Problem 45 - Chemistry An Atoms First Approach

Question

If one hydrogen in a hydrocarbon is replaced by a halogen atom, the number of isomers that exist for the substituted compound depends on the number of types of hydrogen in the original hydrocarbon. Thus there is only one form of chloroethane (all hydrogens in ethane are equivalent), but there are two isomers of propane that arise from the substitution of a methyl hydrogen or a methylene hydrogen. How many isomers can be obtained when one hydrogen in each of the compounds named below is replaced by a chlorine atom? a. \(n\) -pentane c. 2,4 -dimethylpentane b. 2 -methylbutane d. methylcyclobutane

Step-by-Step Solution

Answer

By replacing one hydrogen atom with a chlorine atom in each of the given compounds, we obtain the following number of isomers: a. n-pentane: \(1\) isomer b. 2-methylbutane: \(2\) isomers c. 2,4-dimethylpentane: \(2\) isomers d. methylcyclobutane: \(3\) isomers

Step 1: a. n-pentane

First, let's draw the structure of n-pentane, a linear hydrocarbon with five carbon atoms and twelve hydrogen atoms. All hydrogen atoms in n-pentane are equivalent, so replacing any one of them with a chlorine atom will produce the same chlorinated compound. Thus, there is only 1 isomer obtained when one hydrogen is replaced by a chlorine atom in n-pentane.

Step 2: b. 2-methylbutane

Now let's draw the structure of 2-methylbutane which is an isomer of pentane with a methyl group at the second carbon atom. There are two types of hydrogen atoms in 2-methylbutane: 1. Methyl hydrogen (9 atoms) 2. Methine hydrogen (2 atoms) Replacing a methyl hydrogen or a methine hydrogen with a chlorine atom will result in different isomers. Therefore, we obtain 2 isomers of 2-methylbutane when one hydrogen is replaced by a chlorine atom.

Step 3: c. 2,4-dimethylpentane

Next, let's draw the structure of 2,4-dimethylpentane which has additional methyl groups attached to the second and fourth carbon atoms. There are two types of hydrogen atoms in 2,4-dimethylpentane: 1. Methyl hydrogen (12 atoms) 2. Methine hydrogen (4 atoms) Replacing a methyl hydrogen or a methine hydrogen with a chlorine atom will result in different isomers. Therefore, we obtain 2 isomers of 2,4-dimethylpentane when one hydrogen is replaced by a chlorine atom.

Step 4: d. methylcyclobutane

Finally, let's draw the structure of methylcyclobutane which is a cyclobutane ring with a methyl group attached. There are three types of hydrogen atoms in methylcyclobutane: 1. Methyl hydrogen (3 atoms) 2. Methine hydrogen (1 atom) 3. Bridgehead hydrogen (4 atoms) Replacing any of these hydrogen atoms with a chlorine atom will result in different isomers. Therefore, we obtain 3 isomers of methylcyclobutane when one hydrogen is replaced_by a chlorine atom. In conclusion, we can form the following number of isomers for each given compound by replacing one hydrogen atom with a chlorine atom: a. n-pentane: 1 isomer b. 2-methylbutane: 2 isomers c. 2,4-dimethylpentane: 2 isomers d. methylcyclobutane: 3 isomers