5 Key excerpts on "Chemical Properties of Benzene"

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  eBook - ePub

  BIOS Instant Notes in Chemistry for Biologists

  • J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
  • 2020(Publication Date)
  • Taylor & Francis

    (Publisher)

  Section K - Aromatic Compounds Passage contains an image

  K1 Aromaticity

  DOI: 10.1201/9780203079522-43

  Key Notes

  Benzene	Benzene is an unsaturated molecule and, as such, would be expected to undergo reactions similar to those of other unsaturated hydrocarbons such as alkenes and alkynes. However, benzene is relatively inert, and when it does react favors substitution reactions over addition reactions. The unexpected chemical and physical properties of benzene may be explained by the concept of pi electron delocalization. Benzene is the classic example of an aromatic compound. The term aromatic is applied as benzene, and other ring systems that have similar delocalized pi systems, is fragrant.
  Molecular orbital description of benzene	Benzene is a planar molecule in which all of the bond angles about the carbon atoms are 120°. This bond angle is what would be expected for an sp2 hybridized carbon atom, and therefore means that at each of the six carbon atoms there is a singly occupied p-orbital. These p-atomic orbitals overlap to form six pi molecular orbitals. The molecular orbital picture of benzene helps explain the special stability of this molecule.
  Definition of aromaticity	In 1931 the physicist Erich Hückel carried out a series of calculations based on the molecular orbital picture of benzene, but extended this to cover all planar monocyclic compounds in which each atom had a p-orbital. The results of his work suggested that all such compounds containing (4n + 2) pi electrons should be stabilized through delocalization and therefore should also be termed aromatic.
  Related topics	(I3) Factors affecting reactivity	(K2) Natural aromatics

  Benzene

  The study of the class of compounds now referred to as aromatics began in 1825 with the isolation of a compound, now called benzene, by Michael Faraday. At this time the molecular formula of benzene, C6 H6 , was thought quite unusual due to the low ratio of hydrogen to carbon atoms. Within a very short time the unusual properties of benzene and related compounds began to emerge. During this period, for a compound to be classified as aromatic it simply needed to have a low carbon to hydrogen ratio and to be fragrant; most of the early aromatic compounds were obtained from balsams, resins or essential oils. It was sometime later before Kekulé and coworkers recognized that these compounds all contained a six-carbon unit that remained unchanged during a range of chemical transformations. Benzene was eventually recognized as being the parent for this new class of compound. In 1865 Kekulé proposed a structure for benzene; a six-membered ring with three alternating double bonds (Figure 1 ). However, if such a structure were correct then the addition of two bromine atoms to adjacent carbons would result in the formation of two isomers of 1,2-dibromobenzene (Figure 1 ). Only one compound has ever been found. To account for this apparent anomaly Kekulé suggested that these isomers were in a state of rapid equilibrium (Figure 1

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  eBook - ePub

  BIOS Instant Notes in Organic Chemistry

  • Graham Patrick(Author)
  • 2004(Publication Date)
  • Taylor & Francis

    (Publisher)

  SECTION I — AROMATIC CHEMISTRY

  I1 Aromaticity

  Key Notes

  Definition	Aromatic compounds such as benzene are more stable than suggested from their structure. They undergo reactions which retain the aromatic ring system, and behave differently from alkenes or polyenes.
  Hückel rule	Aromatic compounds are cyclic and planar with sp 2 hybridized atoms. They also obey the Hückel rule and have (4n + 2) π electrons where n = 1, 2, 3, ... Aromatic systems can be monocyclic or polycyclic, neutral, or charged.
  Related topic	(A4) sp 2 Hybridization

  Definition

  The term aromatic was originally applied to benzene-like structures because of the distinctive aroma of these compounds, but the term now means something different in modern chemistry. Aromatic compounds undergo distinctive reactions which set them apart from other functional groups. They are highly unsaturated compounds, but unlike alkenes and alkynes, they are relatively unreactive and will tend to undergo reactions which involve a retention of their unsaturation. We have already discussed the reasons for the stability of benzene in Section A4 . Benzene is a six-membered ring structure with three formal double bonds (Figure 1a ). However, the six π electrons involved are not localized between any two carbon atoms. Instead, they are delocalized around the ring which results in an increased stability. This is why benzene is often written with a circle in the center of the ring to signify the delocalization of the six π electrons (Figure 1b ). Reactions which disrupt this delocalization are not favored since it means a loss of stability, so benzene undergoes reactions where the aromatic ring system is retained. All six carbon atoms in benzene are sp 2 hybridized, and the molecule itself is cyclic and planar — the planarity being necessary if the 2p atomic orbitals on each carbon atom are to overlap and result in delocalization.

  Figure 1. Representations of benzene.

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  eBook - ePub

  Handbook of Industrial Hydrocarbon Processes

  • James G. Speight(Author)
  • 2010(Publication Date)
  • Gulf Professional Publishing

    (Publisher)

  aliphatic hydrocarbons).

  Benzene and other aromatic compounds can have substituents. When benzene itself is a substituent, it is called a phenyl group. Benzene is typically drawn in such a way that the hybrid between the resonance structures is emphasized:

  However, not every conjugated cyclic system is aromatic since not all are stabilized by resonance, mainly due to differences in filling molecular orbitals with electrons.

