What Are Aromatic Derivatives?
Sep. 27, 2020
Aromatic derivatives, usually refers to hydrocarbons containing a benzene ring structure in the molecule. It is a kind of closed chain and has the basic structure of benzene ring.
Most of these compounds found in the early history have aromatic taste, so these hydrocarbons are called aromatic hydrocarbons, and the later discovered hydrocarbons that do not have aromatic taste also use this name.
Structure and expression of benzene
(1) The structure of benzene
Modern physical methods prove that the six carbon atoms and six hydrogen atoms of a benzene molecule are in the same plane, so it is a plane molecule. The six carbon atoms form a regular hexagon. The carbon bond length is equal, about 140 pm, between single and double bonds. The carbon-hydrogen bond length is 108pm, and all bond angles are 120°.
(2) The aromaticity of benzene
From the structural point of view, benzene has a planar cyclic structure, the bond length is completely equalized, and the hydrocarbon ratio is 1. In terms of properties, benzene has special stability: the heat of hydrogenation of cyclohexene ΔH=-120kJ/mol, the heat of hydrogenation of 1,3-cyclohexadiene ΔH=-232kJ/mol (due to the increase of its conjugated double bond To improve its stability). The heat of hydrogenation of benzene ΔH=-208kJ/mol. When 1,3-cyclohexadiene loses two hydrogens and becomes benzene, it not only does not absorb heat, but emits a small amount of heat. This shows that benzene is much more stable than the corresponding cyclohexatrienes. When 1,3-cyclohexadiene becomes benzene, the molecular structure has undergone a fundamental change, which has led to the formation of a stable system.
Benzene is difficult to oxidize and add, and is prone to electrophilic substitution reaction, which is obviously different from ordinary olefins.
Benzene also has special spectral characteristics. The hydrogen on the benzene ring is in the low field of nuclear magnetic resonance.
The above characteristics show that benzene has typical aromatic characteristics.
(3) Benzene expression
How to express the structure of benzene? Since the British physicist and chemist Farady M (Faraday) first separated benzene from illuminating gas in 1825, people have been exploring the expression of benzene structure. Scientists have put forward various hypotheses about the structural formula of benzene; the more representative structural formulas of benzene are:
The structure of benzene and its expression have been discussed for more than 140 years. Although various opinions have been put forward, satisfactory results have not yet been obtained and further discussion is needed.
Structure of biphenyl
The simplest biphenyl is biphenyl. In biphenyl, each benzene ring maintains the structural properties of benzene. The single bond connecting the two benzene rings can rotate freely, but when the four ortho-position hydrogen atoms of the biphenyl are replaced by relatively large groups, the rotation of the single bond will be hindered, and a pair of Photoactive isomers.
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