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The following question appeared in the Cambridge International AS and A level Chemistry Coursebook:

Choose from redox, substitution, elimination, addition and hydrolysis to give the type of reaction for the following: $$\ce{C2H6(g) + 2Cl2(g) -> C2H4Cl2(l) + 2HCl(g)}$$

The answer given by the answer key was substitution since $\ce{Cl2}$ replaces $\ce{H2}$.

However, why is redox not a suitable word to describe the reaction? After all, the oxidation number of $\ce{C}$ in the reactants is $-3$, while it is oxidised to become $-1$ in the products. Furthermore, the oxidation number of $\ce{Cl}$ in the reactants is $0$ while it is reduced to $-1$ in the products.

Hence, by that logic, isn't the reaction also a redox reaction since $\ce{Cl}$ is reduced and $\ce{C}$ is oxidised?

Thanks in advance for any help.

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You are right. The question is definitely misleading. If it would ask for the mechanism, (radical) substitution would be the correct term, as a reduction does not say anything about the mechanism. But "type of reaction" can basically mean anything, and in this case both answers are correct.

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  • $\begingroup$ Electrophilic substitution? This mechanism is likely free radical substitution. An alkane is reacting. $\endgroup$ – Tan Yong Boon Feb 19 '18 at 9:55
  • $\begingroup$ Sorry, my bad. Edited the answer. The main intention of the answer of course still holds. $\endgroup$ – vk_s Feb 19 '18 at 16:37

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