Tropone
Encyclopedia
Tropone or 2,4,6-cycloheptatrien-1-one is an organic compound
with some importance in organic chemistry
as a non-benzenoid aromatic
. The compound consists of a ring of seven carbon atoms with three conjugated
alkene
groups and a ketone
group. The related compound tropolone
(2-hydroxy-2,4,6-cycloheptatrien-1-one) has an additional alcohol
(or an enol
including the double bond) group next to the ketone.
The tropone moiety can be found in biomolecule
s such as colchicine
, stipitatic acid and hinokitiol
.
Tropone has been known since 1951 and is also called cycloheptatrienylium oxide. The name tropolone was coined by M. J. S. Dewar in 1945 in connection to perceived aromatic properties.
group is polarized
with a partial positive charge on the carbon atom (A) and a partial negative charge on oxygen. In an extreme case the carbon atom has a full positive charge (B) forming a tropylium ion
ring which is an aromatic 6 electron system (C).
Tropolone is acidic (E) with a pKa
of 7 which is in between that of phenol
(10) and benzoic acid
(4). The increased acidity compared to phenol is due to regular resonance stabilization. Tropones and to a lesser extent tropolones are also basic (D) and this is very much due to aromatic stabilization. This property can be observed in the ease of salt formation with acids. The dipole moment for tropone is 4.17 D
compared to a value of only 3.04 D for cycloheptanone
, which can also be taken as evidence for aromaticity.
of tropones and its derivatives. Two selected methods for the synthesis of tropone are by selenium oxide
oxidation of cycloheptatriene
and indirectly from tropinone
by a Hofmann elimination
and a bromination.
Two methods for the synthesis of tropolone are by bromination of 1,2-cycloheptanedione with N-bromosuccinimide
followed by dehydrohalogenation
at elevated temperatures and by acyloin condensation
of the ethyl ester
of pimelic acid
the acyloin
again followed by oxidation by bromine.
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
with some importance in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
as a non-benzenoid aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
. The compound consists of a ring of seven carbon atoms with three conjugated
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
groups and a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
group. The related compound tropolone
Tropolone
Tropolone is a derivative of tropone with a hydroxyl group in the 2-position.Two methods for the synthesis of tropolone are by bromination of 1,2-cycloheptanedione with N-bromosuccinimide followed by dehydrohalogenation at elevated temperatures and by acyloin condensation of the ethyl ester of...
(2-hydroxy-2,4,6-cycloheptatrien-1-one) has an additional alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
(or an enol
Enol
Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Alkenes with a hydroxyl group on both sides of the double bond are called enediols. Deprotonated anions of enols are called enolates...
including the double bond) group next to the ketone.
The tropone moiety can be found in biomolecule
Biomolecule
A biomolecule is any molecule that is produced by a living organism, including large polymeric molecules such as proteins, polysaccharides, lipids, and nucleic acids as well as small molecules such as primary metabolites, secondary metabolites, and natural products...
s such as colchicine
Colchicine
Colchicine is a medication used for gout. It is a toxic natural product and secondary metabolite, originally extracted from plants of the genus Colchicum...
, stipitatic acid and hinokitiol
Hinokitiol
Hinokitiol is a natural monoterpenoid found in the wood of trees in the family Cupressaceae. It is a tropolone derivative and one of the thujaplicins. Hinokitiol has inhibitory effects on Chlamydia trachomatis and may be clinically useful as a topical drug.. It can be readily extracted from...
.
Tropone has been known since 1951 and is also called cycloheptatrienylium oxide. The name tropolone was coined by M. J. S. Dewar in 1945 in connection to perceived aromatic properties.
Properties
Dewar in 1945 proposed that tropones could have aromatic properties. The carbonylCarbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
group is polarized
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
with a partial positive charge on the carbon atom (A) and a partial negative charge on oxygen. In an extreme case the carbon atom has a full positive charge (B) forming a tropylium ion
Tropylium ion
In organic chemistry, the tropylium ion is an aromatic species with a formula of [C7H7]+. Its name derives from the molecule tropine . Salts of the tropylium cation can be stable, e.g. tropylium tetrafluoroborate...
ring which is an aromatic 6 electron system (C).
