AROMETIC COMPOUNDS (AROMETICITY)
Aromaticity
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. It can also be considered a manifestation of cyclic delocalization and of resonance.
This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. This commonly-seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see "History" section below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.
Theory
As is standard for resonance diagrams, a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond.
A better representation is that of the circular π bond (Armstrong's inner cycle), in which the electron density is evenly distributed through a π bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.
The single bonds are formed with electrons in line between the carbon nuclei—these are called sigma bonds. Double bonds consist of a sigma bond and a π bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals.
Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.
History
The first known use of the word "aromatic" as a chemical term—namely, to apply to compounds that contain the phenyl radical—occurs in an article by August Wilhelm Hofmann in 1855. If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicating olfactory character to apply to a group of chemical substances, only some of which have notable aromas. It is the case, however, that many of the most odoriferous organic substances known are terpenes, which are not aromatic in the chemical sense. But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation indexes than many aliphatic compounds, and Hofmann may not have been making a distinction between the two categories.
The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Over the next few decades, most chemists readily accepted this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry. However, it was always puzzling that this purportedly highly-unsaturated molecule was so unreactive toward addition reactions.
The discoverer of the electron J. J. Thomson, in 1921 placed three equivalent electrons between each carbon atom in benzene.
An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925) to coin the term aromatic sextet as a group of six electrons that resists disruption.
In fact, this concept can be traced further back, via Ernest Crocker in 1922, to Henry Edward Armstrong, who in 1890, in an article entitled The structure of cycloid hydrocarbons, wrote the (six) centric affinities act within a cycle...benzene may be represented by a double ring (sic) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition.
Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron, which was only to be discovered seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution, proceeding (third) through a Wheland intermediate, in which (fourth) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle, thus anticipating Eric Clar's notation. It is argued that he also anticipated the nature of wave mechanics, since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring (much as the distribution of the electric charge in a body is altered by bringing it near to another body).
The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.
Aroma compound
An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor. A chemical compound has a smell or odor when two conditions are met: the compound needs to be volatile, so it can be transported to the olfactory system in the upper part of the nose, and it needs to be in a sufficiently high concentration to be able to interact with one or more of the olfactory receptors.
Aroma compounds can be found in food, wine, spices, perfumes, fragrance oils, and essential oils. For example, many form biochemically during ripening of fruits and other crops. In wines, most form as byproducts of fermentation. Odorants can also be added to a dangerous odorless substance, like natural gas or hydrogen, as a warning. As well many of the aroma compounds plays a significant role in the production of flavorants, which are used in the food service industry to flavor, improve and increase the appeal of their products.
Aroma compounds classified by functional group
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. It can also be considered a manifestation of cyclic delocalization and of resonance.
This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. This commonly-seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by Kekulé (see "History" section below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.
Theory
As is standard for resonance diagrams, a double-headed arrow is used to indicate that the two structures are not distinct entities, but merely hypothetical possibilities. Neither is an accurate representation of the actual compound, which is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond.
A better representation is that of the circular π bond (Armstrong's inner cycle), in which the electron density is evenly distributed through a π bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring.
The single bonds are formed with electrons in line between the carbon nuclei—these are called sigma bonds. Double bonds consist of a sigma bond and a π bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals.
Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.
History
The first known use of the word "aromatic" as a chemical term—namely, to apply to compounds that contain the phenyl radical—occurs in an article by August Wilhelm Hofmann in 1855. If this is indeed the earliest introduction of the term, it is curious that Hofmann says nothing about why he introduced an adjective indicating olfactory character to apply to a group of chemical substances, only some of which have notable aromas. It is the case, however, that many of the most odoriferous organic substances known are terpenes, which are not aromatic in the chemical sense. But terpenes and benzenoid substances do have a chemical characteristic in common, namely higher unsaturation indexes than many aliphatic compounds, and Hofmann may not have been making a distinction between the two categories.
The cyclohexatriene structure for benzene was first proposed by August Kekulé in 1865. Over the next few decades, most chemists readily accepted this structure, since it accounted for most of the known isomeric relationships of aromatic chemistry. However, it was always puzzling that this purportedly highly-unsaturated molecule was so unreactive toward addition reactions.
The discoverer of the electron J. J. Thomson, in 1921 placed three equivalent electrons between each carbon atom in benzene.
