Question

Free-radical halogenation can occur with chlorine and a source of direct UV radiation or sunlight. Chlorination of 2,4-dimethylpentane via radical halogenation leads to the formation of all three of the below products. Estimate the relative percentages of each product that will be formed using this means of halogenation. Presume that 1 equivalent of chlorine is used.

Answer 1

40 % A, 26 % B, and 34 % C

Step-by-step explanation:

2,4-Dimethylpentane (1) has three sets of equivalent H atoms:

• Four CH₃ groups (12 H, 1°)
• One CH₂ group (2 H, 2°)
• Two CH groups (2 H, 3°)

Thus, replacement of an H atom can give three products:

• 1-Chloro-2,4-dimethylpentane (A)
• 3-Chloro-2,4-dimethylpentane (B)
• 2-Chloro-2,4-dimethylpentane (C)

The relative reactivities of the three types of hydrogen atoms to substitution by Cl are

1°:2°:3° = 1:3.9:5.2

We can predict the distribution of monochloro products by combining statistics with the reactivity factors.

`\begin{array}{cccrccr} & & \textbf{Rel.} & \textbf{Rel.}\; & \textbf{Fraction} & \mathbf{\%}&\\\textbf{Type} & \textbf{No.} & \textbf{Rate} & \textbf{Amt.} & \textbf{Yield} & \textbf{Yield}& \\1^(\circ) & 12 & 1 & 12.0 & (12.0)/(30.2) & 40 \\\\2^(\circ) & 2 &3.9 & 7.8 & (7.8)/(30.2) & 26 &\\\\3^(\circ) & 2 & 5.2 & 10.4 &(10.4 )/(30.2)&34 &\\&& \textbf{Tot.} & \mathbf{30.2} && \mathbf{100} &\\\end{array}\\`

The product mixture contains 40 % A, 26% B, and 34 %C.