Org 5 - Aromatics

Question 1

What is an aromatic compound ?

Answer 1

Aromatics are cyclic compounds with unusual stability due to cyclic delocalisation and resonance.

Question 2

Explain the π electrons in aromatic compounds

Answer 2

The π electrons are delocalised over the entire ring and are therefore stabilised over the structure.

Question 3

Explain benzene and its bond length

Answer 3

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.

Question 4

What is the bond angle of benzene ?

Answer 4

120^o

All bonds are of equal length.

Question 5

Explain/Show the resonance structures of Benzene ?

Answer 5

Question 6

Show the resonance structure of Phenol

Answer 6

Question 7

Show the resonance structure of Toluene ?

Answer 7

Question 8

Show the resonance structure of Aniline

Answer 8

Question 9

Show the resonance structure of Nitrobenzene

Answer 9

Question 10

Show the structure of Benzoic acid

Answer 10

Question 11

Describe the system used to name di-substituted Benzenes

Answer 11

If there are more than two substituents on the aromatic ring, the Ortho-Meta-Para system is used.

Question 12

What is Hückel’s rule ?

Answer 12

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.

Question 13

What is the criteria for Hückel’s rule ?

Answer 13

1. The molecule is cyclic
2. The molecule is Planar
3. The molecule is fully conjugated
4. The molecule has 4n+2π electrons

(If rule 4 is broken, the molecule is anti-aromatic.

Question 14

Name an aromatic and an anti-aromatic compound

Answer 14

Question 15

What is the consequence to an aromatic compound, if the number of ‘p’ orbitals and π electrons is different ?

Answer 15

The molecule can still be aromatic, a cation or anion.

E.g. Cyclopentadiene, due to the lone pair, there are 6 delocalised π electrons.

Question 16

What does aliphatic mean ?

Answer 16

Organic compounds whose carbon atoms are linked in open chains (straight or branched) rather than containing a benzene ring (aromatic).

Question 17

Name five examples of electrophilic aromatic substitution

Answer 17

1. Aromatic halogenation
2. Aromatic nitration
3. Aromatic sulfonation
4. Friedel-Crafts Alkylation
5. Friedel-Crafts Acylation

N.B. A powerful electrophile is always required due to the stability of the aromatic ring.

Question 18

Explain aromatic halogenation, why is Fl^- the odd one out ?

Answer 18

• 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

Question 19

Explain aromatic nitration

Answer 19

• The aromatic ring can be nitrated when reacted with a mixture of nitric and sulfuric acid

Question 20

Explain aromatic sulfonation

Answer 20

• Aromatic rings can react with a mixture of sulfuric acid and sulfur trioxide to form sulfonic acid.
• The electrophile is either HSO₃, or SO₃.

Question 21

Explain Friedel-Crafts Alkylation

Answer 21

• Electrophilic aromatic substitution - the benzene ring is alkylated when it reacts with an alkyl halide

Question 22

Explain the limitations to Friedel-Crafts Alkylation

Answer 22

• 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

Question 23

Explain Friedel-Crafts Acylation

Answer 23

• 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.

Question 24

When do aromatic rings become poly-substituted ?

Answer 24

When the product is more reactive than the starting material (E.g. Flourobenzene). Otherwise, mono-substitution is observed.

Question 25

What determines the position of the electrophile in electrophilic aromatic substitution ?

Answer 25

• 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.

Question 26

What are the 3 classes of substituents that determine electrophilic ring position ?

Answer 26

1. O-P (Ortho-Para) diverting activators
2. O-P diverting deactivators
3. Meta-directing de-activators

Question 27

What is the activating group in terms of aromatics ? What do they do ?

Answer 27

• 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.

Question 28

What are O-P activators in terms of aromatic chemistry ?

Answer 28

• 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.

Question 29

What is a deactivating group in terms of aromatic chemistry ? What do they do ?

Answer 29

• 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.

Question 30

What are Meta-directors in terms of aromatic chemistry ?

Answer 30

• 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.

Question 31

What reactions occur with the alkyl-benzene side chain ?

Answer 31

• Oxidation (E.g. Alkyl Benzene –> Benzoic acid)
• Bromination
• Reduction

Question 32

Explain the term ‘Conjugation’

Answer 32

When there is only one single bond inbetween the double bond and the carbocation, providing the opportunity for resonance stabilisation.