Question Video: Comparing the Reactivities of Halogens in Alkane Substitution Reactions | Nagwa Question Video: Comparing the Reactivities of Halogens in Alkane Substitution Reactions | Nagwa

Question Video: Comparing the Reactivities of Halogens in Alkane Substitution Reactions Chemistry • Third Year of Secondary School

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Which of the following substitution reactions is slowest at room temperature and pressure? [A] Option A [B] Option B [C] Option C [D] Option D

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Video Transcript

Which of the following substitution reactions is slowest at room temperature and pressure?

A substitution reaction is a type of reaction where a part of a molecule is removed and replaced by something else. We can see that each of the substitution reactions provided involves methane, a one-carbon alkane. In each reaction, the methane is reacted with a different diatomic halogen. Over the course of a substitution reaction between an alkane and a halogen, a carbon–hydrogen bond and the halogen–halogen bond must break. This allows for the formation of a new carbon–halogen bond and a new hydrogen–halogen bond, producing a haloalkane and a hydrogen halide.

The breaking of the halogen–halogen bond is a critical rate determining step in this reaction. The higher the bond dissociation energy of this bond, the higher the activation energy of the reaction. In general, a reaction with a higher activation energy will take longer to occur at room temperature without a catalyst, such as heat or ultraviolet light. Let’s take a look at the bond dissociation energy for each of the given halogens.

As we can see, iodine has the lowest bond dissociation energy and chlorine has the highest. So, based on bond dissociation energy alone, we might think that the reaction involving chlorine is the slowest and the reaction involving iodine is the fastest, since breaking the chlorine–chlorine bond requires the most energy and breaking the iodine–iodine bond requires the least. But the breaking of the halogen–halogen bond is just one part of the reaction. The rate of this reaction significantly depends on the reactivity of the halogen involved.

When we compare the reactivity of the halogens, we find that iodine is the least reactive and fluorine is the most reactive. In fact, fluorine is so reactive that these substitution reactions are often explosive. The reactions involving bromine and chlorine are less reactive and can be initiated with very little energy input, often in the form of ultraviolet light. The reaction involving iodine, however, is not energetically favorable. So, iodine does not generally undergo substitution reactions with alkanes.

While the trend in bond dissociation energy suggests that the reaction involving chlorine is the slowest and iodine is the fastest, the reactivity of the halogens proves to be more important when determining the rate of reaction for these substitution reactions. So, the reaction involving iodine is actually the slowest, while the reaction involving fluorine is the fastest.

Therefore, the substitution reaction that is the slowest at room temperature and pressure is the reaction between methane and iodine, answer choice (C).

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