Question

What is wrong with the following names? Give the correct name for each compound. a. 2 -ethylpropane b. 5 -iodo- 5,6 -dimethylhexane c. cis-4-methyl-3-pentene d. 2 -bromo- 3 -butanol

Short answer

a. The correct name is 2-methylbutane. b. The correct name is 1-iodo-2,3-dimethylpentane. c. The correct name is cis-3-methyl-2-pentene. d. The correct name is 1-bromo-2-butanol.

Step-by-step solution

a. 2-ethylpropane

Here, the problem is that ethyl group is not at position 2 in propane since propane only has 3 carbons. The correct name for this compound should be 2-methylbutane, as it has four carbons in the main chain with a methyl group at the second carbon.

b. 5-iodo-5,6-dimethylhexane

This name implies a main chain of hexane (6 carbons), with an iodo substituent at position 5, and two methyl substituents at positions 5 and 6. However, the numbering is incorrect, as the substituents should be numbered from lowest to highest. The correct name for this compound is 1-iodo-2,3-dimethylpentane.

c. cis-4-methyl-3-pentene

In this case, the given name correctly indicates the presence of a double bond, a methyl substituent, and the 'cis' configuration. However, the numbering is incorrect, as the double bond should be assigned the lowest possible number. The correct name for this compound is cis-3-methyl-2-pentene.

d. 2-bromo-3-butanol

This name suggests that there is a bromo substituent at position 2 and a hydroxyl group at position 3 on a butane chain. However, the hydroxyl group should have priority in numbering as it is a functional group. The correct name for this compound is 1-bromo-2-butanol.

Key concepts

IUPAC Naming Rules

The IUPAC (International Union of Pure and Applied Chemistry) naming rules are essential for providing a systematic way to name chemical compounds. These rules ensure that each compound has a unique name, which prevents confusion.

• Main Chain Selection: Choose the longest continuous chain of carbon atoms as the parent hydrocarbon.
• Numbering: Assign numbers from either end of the chain to give substituents and unsaturations (like double bonds) the lowest possible numbers.
• Functional Groups: Identify the functional groups and number them according to their naming priority, which I'll discuss further below.
• Substituents: Name and number any substituents or branches attached to the main chain using prefixes like methyl, ethyl, etc.

It's important to apply these rules systematically to avoid mistakes, as seen in our exercise.

Alkane Structure

Alkanes represent the simplest form of hydrocarbons containing single bonds between carbon atoms, commonly referred to as "saturated hydrocarbons." Structurally, they can be categorized by the number of carbon atoms and the arrangement in the chain.

• Linear Alkanes: Such as propane or butane, they have straight carbon chains.
• Branched Alkanes: These have branches like a methyl group attached to the main chain, thereby changing the name (e.g., methylbutane instead of ethylpropane).

A crucial part of identifying alkane structures is choosing the longest carbon chain, which becomes the main chain when applying IUPAC rules. This affects the naming, as the main chain length determines the base name of the compound.

Substituent Positioning

Correct positioning of substituents on the carbon chain is vital for proper naming. In the IUPAC system, numbers are used to specify the position of each substituent. These positions are crucial in ensuring the compound's name is correct and informative.

• Number from the End: Always number the carbon chain from the end nearest to the first substituent.
• Avoid Unnecessary Positions: Ensure substituents are given the lowest possible numbers to avoid duplicating or unnecessary numbers in the name.

In our exercise, numbers were incorrectly assigned, which led to incorrect names. This highlights why accurate substituent positioning is key for correctly applying IUPAC nomenclature.

Functional Group Priority

Functional groups are specific groups of atoms within molecules that dictate how the compound behaves in chemical reactions. When naming compounds, certain functional groups take precedence in the numbering of the carbon chain.

• Priority in Numbering: Functional groups, like hydroxyl (-OH) or halogens (e.g., bromo), often have numbering precedence over alkyl substituents because they define the main functional behavior of the compound.
• Affect on Naming: The highest priority group gets the smallest number possible, which sometimes means renaming the compound to correctly identify its properties.

This concept was applied incorrectly in exercise 4, leading to a re-evaluation of the naming to 1-bromo-2-butanol. Understanding the priority order is essential in organic nomenclature.