Org 5 - Aromatics Flashcards Preview

GAMSAT March 2016 > Org 5 - Aromatics > Flashcards

Flashcards in Org 5 - Aromatics Deck (33):
1

What is an aromatic compound ? 

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

2

Explain the π electrons in aromatic compounds 

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

3

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. 

4

What is the bond angle of benzene ? 

120o

All bonds are of equal length. 

5

Explain/Show the resonance structures of Benzene ? 

6

Show the resonance structure of Phenol 

7

Show the resonance structure of Toluene ? 

8

Show the resonance structure of Aniline 

9

Show the resonance structure of Nitrobenzene 

10

Show the structure of Benzoic acid 

11

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. 

12

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. 

13

What is the criteria for Hückel's rule ? 

  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. 

14

Name an aromatic and an anti-aromatic compound 

15

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. 

16

What does aliphatic mean ? 

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

17

Name five examples of electrophilic aromatic substitution

  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. 

18

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-, Clor I- to produce mono-substituted products. 
  • Fl- is very reactive and produces multi-substitued products 

19

Explain aromatic nitration 

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

 

20

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.

 

21

Explain Friedel-Crafts Alkylation 

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

 

22

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

23

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. 

24

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. 

25

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. 

26

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

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

27

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. 

28

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. 

29

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. 

30

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. 

31

What reactions occur with the alkyl-benzene side chain ? 

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

32

Explain the term 'Conjugation'

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

33