Organic 3
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Describe the modern structure of benzene
• formula of C6H6 • six C atoms in a hexagonal ring and one H atom bonded by single sigma bonds. • 6 p electrons delocalised in a ring structure above and below the plane of carbon atoms.
Describe kekule’s structure of benzene
• alternate single and double covalent structures between carbon atoms (starts with double)
What does it suggest if benzene is a planar molecule?
• all C-C bonds are the same length, and have a bond length and strength somewhere between the C-C and C=C bond
Explain how X‑ray diffraction shows that benzene has a delocalised structure and not a Kekulé structure
• Shows that all C-C bonds are the same length • in the Kekule structure, C=C bonds would have been shorter.
describe the enthalpy of hydrogenation of the kekule vs benzene vs cyclohexane structure, and how it shows that benzene is more stable than both.
• cyclohexane enthalpy = -120 so kekule will be 3x-120= -360 (expected) But in reality, the value ended up being around -208, so less exothermic than expected. When represented on an energy level diagram, it shows that benzene is the most thermodynamically stable.
What type of reactions does benzene usually go through? Why?
• electrophilic substitutions • electron density is not high enough to induce a dipole, so cannot go through addition reactions.
why is methylbenzene more reactive than benzene?
• methyl side group releases electrons into the delocalised system making it more attractive to nucleophiles.
Name the following for the reaction with benzene and halogens. - change in functional group - reagent - conditions - mechanism
• benzene to halobenzene • halogen • iron halide catalyst • Electrophilic substitution
Name some observations when benzene burns. What effects this observation?
• burns with a very sooty flame • carbon to H ratio - the lower this is, the more sooty the flame will be.
Give the equations to form electrophiles Cl+ and Br+ from AlCl3 and FeBr3
• AlCl3 + Cl2 —> AlCl4- + Cl+ • FeBr3 + Br2 —> FeBr4- + Br+
Describe the mechanism for the halogenation of benzene
• curly arrow going from middle of electron ring to electrophile (the Br+/Cl+ etc) • Curly arrow going from MIDDLE of bond to electron ring (which now has a + sign in the middle with only 3/4 of it actually drawn.) • Forms a halobenzene (with halogen sticking out from an edge of a benzene ring)
Name the following for the nitration of benzene - change in functional group - reagent - conditions - mechanism
• benzene —> nitrobenzene • Conc nitric acid in the presence of conc H2SO4 (catalyst) • 60 Degrees • Electrophilic substitution
Give the equation to form the electrophile NO2+ for nitration of benzene.
• HNO3 + 2H2SO4 —> NO2+ + 2HSO4- + H3O+
What is the usefulness of the nitrification of benzene?
• important to manufacture explosive material, or formation of amines which are used to manufacture dyestuffs.
Name the following for the hydrogenation of benzenes - change in functional group - reagent - conditions - type of reaction
• benzene –> cyclohexane • Hydrogen • nickel catalyst, 30 atm, 200 degrees. • addition and reduction
Name the following for the Friedal Crafts Alkylation - change in functional group - reagent - conditions - mechanism
• benzene to alkyl benzene • chloroalkane in the presence of anhydrous aluminium chloride catalyst. • heat under reflux • electrophilic substitution
give the equation for the electrophile CH3CH2+
AlCl3 + CH3CH2Cl —-> Ch3CH2+ + AlCl4-
Name the following for the Friedal Crafts acylation - change in functional group - reagent - conditions - mechanism
• benzene to phenyl ketone • acyl chloride in presence of a anhydrous AlCl3 catalyst • heat under reflux • electrophilic substitution
Give the equation to form the electrophile CH3C=O+
AlCl3 + CH3COCl –> CH3CO+ + AlCl4-
Why are Friedel Craft reactions important/useful?
• produce a reactive functional group on the benzene ring, which is otherwise unreactive, which can then be used to produce dyestuffs etc. • this allows the molecule to carry out subs in milder conditions.
Name 2 properties of phenols
• very weakly acidic • will react with sodium metal and NaOH but NOT Na2CO3
Name the following for the reaction of phenol with Bromine - change in functional group - reagent - conditions - observations
• no change - will only add Brs. • Bromine water • room temp • white solid forms
Compare the reaction of benzene with bromine and phenol with bromine
• phenol does not need a FeBR3 catalyst • goes through MULTIPLE substitution reactions, whereas benzene will only undergo a single substitution reaction.
Name 3 uses of phenols
• plastics • antiseptics • paint resins