Question

The positions of substituents along the hydrocarbon framework of a molecule are indicated by the ________ of the carbon atom to which the substituents are attached.

Short answer

The positions of substituents along the hydrocarbon framework of a molecule are indicated by the \(locant\) of the carbon atom to which the substituents are attached.

Step-by-step solution

Define substituents

Substituents are atoms or groups of atoms that replace hydrogen atoms in a hydrocarbon molecule. These substituents can be functional groups, halogens, or other hydrocarbons.

Understand the hydrocarbon framework

The hydrocarbon framework is the arrangement of carbon atoms in a molecule. It can be linear, branched, or cyclic. The carbon atoms are numbered to indicate their position in the molecule.

Identify the term that shows the position of substituents

The positions of substituents along the hydrocarbon framework of a molecule are indicated by the number of the carbon atom to which the substituents are attached. This number is called the locant. So, the correct answer is: The positions of substituents along the hydrocarbon framework of a molecule are indicated by the locant of the carbon atom to which the substituents are attached.

Key concepts

Substituents in Organic Molecules

In the realm of organic chemistry, a substituent is akin to a guest taking up residence in a house already occupied by a hydrocarbon framework. Specifically, a substituent is an atom or group of atoms that has replaced one or more hydrogen atoms in a hydrocarbon molecule. The presence of these substituents fundamentally alters the identity and properties of the original molecule. They are not mere visitors but integral components that contribute to the molecule's unique characteristics.

For example, consider a simple hydrocarbon like ethane, which consists only of carbon and hydrogen atoms. When a chlorine atom substitutes one hydrogen atom, the resulting molecule is chloroethane, a compound with distinct chemical and physical properties from its parent hydrocarbon. Substituents can be as simple as a single halogen atom (such as chlorine or bromine) or as complex as entire functional groups like hydroxyl (-OH), methyl (-CH3), or carboxyl (-COOH).

Understanding substituents is crucial for several reasons:

Hydrocarbon Framework

Imagine a hydrocarbon framework as a skeleton that gives an organic molecule its shape and structure. This framework is entirely composed of carbon atoms that can be connected in various configurations: chains (linear structures), branches (side chains off the main chain), and rings (cyclic structures). The connections between carbon atoms are bonds, which can be single, double, or triple, referring to the number of shared electron pairs.

The simplest hydrocarbons are alkanes with single bonds, alkenes with at least one double bond, and alkynes with at least one triple bond. The complexity increases as we introduce branches and rings, creating a vast array of possible structures. The orientation and sequence of these carbon atoms lay the foundation for the properties of the molecule and how it will interact with other molecules.

Importance of the Hydrocarbon Framework:

• Determines the basic shape and size of the molecule.
• Influences the molecule's physical properties, such as boiling and melting points.
• Affects the molecule's reactivity and the types of chemical reactions it can undergo.
• Forms the backbone onto which substituents are attached.

Locant

The concept of a locant in organic chemistry is akin to a house number in an address; it specifies the exact location of a substituent on the hydrocarbon framework. It's a numerical identifier that is assigned to a carbon atom within a molecule to indicate the position of a substituent attached to that carbon.

This positional numbering is crucial for unambiguous identification of compounds, as different locants can lead to different compounds with varying properties. For instance, 1-butanol and 2-butanol are isomers with the alcohol functional group (-OH) attached to different carbons within the four-carbon chain, resulting in distinct chemical behavior.

Locants are determined by rules set out by the International Union of Pure and Applied Chemistry (IUPAC) to ensure that every unique compound has a clear, universally understood name. The assignment of these locants follows specific guidelines to minimize the numbers used in the name and to provide clarity on the location of each substituent:

• Numbering starts from the end nearest to the first substituent.
• When multiple substituents are present, a comma separates individual locants.
• Di, tri, and tetra prefixes are used for identical substituents, followed by the locants.