Lecture 07, 8

• What is the simplest aromatic hydrocarbon?

  Benzene (C6H6)

  
• How many degrees of unsaturation does benzene have?

  Four degrees of unsaturation
• How does benzene differ from other unsaturated hydrocarbons?

  Benzene does not undergo addition reactions
• What product is formed when benzene undergoes hydrogenation?

  Cyclohexane

  
• What does the hydrogenation of benzene suggest about its structure?

  Benzene has a cyclic structure
• How many double and single bonds would a benzene ring contain to fit its molecular formula?

  Three double bonds and three single bonds
• Who proposed the cyclic structure of benzene with alternating bonds?

  Kekulé
• What limitation does Kekulé's structure have regarding benzene's properties?

  It doesn't explain benzene's low reactivity
• What discrepancy arises from the predicted reactivity of benzene based on Kekulé's structure?

  Benzene does not decolourise bromine water
• How many isomers of dibromobenzene are formed according to Kekulé's structure?

  Three isomers

  
• What technique revealed the structure of benzene as a perfect hexagon?

  X-ray crystallography
• What are the bond angles and lengths in benzene?

  120° angles and 0.140 nm bond length
• What must any structure for benzene account for?

  Six-membered ring, planar, equal bond lengths
• What does the resonance structure of benzene indicate about its bond lengths?

  Bond lengths are between single and double bonds
• How is benzene represented in resonance structures?

  By drawing a circle inside the ring
• What is the bond order of benzene?

  1.5
• What is resonance energy in the context of benzene?

  Difference between predicted and observed stability
• Why is benzene particularly stable?

  Low heat of hydrogenation indicates stability
• How does benzene react with bromine?

  Substitutes hydrogen instead of addition

  
• What are the four criteria for a compound to be aromatic?

  Cyclic, planar, completely conjugated, Hückel's rule
• What is Hückel's rule for aromatic compounds?

  Must contain 4n + 2 π electrons
• What happens to compounds with 4n π electrons?

  They are anti-aromatic and unstable
• What are hydrocarbons with alternating double and single bonds called?

  Annulenes
• How is an annulene named?

  Indicate number of atoms in brackets
• What is the structure of [6]-annulene?

  Benzene
• Why is [10]-annulene not aromatic?

  It is not planar due to strain

  
• How does the stability of aromatic compounds compare to acyclic compounds?

  Aromatic compounds are more stable
• What is the stability of anti-aromatic compounds compared to acyclic compounds?

  They are less stable
• What characterises non-aromatic compounds?

  They lack continuous overlapping p-orbitals
• What is the stability of 1,3-cyclohexadiene compared to cis, cis-2,4-hexadiene?

  Similar in stability
• What is the electron configuration of the cyclopentadienyl cation?

  4 electrons, anti-aromatic
• What is the electron configuration of the cyclopentadienyl anion?

  6 electrons, aromatic
• What is the structure of the tropylium cation?

  Planar carbocation with three double bonds

  
• What does cyclooctatetraene form when it reacts with potassium metal?

  An aromatic dianion

  
• What is naphthalene?

  The simplest fused aromatic hydrocarbon
• How do fused ring hydrocarbons form polycyclic aromatic hydrocarbons?

  By fusing two or more six-membered rings
• What happens to resonance energy as the number of fused rings increases?

  Resonance energy per ring decreases
• What is the relationship between larger polynuclear aromatic hydrocarbons and Br2?

  They will add Br2
• How many delocalised electrons does pyridine have?

  Six delocalised electrons
• What is the role of the non-bonding electrons on nitrogen in pyridine?

  They do not interact with π electrons