5.3.1 Benzene C6H6

C H

Aromatic Hydrocarbons Aromatic = Contains a Benzene ring in its structure Hydrocarbon = Made only of carbon and hydrogen. Aliphatic = chains of carbons. [opposite of aromatic] [branched or unbranched] We have to know about [a] benzene, [b] methyl benzene [c] ethyl benzene

Benzene C6H6 Discovered by Michael Faraday in 1825 Chemical formula suggested it had to be highly unsaturated - 3 double bonds Does not behave like normal unsaturated compounds - i.e. does not decolourise bromine Kekule suggested resonating or rotating alternate double and single bonds. but it did not fit the facts

Kekule’s Structure of Benzene Each C has 4 bonds Each H has 1 bond Bonding Rules Satisfied Double bond moves round or Double bond moves back and forward i.e. resonates

Benzene Bonds Bonds length measured by X-ray diffraction C - C single bond is 0.154 nm [ 1 nm = 1*10-9m] C = C is 0.134 nm C  C in Benzene is 0.139 nm NOT [0.144] half way between single and double bond length so resonating or rotating double bonds do not work

Delocalised Pi bonds C H The 6 valence electrons are shared between the whole molecule not localised into 3 double bonds For convenience the C and H atoms are not shown Ring in centre essential

Sigma and Pi Bonding

Two S orbitals overlap Sigma Bond ơ

Two px orbitals overlap Sigma Bond

Sigma Bonds are sausage shaped They occur when Two s orbitals overlap Two Px orbitals overlap Head on

As the two px orbitals overlap so do the two py orbitals if they are occupied

 Py or Pz orbital Py or Pz orbital Pi Bond

Pi Bonds As the two px orbitals join [forming a sigma bond] The two py orbitals overlap [if they are occupied] They also form a modified electron cloud. Shaped vaguely like the symbol π

Pi Bonds are shaped like the symbol  Twp Py orbitals overlap Occur when Twp Py orbitals overlap Two Pz orbitals overlap side on

Bond Construction Single bonds are Sigma bonds Double bonds consist of one Sigma bond and one Pi Bond Triple bonds consist of one Sigma Bond and two Pi Bonds

Benzene Derivatives In benzene the ring of carbon atoms is called the nucleus Any group attached to the nucleus is called a side chain Methylbenzene (toluene) TNT - trinitrotoluene Ethylbenzene

Other Points about benzene Benzene both Toxic and Carcinogenic (Methylbenzene useful substitute in lab.) Liquid at room temperature Soluble in organic [non-polar] solvents e.g. cyclohexane, propanone Insoluble in water Good solvent for organic substances and covalent substances e.g. iodine Increases octane number

Test What does aromatic mean? [2] Give the “opposite” of aromatic: what does it mean? [2] What 3 aromatic compounds do you know? [3] Draw Kekule’s structure of benzene. [2] Draw the modern structure of benzene. [2] Draw the structure of two other aromatic compounds.[4] Give two dangers of benzene. [2] Describe with reasons the solubility of benzene. [6] Give a use of benzene. [1] What do you call the CH3 in methyl benzene? [1] What does hydrocarbon mean? [2] What does the ring represent? [2] How can you show that benzene is not unsaturated [3]

Test What does aromatic mean? [2] contains benzene ring What is the “opposite” of aromatic and what does it mean? [2] aliphatic / chain What 3 aromatic compounds do you know? [3] benzene / methyl benzene / ethylbenzene Draw Kekule’s structure of benzene. [2] single and double bonds alternate Draw the modern structure of benzene. [2] hexagon with circle inside Draw the structure of the other two aromatic compounds. [4] Methyl and ethylbenzene structures Give two dangers of benzene. [2] poisonous / carcinogenic / combustible

Give two dangers of benzene. [2] poisonous / carcinogenic / combustible Describe with reasons the solubility of benzene. [4] insoluble in water / water polar / benzene not polar / soluble in cyclohexane / both non-polar Give a use of benzene. [1] solvent / petrol additive What do you call the CH3 in methyl benzene? [1] side chain What does hydrocarbon mean? [2] carbon and hydrogen / only What does the ring represent? [2] delocalised / Pi Bond How do we know benzene is not unsaturated [3] doesn’t turn bromine from red/ brown to colourless [27]