An explanation for the exceptional stability of benzene is conventionally attributed to Sir Robert Robinson, who was apparently the first (in 1925) to coin the term aromatic sextet as a group of six electrons that resists disruption.
In fact, this concept can be traced further back, via Ernest Crocker in 1922, to Henry Edward Armstrong, who in 1890, in an article entitled The structure of cycloid hydrocarbons, wrote the (six) centric affinities act within a cycle...benzene may be represented by a double ring (sic) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition.
Here, Armstrong is describing at least four modern concepts. First, his "affinity" is better known nowadays as the electron, which was only to be discovered seven years later by J. J. Thomson. Second, he is describing electrophilic aromatic substitution, proceeding (third) through a Wheland intermediate, in which (fourth) the conjugation of the ring is broken. He introduced the symbol C centered on the ring as a shorthand for the inner cycle, thus anticipating Eric Clar's notation. It is argued that he also anticipated the nature of wave mechanics, since he recognized that his affinities had direction, not merely being point particles, and collectively having a distribution that could be altered by introducing substituents onto the benzene ring (much as the distribution of the electric charge in a body is altered by bringing it near to another body).
The quantum mechanical origins of this stability, or aromaticity, were first modelled by Hückel in 1931. He was the first to separate the bonding electrons into sigma and pi electrons.
Aroma compound
An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor. A chemical compound has a smell or odor when two conditions are met: the compound needs to be volatile, so it can be transported to the olfactory system in the upper part of the nose, and it needs to be in a sufficiently high concentration to be able to interact with one or more of the olfactory receptors.
Aroma compounds can be found in food, wine, spices, perfumes, fragrance oils, and essential oils. For example, many form biochemically during ripening of fruits and other crops. In wines, most form as byproducts of fermentation. Odorants can also be added to a dangerous odorless substance, like natural gas or hydrogen, as a warning. As well many of the aroma compounds plays a significant role in the production of flavorants, which are used in the food service industry to flavor, improve and increase the appeal of their products.
Aroma compounds classified by functional group
Alcohols
Benzyl alcohol (oxidises to benzaldehyde, almond)
Ethyl maltol (sugary, cooked fruit)
Furaneol (strawberry)
1-Hexanol (herbaceous, woody)
cis-3-Hexen-1-ol (fresh cut grass)
Menthol (peppermint)
Aldehydes
Acetaldehyde (pungent)
Benzaldehyde (marzipan, almond)
Hexanal (green, grassy)
Cinnamaldehyde (cinnamon)
Citral (lemongrass, lemon oil)
cis-3-Hexenal (green tomatoes)
Furfural (burnt oats)
Neral (citrus, lemongrass)
Vanillin (vanilla)
Amines
Cadaverine (rotting flesh)
Indole (jasmine flowery, feces)
Putrescine (rotting flesh)
Pyridine (very unpleasant)
Skatole (bad breath, feces)
Substituted pyrazines: 2-ethoxy-3-isopropylpyrazine, 2-methoxy-3-sec-butylpyrazine, 2-methoxy-3-methylpyrazine (toasted seeds of fenugreek, cumin, and coriander)
Alkylpyrazines
Methoxypyrazines
Trimethylamine (fish)
Esters
Ethyl acetate (fruity, solvent)
Ethyl butanoate (fruity) - also known as ethyl butyrate
Ethyl decanoate - also known as ethyl caprate
Ethyl hexanoate - also known as ethyl caproate
Ethyl octanoate - also known as ethyl caprylate
Hexyl acetate (apple, floral, fruity)
Isoamyl acetate (banana)
Methyl butanoate (apple, fruity) - also known as methyl butyrate
Methyl salicylate (oil of wintergreen)
Pentyl butanoate (pear, apricot)
Pentyl pentanoate (apple, pineapple)
Sotolon (maple syrup, curry, fenugreek)
