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ARENES: BENZENE

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A very concise study material for Edexcel A2 Level. Few related past paper questions were incorporated for practice.
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  • VIHS/DEPARTMENT OF CHEMISTRY Page 1

    5.3 ARENES: BENZENE 2014

    Syllabus specification

    Arenes: benzene

    a. use thermochemical, x-ray diffraction and infrared data as evidence for the structure and

    stability of the benzene ring.

    Students may represent the structure of benzene as

    or

    as appropriate in equations and mechanisms

    b. describe the following reactions of benzene, limited to:

    i) combustion to form a smoky flame.

    treatment with:

    ii) bromine.

    iii) concentrated nitric and sulfuric acids.

    iv) fuming sulfuric acid.

    v) halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts

    reaction).

    vi) addition reactions with hydrogen.

    c. describe the mechanism of the electrophilic substitution reactions of benzene in

    halogenation, nitration and Friedel-Crafts reactions including the formation of the

    electrophile.

    d. carry out the reactions in 5.4.1b where appropriate (using methylbenzene or

    methoxybenzene).

    e. carry out the reaction of phenol with bromine water and dilute nitric acid and use these

    results to illustrate the activation of the benzene ring.

    Introduction:

    Arenes are hydrocarbons with a ring or rings of carbon atoms in which there are delocalised

    electrons. Benzene, the simplest arene with a molecular formula C6H6, is an important and

    useful chemical which is obtained by the catalytic reforming of fractions from crude oil.

    Arenes are sometimes called aromatic compounds.

    Study of the structure of benzene is an another example that shows how scientific models

    develop in response to new evidence. This links to further investigations of the models that

    chemists use to describe the mechanisms of organic reactions.

  • VIHS/DEPARTMENT OF CHEMISTRY Page 2

    5.3 ARENES: BENZENE 2014

    General properties of benzene

    It is a Colourless liquid with a characteristic odour.

    Boils at 80oC and freezes at 6oC.

    Immiscible with water but soluble in organic solvent.

    Gives smoky luminous sooty flame on burning.

    Structure of benzene:

    Benzene, C6H6, is a cyclic compound that has six carbon atoms in a hexagonal ring. Several

    structures for benzene have been proposed. Early theories suggested that there were

    alternative single and double bonds between the carbon atoms(fig 5.3.1), but this did not fit

    with later experimental evidence. It was shown that all the carbon-carbon bonds are the

    same length and that the molecule is planar.

    Two modern theories are used to explain the structure.

    The Kekule version assumes that benzene is a resonance hybrid between

    the two structures as given below. This model can be used to explain many

    chemical properties and reaction of benzene.

    Fig 5.3.2 The displayed formula of kekules benzene ring structure

    The other theory assumes that each sp2 hybridized carbon atom is joined by

    a - (sigma) bond to each of its two neighbours, and by a third - sigma bond

    to s-orbital of hydrogen atom forming a hexagonal planar ring. The fourth

    bonding electron is in p-orbital(called as non-hybrid porbital) in the right

    angle to the planar of - (sigma) bonds. This p-orbital overlap side way, and

    the six p-orbitals overlap above and below the plane of the ring of carbon

    atoms. This produces a delocalised -(pi)bonding system of electrons, as in:

    Fig. 5.3.1 Simplified structure of benzene

  • VIHS/DEPARTMENT OF CHEMISTRY Page 3

    5.3 ARENES: BENZENE 2014

    Fig. 5.3.3 The delocalisation of the electrons in the -bonds of the

    symmetrical six-membered ring structure of benzene

    Evidences for structure and extra stability of benzene

    (i) Thermochemical evidence: via enthalpy of hydrogenation.

    Benzene is more stable than cyclohexatriene, which is the theoretical compound

    with three single and three localised double carbon-carbon bonds. The amount by

    which it is stabilised can be calculated from the enthalpies of hydrogenation.

    For example, the enthalpy of hydrogenation of one mole cyclohexene is -120 kJ.

    + H2(g) H = -120 kJ mol1

    Cyclohexene Cyclohexane

    Therefore , H for the addition to three localised double bonds in cyclohexatriene

    would be 3 x (-120) = -360 kJ mol1. However for benzene:

    + 3H2(g) H = -208kJ mol1

    Benzene cyclohexane

    Thus,152 kJ less energy is given out because of benzenes unique structure. This is

    called the delocalisation stabilisation energy or resonance energy and can be

    shown in an enthalpy-level diagram.

