Org 5 - Aromatics Flashcards
What is an aromatic compound ?
Aromatics are cyclic compounds with unusual stability due to cyclic delocalisation and resonance.
Explain the π electrons in aromatic compounds
The π electrons are delocalised over the entire ring and are therefore stabilised over the structure.
Explain benzene and its bond length
Benzene is the simplest aromatic hydrocarbon.
It is known to have only one type of C-C bond, with a length between that of a single and double bond.
What is the bond angle of benzene ?
120o
All bonds are of equal length.
Explain/Show the resonance structures of Benzene ?
Show the resonance structure of Phenol
Show the resonance structure of Toluene ?
Show the resonance structure of Aniline
Show the resonance structure of Nitrobenzene
Show the structure of Benzoic acid
Describe the system used to name di-substituted Benzenes
If there are more than two substituents on the aromatic ring, the Ortho-Meta-Para system is used.
What is Hückel’s rule ?
A criteria for determining whether or not a compound is aromatic. If a compound does not meet all the criteria, it is likely not aromatic.
What is the criteria for Hückel’s rule ?
- The molecule is cyclic
- The molecule is Planar
- The molecule is fully conjugated
- The molecule has 4n+2π electrons
(If rule 4 is broken, the molecule is anti-aromatic.
Name an aromatic and an anti-aromatic compound
What is the consequence to an aromatic compound, if the number of ‘p’ orbitals and π electrons is different ?
The molecule can still be aromatic, a cation or anion.
E.g. Cyclopentadiene, due to the lone pair, there are 6 delocalised π electrons.
What does aliphatic mean ?
Organic compounds whose carbon atoms are linked in open chains (straight or branched) rather than containing a benzene ring (aromatic).
Name five examples of electrophilic aromatic substitution
- Aromatic halogenation
- Aromatic nitration
- Aromatic sulfonation
- Friedel-Crafts Alkylation
- Friedel-Crafts Acylation
N.B. A powerful electrophile is always required due to the stability of the aromatic ring.
Explain aromatic halogenation, why is Fl- the odd one out ?
- The benzene ring acts as an electron donor (nucleophile) in most cases.
- Reacts with Br-, Cl- or I- to produce mono-substituted products.
- Fl- is very reactive and produces multi-substitued products
Explain aromatic nitration
- The aromatic ring can be nitrated when reacted with a mixture of nitric and sulfuric acid
Explain aromatic sulfonation
- Aromatic rings can react with a mixture of sulfuric acid and sulfur trioxide to form sulfonic acid.
- The electrophile is either HSO3, or SO3.
Explain Friedel-Crafts Alkylation
- Electrophilic aromatic substitution - the benzene ring is alkylated when it reacts with an alkyl halide
Explain the limitations to Friedel-Crafts Alkylation
- The reaction does not proceed on an aromatic ring that has a strongly deactivating substituent group
- The product is more reactive than the starting material leading to poly-alkylation
- Skeletal rearrangement of the alkyl group sometimes occurs, producing a more stable cation
Explain Friedel-Crafts Acylation
- An eletrophilc aromatic substitution in which the benzene ring is acylated.
- The electrophile is an acyl cation generated by the reaction between the acyl halide and AlCl3.
- The product is less stable than the starting material, so monosubstitution is observed.
When do aromatic rings become poly-substituted ?
When the product is more reactive than the starting material (E.g. Flourobenzene). Otherwise, mono-substitution is observed.
What determines the position of the electrophile in electrophilic aromatic substitution ?
- The nature of the 1st substituent on the ring determines the position of the 2nd.
- Substituents are classified, which indicated where the electrophile will end up.
What are the 3 classes of substituents that determine electrophilic ring position ?
- O-P (Ortho-Para) diverting activators
- O-P diverting deactivators
- Meta-directing de-activators
What is the activating group in terms of aromatics ? What do they do ?
- If a substituted benzene reacts more rapidly than a benzene alone, the substituent group is said to be an activating group
- Activating groups can donate electrons to the ring, therefore the ring is more attractive to an electrophile.
What are O-P activators in terms of aromatic chemistry ?
- All activating groups are O-P directors e.g. OH, NH2, OR, OCR, and alkyl groups.
- A small percentage of electrophiles will still add at a meta position, but O-P are favoured.
What is a deactivating group in terms of aromatic chemistry ? What do they do ?
- If a substituted benzene reacts more slowly than the benzene alone, the substituent group is said to be deactivating.
- Deactivating groups withdraw electrons from the ring, therefore the ring is less attractive to an electrophile.
What are Meta-directors in terms of aromatic chemistry ?
- All deactivating groups are meta-directors (except weakly deactivating hallides - which are O-P directors).
- E.g. NO2, SO2, CN, SO3H, COOH
- Without any substituents, the sigma positive will be at the O-P position, therefore the electrophile avoids it, and favours attack at the meta position.
What reactions occur with the alkyl-benzene side chain ?
- Oxidation (E.g. Alkyl Benzene –> Benzoic acid)
- Bromination
- Reduction
Explain the term ‘Conjugation’
When there is only one single bond inbetween the double bond and the carbocation, providing the opportunity for resonance stabilisation.
