Which of the following reactions is an example of a substitution reaction? (A) Chloroethane plus sodium hydroxide react to form ethene plus sodium chloride plus water. (B) Propene plus hydrogen react to form propane. (C) Butan-2-ol plus six oxygen react to form four carbon dioxide plus five water. (D) Ethanoic acid plus ethanol react to form ethyl ethanoate plus water. Or (E) pentane plus chlorine react to form three chloropentane plus hydrogen chloride.
A substitution reaction is a type of reaction where part of a molecule is removed and replaced with something else. One of the most common examples of this is the substitution reaction of alkanes with elemental halogens. And the halogens that you typically see used in the substitution reactions are fluorine, chlorine, and bromine. It’s important to note that we have not included iodine even though it’s a halogen. That’s because iodine is not used in this type of substitution reaction.
In an alkane substitution reaction, an alkane is reacted with a halogen, and this forms a haloalkane and a hydrogen halide molecule. In one example of this type of reaction, methane, or CH4, is reacted with elemental fluorine or F2. And in this example, one of the fluorine atoms from the halogen molecule replaces one of the hydrogen atoms from the methane molecule. And this monosubstitution reaction gives the product CH3F or fluoromethane, which is a haloalkane. And when this takes place, the molecule HF or hydrogen fluoride is also created. And this type of reaction can occur whenever you react a halogen, such as chlorine, fluorine, or bromine, with any alkane.
And with this information, we should be able to revisit and answer the question. Which of the following reactions is an example of a substitution reaction? Answer choice (A) reacts chloroethane, which is an alkyl halide, with sodium hydroxide, a base. And this gives the alkene product ethene as well as sodium chloride and water. And this type of reaction is known as an elimination reaction, not a substitution reaction. And so, we know that answer choice (A) is not the correct answer.
Answer choice (B) reacts the alkene propene with elemental hydrogen, and this reaction forms propane, an alkane. In this reaction, we are adding two hydrogen atoms across a double bond, which is why it is known as an addition. So, because we know that there are no atoms being substituted in this reaction, we can say that answer choice (B) is not the correct answer.
Answer choice (C) reacts butan-2-ol with oxygen, and this reaction forms the products of carbon dioxide and water. This type of oxidation process is a common reaction known as combustion. And again, since no atoms are being substituted, we can eliminate answer choice (C).
Answer choice (D) shows ethanoic acid reacting with ethanol. And when a carboxylic acid reacts with an alcohol, an ester is formed, in this case ethyl ethanoate and water. This process is known as an esterification reaction and not a substitution reaction. And so, we can eliminate answer choice (D).
And that leaves us with answer choice (E). Here, we are reacting pentane and alkane with chlorine, a halogen. And as we’ve already discussed, when you react an alkane with a halogen, you form a haloalkane and a hydrogen halide. In this case, we’re forming 3-chloropentane and hydrogen chloride. And because, in this example, we have substituted one of the hydrogen atoms from pentane with a chlorine atom, we can correctly identify this as a substitution reaction. And this means that the correct answer to “Which of the following reactions is an example of a substitution reaction?” is answer choice (E).