Do you ever get impatient waiting for your fruit to ripen? Don't worry, there's an easy solution! Just pop your unripe fruit, like kiwis or avocados, in a paper bag with a banana. Bananas release a hormone called ethene which speeds up the ripening process. Ethene is actually a type of organic compound called an alkene.

So, what are alkenes exactly? They're organic compounds that have a double bond between two carbon atoms. The general formula for alkenes is CnH2n. In this article, we'll explore alkene nomenclature and isomerism, as well as the properties of alkenes. We'll also dive into how alkenes are produced and how they can be tested for. But how do alkenes compare to alkanes, another type of organic compound? We'll explore the differences between the two. And to wrap things up, we'll give some examples of alkenes in action. Overall, alkenes play an important role in organic chemistry, and understanding their properties and uses can be really helpful.

Alkene definition

Alkenes, which are also known as olefins, are a type of hydrocarbon. Hydrocarbons are organic molecules composed solely of carbon and hydrogen atoms. What makes alkenes different from other hydrocarbons is that they contain at least one carbon-carbon (C=C) double bond, making them unsaturated.

In simpler terms, alkenes are composed of carbon and hydrogen atoms with at least one C=C double bond. They are used in the production of polymers like polystyrene and PVC, and can also be found in everyday products such as antifreeze and paints. Understanding the properties and uses of alkenes is important in the fields of chemistry and engineering.

Alkene general formula

Alkenes are part of a homologous series, which is defined by their C=C double bond. The general formula for alkenes with just one C=C double bond is CnH2n, where n represents the number of carbon atoms in the molecule. To determine the number of hydrogen atoms in an alkene, simply double the number of carbons.

For example, let's consider propene, an alkene with 3 carbon atoms. Using the general formula CnH2n, we can see that n equals 3 for propene. Therefore, propene has (2 x 3) = 6 hydrogen atoms. To learn more about homologous series and their properties, check out resources on organic compounds. Understanding these concepts is crucial in the study of organic chemistry.

Alkene nomenclature

Alkenes are named using the suffix -ene and standard nomenclature rules. A number between the root name and the suffix indicates the position of the double bond within the chain, just like numbers are used to show the position of other side chains or functional groups.

Let's consider an example.

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We can see that this molecule contains a carbon backbone four atoms long, a methyl side chain, and a C=C double bond. This means that it takes the prefix methyl-, the suffix -ene and the root name -but-.To show the position of the side chain and the C=C double bond, we use numbers. If we number the carbons from both directions, either the methyl group is attached to carbon 3 and the double bond is joined to carbon 1, or the methyl group is joined to carbon 2 and the double bond is attached to carbon 3. If we add those numbers up, we get 3 + 1 = 4 or 3 + 2 = 5. Remember the ‘lowest numbers’ rule - we want any constituents on the molecule to be attached to the lowest-numbered carbons possible. So in this case, we number the carbon atoms from right to left.

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For more information on nomenclature, see Organic Nomenclature.

Alkene isomerism

Alkenes show three types of isomerism.

Chain isomerism. Position isomerism. Geometric isomerism

Chain isomerism

Chain isomerism is a type of structural isomerism. Structural isomers are molecules with the same molecular formula but different structural formulae. In the case of chain isomers, these molecules have different arrangements of the hydrocarbon chain. They might have side chains in different places, for example, or perhaps side chains of different lengths. For example, 3-methylbut-1-ene and pent-1-ene are chain isomers - count the number of carbon atoms to make sure.

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Positional isomerism

Positional isomerism is also a type of structural isomerism. In this case, the functional group differs in its position within the carbon chain. For example, but-1-ene and but-2-ene are position isomers.

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Geometric isomerism

Geometric isomerism is a type of stereoisomerism that occurs when two different groups are attached to each of the atoms involved in a double bond. The double bond restricts rotation of the molecule, resulting in different spatial arrangements of atoms. Stereoisomers have the same structural formula but different spatial arrangements.

To name geometric isomers, each carbon in the C=C double bond is examined, and the two atoms directly attached to it are analyzed. The group containing the atom with a higher molecular mass is assigned first priority. If the highest priority groups from each carbon are on the same side of the double bond, the molecule is known as the Z-isomer. If the highest priority groups are on opposite sides of the double bond, the molecule is known as the E-isomer. The E- and Z-isomers are also referred to as trans- and cis-isomers, respectively. For instance, consider but-2-ene. Each carbon atom involved in the C=C double bond is joined to a -CH3 group and a hydrogen atom. In both cases, the -CH3 group takes higher priority. As a result, but-2-ene exhibits geometric isomerism, with the two possible isomers being cis-but-2-ene (Z-but-2-ene) and trans-but-2-ene (E-but-2-ene).

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It should be noted that in the molecule on the right, the -CH3 groups are on the same side of the double bond, making it the Z-isomer.

