4 Key excerpts on "Electrophilic Substitution of Benzene"

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  eBook - ePub

  BIOS Instant Notes in Organic Chemistry

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

    (Publisher)

  + ) with the aromatic ring remaining intact. Therefore, one electrophile replaces another and the reaction is known as an electrophilic substitution. (At this stage we shall ignore how the bromine cation is formed.)

  Figure 1. Electrophilic Substitution of Benzene.

  Mechanism

  In the mechanism (Figure 2 ) the aromatic ring acts as a nucleophile and provides two of its π electrons to form a bond to Br+ . The aromatic ring has now lost one of its formal double bonds resulting in a positively charged carbon atom. This first step in the mechanism is the same as the one described for the electrophilic addition to alkenes, and so the positively charged intermediate here is equivalent to the carbocation intermediate in electrophilic addition. However in step 2, the mechanisms of electrophilic addition and electrophilic substitution differ. Whereas the carbocation intermediate from an alkene reacts with a nucleophile to give an addition product, the intermediate from the aromatic ring loses a proton. The C–H σ bond breaks and the two electrons move into the ring to reform the π bond, thus regenerating the aromatic ring and neutralizing the positive charge on the carbon. This is the mechanism undergone in all electrophilic substitutions. The only difference is the nature of the electrophile (Figure 3 ).

  Figure 2. Mechanism of electrophilic substitution. Figure 3. Examples of electrophiles used in electrophilic substitution.

  Intermediate stabilization

  The rate-determining step in electrophilic substitution is the formation of the positively charged intermediate, and so the rate of the reaction is determined by the energy level of the transition state leading to that intermediate. The transition state resembles the intermediate in character and so any factor stabilizing the intermediate also stabilizes the transition state and lowers the activation energy required for the reaction. Therefore, electrophilic substitution is more likely to take place if the positively charged intermediate can be stabilized. Stabilization is possible if the positive charge can be spread amongst different atoms — a process called delocalization. The process by which this can take place is known as resonance (Figure 4 ) — see also Sections H11 , G2 , and G5

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  eBook - ePub

  Biochemistry

  An Organic Chemistry Approach

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

    (Publisher)

  This type of reaction is labeled electrophilic aromatic substitution and it accounts for most of the chemistry of benzene and its derivatives. This reaction is labeled S E Ar. A carbocation intermediate is formed in a S E Ar reaction called a Wheland intermediate. Such intermediates have also been called Meisenheimer adducts and even σ-adducts. The modern term, which will be used exclusively, is an arenium ion. The aromatic stability of benzene is disrupted to form a resonance-stabilized arenium ion, as shown in Figure 9.4, and the aluminate anion (AlCl 3 Br −) can remove a proton from the carbon bearing the bromine to generate a new C=C unit, regenerating the aromatic benzene ring to yield the final product, bromobenzene. Conversion of an arenium ion to a benzene derivative is f ormally an E1 reaction, and the energetic driving force for losing this proton (for the E1 reaction) is regeneration of the aromatic ring. From a mechanistic viewpoint, an electrophilic aromatic substitution is in reality two reactions. The first is the Lewis acid-base reaction of benzene with Br +, which is generated by the reaction of bromine with the Lewis acid. The second is an E1-type reaction to yield the substitution product, and it is an acid-base reaction. The reaction of benzene and bromine in the presence of aluminum chloride to yield bromobenzene is a S E Ar reaction. FIGURE 9.4 Mechanism of electrophilic aromatic substitution with benzene and bromine. Many Lewis acids other than ferric bromide can be used, including aluminum bromide (AlBr 3), aluminum chloride (AlCl 3), boron trifluoride (BF 3), or ferric oxide (Fe 2 O 3). Indeed, old lab experiments for this reaction suggest that adding a “rusty nail” (rust is ferric oxide) to benzene, and then slowly adding bromine, will yield bromobenzene

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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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  Water Quality Data

  Analysis and Interpretation

  • Arthur Hounslow(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  The benzene ring has six identical bonds, neither single nor double bonds. The ring has been drawn with alternating single and double bonds. In recent texts the benzene ring is drawn as a hexagon enclosing a circle, indicating the ring of delocalized electrons. This gives rise to the two symbols used for the benzene ring, namely:

  In some literature it is designated as C6 H5 — when acting as a substituent.

  IUPAC Naming of Aromatics

  The basis for naming monocyclic aromatic compounds is to specify what atoms are on each of the six carbon atoms making up the benzene ring.

  Most monosubstituted derivatives of benzene are named as benzene derivatives. The two major exceptions are phenol and aniline, which will be discussed later. On the other hand, all the disubstituted benzenes are named as derivatives of benzene with locator numbers indicating the position of the substituted groups. Other methods of naming aromatics are in common use and must also be known. The three major alternative methods are

  1. Numeric system (standard IUPAC).

  The benzene ring carbon atoms are numbered from 1 to 6, starting with one substituent and continuing either clockwise or counterclockwise so that the lowest numbers for each substituent are obtained. In naming complex compounds with functional groups (see later), the numbering starts with the functional group of highest nomenclature priority.

  2. Specific names based on long usage.

  3. Based on the ortho, meta, and para system.

  ortho-, abbreviated ο- , (1, 2-),

  meta-, abbreviated m -, (1, 3-), and

  para-, abbreviated p- , (1, 4-), benzene (or benzene derivative).

  This system is commonly used for disubstituted benzene derivatives. It should be emphasized that the o, m , and ρ prefixes are lowercase letters.

  Aromatic Combining Forms

  Combining names for two common benzene containing groups are

  Aromatics Commonly Found in Groundwater

  An important group of aromatic compounds includes benzene, toluene, ethylbenzene, and xylenes that are often called BTEX. They occur in gasoline and related petroleum fractions. Because of their high water solubility they present an immediate hazard to drinking water supplies. The solubilities of these compounds are given in Table 7.7 . A summary of hydrocarbon nomenclature is given in Table 7.8

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