Video Transcript
When ethane and bromine are mixed and illuminated by ultraviolet light, they react together in a substitution reaction. The incomplete balanced equation for this reaction is shown. C2H6 plus Br2 react to form X plus HBr. Determine the stoichiometry and molecular formula of the product X.
A substitution reaction is defined as a type of reaction where a part of a molecule is removed and replaced with something else. These type of reactions can involve the substitution of a single atom like bromine, chlorine, or iodine. The reactions can also involve full functional groups, such as diazo or tosyl groups. A functional group is a group of atoms within a molecule in a particular bonding arrangement. Diazo and tosyl groups, for example, are groups of multiple atoms that have weak bonds to carbon and are easily displaced in substitution reactions.
In this question, we are reacting ethane, which is an alkane, with bromine, which is a halogen. And this is an example of a substitution reaction of alkanes with elemental halogen, which is the most common substitution reaction that we perform on alkanes. In these reactions, an alkane is reacted with elemental halogen to form a haloalkane plus a hydrogen halide. In a monosubstitution reaction, one hydrogen atom from each of the alkane molecules will be substituted, which is why it’s called a monosubstitution reaction. And this gives a monohalogenated alkane and hydrogen halide as product.
One such example is the reaction of propane with elemental chlorine. In this reaction, one of the hydrogens of propane — we’ll choose a terminal hydrogen for this example — is substituted by one of the chlorine atoms in elemental chlorine. And this gives the products 1-chloropropane and hydrogen chloride. At this point, it’s important to reiterate that we chose the terminal hydrogen to substitute for this example. But this reaction could’ve occurred on a nonterminal carbon as well.
In the reaction above of ethane with elemental bromine, we can identify that we’re performing a monosubstitution reaction. This is because we’re reacting one stoichiometric equivalent of each of the starting materials. We can also see that we’re forming one equivalent of HBr as a product. At this point, we can compare the reaction in question with the example reaction shown below. In both reactions, we have one equivalent of alkane reacting with one equivalent of halogen. And our products are one equivalent of hydrogen halide, but we’re missing our haloalkane product. This means that X must represent the haloalkane product of a monosubstitution reaction.
And so if we substitute one of the hydrogen atoms from ethane with one of the bromine atoms from elemental bromine, we should be able to answer the question. When ethane and bromine are mixed and illuminated by ultraviolet light, they react together in a substitution reaction. The incomplete balanced equation for this reaction is shown. C2H6 plus Br2 react to form X plus HBr. Determine the stoichiometry and molecular formula of the product X. And the correct answer is C2H5Br, with a stoichiometric coefficient of one.
