Unit 4.2 Aromaticity (og) Flashcards
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
