Question

No number is needed to identify the location of the double bond in \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\), propene. Why?

Short answer

The double bond in propene is always between the first and second carbon atoms, regardless of the direction of numbering, due to the molecule's symmetry and the IUPAC naming rule of using the smallest possible numbers for double bond locations.

Step-by-step solution

Understand the Structure of Propene

Propene is a three-carbon alkene with a molecular formula of \(C_3H_6\). Recognize that the first carbon (\(\text{C-}1\)) is bonded to three hydrogens, and the second carbon (\(\text{C-}2\)) is double-bonded to the third carbon (\(\text{C-}3\)), which in turn is bonded to two hydrogens.

Identify the Double Bond and its Location

In propene, there's only one possible location for a double bond, that is between \(C-1\) and \(C-2\), or equivalently, between \(C-2\) and \(C-3\), given the molecule's symmetrical nature. In larger alkenes, numbering becomes critical to identify the position of the double bond.

Explain the IUPAC Naming Convention for the Double Bond

According to IUPAC naming rules, the double bond in alkenes is indicated by the smallest possible number. For propene, regardless of which end of the chain one starts counting from, the double bond will always be between the first and second carbons. Therefore, there is no need to signify the location with a number, as it is understood to be between carbons 1 and 2.

Key concepts

Alkene Naming Conventions

Alkene naming conventions are systematic methods for naming organic compounds that contain at least one carbon-carbon double bond. Alkenes are a subset of hydrocarbons and follow specific rules to ensure a clear and universal understanding of their structure. In general, the name of an alkene is derived from the number of carbon atoms in the longest carbon chain that contains the double bond, followed by the Greek suffix '-ene' to indicate the presence of a double bond.

For instance, when naming an alkene with three carbon atoms like the molecule in our exercise, propene, we start with the prefix 'prop-' (which signifies three carbon atoms) followed by the suffix '-ene' (which indicates the presence of a double bond). Therefore, the combined name is 'propene'. This naming convention also includes the identification of the position of the double bond when necessary, which leads us to our next concept, IUPAC nomenclature.

IUPAC Nomenclature

IUPAC nomenclature is the standardized system for naming chemical compounds established by the International Union of Pure and Applied Chemistry. The goal of IUPAC naming is to provide a unique and unambiguous name to each chemical compound. For alkenes, IUPAC rules state that the chain should be numbered from the end nearest the double bond, with the lowest possible number assigned to the double bond. This is critical in the naming process as it provides information about the structure of the molecule.

For example, if we had a longer-chain alkene, say a six-carbon chain with a double bond starting on the second carbon, it would be named 'hex-2-ene'. However, with smaller molecules like propene, as our exercise illustrated, there is only one possible position for the double bond. Therefore, there is no need for a number in the name, because there is no ambiguity about the location of the double bond.

Molecular Structure

The molecular structure of a compound refers to the three-dimensional arrangement of atoms within the molecule. Understanding this structure is pivotal in chemistry, as it influences the physical and chemical properties of the substance. In the context of alkenes, like propene, the structural focus is on the carbon atoms and how they are bonded.

A double bond in an alkene consists of one sigma bond (σ) and one pi bond (π), with the pi bond being responsible for the characteristic reactivity of alkenes. The presence of the double bond also imposes a planar geometry around it, restricting rotation and giving rise to cis-trans isomerism in alkenes with four different groups. However, in our example of propene, because the double bond is at the end of the chain, such isomerism is not applicable.

Double Bond Location

The location of the double bond within a molecule greatly affects the molecule's reactivity and properties. In alkenes, the double bond's position is indicated with the smallest possible number in the carbon chain, as stipulated by IUPAC nomenclature. This number goes before the suffix '-ene' in the name.

However, in the case of simple molecules such as propene, the symmetry of the molecule renders the numerical identification of the double bond unnecessary. As explained earlier, the double bond is understood to be between the first and second carbon atoms since counting from either end of the propene molecule leads to the same result. As alkenes become larger and more complex, proper double bond location identification becomes crucial for accurate naming and understanding of the molecule's reactivity and interaction with other substances.