-41 (color negative film) stabilizer in the final wash step in photographs, as well as in the process E-6 pre-bleach step, to obviate the need for it in the final wash.
The following ketones are used, 1) Acetone
It is used as a solvent for most plastics and synthetic fibers including those used in laboratory bottles made of polystyrene, polycarbonate and some types of polypropylene.
It is used as a photo initiator in UV-curing applications such as inks, imaging, and clear coatings in the printing industry. It also prevents ultraviolet light from damaging scents and colors in products such as perfumes and soaps. It can also be added to the plastic packaging as a UV blocker. Without it, opaque or dark packaging would be required.
The carboxylic acids used in the experiment are, 1) Acetic acid
It is one of the simplest carboxylic acids. It is an important chemical reagent and industrial chemical, mainly used in the production of cellulose acetate.
2) Benzoic Acid
Its salts are used as a food preservative and an important precursor for the synthesis of many other organic substances.
3) Picric acid
It is one of the most acidic carboxylic acid.
The following phenols are used in the experiment, 1) Phenol
Its major uses involve its conversion to plastics or related materials. Phenol and its chemical derivatives are key for building polycarbonates, epoxies, Bakelite, nylon, detergents, herbicides such as phenoxy herbicides, and a large collection of pharmaceutical drugs.
First on the line are aldehydes and ketones, as shown below aldehydes and ketones share the carbonyl functional group which features carbon doubly bonded to oxygen. In the case of ketones there are two carbon atoms bonded to the carbonyl carbon and no hydrogen. In the case of aldehydes there is at least one hydrogen bonded to the carbonyl carbon, the other attachment may be to a carbon or hydrogen. In all cases, the carbons are attached to the carbonyl carbon maybe aliphatic or aromatic. Since they share the carbonyl group, aldehydes and ketones share much of their chemistry.
Precisely, ketones and aldehydes are functional isomers. Furthermore, they can be prepared by hydration of alkynes, Friedel Craft’s Acylation and oxidation of primary alcohol and secondary alcohol. However, ketones have lower boiling point compare to aldehydes.
Aldehydes and ketones both undergo nucleophilic addition. However, the reactivity of carbonyl compounds to nucleophiles depends on many factors as it is affected by both electronic and steric effect. One of the factors is inductive and resonance effects on the magnitude of partial positive charge on the carbonyl carbon.
Thus, the more electron poor the carbonyl carbon, the more it is susceptible for a nucleophilic attack. Lastly is the effect of steric strain on the stability of the transition states. The less crowded the transition state, the more stable product formed and the lower the activation energy, thus faster reaction.
Generally, aliphatic aldehydes are more reactive than aliphatic ketones however, compare to aromatic aldehydes and ketones aliphatic aldehydes and ketones are more reactive. It is because the aromatic aldehydes and ketones have greater electron donating groups.
In the experiment, 2,4-Dinitrophenylhydrazine is used as a qualitative test for aldehydes and ketones. Formation of large amount of yellow to red, insoluble, 2,4-Dinitrophenylhydrazone product indicates a positive test.