Problem 19 - Chemistry An Atoms First Approach

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 c are incorrect. Give the correct names for these hydrocarbons.

Step-by-Step Solution

Answer

The structural formulas for the given hydrocarbons are as follows: a. 3-isobutylhexane (Incorrect name, the correct name is 3-methylheptane): \( CH_{3}CH_{2}CH(CH_{2}CH(CH_{3})_{2})CH_{2}CH_{2}CH_{2}CH_{3} \) b. 2,2,4-trimethylpentane (isooctane): \( CH_{3}-C(CH_{3})_{2}CH_{2}C(CH_{3})_{2}CH_{3} \) c. 2-tert-butylpentane (Incorrect name, the correct name is 2,2,3-trimethylbutane): \( CH_{3}C(CH_{2}CH_{3})C(CH_{3})_{3} \) The correct names for parts a and c are: a: 3-methylheptane c: 2,2,3-trimethylbutane

Step 1: 3 -isobutylhexane

Draw the hexane chain with six carbon atoms, and then attach an isobutyl group, which contains four carbons, to the 3rd carbon of the hexane chain. Hexane: \(-CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{3}\) Isobutyl: \(-CH_{2}CH(CH_{3})_{2}\) Result: \(CH_{3}CH_{2}CH(-CH_{2}CH(CH_{3})_{2})CH_{2}CH_{2}CH_{3}\)

Step 2: 2,2,4 -trimethylpentane (isooctane)

Draw the pentane chain with fives carbon atoms, and then attach three methyl groups to carbons 2 and 4. Pentane: \(-CH_{3}CH_{2}CH_{2}CH_{2}CH_{3}\) Methyls: \(-CH_{3}\) Result: \(-CH_{3}-C(CH_{3})_{2}CH_{2}C(CH_{3})_{2}CH_{3}\)

Step 3: 2 -tert-butylpentane

Draw the pentane chain and then attach a tert-butyl group (with four carbons) to the 2nd carbon of the pentane chain. Pentane: \(-CH_{3}CH_{2}CH_{2}CH_{2}CH_{3}\) tert-Butyl: \(-C(CH_{3})_{3}\) Result: \(-CH_{3}C(CH_{2}CH_{2}CH_{2}CH_{3})C(CH_{3})_{3}\)

Step 4: Correct names for 3 -isobutylhexane and 2 -tert-butylpentane (parts a and c)

a. We need to change the name of 3-isobutylhexane to its correct IUPAC name, which should be 3-methylheptane. For 3 -isobutylhexane, check the order of carbons: Longest chain: \(CH_{3}CH_{2}CH_{2}CH_{2}CH_{2}CH_{2}CH_{3}\) (Heptane) Position 3: \(-CH_{2}CH(CH_{3})_{2}\) (Methyl group) Correct Name: \(CH_{3}CH_{2}CH(-CH_{2}CH(CH_{3})_{2})CH_{2}CH_{2}CH_{2}CH_{3}\) (3-methylheptane) c. We need to change the name of 2-tert-butylpentane to its correct IUPAC name, which should be 2,2,3-trimethylbutane. For 2 -tert-butylpentane, check the order of carbons: Longest chain: \(-CH_{3}CH_{2}CH_{2}CH_{3}\) (Butane) Position 2: \(-C(CH_{3})_{3}\) (Two methyl groups) Position 3: \(-C(CH_{3})\) (One methyl group) Correct Name: \(-CH_{3}C(CH_{2}CH_{3})C(CH_{3})_{3}\) (2,2,3-trimethylbutane)

Key Concepts

Structural Formula

A structural formula is a depiction of a molecule highlighting the arrangement of atoms and the bonds between them. It helps in understanding how atoms are connected and offers a visual representation, different from simpler chemical formulas like molecular formulas. Structural formulas are essential in organic chemistry as they clearly denote the positioning of functional groups and substituents, which can significantly impact the properties and reactivity of the compound.
For example, when drawing the structural formula of a compound like 3-isobutylhexane, we visualize the main hexane chain,

• Hexane: Consists of six carbon atoms connected in a linear fashion.
• Isobutyl group: Connected to the third carbon in the chain.

By depicting this in a structural formula, we can identify how the isobutyl group alters the basic hexane framework and helps define the compound's unique properties. Structural formulas are instrumental for chemists to convey complex molecules in a simplified drawing, making it easier to understand and communicate molecular structures.

IUPAC Nomenclature

The IUPAC nomenclature is a systematic way of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). This method ensures that each compound has a unique and standard name, which is universally understood without confusion. The process takes into account the longest carbon chain, various substituents, and the specific locations of these groups.
For instance, in the exercise, '3-isobutylhexane' was corrected to '3-methylheptane'. This correction was made by identifying:

• The longest carbon chain, which is heptane (seven carbons).
• The substituent group – in this case, a methyl group – located on the third carbon.

Similarly, '2-tert-butylpentane' should be corrected to '2,2,3-trimethylbutane'. It reflects the arrangement of the main butane chain and the position of methyl groups on the structure at carbon positions 2 and 3. Employing IUPAC nomenclature allows for the consistent and precise naming of compounds, thus making communication among chemists clearer and more effective.

Hydrocarbons

Hydrocarbons are organic compounds made entirely of hydrogen and carbon atoms. They are the simplest form of organic molecules and form the foundation of organic chemistry. These compounds can be classified into different categories, such as alkanes, alkenes, and alkynes, based on the types of bonds between the carbon atoms.
In the exercise, hydrocarbons like '3-isobutylhexane' and '2,2,4-trimethylpentane' are alkanes, which means they only consist of single bonds between carbon atoms.

• Alkanes: Saturated hydrocarbons with single C-C bonds and the general formula C\(_n\)H\(_{2n+2}\).
• Structural isomers: Molecules with the same molecular formula but different structural arrangements, leading to distinct properties.

Alkanes are major components of fuels, like gasoline, and reliable models for understanding complex organic reactions and structures. The variations within hydrocarbons, through different chains and branches of carbon, can significantly alter their physical and chemical behavior, emphasizing the importance of understanding their structural nuances.