Question

Which of the following is the correct IUPAC name? (1) 3-methyl-2-ethyl pentane (2) 3-pentyne (3) ethyl-1-butene (4) Hexabromo ethane

Short answer

3-pentyne

Step-by-step solution

Identify Functional Group and Longest Carbon Chain

Identify whether the compound contains any functional groups and determine the longest continuous carbon chain in the structure. For example, in option (1), the chain should be analysed with branches in mind.

Check for Proper Numbering

Ensure that the carbon chain is numbered in a way that the functional groups and substituents get the lowest possible numbers. Assess each option accordingly.

Ensure Alphabetical Order of Substituents

Verify that the substituents are named and organised in alphabetical order when precede the name of the parent chain.

Verify Functional Group Prioritisation

Check that functional groups are properly prioritised. For instance, alkenes, alkynes, alcohols, etc. need proper recognition in the naming.

Comparing the Options

Compare each option against IUPAC rules as determined in previous steps.For example, in option (1) 3-methyl-2-ethyl pentane, the numbering does not provide lowest locants; it should be named as 2-ethyl-3-methylpentane if it were correct.Option (2) 3-pentyne correctly names an alkyne but verify existence.Option (3) ethyl-1-butene does not follow as there should be a numeric identifier for ethyl's position.Option (4) Hexabromo ethane incorrectly names multiple bromines without proper locant and prefix usage.

Find the Correct Option

Based on validation against IUPAC norms, the correct name can be derived. Option (2) is correct since 3-pentyne correctly follows the rules of proper carbon chain naming and functional group (alkyne) prioritisation.

Key concepts

Functional Groups

In organic chemistry, functional groups are specific groups of atoms within a molecule that determine the molecule's overall chemical properties and reactions.
Common functional groups include alcohols, carboxylic acids, amines, alkenes, and alkynes.
Identifying the functional group is a crucial step in the IUPAC naming process as these groups often take priority when numbering the carbon chain.
For example in the exercise solution, option (2) 3-pentyne, the functional group is the alkyne (triple bond).

Carbon Chain

The carbon chain refers to the longest continuous sequence of carbon atoms in an organic molecule.
Determining the longest carbon chain is a key step in IUPAC nomenclature because the base name of the molecule depends on it.
For instance, in option (1) of the exercise, we look for the longest chain, which is 'pentane' with five carbons.
Always include any carbon atoms that belong to the functional groups, ensuring the chain includes the highest priority groups when possible.

Substituents

Substituents are groups of atoms attached to the main carbon chain but are not part of the main chain itself.
Common examples include methyl (-CH3), ethyl (-C2H5), and bromo (Br-).
In the IUPAC naming process, substituents are named and numbered based on their position on the main carbon chain.
They should be listed in alphabetical order before the name of the parent chain.
For example in the exercise, option (1) 3-methyl-2-ethyl pentane should rather be named 2-ethyl-3-methylpentane to comply with the alphabetical rule.

Chemical Naming Rules

IUPAC nomenclature involves specific rules to ensure each compound's name is unique and informative.
Follow these main steps:

• Identify the longest carbon chain.
• Identify and prioritize functional groups.
• Number the chain to give substituents and functional groups the lowest possible locants.
• List substituents in alphabetical order.

For example, in the given exercise, option (3) 'ethyl-1-butene' is incorrect because there must be a numeric identifier for ethyl's position on the carbon chain.
Correctly applying these rules ensures accurate and systematic naming for all organic compounds.