Aromatic- Radical aromatic reactions

Question 1

What are the two common radical aromatic reactions

Answer 1

1. The Sandmeyer Reaction

2. The Birch Reaction

Question 2

How is a diazonium salt produced

Answer 2

1. Diazotization of anilines using sodium nitrite NaNO2 and aqueous acid at low temperature
2. The nitrite ion is converted to nitrous acid HNO2 by the acid
3. Upon further protonation, it dehydrates to form the highly-reactive nitrosonium ion NO+
4. This cation is then trapped by the lone pair on the amino group of the aniline to give a nitrosamine and then another acid-promoted dehydration occurs to give the diazonium salt.

Question 3

Describe what happens in the Sandmeyer reaction

Answer 3

1. In the presence of various copper (I) salts, aryl diazonium salts undergo a displacement reaction where the nitrogen cation N2+ group is displaced with the counterion of the copper salt.
2. The loss of nitrogen gas N2 is triggered by the presence of a copper (I) salt and is displaced by the counterion of the copper salt.
3. Typically this is chloride Cl- or Br- to give the corresponding haloarene but can also be CN- to give the corresponding benzonitrile

Question 4

What is the Balz-Schiemann modification

Answer 4

1. Modification of Sandmeyer reaction

2. Diazonium salt is heated when X=BF4- the corresponding fluorobenzene is made

Question 5

What is another modification of the Sandmeyer reaction

Answer 5

1. Modification of Sandmeyer reaction

2. When heated in water when X=HSO4- the corresponding phenol is made

Question 6

Why are the Sandmeyer modifications useful

Answer 6

1. Neither can be synthesised by electrophilic aromatic substitution as there are no synthetic equivalents for electrophilic fluorine F+ or electrophilic oxygen O+

Question 7

What are further transformations of diazonium salts

Answer 7

1. Using KI allows access to electron-deficient iodobenzenes

2. Using phosphonic acid H3PO2 to produce the parent hydrocarbon

Question 8

What can nitro, amino and diazonium groups be used for

Answer 8

1. As temporary directing or blocking groups

Question 9

How can benzene be reduced

Answer 9

1. Birch reduction
2. Hydrogenation of benzene is incredibly difficult has it requires substantial amounts of energy to break aromaticity
3. Can be achieved using dissolved metal in liquid ammonia conditions to produce an unconjugated diene

Question 10

Describe what happens in the Birch reduction

Answer 10

1. Sodium or lithium is added to ammonia, the metal becomes oxidised and electrons are released into solution
2. The solvated electrons then add into the low-energy anti-bonding orbitals of the aromatic to produce a dianion, which repel one another so sit on opposite carbons on either side of the ring
3. The dianion is then promptly protonated by an alcohol also present in solution to afford the unconjugated diene
4. Used for benzene and derivatives

Question 11

How does the Birch reduction product vary depending on substituent attached

Answer 11

1. EWG- ipso and para carbons are the ones reduced as the EWG can stabilise one of the negative charges
2. When an EDG is present on the aromatic, one ortho and one meta carbon is reduced as EDG group destabilises the dianion intermediated forcing the charges away from substituent