Question

Draw the major monobromination product when the following alkane is subjected to radical bromination at 25 �C. If there is more than one product, they both may be drawn in the same box.

Draw the major monobromination product when the following alkane is subjected to radical bromination at 25 degree C. If there is more than one product, they both may be draw in the same box.

Answer 1

Concepts and reason

Bromination: The inclusion of bromine atom through addition or substitution reaction in to an organic substrate is called as bromination reaction. The bromination may takes place through ionic mechanism or radical mechanism depends on the substrate and condition. Generally alkanes follows radical mechanism, since ${\rm{C - H}}$ bond is not polar due to absence any functional group.

Homolytic cleavage or Homolysis: Covalent bond break symmetrically to produce radicals, that is the bonded electrons were departed equally with their native atom.

For example:

Heterolytic cleavage: The covalently bonded electron move towards the more electronegative atom and resulted in the breaking of bond in an unsymmetrical manner lead to formation of anion and cation.

For example:

Radical: The atom which contains one unpaired electron is called a radical or a free radical. Most of the radicals have a very short life time. The radical molecule is trivalent. Hybridization of the radical is ${\rm{s}}{{\rm{p}}^{\rm{2}}}$ .

Radical stability depends entirely on the nature of the substituent. The presence of an electron-donating group stabilizes the radical while the electron-withdrawing group destabilizes the radicals.

Fundamentals

Radical reaction : Free radical halogenation mechanism involves three major steps, which are as follows:

a)Initiation step: It involves homolysis of halogens lead to free radical formation. This can be initiated by light or heat.

b)Propagation step: The halogen radical induced radical formation in the alkanes.

c)Termination step: Here, the combination of two free radicals from halogen and alkanes combine to form the final target product.

Stability of radicals is an important factor for the radical mechanism, the stability order was given below, as the number of alkyl group increases the stability of the radical.

${{\rm{3}}^{\rm{o}}}{\rm{ - radical}}\,{\rm{ > }}\,{{\rm{2}}^{\rm{o}}}{\rm{ - radical}}\,{\rm{ > }}\,{{\rm{1}}^{\rm{o}}}{\rm{ - radical}}$

${\rm{Stability}}\,{\rm{of}}\,{\rm{radical}}\, \propto \,{\rm{EDG}}\,\, \propto \frac{{\rm{1}}}{{{\rm{EWG}}}}$

The radical formed during bromination was given below

Initiation step:

Propagation step:

The final termination step lead to the formation the major bromo-methyl cyclohexane compound, the structure of the compound was given below.

Ans:

The major product obtained from the bromination of methyl cyclohexane using ${\bf{B}}{{\bf{r}}_{\bf{2}}}$ in presence of light was given below