Question

Draw a structural formula for each of the following branched alkanes. a. 2,2,4 -trimethyloctane b. 2,3,4 -trimethyloctane c. 3,3,4 -trimethyloctane d. 2,4,4 -trimethyloctane

Short answer

The short answer is: a. 2,2,4-trimethyloctane \(CH_3-CH(CH_3)-CH_2-CH_2-CH_2-CH_2-CH_2-CH_3\) b. 2,3,4-trimethyloctane \(CH_3-CH(CH_3)-CH(CH_3)-CH(CH_3)-CH_2-CH_2-CH_2-CH_3\) c. 3,3,4-trimethyloctane \(CH_3-CH_2-CH(CH_3)-CH(CH_3)-CH_2-CH_2-CH_2-CH_3\) d. 2,4,4-trimethyloctane \(CH_3-CH(CH_3)-CH_2-CH(CH_3)-CH_2-CH_2-CH_2-CH_3\)

Step-by-step solution

Parent Chain Identification

Identify the parent chain indicated by the longest word in the name. Here, 'octane' is the parent chain which tells us that there are eight carbon atoms in the parent chain.

Identify and Number Branches

Identify the branches with their respective positions on the parent chain. The prefix 'tri' indicates three branches, and 'methyl' tells us that they have only one carbon atom. The numbers before the prefix (2,2,4, etc.) indicate the positions of these branches on the parent chain.

Step a: Draw the Structural Formula of 2,2,4-trimethyloctane

1. Draw a continuous chain of 8 carbon atoms (parent chain). 2. At the 2nd and 4th carbon atoms, add a methyl (CH3) group. 3. At the 2nd carbon, add another methyl group. The structural formula of 2,2,4-trimethyloctane is shown below: CH3 | CH3-CH-CH2-CH2-CH2-CH2-CH2-CH3 | CH3

Step b: Draw the Structural Formula of 2,3,4-trimethyloctane

1. Draw a continuous chain of 8 carbon atoms (parent chain). 2. Add a methyl (CH3) group to the 2nd, 3rd, and 4th carbon atoms. The structural formula of 2,3,4-trimethyloctane is shown below: CH3-CH-CH-CH-CH2-CH2-CH2-CH3 | | CH3 CH3

Step c: Draw the Structural Formula of 3,3,4-trimethyloctane

1. Draw a continuous chain of 8 carbon atoms (parent chain). 2. Add a methyl (CH3) group to the 3rd and 4th carbon atoms. 3. At the 3rd carbon, add another methyl group. The structural formula of 3,3,4-trimethyloctane is shown below: CH3-CH2-CH-CH-CH2-CH2-CH2-CH3 | | CH3 CH3

Step d: Draw the Structural Formula of 2,4,4-trimethyloctane

1. Draw a continuous chain of 8 carbon atoms (parent chain). 2. Add a methyl (CH3) group to the 2nd and 4th carbon atoms. 3. At the 4th carbon, add another methyl group. The structural formula of 2,4,4-trimethyloctane is shown below: CH3 | CH3-CH-CH2-CH-CH2-CH2-CH2-CH3 | CH3

Key concepts

Parent Chain Identification

The first step in drawing structural formulas for branched alkanes is to identify the primary framework of the molecule, known as the parent chain. This is the longest continuous sequence of carbon atoms that forms the backbone of the compound.

In our examples, the parent chain is 'octane', indicating an 8-carbon alkane. While identifying the parent chain, one must ensure that it contains the maximum number of substituents or branches. This is crucial for the correct naming and representation of the molecule. If there's more than one option for a longest sequence, choose the one that encompasses the most branching points, thus reflecting the molecule's complexity accurately.

It's important to visualize the structure, as sometimes the longest chain isn't immediately obvious and may not be straight.

Alkane Nomenclature

Once the parent chain is determined, the next step is to understand the naming process, or nomenclature, of alkanes. Alkane names consist of a prefix, indicating the number of carbons in the parent chain, and a suffix, '-ane', which indicates that the compound is an alkane, a type of hydrocarbon where all the bonds between carbons are single bonds.

Numbering the Carbon Atoms

After establishing the parent chain, we number the carbon atoms usually starting from the end nearest to a substituent (a group replacing a hydrogen on the parent chain). This ensures that the substituents receive the lowest possible numbers.

Identifying and Naming Substituents

Each branch or substituent is named according to the number of carbon atoms it contains, preceded by a number indicating its position on the parent chain. In our example, 'tri' suggests three substituents, and 'methyl' indicates that these are -CH3 groups. The numbers before 'trimethyl' give the carbon positions where these methyl groups are attached to the parent chain.

Drawing Structural Formulas

Creating the structural formula of a branched alkane requires a clear and methodical approach. Begin with drawing the backbone of the parent chain as a series of connected lines, each representing a carbon-carbon bond. Then, add the branches at the specified carbon atoms using the correct positional numbers.

For instance, to depict a methyl group (-CH3), you would draw a line from the appropriate carbon atom on the main chain to a 'CH3' notation, representing the carbon atom of the methyl group and its three associated hydrogen atoms. Consistency in representation – ensuring that all similar branches are drawn in the same fashion – enhances clarity.

An accurate representation of the structural formula is not only essential for understanding the molecule's properties but also for physical and chemical analyses. A correctly drawn structure reveals the molecule's geometry and can aid in predicting reactivity patterns and physical properties.