Tropolone is acidic (E) with a pKa
PKA
PKA, pKa, or other similar variations may stand for:* pKa, the symbol for the acid dissociation constant at logarithmic scale* Protein kinase A, a class of cAMP-dependent enzymes* Pi Kappa Alpha, the North-American social fraternity...
of 7 which is in between that of phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
(10) and benzoic acid
Benzoic acid
Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
(4). The increased acidity compared to phenol is due to regular resonance stabilization. Tropones and to a lesser extent tropolones are also basic (D) and this is very much due to aromatic stabilization. This property can be observed in the ease of salt formation with acids. The dipole moment for tropone is 4.17 D
Debye
The debye is a CGS unit of electric dipole momentElectric dipole moment is defined as charge times displacement: Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb10-10 statcoulomb is approximately 0.2083...
compared to a value of only 3.04 D for cycloheptanone
Cycloheptanone
Cycloheptanone, 6CO, is a cyclic ketone also referred to as suberone. It is a colourless volatile liquid. Cycloheptanone is used as a precursor for the synthesis of pharmaceuticals.- Synthesis :...
, which can also be taken as evidence for aromaticity.
Synthesis
Numerous methods exist for the organic synthesisOrganic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
of tropones and its derivatives. Two selected methods for the synthesis of tropone are by selenium oxide
Selenium oxide
Selenium oxide may refer to either of the following compounds:*Selenium dioxide, SeO2*Selenium trioxide, SeO3...
oxidation of cycloheptatriene
Cycloheptatriene
Cycloheptatriene is an organic compound with the formula C7H8. This colourless liquid has been of recurring theoretical interest in organic chemistry. It is a ligand in organometallic chemistry and as a building block in organic synthesis...
and indirectly from tropinone
Tropinone
Tropinone is an alkaloid, famously synthesised in 1917 by Robert Robinson as a synthetic precursor to atropine, a scarce commodity during World War I. Tropinone and the alkaloids cocaine and atropine all share the same tropane core structure.-Synthesis:...
by a Hofmann elimination
Hofmann elimination
Hofmann elimination is a process where an amine is reacted to create a tertiary amine and an alkene by treatment with excess methyl iodide followed by treatment with silver oxide, water, and heat.After the first step, a quaternary ammonium iodide salt is created...
and a bromination.
Two methods for the synthesis of tropolone are by bromination of 1,2-cycloheptanedione with N-bromosuccinimide
N-Bromosuccinimide
N-Bromosuccinimide or NBS is a chemical reagent which is used in radical substitution and electrophilic addition reactions in organic chemistry. NBS can be considered a convenient source of cationic bromine.-Preparation:...
followed by dehydrohalogenation
Dehydrohalogenation
Dehydrohalogenation is an organic reaction from which an alkene is obtained from an alkyl halide . It is also called a β-Elimination reaction and is a type of elimination reaction....
at elevated temperatures and by acyloin condensation
Acyloin condensation
Acyloin condensation is a reductive coupling of two carboxylic esters using metallic sodium to yield an α-hydroxyketone, also known as an acyloin....
of the ethyl ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
of pimelic acid
Pimelic acid
Pimelic acid is the organic compound with the formula HO2C5CO2H. Derivatives of pimelic acid are involved in the biosynthesis of the amino acid called lysine. Pimelic acid is one methylene longer than a related dicarboxylic acid, adipic acid, a precursor to many polyesters and polyamides...
the acyloin
Acyloin
Acyloins are a class of organic compounds in organic chemistry sharing a common functional group consisting of a hydroxyl group placed on the α-position of a carbonyl group.- Nomenclature :Common types of ketols include:...
again followed by oxidation by bromine.
Reactions
- Tropone isomerizes to benzoic acidBenzoic acidBenzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
with potassium hydroxidePotassium hydroxidePotassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
at elevated temperature. Many derivatives also isomerize to the corresponding arene compounds. - Tropone reacts in electrophilic substitutionElectrophilic substitutionElectrophilic substitution reactions are chemical reactions in which an electrophile displaces a group in a compound, typically but not always hydrogen. Electrophilic aromatic substitution is characteristic of aromatic compounds and is an important way of introducing functional groups onto benzene...
for instance with bromineBromineBromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
but the reaction proceeds through the 1,2-addition product and is not an electrophilic aromatic substitutionElectrophilic aromatic substitutionElectrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
.
-
- Tropone derivatives also react in nucleophilic substitutionNucleophilic substitutionIn organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...
very much like in nucleophilic aromatic substitutionNucleophilic aromatic substitutionright|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...
.
- Tropone derivatives also react in nucleophilic substitution
-
- Tropone is a dieneDieneIn organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
in a Diels-Alder reactionDiels-Alder reactionThe Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
for instance with maleic anhydrideMaleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour.... - Tropone is also found to react in an [8+3]annulationAnnulationAnnulation in organic chemistry is a chemical reaction in which a new ring is constructed on another molecule ....
with a cinnamic aldehyde
- Tropone is a diene