Question

What is an alkene? What structural feature characterizes alkenes? Give the general formula for alkenes.

Short answer

An alkene is an unsaturated hydrocarbon containing a carbon-carbon double bond (\(C=C\)). The presence of this double bond, comprising one sigma bond and one pi bond, characterizes alkenes and results in restricted rotation around the bond, leading to geometric isomers (cis and trans configurations). The general formula for alkenes is \(C_nH_{2n}\), where n represents the number of carbon atoms in the molecule.

Step-by-step solution

Definition of Alkene

An alkene is a type of hydrocarbon that contains a carbon-carbon double bond (\(C=C\)). Alkenes are unsaturated hydrocarbons, meaning they have fewer hydrogen atoms compared to the maximum possible number for a given carbon chain.

Structural Feature Characterizing Alkenes

The key structural feature that characterizes alkenes is the presence of a carbon-carbon double bond (\(C=C\)). This double bond consists of one sigma bond and one pi bond. Due to the presence of the double bond, alkenes have restricted rotation around the bond, resulting in different geometric isomers (cis and trans configurations).

General Formula for Alkenes

The general formula for alkenes is \(C_nH_{2n}\), where n refers to the number of carbon atoms present in the molecule. This formula represents the relationship between the number of carbon atoms and the number of hydrogen atoms within an alkene.

Key concepts

Hydrocarbon

Let's begin by exploring hydrocarbons, the fundamental organic compounds from which alkenes are derived. Hydrocarbons are compounds consisting of only carbon and hydrogen atoms. These are the simplest forms of organic compounds and serve as the building blocks for more complex molecules. They're found in many sources, like crude oil and natural gas, and are used to create a variety of materials including fuels, plastics, and pharmaceuticals. Hydrocarbons can be categorized based on their structure and the types of bonds between carbon atoms; they come in various forms, including alkanes with single bonds, alkenes with double bonds, and alkynes with triple bonds.

Carbon-Carbon Double Bond

A distinctive feature of alkenes is the carbon-carbon double bond, denoted as \(C=C\). This bond consists of one sigma bond, which allows for the free rotation of carbon atoms, and one pi bond, which restricts this rotation. The pi bond is formed by the sideways overlap of p orbitals across the two carbon atoms. This restriction leads to unique properties such as the potential for geometric isomerism, where molecules have the same formula but different spatial orientations, commonly referred to as 'cis' or 'trans' isomers. The presence of a carbon-carbon double bond also affects the alkene’s reactivity, making it susceptible to chemical reactions such as addition reactions where atoms are added across the double bond.

Unsaturated Hydrocarbons

Alkenes are part of a broader category known as unsaturated hydrocarbons. Unlike saturated hydrocarbons, which have only single bonds between carbon atoms (alkanes), unsaturated hydrocarbons contain one or more multiple (double or triple) bonds. This unsaturation refers to the hydrocarbon molecules' ability to add more hydrogen atoms or other atoms in a reaction. Unsaturated hydrocarbons tend to be more reactive than their saturated counterparts due to the presence of highly reactive multiple bonds. These reactive sites allow for various chemical reactions, making alkenes versatile intermediates in organic synthesis.

General Formula for Alkenes

The general formula for alkenes is an essential tool in understanding these hydrocarbons' makeup. It’s given by \(C_nH_{2n}\), where 'n' represents the number of carbon atoms in the molecule. This formula indicates that for every carbon atom in an alkene, there should be two hydrogen atoms. For example, ethene, the simplest alkene, has the formula \(C_2H_4\). It is critical to note that this formula only applies to alkenes without any rings or branches in their structure. Alkenes with additional complexities may deviate from this general formula, but for straight-chain alkenes, the \(C_nH_{2n}\) rule is a quick way to identify or construct the molecular formula.