Q77. Question: The triphenylmethyl ra... [FREE SOLUTION]

91影视

Chapter 15: Q77. (page 570)

Question: The triphenylmethyl radical is an unusual persistent radical present in solution in equilibrium with its dimer. For 70 years the dimer was thought to be hexaphenylethane, but in 1970, NMR data showed it to be A.
a. Why is the triphenylmethyl radical more stable than most other radicals?
b. Use curved arrow notation to show how two triphenylmethyl radicals dimerize to form A.
c. Propose a reason for the formation of A rather than hexaphenylethane.d. How could 1 H and 13C NMR spectroscopy be used to distinguish between hexaphenylethane and A?

Short Answer

Expert verified

Answer

a. Triphenylmethyl radical has more number of resonance structures.

b. Dimerization of triphenyl methyl radical

c. Hexaphenyl ethane has more steric hindrance than Compound A.

d. Due to the quinonoid ring in compound A, the proton and carbon-13 NMR of compound A will have more peaks.

Step by step solution

Dimerization

The formation of an adduct by the addition reaction of two molecules of the same compound is called dimerization.

A bond between two monomers of a dimer can be strong or weak.

If two oppositely charged species form a dimer, it is called a Bjerrum pair.

Dimerization usually occurs when the monomers are unstable. Then they will have high reactivity, and when come in contact with a solution and react to form a dimer.

Step 2: Dimerization of free radicals

Free radicals are species with an unpaired electron in the valence shell. It can be an atom, molecule, or ion.

It is generally formed during the homolytic cleavage of larger molecules.

Due to the presence of an incomplete shell with an unpaired electron, they are highly reactive. They react with other molecules or dimerize with themselves.

Due to their high reactivity, they can act as initiators in many chemical reactions.

Dimerization of triphenyl methyl cation

a. Triphenyl methyl radical is a carbon radical attached to three phenyl rings. Due to this, the radical shows extensive resonance. This is because all three rings can take part in resonance.

The total number of resonating structures for triphenylmethyl radical is 10. This makes the radical more stable than any other radical formed in the solution.

b. The dimerization includes resonance in radical to form a quinonoid structure and reaction with another radical.

Dimerization of triphenyl methyl radical

c. In hexaphenyl ethane all the six phenyl groups in the dimer are in close proximity. This increases the steric hindrance in the dimer and thus reduces the stability. Therefore, higher energy is required for the formation of this dimer.

This causes the formation of A to be higher than hexaphenyl ethane.

d. Proton NMR spectra-

Hexaphenyl ethane contains only one type of proton which is the proton in the ring. Therefore, ¹H NMR of this compound shows only one strong peak.

Compound A contains more than 1 peak due to the difference in the chemical environment of hydrogen in the quinonoid ring.

¹³C NMR spectra-

Due to the presence of the quinonoid ring, the ¹³C NMR of compound A will have more peaks than the spectra of hexaphenyl ethane.