Nomenclature, Benzene-Structure and Aromaticity

C6H6 is the chemical formula for benzene, which is a hydrocarbon. It is made up of six carbon atoms that are arranged in a benzene ring, and each carbon atom has one hydrogen atom linked to it. Many benzene compounds can be created by substituting one or more hydrogen atoms with a functional group. We can also prefix the substituent name to the term benzene when designating the substituted benzene compounds. 

Benzene (C6H6) is the most basic member of the aromatic hydrocarbons’ family. These compounds include ring structures and bonding that must be characterised using valence bond theory’s resonance hybrid idea or molecular orbital theory’s delocalization notion.

There are several benzene derivatives. Many different substituents can be used to substitute hydrogen atoms. Aromatic compounds are more prone to substitution reactions over addition events because the delocalized chemical bonds are preserved whenever one of the hydrogens is replaced with another substituent. 

Nomenclature  

Because of their unique scents, benzene-like compounds were previously referred to as aromatic hydrocarbons. An aromatic compound is now defined as any chemical with a benzene ring or some benzene-like characteristics (but not necessarily a strong smell).  The existence of one or more benzene rings inside the structure of the aromatic compounds makes them easy to identify.

Nomenclature of Mono Substituted Benzenes

Because a single aromatic molecule might have numerous alternative names (including common & systematic names) linked with its structure, unlike aliphatic organics, the naming of benzene-derived substances can indeed be complicated. Aromatic compound nomenclature frequently uses common names. Some of the most commonly used common names are still allowed by IUPAC. This benzene base name is replaced by these popular names. Toluene is the basic name for methylbenzene, whereas phenol is the basis name for hydroxy phenol. It’s crucial to know how to recognize these structures because they’ll be used in the naming of more complicated compounds.

Nomenclature of Disubstituted Benzenes

Three unique positional isomers can exist with disubstituted benzenes and must be indicated in the chemical name. Although numbering could be used to show the relative positions of the two substituents, prefixes are far more commonly employed to name the compounds. These prefixes are commonly shortened with a single character and are emphasised. 

They are described as follows:

• 1,2- ortho: (next to each other in a benzene ring)
• 1,3-meta (m): (separated by one carbon in a benzene ring)
• 1,4-para (p): (across from each other in a benzene ring) 

The compound is given a distinct parent name rather than benzene. These compounds are given the following names: Name of the constituent + Name of the parent chain is the position prefix. 

Nomenclature of Benzenes with >3 Substituents 

When 3 substituents are present, the ortho, meta, and para positional prefixes are no longer sufficient, and a ring numbering scheme must be used. In addition, this substituent is assigned to first place in the numbering scheme. The remaining substituents are numbered to get the lowest feasible total. The substituents are assigned a position number and named alphabetically in the compound’s name. Remember that the prefixes di-, tri-, and tetra- are still used to confirm the presence of multiples of the same substituent, but they are omitted for alphabetical listing. 

Structure of Benzene  

The low hydrogen-carbon ratio (1:1) of benzene (C6H6) led researchers to assume it had double/triple bonds when it was initially found. This reaction was used to add bromine (Br2) to benzene because double & triple bonds add bromine (Br2) quickly. Surprisingly, benzene did not react with bromine at all. Furthermore, when benzene is pushed to combine with bromine by adding a catalyst, it conducts substitution reactions instead of the alkene-typical addition events. The six-carbon benzene ring is particularly resilient to chemical change, according to these investigations. 

The currently accepted form of benzene as a hexagon, a planar ring of carbons containing alternating carbon-carbon double bonds was eventually selected, and the system’s outstanding chemical stability was attributed to the conjugated cyclic triene’s particular resonance stabilisation. Because benzene is a mixture of two structurally & energetically equivalent resonance types indicating the continual cyclic conjugation of double bonds, no single structure can accurately describe it.

Aromaticity of Benzene  

Measurements of the heat generated when double bonds inside a six-carbon ring were hydrogenated (hydrogen is supplied catalytically) to create cyclohexane. Like a common product provided evidence for benzene’s improved thermodynamic stability. 

The introduction of hydrogen in cyclohexene yields cyclohexane, which emits 28.6 kcal/mole of heat. On full hydrogenation, a cyclohexadiene should release 57.2 kcal per mole, and 1,3,5-cyclohexatriene should release 85.8 kcal per mole if this value represents the energy cost of inserting one double bond together into a six-carbon ring. The comparative thermal stability of the substances would be reflected by these hydrogenation temperatures.

Benzene, on the other hand, is 36 kcal/mole higher stable than predicted. Aromaticity is the term for this type of stability improvement, and aromatic substances are molecules having aromaticity. The most popular aromatic chemical is benzene, although there are many more. The lack of reactivity of benzene in comparison to other alkenes is due to aromatic stabilisation. 

Conclusion

Ortho (1,2), meta (1,3), and para (1,4) are common names for disubstituted benzene compounds (1,4). Toluene, phenol, benzoic acid, and benzaldehyde are some of the typical benzene derivative names used by IUPAC. Phenyl refers to a benzene group that has been given a name as a substituent. Benzyl is benzene with a CH2 like a substituent group.

Because of its aromatic stability, benzene does not experience the same reactions as alkenes. To allow the pi electrons to delocalize, aromatic compounds must contain all ring atoms on the same plane. Heats of hydrogenation could be utilised to demonstrate benzene’s unique stability when compared to a theoretical cyclohexatriene structure.