Question

The following are improper IUPAC names. Determine what is incorrect or ambiguous about the name and give the correct name. a. 2 -methyl-3-propylpentane b. 3-methyl-3-chloropentane c. \(2,3,3,7,7\) -pentamethyloctane d. 3-(1,1-dimethylethyl)pentane

Short answer

a. 3-methyl-4-ethylhexane b. 1-chloro-3-methylpentane c. 2,2,6,6,7-pentamethyloctane d. 3-tert-butylhexane.

Step-by-step solution

Analyzing the Given Name (a)

The compound name is 2-methyl-3-propylpentane. Upon decomposing it, it is clear that the longest chain chosen is pentane, however, the longest continuous carbon chain should be considered first. The longest chain here is actually hexane, which means the base name is incorrect.

Correcting the Structure (a)

Identify the correct longest carbon chain, and then determine the positions and types of substituents. With a hexane chain, the correct IUPAC name for the structure is 3-methyl-4-ethylhexane.

Analyzing the Given Name (b)

The name 3-methyl-3-chloropentane indicates the presence of a methyl and a chloro group both on the third carbon. Here, the issue is determining the position priority and the numbering of the chain should be such that the substituents have the lowest possible numbers.

Correcting the Structure (b)

For the molecule, the longest chain is pentane correctly. Re-evaluating the numbering for substituents gives 1-chloro-3-methylpentane, aligning with correct priority rules.

Analyzing the Given Name (c)

The compound is named as 2,3,3,7,7-pentamethyloctane. The name correctly identifies the longest chain as octane but the substituents aren't minimizing the numbers correctly.

Correcting the Structure (c)

For minimal numbers, the numbering should start from the right end leading to 2,2,6,6,7-pentamethyloctane.

Analyzing the Given Name (d)

The name 3-(1,1-dimethylethyl)pentane describes a tert-butyl group incorrectly drawn as a substituent. Considering the base chain gives an incorrect longest chain.

Correcting the Structure (d)

Recognize the substituent as tert-butyl which is on the third carbon in a hexane chain. Therefore, the corrected IUPAC name should be 3-tert-butylhexane.

Key concepts

Longest Carbon Chain

When naming organic compounds, the longest carbon chain is a vital concept in IUPAC nomenclature. This chain forms the base of the compound's name and determines the prefix of the alkane name. To correctly identify the longest carbon chain, one must:

• Start by counting the maximum number of carbon atoms linked in a continuous, unbroken sequence.
• Consider all possible chains to ensure that the longest one is chosen.
• Keep in mind that the longest chain might not always be a straight line; it could zigzag or bend in the structure.

By accurately identifying the longest chain, the base name can be set correctly, such as choosing 'hexane' instead of 'pentane' when there are six continuous carbons rather than five.

Substituent Positioning

Substituent positioning is critical in ensuring chemical names reflect the true structure of specific compounds. Substituents are groups of atoms attached to the main chain and must be accurately placed. To do this, follow these guidelines:

• Identify all substituents along the carbon chain.
• Assign each substituent to the specific carbon it is attached to.
• Substituents should be named according to their chemical structure, like 'methyl', 'chloro', or 'ethyl'.

Correct positioning not only aids in deducing the structural formula but also in understanding the compound's potential reactivity and properties.

Chain Numbering

Chain numbering is a fundamental aspect to correctly name organic molecules. It ensures that substituents receive the lowest possible numbers, reducing any ambiguity. Here’s how to approach it:

• Begin numbering from the end of the chain closest to a substituent.
• If two substituents are equidistant, give preference to alphabetical order or the higher priority group as per IUPAC rules.
• Re-numbering may be necessary if initial counting doesn’t yield the smallest numbers.

Accurate numbering helps eliminate confusion and ensures that the chemical name is precise and accepted universally.

Substituent Priority

Substituent priority plays a major role in determining the final naming of compounds. Prioritizing substituents correctly is key to following proper IUPAC nomenclature rules:

• Substituents are ranked based on a standard set of rules, such as electronegativity and atomic number.
• Functional groups can often trump simpler alkyl chains in priority.
• When choosing which substituent to give the lowest number, priority can influence which end of the carbon chain you start numbering from.

By understanding and applying substituent priority, the naming of compounds achieves an order respected by chemists worldwide, facilitating clearer communication and fewer mistakes.