Question

Name the following alkenes. (a) \(\mathrm{CH}_{2}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

(a) \(\mathrm{CH}_{2}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\) Answer: (a) 1-methylpropene (b) 2,3-dimethylbutene (c) 2-pentene

Step-by-step solution

Identify the parent chain

For each alkene, the parent chain is the longest continuous chain of carbon atoms that contains the double bond(s). This chain will form the base name of the alkene. (a) \(\mathrm{CH}_{2}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Parent chain: C2 (b) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Parent chain: C4 (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\) Parent chain: C5

Indicate the position of the double bond

The position of the double bond(s) is indicated by the lower numbered carbon atom participating in the double bond. In each example, we'll identify the position of the double bond. (a) \(\mathrm{CH}_{2}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Double bond position: carbon 1 (b) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Double bond position: carbon 2 (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\) Double bond position: carbon 2

Name the alkenes

Finally, we'll name the alkenes using the IUPAC rules. The parent chains provide the base names, to which we add the "ene" suffix and the position of the double bond. (a) \(\mathrm{CH}_{2}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Name: 1-methylpropene (b) \(\left(\mathrm{CH}_{3}\right)_{2}-\mathrm{C}=\mathrm{C}-\left(\mathrm{CH}_{3}\right)_{2}\) Name: 2,3-dimethylbutene (c) \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\) Name: 2-pentene

Key concepts

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) nomenclature is a systematic method of naming organic chemical compounds. When it comes to alkenes, the name reflects their molecular structure, including the length of the carbon chain, the position of the double bonds, and the presence of any substituents (additional groups or atoms attached to the carbon chain).

The process begins by identifying the longest carbon chain that contains the double bond, known as the parent chain. Its length determines the root of the compound's name. For instance, a two-carbon chain is named 'eth-', a three-carbon 'prop-', and so on. The suffix '-ene' is added to denote the presence of a double bond. The location of the double bond is indicated by a number placed before the alkene name, representing the lower numbered carbon atom involved in the double bond. If there are substituents, their names and positions on the chain are also included in the compound name, using prefixes like 'methyl-' for CH3 or 'ethyl-' for C2H5, and numbers to show their positions.

Organic Chemistry

Organic chemistry is the branch of chemistry that studies carbon-based compounds and their reactions. It encompasses a vast array of substances including, but not limited to, alkenes, which are hydrocarbons that contain at least one carbon-to-carbon double bond. These compounds are ubiquitous in nature and serve as building blocks for a myriad of organic substances, from simple hydrocarbons to complex biomolecules.

Understanding the structure and nomenclature of organic molecules, such as alkenes, is of paramount importance for discussing their properties and reactions. Organic molecules can differ greatly in size and complexity, and their names provide chemists with detailed information about their structures. The step-by-step procedure utilized in naming alkenes, as outlined in textbook solutions, is a practical application of organic chemistry principles.

Double Bond Configuration

Alkenes are characterized by their carbon-to-carbon double bonds. The double bond has both a sigma (\f\(\b\fsigma\fb\f\)) bond, formed by the overlap of orbitals directly between the bonding atoms, and a pi (\f\(\b\fpi\fb\f\)) bond, resulting from the side-to-side overlap of p-orbitals. This pi bond is what gives the alkene its unique reactivity.

Double bonds are also crucial in determining the compound's geometry. They introduce rigidity because the atoms involved in a double bond cannot rotate freely around the bond axis, leading to distinct cis and trans isomers if substituents are present. For the IUPAC naming, however, the focus is on the bond's position within the molecule. The double bond's location is a critical aspect of the alkene's identity and affects both its physical properties and its chemical reactions.

Alkene Structural Formula

The structural formula of an alkene shows the arrangement of atoms within the molecule. It details not just the carbon skeleton but also the specific placement of the double bonds and any substituents attached to the carbon chain.

For example, the structural formula of an alkene enables us to visualize the molecule's geometry, such as whether the substituents are on the same side (cis) or opposite sides (trans) of the double bond. When writing the IUPAC name, one must consider the structural formula to properly identify the longest chain, the correct position of the double bond, and the location and type of substituents. A full understanding of the structural formula is fundamental for naming alkenes and predicting their chemical behavior.