  Benzene is obviously an unsaturated hydrocarbon because it has far less hydrogen than the equivalent saturated hydrocarbon: cyclohexane, C6 H12 . But benzene is too stable to be an alkene or alkyne. Alkenes and alkynes rapidly add bromine (Br2 ) to the C=C or CC bonds, whereas benzene only reacts with bromine in the presence of a catalyst: ferric bromide (FeBr3 ). Furthermore, when benzene reacts with Br2 in the presence of FeBr3 , the product of this reaction is a compound in which a bromine atom has been substituted for a hydrogen atom, not added to the compound in the way an alkene adds bromine:

  Other compounds were eventually isolated from coal that had similar properties. Their formulas suggested the presence of multiple C=C bonds, but these compounds were not reactive enough to be alkenes.

  The structure of benzene was a recurring problem throughout most of the nineteenth century. The first step toward solving this problem was taken by Friedrich August Kekulé in 1865. (Kekulé’s interest in the structure of organic compounds may have resulted from the fact that he first enrolled at the University of Giessen as a student of architecture.) One day, while dozing before a fire, Kekulé dreamed of long rows of atoms twisting in a snakelike motion until one of the snakes seized hold of its own tail. This dream led Kekulé to propose that benzene consists of a ring of six carbon atoms with alternating C–C single bonds and C=C double bonds. Because there are two ways in which these bonds can alternate, Kekulé proposed that benzene was a mixture of two compounds in equilibrium.

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  eBook - ePub

  Handbook of Industrial Hydrocarbon Processes

  • James G. Speight(Author)
  • 2019(Publication Date)
  • Gulf Professional Publishing

    (Publisher)

  Aromatic hydrocarbon derivatives are derived from benzene. Group members have six free valence electrons which are distributed in a circle in the form of a charged cloud. Because of the presence of these valence electrons, we can predict that the reactivity of these aromatic compounds will be similar to other unsaturated hydrocarbon derivatives. However, benzene is much less reactive than other unsaturated hydrocarbon derivatives. Only at high temperatures and in the presence of a catalyst can benzene take on another hydrogen atom. When it does, cyclohexane is the resultant product.

  5. Physical properties

  Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter (Howard and Meylan, 1997 ; Yaws, 1999 ). The three states of matter are: solid, liquid, and gas. The melting point and boiling point are related to changes of the state of matter. All matter may exist in any of three physical states of matter. A physical change takes place without any changes in molecular composition. The same element or compound is present before and after the change. The same molecule is present throughout the changes. Physical changes are related to physical properties since some changes require a change in the three-dimensional structure of the molecule.

  Physical properties that are of interest in the current context include: (i) boiling point, (ii) density and specific gravity, (iii) dew point, (iv) flash point and ignition temperature, (v) melting point, and (vi) vapor density. These properties are listed in alphabetical order rather than attempt to assign importance to any individual property. The properties present indications the behavior of hydrocarbon derives as determined by application of standard test method (Speight, 2015 ).

  The physical properties of alkene derivatives are similar to those of the alkane derivatives. The boiling points of straight-chain alkenes increase with increasing molar mass, just as with alkanes. For molecules with the same number of carbon atoms and the same general shape, the boiling points usually differ only slightly, just as would be expected for chemicals in which whose molar mass differs by only one to two hydrogen atoms (i.e., RCH2 CH

  CH2 compared to RCH2 CH2 CH3

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  eBook - ePub

  Biochemistry

  An Organic Chemistry Approach

  • Michael B. Smith(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  9  Aromatic Compounds and Heterocyclic Compounds

  Benzene is a special type of hydrocarbon. Derivatives are known by replacing the hydrogen atoms of benzene with substituents and/or functional groups. There are hydrocarbons related to benzene that have two, three, or more rings fused together (polycyclic compounds). The unifying concept of all these molecules is that they are aromatic, which means that they are especially stable with respect to their bonding and structure.

  9.1 Benzene and Aromaticity

  Benzene is a hydrocarbon with the formula C6 H6 . It is the parent of a large class of compounds known as aromatic hydrocarbons. The structure and chemical reactivity of aromatic hydrocarbons are so unique that benzene derivatives are given their own nomenclature system. This discussion will begin with the unique structure of benzene.

  The structure of benzene is shown in Figure 9.1 . It is known that the C—C bond length in ethane is 1.53 Å (153 pm), and the C—C bond length is 1.536 Å (153.6 pm) in cyclohexane. The bond distance for the C=C bond in ethene is 1.34 Å (134 pm). These data indicate that a C=C unit has a shorter bond distance than a C—C unit. If benzene has a structure with both single and double bonds, it should have three longer C—C single bonds and three shorter C=C units. It would then be called cyclohexatriene. It has been experimentally determined that all six carbon–carbon bonds have a measured bond distance of 1.397 Å (139.7 pm), a value that lies in between those for the C–C bond in an alkane and the C=C unit of an alkene. This observation means that the C–C bonds in benzene are not single bonds, nor are they C=C double bonds where the π-electrons are localized between two carbons in a π-bond. This molecule is not cyclohexatriene, it is benzene . Each carbon in benzene is sp2 hybridized, however, which means each has a trigonal planar geometry. The planar geometry is seen more clearly in the molecular model of benzene in Figure 9.1

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