An enone is an unsaturated functional group consisting of a conjugated system of an alkene and ketonic group. Enal, on the other hand, is a similar unsaturated functional group with alkene and aldehydic groups attached.
Alpha-beta unsaturated carbonyl compounds
The carbonyl compounds having unsaturation in alpha-beta (α-β) positions, where the unsaturation corresponds to a double bond are termed α-β unsaturated carbonyl compounds. They have a general structure of ( O=C-Cα = Cβ– ). These compounds are more susceptible to being attacked on β carbon rather than the carbonyl carbon itself, the usual case.
The reactivity of alpha-beta unsaturated carbonyl compounds is called vinylogous reactivity which is unusual from other carbonyl compounds.
Enones are chemical compounds with alpha-beta unsaturation along with a carbonyl group. The pi electrons of the alkene functional group and that of carbonyl are in conjugation with each other bringing excess stability.
Preparation of Enones
Enones are prepared by the following list of different methods:
- Enol-Dienol tautomerization in a solution
- Dehydrogenation of saturated ketones (via palladium-catalyzed dehydrogenation)
- Aldol condensation
- Knoevenegal condensation
- Meyer-Schuster rearrangement
- Selenoxide elimination (Related to Cope reaction)
- Rauhut-Currier reaction (dimerization)
The Pauson-Khand cyclization reaction prepares cyclic enones. Specifically, cyclic pent-enones are prepared by the Nazarov cyclization reaction.
reactions of Enones
Enones are electrophilic on beta as well as carbonyl carbon. This makes the reactivity of enones considerably high against nucleophiles. This electrophilicity makes the alkene part a good dienophile leading to the Diels-Alder reaction. Their extended conjugation results in a polymerization reaction which leads to the formation of industrially important α-β unsaturated carbonyl polymers.
Enones can be activated using Lewis acids which bind to the carbonyl oxygen and make complexes.
Woodward Fieser rules for Enones
The wavelength absorption region shift regarding UV-vis spectroscopy is observed in enones. The reason for these shifts is the addition of certain groups e.g Br, alkyl, etc rendering the absorption chromophores to change. A set of rules have been placed for this shift calculation and proper identification of enones in UV-visible spectroscopy. This set of rules is termed Woodward Fieser’s rules for enones.
Examples of Enones
Methyl vinyl ketone (Butenone) is the simplest ones of enones. Mesityl oxide and phorone and isophorone are prepared by the condensation of acetone. The chain lengths can change as desired to make a large number of enones.
Enals are the aldehydic derivative of the carbonyl compounds having alpha-beta unsaturation. The resonance of carbonyl and alkene pi electrons also exists in enals but the difference is in the environment of the carbonyl group. The side of carbonyl-carbon containing alkyls in enones contains only hydrogen in enals.
Preparation of Enals
- Dehydrogenation of saturated aldehydes (via palladium-catalyzed dehydrogenation)
- Continuous flow preparation using Vilsmeier reagent
- Selenium stabilized carbanions
Reactions of Enals
Aldehydes being the versatile and highly reactive class of compounds in organic chemistry, make enal very much reactive. Although there is a matter of resonance stabilization due to alternate pi bonds in the alkene-carbonyl neighborhood similar to that of enones. Still, two probable electrophilic centers are available in enal structures, the carbonyl carbon, and the β carbon making them highly susceptible to nucleophiles.
- Stereoselective synthesis of spirocyclopentanones via N-heterocyclic carbene catalyzed reactions of enals and dienones is the newest research regarding the reaction of enals and enones. To view this paper click here.
Examples of Enals
Acrolein (prop-2-enal) is the simplest enal. Propenal, Butenal, Pentenal, Cis, 3-hexenal, and cinnamaldehyde, etc are further enals.
Further α-β unsaturated carbonyl compounds
Methyl acrylate, the methyl ester of acrylic acid
Acryl amide, a precursor of polyacrylamide
Maleic acid or cis betendioic acid
Fumaric acid, a food additive and a precursor to the polymeric resins
Acryloyl chloride, a monomer, and a chemical intermediate
Chalcone, a biologically important α-β unsaturated ketone
E-Crotonaldehyde, a ubiquitous environmental pollutant
Cyclohexenone, a versatile intermediate important for the pharmaceutical industry
Testosterone, a male sex hormone
Cinnamaldehyde, the flavor, and odor of cinnamon
Paraquinone, a highly reactive pharmaceutical agent
Squaric acid, or quadratic acid
Tricarbonyl enone iron complex [ (Benzyledeneacetone)iron tricarbonyl complex]
Alpha-beta unsaturated ketone and aldehydes have resonance among pi electrons of carbonyl and alkene carbons.
Reduction of alpha-beta unsaturated carbonyl compounds.
The Pd/C (Palladium charcoal) catalyzed hydrogenation can reduce alpha-beta unsaturated carbonyl compounds.
Alpha-beta unsaturated aldehyde reactions
Alpha-beta unsaturated aldehydes are very reactive electrophilic groups. They react with nucleophilic carbons in no time.
Several catalysts can be used to reduce enones like,
- Ir (III) diamine catalyst O (1,2 reduction)
- Pd/P (ter-But)3 (1,4 reduction)
- Potassium borohydride
- Luche reduction mechanism
What is alpha-beta unsaturated ester?
Maleic acid is the alpha-beta unsaturated ester.
How do you make alpha-beta and unsaturated carbonyl compounds?
Alpha-beta unsaturated carbonyl compounds are synthesized usually by aldol condensation reaction. However, there are many other methods listed above in the article.
What do you mean by alpha-beta unsaturated carbonyl compounds?
The carbonyl compounds having unsaturation (double bond) present at the alpha-beta position are termed α-β unsaturated carbonyl compounds.
What is the correct order of reactivity of the carbonyl compounds?
The general order of reactivity among formaldehyde, aldehydes, and ketones is as follows.
HCOH < RCOH > RCOR
What is alpha beta-unsaturated aldehyde?
An aldehyde with unsaturation between alpha and beta carbons is called an alpha-beta unsaturated aldehyde.
What is alpha and beta hydrogen?
The hydrogen atom present at the alpha position of a chemical compound is called alpha hydrogen. It is the first position after a functional group or identity. Similarly, beta hydrogen is the hydrogen atom present on the carbon atom present next to the alpha carbon.