Strawberry aldehyde (strawberry)
Fructone (fruity, apple-like)
Ethers
Anethole (liquorice, anise seed, ouzo, fennel)
Anisole (anise seed)
Eugenol (clove oil)
2,4,6-Trichloroanisole (cork taint)
Ketones
Dihydrojasmone (fruity woody floral)
Oct-1-en-3-one (blood, metallic, mushroom-like)
2-Acetyl-1-pyrroline (fresh bread, jasmine rice)
6-Acetyl-2,3,4,5-tetrahydropyridine (fresh bread, tortillas, pop corn)
Lactones
gamma-Decalactone intense peach flavor
gamma-Nonalactone coconut odor, popular in suntan lotions
delta-Octalactone creamy note
Jasmine lactone powerful fatty fruity peach and apricot
Massoia lactone powerful creamy coconut
Benzyl alcohol (oxidises to benzaldehyde, almond)
Ethyl maltol (sugary, cooked fruit)
Furaneol (strawberry)
1-Hexanol (herbaceous, woody)
cis-3-Hexen-1-ol (fresh cut grass)
Menthol (peppermint)
Aldehydes
Acetaldehyde (pungent)
Benzaldehyde (marzipan, almond)
Hexanal (green, grassy)
Cinnamaldehyde (cinnamon)
Citral (lemongrass, lemon oil)
cis-3-Hexenal (green tomatoes)
Furfural (burnt oats)
Neral (citrus, lemongrass)
Vanillin (vanilla)
Amines
Cadaverine (rotting flesh)
Indole (jasmine flowery, feces)
Putrescine (rotting flesh)
Pyridine (very unpleasant)
Skatole (bad breath, feces)
Substituted pyrazines: 2-ethoxy-3-isopropylpyrazine, 2-methoxy-3-sec-butylpyrazine, 2-methoxy-3-methylpyrazine (toasted seeds of fenugreek, cumin, and coriander)
Alkylpyrazines
Methoxypyrazines
Trimethylamine (fish)
Esters
Ethyl acetate (fruity, solvent)
Ethyl butanoate (fruity) - also known as ethyl butyrate
Ethyl decanoate - also known as ethyl caprate
Ethyl hexanoate - also known as ethyl caproate
Ethyl octanoate - also known as ethyl caprylate
Hexyl acetate (apple, floral, fruity)
Isoamyl acetate (banana)
Methyl butanoate (apple, fruity) - also known as methyl butyrate
Methyl salicylate (oil of wintergreen)
Pentyl butanoate (pear, apricot)
Pentyl pentanoate (apple, pineapple)
Sotolon (maple syrup, curry, fenugreek)
Strawberry aldehyde (strawberry)
Fructone (fruity, apple-like)
Ethers
Anethole (liquorice, anise seed, ouzo, fennel)
Anisole (anise seed)
Eugenol (clove oil)
2,4,6-Trichloroanisole (cork taint)
Ketones
Dihydrojasmone (fruity woody floral)
Oct-1-en-3-one (blood, metallic, mushroom-like)
2-Acetyl-1-pyrroline (fresh bread, jasmine rice)
6-Acetyl-2,3,4,5-tetrahydropyridine (fresh bread, tortillas, pop corn)
Lactones
gamma-Decalactone intense peach flavor
gamma-Nonalactone coconut odor, popular in suntan lotions
delta-Octalactone creamy note
Jasmine lactone powerful fatty fruity peach and apricot
Massoia lactone powerful creamy coconut
Terpenes
Camphor (Cinnamomum camphora)
Citronellol (rose)
Linalool (floral, citrus, coriander)
Nerol (sweet rose)
Nerolidol (wood, fresh bark)
alpha-Terpineol (lilac)
Thujone (juniper, common sage, Nootka cypress, and wormwood)
Thymol (Thyme-like)
Thiols
Ethanethiol, formerly called Ethyl mercaptan (Durian or leek, added to natural gas)
Grapefruit mercaptan (grapefruit)
Methanethiol, formerly called Methyl mercaptan (added to natural gas)
Miscellaneous compounds
Methylphosphine and dimethylphosphine (garlic-metallic, two of the most potent odorants known)
Nerolin (orange flowers)
Tetrahydrothiophene (added to natural gas)
Camphor (Cinnamomum camphora)
Citronellol (rose)
Linalool (floral, citrus, coriander)
Nerol (sweet rose)
Nerolidol (wood, fresh bark)
alpha-Terpineol (lilac)
Thujone (juniper, common sage, Nootka cypress, and wormwood)
Thymol (Thyme-like)
Thiols
Ethanethiol, formerly called Ethyl mercaptan (Durian or leek, added to natural gas)
Grapefruit mercaptan (grapefruit)
Methanethiol, formerly called Methyl mercaptan (added to natural gas)
Miscellaneous compounds
Methylphosphine and dimethylphosphine (garlic-metallic, two of the most potent odorants known)
Nerolin (orange flowers)
Tetrahydrothiophene (added to natural gas)
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