Question

Draw the structural formula for each of the following. a. 3 -isobutylhexane b. \(2,2,4\) -trimethylpentane, also called isooctane. This substance is the reference ( 100 level) for octane ratings. c. 2 -tert-butylpentane d. The names given in parts a and \(\mathrm{c}\) are incorrect. Give the correct names for these hydrocarbons.

Short answer

The corrected names and structural formulas for the given hydrocarbons are: a. 3-methylheptane: \(CH_3 - CH_2 - CH(CH_3) - CH_2 - CH_2 - CH_2 - CH_3\) b. 2,2,4-trimethylpentane (isooctane): \(CH_3 - C(CH_3)_2 - CH_2 - CH(CH_3) - CH_3\) c. 3-tert-butylpentane: \(CH_3 - CH_2 - C(CH_3)_3 - CH_2 - CH_3\)

Step-by-step solution

Drawing the structural formula for 3-isobutylhexane (incorrect name)

The given name is not accurate as per standard naming conventions, so we will correct the name first. The correct name for this hydrocarbon is 3-methylheptane. Now, let's draw its structural formula. 1. Draw a heptane chain, which has 7 carbon atoms in a straight chain. 2. Add a methyl group (CH3) on the 3rd carbon atom. Here is the structural formula for 3-methylheptane: \(CH_3 - CH_2 - CH(CH_3) - CH_2 - CH_2 - CH_2 - CH_3\)

Drawing the structural formula for 2,2,4-trimethylpentane

This hydrocarbon is also known as isooctane. To draw its structural formula: 1. Draw a pentane chain, which has 5 carbon atoms in a straight chain. 2. Add a methyl group (CH3) to each of the 2nd and 4th carbon atoms. 3. Add another methyl group (CH3) to the 2nd carbon atom. Here is the structural formula for 2,2,4-trimethylpentane (isooctane): \(CH_3 - C(CH_3)_2 - CH_2 - CH(CH_3) - CH_3\)

Drawing the structural formula for 2-tert-butylpentane (incorrect name)

The given name is not accurate according to standard naming conventions, so we will correct the name first. The correct name for this hydrocarbon is 3-tert-butylpentane. Now, let's draw its structural formula. 1. Draw a pentane chain, which has 5 carbon atoms in a straight chain. 2. Add a tert-butyl group (C(CH3)3) to the 3rd carbon atom. Here is the structural formula for 3-tert-butylpentane: \(CH_3 - CH_2 - C(CH_3)_3 - CH_2 - CH_3\)

Correcting the names of hydrocarbons

As we have identified earlier, the incorrect hydrocarbon names given in the exercise are: a. 3-isobutylhexane - The correct name is 3-methylheptane. c. 2-tert-butylpentane - The correct name is 3-tert-butylpentane. These corrected names have been used in their respective structural formula drawings.

Key concepts

IUPAC nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) developed a systematic method to name organic compounds, which applies to a wide range of substances, including alkanes. This system is important to ensure that each compound has a unique and universally accepted name. It relies on identifying the principal chain, which is the longest continuous carbon atom chain in the molecule.

For instance, when naming a molecule like 3-methylheptane, we start by identifying the longest chain of carbon atoms, in this case, heptane (7 carbons). Then, substituents are identified and numbered in such a way that they are attached to the carbon closest to the end of the chain. This is why we name it 3-methylheptane, indicating a methyl group attached to the third carbon.

In situations where multiple substituents are present, as in 2,2,4-trimethylpentane, they must be listed alphabetically and the position numbers should be minimized to guide the placement of each substituent along the main chain. Understanding IUPAC nomenclature is crucial for clear communication in organic chemistry.

alkanes

Alkanes are the simplest type of hydrocarbons, only composed of carbon and hydrogen atoms with single covalent bonds. They are generally known for their saturated property, meaning each carbon atom is saturated with hydrogen atoms. This structure makes them quite stable and non-reactive under normal conditions.

The general formula for alkanes is given by \( C_nH_{2n+2} \), where \( n \) is the number of carbon atoms. For example, methane is the simplest alkane with one carbon atom ( = 1d = 't the first alkane), and it follows the formula \( CH_4 \).

The line of alkanes extends into larger and more complex structures like propane, butane, and octane. As we explore further, each successive alkane comes with an additional \( CH_2 \) unit in its structure. Alkanes also form the backbone for more complex hydrocarbon chains, making them a fundamental concept in organic chemistry.

isooctane

Isooctane is a specifically branched isomer of octane and is significant in the petrochemical industry as a reference standard for octane ratings. Its systematic IUPAC name is 2,2,4-trimethylpentane, referring to its structure, having a pentane chain with three methyl groups attached — two at the 2nd position and one at the 4th position.

The formula for isooctane can be expressed as \( C_8H_{18} \), and its importance stems from its ability to resist autoignition better than many other hydrocarbons. This characteristic is crucial for evaluating the performance of fuels, especially gasoline, where a higher octane rating implies better knocking resistance in engines.

Isooctane represents a blend of chemical simplicity and technological importance, making it both a subject of study in chemistry and a practical determinant in engineering applications.

organic chemistry problem-solving

Solving organic chemistry problems often involves a combination of understanding molecular structures, applying nomenclature rules, and drawing upon a foundational knowledge of hydrocarbons. The first step in tackling such a problem is usually to correctly interpret the provided molecular name or formula. This involves breaking down the problem into smaller, more manageable components.

For example, if tasked with drawing a structure like 3-methylheptane, one must first identify the primary seven-carbon chain (heptane) and determine the correct point of attachment for any substituents, such as the methyl group on the third carbon.

As shown in correcting names like 3-isobutylhexane to 3-methylheptane, the resolution involves understanding the numbering and naming conventions according to IUPAC standards. These conventions ensure clarity and precision, providing a universal language in the field of chemistry. Such problem-solving skills are essential in synthesis and application, bridging the gap between theory and practical use.