Unit 4.2 Aromaticity (og)
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Describe the term aromaticity
• Cyclic (ring-shaped) • Planar (flat) • Molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms ^what da faq
What are arenes?
Ring structures
What is Benzene?
• A liquid • Colourless • Flammable • Sweet and gasoline-like odour
How can Benzene (a useful chemical substance) be toxic?
• Continuous exposure = cancerous effects • It’s a carcinogen
What is a carcinogen?
Chemical/agent that can potentially cause cancer
Describe 5 key facts of Benzene?
• C6H6 • Planar • Contains 3 C=C (π) double bonds and 3 C-C (σ) single bonds • Hexagonal ring • Trigonal planar about each carbon
Must learn how to draw benzene … and perhaps acknowledge it’s 3 other representations
Sweet …. yayaya
What does the circle within benzene represent?
3 π bonds aka 6 π e-
What are aliphatic compounds?
Alkanes and alkenes
How can arenes (benzene) differ from aliphatic compounds (alkanes and alkenes)?
• Possess one/more rings of carbon atoms • Bonding electrons are delocalised That’s why I said benzene in brackets
Show me how benzene differ from alkanes or alkenes (pretty much explains the circle inside hexagon)
• Normal sigma bond (standard covalent bond) • Formed between each pair of carbons • Each containing 2 electrons BUT THEN • Remaining 6 electrons (1 from each carbon) • Occupy p-orbitals • Which they overlap to form a π bond In Benzene’s case, bond extends over all 6 carbon atoms
What are 3 consequences of Benzene’s delocalisation of some of the bonding electrons?
• Benzene much more stable than expected • Extra stability = benzene less readily undergo addition reactions • More loosely held electrons = open to attack by electrophiles electrophilic substitution type shi
Explain the consequence of Benzene being much more stable than expected
• Actual enthalpy of formation of benzene = 82 kJmol-1 (Enthalpy of Hydrogenation experiments) • Enthalpy of Hydrogenation of cyclohexene is -120 kJmol-1 ^^ feel like kinda useless? Perhaps know just the top right? • Contained 3 alkene double bonds ∴ expected enthalpy of hydrogenation to cyclohexane to be 3x -360kJmol-1 • Ended up being -210 kJmol-1 • Difference between 2 values = 150 kJmol-1 And is therefore the stabilisation energy (delocalisation energy) of benzene EXPLAINS WHY IT’S RESISTANT TO ADDITION REACTIONS
2 aromatic properties of Benzene?
• Resistance to addition (Extra stable due to delocalisation) • Electrophilic substitution (Characteristic reaction of benzene is electrophilic substitution)
How will Benzene only undergo electrophilic substitution?
The use of a halogen carrier
What do u call benzene that contains an OH group?
Phenol (hydroxybenzene)
2 significant difference between Phenol and Benzene?
• Phenol more reactive than Benzene • No assistance undergoing electrophilic substitution (Lone pairs of O2 will overlap with delocalised ring system) Refer to booklet D:
What do u call benzene that contains Methyl group?
Methylbenzene (Toluene)
Significant difference between Methylbenzene and Benzene? (Same as with Phenol)
• More reactive than Benzene
State all the reactions of Benzene
• Nitration of Benzene • Friedel-Crafts alkylation of Benzene • Friedel-Crafts Acylation of Benzene • Halogenation of Benzene
You must learn how to draw all the reactions of Benzene
It’ll take a while….
Explain nitration of Benzene?
Err
Explain Friedel-Crafts alkylation of Benzene?
Uhh
Explain Friedel-Crafts acylation of Benzene?
Umm