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An atom or group of atoms containing odd or unpaired electrons is known as free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond

In a free-radical chain reaction, free radicals are generally created in the initiation step. A free radical and a reactant are combined to yield a product in the propagation step. Lastly, the number of free radicals generally decreases in the termination step.

BDE is the amount of enthalpy required to break a bond homolytically in such a way that each bonded atom retains one of the bond’s two electrons.

Mathematically,

${\mathrm{Δ\pm á}}^0=∑(BDE  of  bonds  broken)-∑(BDE  of  bonds  formed)$

Primary hydrogen is hydrogen that resides on a carbon which is only attached to one carbon atom.

Secondary hydrogen is hydrogen that resides on a carbon which is only attached to two carbon atoms.

(a)

abstraction of primary hydrogen by a fluorine radical

$\begin{array}{rcl}& & \text{breaking}  \text{primary}  \text{C}-\text{H}  \text{bond:} \text{410}  \text{kJ/mol}\\ & & \text{forming}     \text{F}-\text{H}  \text{bond}  \text{:}  -\text{569}   \text{kJ/mol}\\ & & \mathit{Δ}\mathbf{\text{H}}\mathbf{ }\mathbf{\text{=}}\mathbf{ }\mathbf{\text{410}}\mathbf{ }\mathbf{\text{kJ/mol + (}}\mathbf{-}\mathbf{\text{569kJ/mol)}}\mathbf{ }\mathbf{\text{=}}\mathbf{ }\mathbf{-}\mathbf{\text{159}}\mathbf{ }\mathbf{ }\mathbf{\text{kJ/mol}}\\ & & \end{array}$

abstraction of secondary hydrogen by fluorine radical

$$\begin{gathered} \text{breaking}  \text{secondary}  \text{C}-\text{H}  \text{bond:} 397  \text{kJ/mol} \\ \text{forming}     \text{F}-\text{H}  \text{bond}  \text{:}  -\text{569}   \text{kJ/mol} \\ \mathit{Δ}\mathbf{\text{H}}\mathbf{ }\mathbf{\text{=}}\mathbf{ }\mathbf{397}\mathbf{ }\mathbf{\text{kJ/mol + (}}\mathbf{-}\mathbf{\text{569kJ/mol)}}\mathbf{ }\mathbf{\text{=}}\mathbf{ }\mathbf{-}\mathbf{172}\mathbf{ }\mathbf{ }\mathbf{\text{kJ/mol}} \end{gathered}$$

(b) Free-radical fluorination is nearly non-selective.

(c)

products from free-radical fluorination of propane

The second reaction has more negative change in enthalpy which implies that it is more feasible. Secondary carbon is more easily fluorinated.