  • VIHS/DEPARTMENT OF CHEMISTRY Page 4

    5.3 ARENES: BENZENE 2014

    Cyclohexatriene

    H = -360 kJ mol-1 H = -152 kJ (resonance energy)

    Benzene

    H = -208 kJ mol-1

    Cyclohexane, C6H12

    Fig.5.3.4 Enthalpy-level diagram for the hydrogenation of benzene and cyclohexatriene.

    Thermo-chemical evidence: via bond enthalpies

    The amount by which benzene is stabilised can also be calculated from average

    bond enthalpies. The enthalpy of formation of gaseous benzene is +83 kJ mol-1.

    The value for the theoretical molecule cyclohexatriene can be found using the

    Hesss law cycle below:

    6C(s) + 3H2(g) C6H6(g)

    6C(g) + 3H2(g) 6C(g) + 6H(g)

    Step 1 equals 6 x enthalpy of atomisation of carbon(Hatm[C(s)]) = 6 x (+715)

    = +4290 kJ

    Step 2 equals 3 x HH bond enthalpy = 3 x (+436) = + 1308kJ

    Step 3 equals enthalpy change of bonds made, which is calculated as below

    Three CC = 3 x (-348) = -1044 kJ

    Three C=C = 3 X (-612) = -1836 kJ

    Six CH = 6 x (-412) = -2472 kJ

    Total = - 5362 kJ

    Enthalpy

    kJmol-1

    Hf

    Step 1

    Step 2

    Step 3

  • VIHS/DEPARTMENT OF CHEMISTRY Page 5

    5.3 ARENES: BENZENE 2014

    Hence the Hf of cyclohexatriene = Hstep 1 + Hstep 2 + Hstep 3

    = +4290 + 1308 +(-5352)

    = +246 kJ mol-1.

    The actual enthalpy of formation of gaseous benzene is +83 kJ mol-1. The value

    calculated above is 163 kJ more and approximately equals the resonance energy of

    benzene. Hence, the structure with the delocalised electron system is energetically

    more stable.

    X-ray diffraction evidence

    X-ray diffraction shows the position of the centre of atoms. If the diffraction pattern of

    benzene is analysed, it clearly shows that all the bond lengths between the carbon

    atoms are the same. Which is not the same in the case of cyclohexene.

    Table 1. comparison of bond length in benzene and cyclohexene.

    Bond Bond length/nm

    All the six carbon-carbon bonds in

    benzene

    0.140

    Carbon-carbon single bond in

    cyclohexene

    0.154

    Carbon-carbon double bond in

    cyclohexene

    0.134

    Fig.5.3.5 Electron density map of

    benzene.

    Electrons are equally distributed

    over six carbon atoms due to

    delocalisation of the pi- bonding

    electron system.

    If benzene has cyclohexatriene

    structure, equal distribution of

    electrons cannot be seen on the

    carbon ring.

    Thus, benzene is thermodynamically

    more stable due to its delocalized pi-

    bonding system.

    0.140 nm

  • VIHS/DEPARTMENT OF CHEMISTRY Page 6

    5.3 ARENES: BENZENE 2014

    Infra red evidence:

    Comparison of the infrared spectrum of aromatic compounds with those of aliphatic

    compounds containing a C=C group showed slight differences. The CH stretching

    vibration in benzene is at 3036cm-1 and the C=C stretching is at 1479cm-1, whereas

    the equivalent vibrations in an aliphatic compound such as cyclohexene are at 3023

    and 1438cm-1.

    Naming benzene derivatives.

    The derivatives of benzene are named either as substituted products of benzene or

    as compounds containing the phenyl group, C6H5. The names and structures of

    some derivatives of benzene are given below.

    Systematic name Substituent group Structure

    Chlorobenzene Chloro, -Cl C6H5-Cl

    Nitrobenzene Nitro, -NO2 C6H5-NO2

    Methylbenzene Methyl,-CH3 C6H5-CH3

    Phenol Hydroxyl, -OH C6H5-OH

    Phenylamine Amine, -NH2 C6H5-NH2

    Phenylethanone Ethanone,-COCH3 C6H5-COCH3

    Phenylmethanol Methanol,-CH2OH C6H5-CH2OH

    When more than one hydrogen atom is substituted, numbers are used to indicate the

    positions of substituent on the benzene ring. The ring is usua

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