Naming Other Substituents: Videos & Practice Problems

Alkyl halides are organic compounds where a halogen acts as a substituent on an alkyl group, with fluorine, chlorine, bromine, and iodine being named fluoro, chloro, bromo, and iodo, respectively. Their position on the parent chain, which could vary in size and structure, must be specified in the compound's name. Additionally, a phenyl group represents a benzene ring (

$C_6{H}_5$

) attached as a substituent to a larger alkane, alkene, or alkyne chain. The phenyl group's naming involves stating its position followed by the name of the parent chain. Understanding the nomenclature of these substituents is crucial for identifying the structure and composition of organic compounds.

Alkyl Halides

An alkyl halide consists of an alkyl group bonded to a halogen. In organic chemistry, halogens such as fluorine, chlorine, bromine, and iodine are treated as substituents on a carbon chain. When naming these compounds, the halogens are designated with specific prefixes: fluorine is referred to as fluoro, chlorine as chloro, bromine as bromo, and iodine as iodo.

When naming an alkyl halide, it is essential to identify the parent chain, which can vary in structure, such as being a six-membered chain or cyclic. The position of the halogen substituents must be indicated numerically based on their location on the parent chain. This systematic approach to naming ensures clarity in identifying the structure and composition of organic compounds, highlighting the role of halogens as substituents within the molecular framework.

Naming Other Substituents Example

To determine the systematic name of an alkane, the first step is to identify the longest continuous carbon chain, known as the parent chain. In this case, the longest chain consists of nine carbon atoms, which designates the alkane as nonane. The numbering of the carbon atoms begins from the end closest to any substituents to ensure the lowest possible numbers are assigned to them.

In this example, there are three substituents: a three-carbon substituent (isopropyl), a one-carbon substituent (methyl), and a bromine atom (bromo). The numbering from the right side gives us the following positions: the isopropyl group is located at carbon 6, the bromine at carbon 3, and the methyl group at carbon 5. The substituents are then named and listed in alphabetical order, regardless of their position numbers.

Thus, the systematic name of the compound is constructed as follows: starting with the substituents in alphabetical order, we have bromo at position 3, isopropyl at position 6, and methyl at position 5. Therefore, the complete name of the alkane is 3-bromo-6-isopropyl-5-methylnonane.

Name the following compound.

• A. 3-fluoro-1-ethylpentane
• B. 3-fluoro-1-ethylcyclopentane
• C. 1-ethyl-3-fluorocyclopentane
• D. 1-ethyl-4-fluorocyclpentane

Answer: (C)

Phenyl Group

The phenyl group is an important concept in organic chemistry, representing a benzene ring that acts as a substituent on a parent hydrocarbon chain. A benzene ring, with the molecular formula C₆H₆, consists of six carbon atoms arranged in a cyclic structure with alternating double and single bonds. When the phenyl group is attached to a longer carbon chain, it is denoted as C₆H₅, since one hydrogen atom is removed to allow for bonding with the parent chain.

The parent chain can be an alkane, alkene, or alkyne, provided it contains more than six carbon atoms. The skeletal structure of benzene features a six-membered ring where five of the carbon atoms are bonded to hydrogen atoms, while the sixth carbon is connected to the longer carbon chain, thus forming the phenyl group.

When naming compounds that include a phenyl group, the convention is to indicate the position of the phenyl substituent followed by the name of the parent chain. This systematic approach helps in identifying the structure and properties of the compound effectively. Understanding the phenyl group is crucial for studying aromatic compounds and their reactivity in various chemical reactions.

Naming Other Substituents Example

To systematically name the given alkane, we first identify the longest carbon chain, which consists of eight carbon atoms, making it an octane. In this case, since the carbon chain exceeds six carbons, it serves as the parent chain, while the benzene ring acts as a substituent. The benzene substituent is referred to as a phenyl group.

Next, we need to consider the substituents attached to the parent chain. There are two methyl groups attached to carbon number 5 of the octane chain. When numbering the carbon chain, we start from the end closest to the substituent, which allows us to assign the correct positions to the substituents. Thus, we number the carbons as follows: 1, 2, 3, 4, 5, 6, 7, 8.

Since we have two identical methyl substituents on carbon 5, we denote this as 5,5-dimethyl. It is important to list substituents alphabetically; therefore, the methyl groups (M) come before the phenyl group (P) in the final name. The complete systematic name of the compound is thus 5,5-dimethyl-1-phenyl-octane.

Draw a structure for the following alkane: 3-tert-butyl-1-phenylheptane.

• A.
• B.
• C.
• D.

Answer: (D)

Here’s what students ask on this topic:

What is the process of naming alkyl halides?

The process of naming alkyl halides, which are compounds consisting of an alkane with one or more halogen atoms (fluorine, chlorine, bromine, or iodine) replacing a hydrogen atom, follows the rules of the IUPAC (International Union of Pure and Applied Chemistry) nomenclature:

1. Identify the Longest Carbon Chain: Find the longest continuous carbon chain that includes the carbon atom bonded to the halogen. This chain determines the parent name of the compound.
2. Number the Chain: Number the carbon atoms in the longest chain starting from the end nearest to the halogen atom. This ensures that the halogen gets the lowest possible number.
3. Name the Halogen as a Prefix: The halogen is named using its prefix form: fluorine becomes fluoro-, chlorine becomes chloro-, bromine becomes bromo-, and iodine becomes iodo-.
4. Assign a Number to the Halogen: The number of the carbon to which the halogen is attached is placed in front of the halogen prefix.
5. Combine the Names: The final name is constructed by combining the numbered halogen prefix with the name of the parent alkane. If there are multiple halogens or different substituents, list them in alphabetical order, using prefixes like di-, tri-, etc.

How to name alkyl halides using IUPAC?

To name alkyl halides (haloalkanes) using the IUPAC system, follow these steps:

1. Identify the Longest Carbon Chain: Find the longest continuous carbon chain in the molecule. This chain will serve as the base name of the alkyl halide, using the standard alkane naming (methane, ethane, propane, etc.).
2. Number the Chain: Assign numbers to the carbon atoms in the main chain starting from the end nearest to the halogen atom. This ensures that the halogen gets the lowest possible number.
3. Name the Halogen: The halogen is treated as a substituent and is given a prefix based on its element: fluorine (fluoro-), chlorine (chloro-), bromine (bromo-), or iodine (iodo-).
4. Locate the Halogen: Use the number from step 2 to indicate the position of the halogen on the main chain. If there are multiple halogens, use prefixes such as di-, tri-, tetra-, etc., to indicate the quantity, and list them in alphabetical order if different types are present.
5. Combine the Elements: Write the name as one word, placing the halogen prefix(es) before the base name of the alkane, and use hyphens to separate numbers.