Naming alkanes with alkyl groups | Organic chemistry | Khan Academy

Summary

In this lesson, the process of naming organic molecules, specifically alkanes, is explored in detail. The focus is on identifying the longest carbon chain, determining the proper prefix for the number of carbon atoms, and correctly labeling substituents such as methyl and propyl groups. The concept of numbering the carbon chain to give the lowest possible number to the substituent is emphasized, along with examples like 2-propylheptane and 6-butyltetradecane. The lesson highlights the importance of applying IUPAC naming conventions logically to ensure clear and precise molecular identification.

Takeaways

• 😀 The first step in naming a branched alkane is to identify the longest continuous chain of carbon atoms, which forms the backbone of the molecule.
• 😀 The number of carbon atoms in the longest chain determines the base name of the alkane (e.g., nonane for 9 carbons, heptane for 7).
• 😀 Substituent groups attached to the main chain are named based on their number of carbon atoms (e.g., methyl for 1 carbon, ethyl for 2).
• 😀 Number the carbon atoms in the longest chain starting from the end nearest to the substituent group to give the lowest possible number to the substituent.
• 😀 A methyl group attached to a chain gives the prefix 'methyl' (e.g., 3-methylnonane), while a propyl group gives 'propyl' (e.g., 2-propylheptane).
• 😀 In naming molecules with multiple branches, ensure the positions of the substituents are listed in numerical order.
• 😀 The prefix '-ane' indicates an alkane with all single bonds (e.g., nonane, heptane, tetradecane).
• 😀 If a molecule can be named in multiple ways, choose the numbering that provides the lowest possible number for the first substituent.
• 😀 The molecule's name is structured as the longest chain name + the substituent(s) + their position(s) on the chain (e.g., 6-butyltetradecane).
• 😀 The naming system ensures clarity and avoids ambiguity, such as choosing between 9-butyltetradecane or 6-butyltetradecane, where the latter is preferred for the lowest substituent number.

Q & A

What is the first step in naming a molecule with branches?

The first step is to identify the longest chain of carbon atoms in the molecule, as this will form the backbone of the molecule and determine its base name (e.g., 'nonane' for a 9-carbon chain).

How do you name a branch or group attached to the longest chain?

The group or branch is named based on its structure. For example, a one-carbon group is called 'methyl,' a two-carbon group is 'ethyl,' and a three-carbon group is 'propyl.' These are added as prefixes to the name of the main chain.

What is the rule for numbering the longest carbon chain in a molecule?

The longest chain is numbered such that the substituent (branching group) is attached to the lowest possible numbered carbon atom. If there are two possible numbering directions, you choose the one where the substituent gets the lower number.

Why is the molecule 'methylnonane' insufficient on its own?

Simply calling the molecule 'methylnonane' is insufficient because it doesn’t specify the position of the methyl group on the nonane chain. Therefore, you must number the chain to indicate where the methyl group is attached, such as '3-methylnonane' or '4-methylnonane,' based on the lowest position.

How do you determine the position of a substituent like the methyl group?

You determine the position of a substituent by numbering the longest chain from both ends and selecting the numbering that gives the substituent the lowest number. This helps ensure clarity in naming molecules with multiple substituents.

What does the prefix 'non-' indicate in the molecule 'methylnonane'?

The prefix 'non-' indicates that the molecule has a chain of 9 carbon atoms. The suffix '-ane' shows that all the bonds between the carbon atoms are single bonds, meaning it's an alkane.

What is the correct name for a molecule with a three-carbon propyl group attached to the second carbon of a heptane chain?

'2-propylheptane' is the correct name for this molecule. 'Heptane' indicates a 7-carbon chain, and 'propyl' signifies a three-carbon group attached at the second position of the chain.

What happens if you try to name a molecule as '9-butyltetradecane'?

Naming the molecule '9-butyltetradecane' would be incorrect because you should always number the chain from the direction that gives the substituent (in this case, the butyl group) the lowest number. The correct name is '6-butyltetradecane,' as the butyl group is at the 6th position if the chain is numbered in the optimal direction.

What does the suffix '-ane' tell you about the molecule?

The suffix '-ane' tells you that the molecule is an alkane, meaning it consists only of single bonds between carbon atoms.

How do you name a molecule with a four-carbon butyl group attached to the sixth carbon of a tetradecane chain?

'6-butyltetradecane' is the correct name. 'Tetradecane' indicates a 14-carbon chain, and the butyl group (a four-carbon group) is attached to the 6th carbon of the chain.