Key Information & Summary
- The aldehydes are organic compounds that have a formyl group, indicated by -CHO. They can be described by the general formula CnH2n
- Ketones are organic compounds of general formula R-CO-R', characterized by the presence of a carbonyl group C = O.
- Primary alcohols, in general, can be easily oxidized by a wide variety of oxidizing agents. With mild oxidant, aldehyde is formed. With stronger oxidant, a ketone is formed.
- The aldehydes can be easily oxidized in the corresponding carboxylic acid.
- Through the use of reducing agents such as lithium aluminium hydride (LiAlH4) aldehydes are easily reduced in the corresponding primary alcohols.
- Ketones can be obtained by direct oxidation of the secondary alcohols.
- Ketones do not undergo oxidation reactions.
- Ketones are in equilibrium with enols.
The aldehydes are organic compounds that has a formyl group, indicated by -CHO. They can be described by the general formula CnH2nO.
Their name derives from "dehydrogenated alcohol", referring to one of the methods of preparation. In nature, they are produced in the processes of fermentation of sugars.
The group C = O is called generically carbonyl (or "carbonyl group"). In aldehydes, a carbonyl group is bound to a hydrogen atom and an alkyl group. In the simplest aldehyde, a hydrogen is bound instead of the alkyl. Its name is formaldehyde (HCHO) and it used as a preservative and as a bactericide.
The carbon atom linked to the oxygen atom has sp2 hybridization at the centre of a roughly equilateral triangle on whose plane lie the oxygen, the hydrogen and the bound atom bound to the carbonyl group.
Oxidations of primary alcohols
Primary alcohols, in general, can be easily oxidized by a wide variety of oxidizing agents. For the synthesis of aldehydes, it is necessary to use milder reagents than the common oxidizing systems, such as KMnO4 or K2Cr2O7. With these reagents, the corresponding carboxylic acids would be obtained directly.
A primary alcohol can be selectively oxidized to an aldehyde by several reactions:
Oxidation of Sarett-Collins: the homonymous reagent is used, which consists of an adduct between Chromium trioxide and pyridine (Py). It can be easily synthesized in the laboratory by the reaction:
CrO3 + 2Py → CrO3 · 2Py
The oxidation of alcohol occurs in good yields and can be summarized as follows:
R-CH2OH + CrO3 · 2Py → R-CHO
Corey Oxidation: Pyridinium chlorochromate (PCC) is used, a complex obtained by mixing chromic anhydride, pyridine and hydrochloric acid. The reaction, generally carried out in organic solvent (for example CH2Cl2), is as follows:
R-CH2OH + PCC → R-CHO
Swern Oxidation: The reaction of primary alcohols with oxalyl chloride (COCl)2 and dimethyl sulphoxide (DMSO) leads to good yields of the corresponding aldehyde.
R-CH2OH + DMSO + (COCl) 2 → RCHO + (CH3) 2S + CO2 + CO + HCl
The functional group of aldehydes gives this category of compounds the opportunity to undergo some typical reactions.
The aldehydes can be recognized by the reaction with 2,4-dinitrophenylhydrazine, which reacts with their functional group to form a yellow-orange precipitate, which can subsequently be analyzed, determining the melting point, to be able to conclude which aldehyde has been analyzed.
The aldehydes can be easily oxidized to the corresponding carboxylic acid by numerous reagents, such as potassium permanganate or potassium dichromate. Since aldehydes are similar to ketones, but with the important difference that they can not be easily oxidized, the oxidation reaction is very useful for distinguishing these two categories. To make this distinction, the Tollens or the Fehling reagents are generally used. The positive result of these tests confirms the presence of aldehydes rather than ketones.
Through the use of reducing agents such as lithium aluminum hydride (LiAlH4) aldehydes are easily reduced in the corresponding primary alcohols. Alternatively it is possible to use sodium borohydride (NaBH4) which is a milder reducing agent and does not reduce the less reactive compounds such as esters, amides and carboxylic acids. It is therefore useful for selectively reducing carbonyl carbon in an aldehyde group and a carboxylic group.
Ketones, are organic compounds of general formula R-CO-R', characterized by the presence of a carbonyl group C = O. The carbon atom, with sp2 hybridization, is directly linked to two hydrocarbon chains. They differ structurally from aldehydes due to the presence of a R' group instead of hydrogen directly linked to the carbonyl. The most simple ketone is acetone, CH3-CO-CH3, in which the carbonyl group is bound to two methyl groups.
The reactivity of ketones is essentially determined by the dipolar nature of the carbonyl group, thus it is shared with aldehydes. One of the main differences between the reactivity of aldehydes and ketones is that the ketones do not undergo to oxidation reactions, while aldehydes can oxidize to carboxylic acids.
Ketones can be obtained by direct oxidation of the secondary alcohols with the use of suitable oxidizing agents:
RR'CH-OH → R-CO-R '+ 2e- + 2H +
Synthesis for hydration of alkynes, suitably catalysed by Hg + in acidic medium:
R-C≡C-R '+ H2O → R-CO-CH2-R'
Synthesis by reaction between a nitrile and a Grignard reagent and consecutive addition of H2O:
R-MgX + R'-C≡N → RR'C = N-MgX
RR'C = N-MgX + 2H2O → R-CO-R '+ NH3 + HO-MgX
Synthesis by reaction between an amide and a Grignard reagent:
R-CO-NR'R '' + R '' '- Mg-X + H + → R-CO-R' '' + R'R''NH + Mg-X+
The typical reaction of ketones is the nucleophilic addition, where a generic nucleophile Nu in acidic environment (in general it is the same nucleophile to free H + in solution) establishes a covalent bond with the carbon of the C = O group. As instance the cyanhydrin formation by addition of HCN:
R-CO-R '+ HCN → RR'C (OH) CN
or the addition of halogens acids (where X represents a generic halogen):
R-CO-R '+ HX → RR'C (OH) X
In an aqueous environment, the ketones with an alpha-hydrogen undergo what is called tautomeric equilibrium. It is a particular form of isomerism typical of some organic compounds. In this specific case, ketones are in equilibrium with enols, compounds of general formula RR'C = CR '' - OH, a phenomenon that takes the name of keto-enol tautomerism. Usually, the keto form is more stable than the enol. This equilibrium allows ketones to be prepared via the hydration of alkynes.
References and further readings:
“Organic chemistry”, Francis A. Carey, ISBN 0-07-117499-0