Question

Draw and name all of the structural isomers possible for an alkyl halide with no branches and the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{Br}_{2}\)

Short answer

The possible structural isomers for the unbranched alkyl halide with molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{Br}_{2}\) are: 1. \(1,2\)-Dibromopentane: \(\mathrm{CH_{3} - CHBr - CHBr - CH_{2} - CH_{3}}\) 2. \(1,3\)-Dibromopentane: \(\mathrm{CH_{3} - CHBr - CH_{2} - CHBr - CH_{3}}\) 3. \(2,3\)-Dibromopentane: \(\mathrm{CH_{3} - CH_{2} - CHBr - CHBr - CH_{3}}\) 4. \(1,4\)-Dibromopentane: \(\mathrm{CH_{3} - CHBr - CH_{2} - CH_{2} - CHBr}\)

Step-by-step solution

Draw the base alkyl structure

Draw an unbranched alkyl structure having 5 carbons, a straight chain of carbon atoms will look like: \(\mathrm{CH_{3} - CH_{2} - CH_{2} - CH_{2} - CH_{3}}\)

Add one bromine atom

Attach the first bromine atom to the carbon chain. The bromine atom can be placed on any carbon atom except for the initial and final carbon atoms (1st and 5th respectively) since it would lead to the same structure if placed on any of these two carbons: - \(\mathrm{CH_{3} - CHBr - CH_{2} - CH_{2} - CH_{3}}\) - \(\mathrm{CH_{3} - CH_{2} - CHBr - CH_{2} - CH_{3}}\) - \(\mathrm{CH_{3} - CH_{2} - CH_{2} - CHBr - CH_{3}}\)

Add the second bromine atom

Now, we'll add the second bromine atom. For each structure created in Step 2, consider placing the second bromine atom on any carbon atom following the same restriction as in step 2, while also avoiding the carbon atom that already has a bromine atom on it. For the first structure in step 2: - \(\mathrm{CH_{3} - CHBr - CHBr - CH_{2} - CH_{3}}\) For the second structure in step 2: - \(\mathrm{CH_{3} - CHBr - CH_{2} - CHBr - CH_{3}}\) - \(\mathrm{CH_{3} - CH_{2} - CHBr - CHBr - CH_{3}}\) For the third structure in step 2: - \(\mathrm{CH_{3} - CHBr - CH_{2} - CH_{2} - CHBr}\)

Name the isomers

Now that we have all the possible structural isomers drawn, we'll name them using IUPAC nomenclature: 1. \(\mathrm{CH_{3} - CHBr - CHBr - CH_{2} - CH_{3}}\): 1,2-Dibromopentane 2. \(\mathrm{CH_{3} - CHBr - CH_{2} - CHBr - CH_{3}}\): 1,3-Dibromopentane 3. \(\mathrm{CH_{3} - CH_{2} - CHBr - CHBr - CH_{3}}\): 2,3-Dibromopentane 4. \(\mathrm{CH_{3} - CHBr - CH_{2} - CH_{2} - CHBr}\): 1,4-Dibromopentane These are all the possible structural isomers of the given molecular formula without branches, \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{Br}_{2}\).

Key concepts

Alkyl Halide

An alkyl halide, also known as a haloalkane, is an organic compound containing one or more halogen atoms (such as fluorine, chlorine, bromine, or iodine) bonded to an alkyl group. The general formula for alkyl halides is RX, where R represents an alkyl group and X represents a halogen.

The structure and reactivity of alkyl halides are significantly influenced by the presence of the electronegative halogen atom. This impacts not only the physical properties, such as boiling and melting points, but also the chemical behavior in reactions, such as nucleophilic substitutions or eliminations.

As seen in the exercise, the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{Br}_{2}\) indicates the presence of two bromine atoms in the compound, classifying it as a dibromoalkane. Different structural isomers can be formed by varying the positions of these bromine atoms on the carbon chain, while maintaining the same molecular formula. Understanding these variations is fundamental when studying reaction mechanisms and synthesis in organic chemistry.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature is a systematic method for naming organic compounds. It provides a standard way to name the structure of molecules based on their chemical composition and the arrangement of atoms. This standardized nomenclature helps to avoid ambiguity and ensure clear communication among chemists.

In the case of alkyl halides, the IUPAC rules dictate that the name reflects the length of the longest continuous chain of carbon atoms, the location and identity of the halogen substituents, and any branching within the molecule. The carbon atoms in the chain are usually numbered from the end nearest to the substituent, and prefixes like 'di-', 'tri-', 'tetra-', etc., indicate the number of identical substituents present.

For example, the exercise provided shows us how to systematically name the structural isomers of dibromopentane using IUPAC rules. The names 1,2-Dibromopentane, 1,3-Dibromopentane, 2,3-Dibromopentane, and 1,4-Dibromopentane all specifically describe the relative positions of bromine atoms on a five-carbon chain.

Molecular Formula

A molecular formula provides a concise way to describe the number and types of atoms in a molecule. It lists the symbols of the elements present in the molecule, followed by subscript numbers indicating the number of each atom present. The molecular formula is vital for identifying the substance and understanding its composition.

When comparing different compounds, the molecular formula can be the same, yet the arrangement of atoms may differ, leading to distinct substances. These are known as isomers. A compelling example is provided by the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{Br}_{2}\), which describes several structural isomers. Each isomer has the same number of carbon, hydrogen, and bromine atoms, but differs in the arrangement of these atoms. Instances like these highlight the importance of structural isomers and why chemists need to interpret molecular formulas beyond just the elemental composition.

To effectively utilize molecular formulas in problem-solving, one must consider not only the possible combinations fitting a given formula but also how permutations can result in different structures with distinct chemical and physical properties.