There is a misconception amongst the scientific community, however, that all aromatics are sweet smelling and that all sweet smelling compounds would have a benzene ring in its structure.
This is false, since non-aromatic compounds, such as camphor, extracted from the camphor laurel tree, release a strong, minty aroma, yet it lacks the benzene ring in its structure See figure 1. On the other hand, benzene itself gives off a rather strong and unpleasant smell that would otherwise invalidate the definition of an aromatic sweet-smelling compound. Despite this inconsistency, however, the term aromatic continues to be used today in order to designate molecules with benzene-like rings in their structures.
Each element within the ring must have a p-orbital that is perpendicular to the ring, hence the molecule is planar. Putting it all together : The historical naming convention for molecules that smelled good is responsible for the term aromatic.
The smell of a compound as we perceive it usually depends on which receptors in the nose will bind the molecule or not. These receptor, being proteins, are usually rather specific for certain shapes, not for bonding patterns. So chlorobenzene might fit into similar receptors as toluene which would lead to at least partly a similar smell, but that is it.
The definition of aromatic is, as Todd pointed out, one based on structure. Some aromatic compounds have similar structures, but there is no way that say furane would occupy the same or similar receptors as benzo a pyrene would.
Therefore, their common feature aromaticity does not lead to a common odour. The theory shape or chemical composition has anything to do with smell can be refuted based on three observations:. Since the advent of genetic technologies, a new theory is on the horizon is the underlying mechanism for smell is based on quantum oscillations.
This theory is derived from information gathered from studying the genome which encodes the proteins responsible for the sense of smell. Todd's answer is correct. This is because benzene's conjugated pi electrons freely resonate within the cyclic ring, thus resulting in its two resonance forms.
Figure 4. The figure to the left shows the two resonance forms of benzene. The delocalized electrons are moved from one carbon to the next, thus providing stabilization energy. Ring structures stabilized by the movement of delocalized electrons are sometimes referred to as arenes. As the electrons in the benzene ring can resonate within the ring at a fairly high rate, a simplified notation is often used to designate the two different resonance forms.
This notation is shown above, with the initial three pi bonds 1, 2 replaced with an inner ring circle 3. Alternatively, the circle within the benzene ring can also be dashed to show the same resonance forms 4. The phenyl group can be formed by taking benzene, and removing a hydrogen from it. The resulting molecular formula for the fragment is C 6 H 5. NOTE: Although the molecular formula of the phenyl group is C 6 H 5 , the phenyl group would always have something attached to where the hydrogen was removed.
Thus, the formula is often written as Ph - R , where Ph refers to the Phenyl group, and R refers to the R group attached to where the hydrogen was removed. Figure 5. Figure demonstrating the removal of hydrogen to form the phenyl group. Different R groups on the phenyl group allows different benzene derivatives to be formed.
Benzene , for this very same reason, can be formed from the phenyl group by reattaching the hydrogen back its place of removal. Thus benzene, similar to phenol, can be abbreviated Ph-H , or C 6 H 6. Figure 7. Left : Epigallocatechin gallate EGCG , an antioxidant found in green teas and its extracts, is famous for its potential health benefits. The molecule is a type of catechin, which is composed of multiple phenol labeled in red units polyphenols - see polycyclic aromatics. Since catechins are usually found in plant extracts, they are often referred as plant polyphenolic antioxidants.
As you can see above, these are only some of the many possibilities of the benzene derived products that have special uses in human health and other industrial fields. Unlike aliphatic organics, nomenclature of benzene-derived compounds can be confusing because a single aromatic compound can have multiple possible names such as common and systematic names be associated with its structure. In these sections, we will analyze some of the ways these compounds can be named.
Some common substituents, like NO 2 , Br, and Cl, can be named this way when it is attached to a phenyl group. Long chain carbons attached can also be named this way. The general format for this kind of naming is:. Since there is only one substituent on the benzene ring, we do not have to indicate its position on the benzene ring as it can freely rotate around and you would end up getting the same compound.
Figure 8. Example of simple benzene naming with chlorine and NO 2 as substituents. Figure 9. More complicated simple benzene naming examples - Note that standard nomenclature priority rules are applied here, causing the numbering of carbons to switch. See Nomenclature of Organic Compounds for a review on naming and priority rules.
Instead of using numbers to indicate substituents on a benzene ring, ortho- o- , meta- m- , or para p- can be used in place of positional markers when there are two substituents on the benzene ring disubstituted benzenes.
They are defined as the following:. Using the same example above in figure 9a 1,3-dichlorobenzene , we can use the ortho-, meta-, para- nomenclature to transform the chemical name into m-dichlorobenzene, as shown in the figure below. Figure Transformation of 1,3-dichlorobenzene into m-dichlorobenzene. Here are some other examples of ortho-, meta-, para- nomenclature used in context:. However, the substituents used in ortho-, meta-, para- nomenclature do not have to be the same.
For example, we can use chlorine and a nitro group as substituents in the benzene ring. In conclusion, these can be pieced together into a summary diagram, as shown below:. In addition to simple benzene naming and OMP nomenclature, benzene derived compounds are also sometimes used as bases.
The concept of a base is similar to the nomenclature of aliphatic and cyclic compounds, where the parent for the organic compound is used as a base a name for its chemical name.
Related questions What functional groups are found in proteins? What functional groups are found in the structure of melatonin? What functional groups are present in carbohydrates? What is the structure of the functional group and the condensed formula for 4,4,5-triethyl What reactants combine to form 3-chlorooctane? Here is a list of some important aroma compounds. Some occur naturally, while others are synthetic:. Among the "smelliest" of the odorants are methyl phosphine and dimethyl phosphine, which can be detected in extremely low amounts.
The human nose is so sensitive to thioacetone that it can be smelled within seconds if a container of it is opened hundreds of meters away. The sense of smell filters out constant odors, so a person becomes unaware of them after continuous exposure.
However, hydrogen sulfide deadens the sense of smell. Initially, it produces a strong rotten egg smell, but binding of the molecule to odor receptors prevents them from receiving additional signals. In the case of this particular chemical, the loss of sensation can be deadly, as it is extremely toxic. Odorants are used to make perfumes, to add odor to toxic, odorless compounds e. According to Yamazaki et al.
MHC can be detected via scent. Studies in humans support this connection, noting it's also affected by the use of oral contraceptives. Whether an odorant occurs naturally or is produced synthetically, it may be unsafe, especially in high concentrations. Many fragrances are potent allergens. The chemical composition of fragrances is not regulated the same from one country to another. In the United States, fragrances in use before the Toxic Substances Control Act of were grandfathered in for use in products.
New aroma molecules are subject to review and testing, under the oversight of the EPA. Actively scan device characteristics for identification. Use precise geolocation data.
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