Question

Name each of the following alkenes: a. \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) C=CCCCC c. \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

a. 2-hexene, b. propene, c. 2-pentene

Step-by-step solution

- Identify the Longest Carbon Chain

Find the longest continuous chain of carbon atoms that includes the double bond for each molecule. This determines the root name of the alkene.

- Number the Carbon Chain

Number the carbon atoms in the longest chain starting from the end nearest the double bond. This gives the double bond the lowest possible number.

- Name the Alkene a

Based on step 1 and 2, the longest chain for compound a is six carbons, so the root is 'hexene'. The double bond starts at the second carbon, so the name is '2-hexene'.

- Name the Alkene b

The compound b has a three-carbon chain with a double bond starting at the first carbon, hence the name is 'propene'.

- Name the Alkene c

For compound c, the longest chain containing the double bond is five carbons long, which yields the root 'pentene'. The double bond starts at the second carbon, resulting in the name '2-pentene'.

Key concepts

Alkene Nomenclature

Understanding the nomenclature of alkenes is essential for identifying and communicating the structure of compounds with carbon-carbon double bonds. The name of an alkene is composed of a prefix, which indicates the number of carbons in the longest chain, and a suffix, typically '-ene', which defines the molecule as an alkene.

Prefixes such as 'meth-', 'eth-', 'prop-', and 'but-', correspond to chains of one, two, three, and four carbon atoms, respectively. Additional elements or functional groups attached to the main carbon chain are named as prefixes to the alkene name and are ordered alphabetically.

Carbon Chain Identification

When determining the structure of an alkene, the first task is to identify the longest continuous carbon chain that includes the double bond. This chain forms the base of the compound's name and indicates the primary framework. If multiple chains of equal length exist, the chain with the most substituents takes precedence.

The ability to correctly identify the primary carbon chain is a fundamental skill in organic chemistry. Accurate identification ensures proper naming and better understanding of the compound's reactivity and properties.

IUPAC Naming System

The International Union of Pure and Applied Chemistry (IUPAC) has established a standardized naming system to provide a clear and universally understood language for chemical compounds. IUPAC rules for naming alkenes involve several steps: identifying the longest carbon chain, numbering it for the lowest locants for the double bond and substituents, and naming any substituents.

Applying this system results in unambiguous names that convey the structure and isomers of compounds. For example, the '2-' in '2-hexene' tells us that the double bond starts with the second carbon in the chain, distinguishing it from other hexenes with the double bond in different locations.

Double Bond Positioning

The location of the double bond in an alkene is indicated by a number placed before the root name. In accordance with the IUPAC system, the carbon chain should be numbered from the end closest to the double bond, ensuring that the double bond is given the lowest possible locant. This locant is crucial as it can define different isomers with unique properties.

For instance, '1-butene' and '2-butene' are two distinct compounds, with the numerical prefix indicating the starting point of the double bond relative to the end of the carbon chain nearest to it. Notably, the prefix is omitted if the double bond is positioned at the first carbon, as it is inferred.