When combining structural isomerism and stereoisomerism, the number of potential isomers of alkenes increases rapidly as the length of the carbon chain increases. For instance, C5H10 has only six alkene isomers, while C12H24 has 2,281 alkene isomers, and C31H62 has 193,706,542,776 alkene isomers.

Alkenes share some similarities with alkanes in terms of their properties. They are similar in mass and contain only non-polar bonds, resulting in the presence of only van der Waals forces between molecules. However, the presence of the C=C double bond makes alkenes more reactive than alkanes. The properties of alkenes include their solubility, melting and boiling points, shape, and reactivity. To learn more about stereoisomers and assigning priority, refer to resources on isomerism. It is important to note the differences between alkanes and alkenes, which will be discussed in more detail later in the article.

Solubility

Alkenes are insoluble in water. Because they contain only non-polar bonds, they cannot bond to polar water molecules. However, they are soluble in other organic solvents.

Melting and boiling points

Alkenes have relatively low melting and boiling points. This is because the weak van der Waals forces between molecules do not require much energy to overcome. As the number of carbon atoms increases, boiling point increases, and as branching of the molecule increases, boiling point decreases.

For example, but-1-ene (C4H8) has a higher boiling point than propene (C3H6) as it has a longer carbon chain with more atoms.

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In addition, but-1-ene has a higher boiling point than methylpropene (also C4H8). Although they have the same number of carbon atoms, methylpropene is branched, and so has weaker intermolecular forces between molecules.

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For more information on the effect of van der Waals forces on physical properties, see Alkanes.

Shape

Alkenes are trigonal planar molecules. They have an angle of roughly 120° between each bond.

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Reactivity

Alkenes are relatively reactive due to the C=C double bond being an area of high electron density and attracting electrophiles.

Electrophiles are molecules or atoms that contain a positive ion or δ+ atom with an empty orbital that can accept an electron pair. Therefore, alkenes frequently undergo electrophilic addition reactions. Examples of this include hydration with steam and phosphoric acid catalyst to form an alcohol, reaction with a hydrogen halide to form a halogenoalkane, reaction with a halogen to form a halogenoalkane, and hydrogenation in the presence of a nickel catalyst to form an alkane.

Alkenes also react in other ways, such as oxidation with KMnO4 to form varying products and addition polymerization to form polymers. For more information on electrophilic addition and oxidation reactions, visit resources on reactions of alkenes. Addition polymerization is covered in greater depth in polymerization reactions.

Producing alkenes

We'll now move on to learn about how you produce alkenes. This is done in a variety of different ways: Cracking of alkanes. Elimination of a halogenoalkane using hot ethanolic NaOH or KOH. Dehydration of an alcohol, using hot Al2O3 or a concentrated acid. You'll find all the necessary information you need to know about these reactions in Cracking (Chemistry), Elimination Reactions, and Reaction of Alcohols respectively.

Testing for alkenes

Testing for alkenes relies on an electrophilic addition reaction, like the ones we mentioned above. The process is simple: Shake an unknown substance with orange-brown bromine water (Br2). If the solution decolourises, there is an alkene present. This is because the bromine water adds to the double bond, forming a dihalogenoalkane.

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Alkanes and alkenes

Throughout this article, we have discussed alkanes and their similarities and differences with alkenes. Alkanes do not contain any C=C double bonds, but instead contain only C-C and C-H single bonds.

Some examples of alkenes include ethene (C2H4), which is the simplest alkene and the most produced organic compound globally. It is used in the production of poly(ethene) and can be turned into ethanol. Other alkenes, such as butene (C4H8), are used in fuels, paints, and detergents. Alkenes are also used to make polymers like Teflon and PVC, as well as alkene derivatives used to produce anti-freeze. For more information on alkenes and their properties and reactions, refer to additional resources on the topic.

Key takeaways about alkenes include:

• Alkenes are unsaturated hydrocarbons containing one or more carbon-carbon double bond (C=C).
• They are named using the suffix -ene and standard nomenclature rules.
• Alkenes can exhibit position, chain, and geometric isomerism due to their C=C double bond. Geometric isomers are named using E/Z or cis/trans notation.
• Alkenes have similar solubility and boiling points to comparable alkanes, but are more reactive due to their C=C double bond.
• Alkenes can undergo electrophilic addition reactions and can be tested for using an electrophilic addition reaction involving bromine water (Br2).
• Examples of alkenes include ethene (C2H4) and butene (C4H8). They are used in a variety of products, from polymers to paints.

For additional information on alkenes and their properties, consult relevant resources on the topic.

Alkenes

What is an alkene?

An alkene, also known as an olefin, is an unsaturated hydrocarbon containing one of more carbon-carbon double bonds (C=C).

Are alkenes saturated or unsaturated?

Alkenes are unsaturated hydrocarbons.

What are alkenes used for?

Alkenes are used to make polymers like polystyrene and PVC, and are found in products such as antifreeze and paints.

What are the differences between alkenes and alkanes?

Alkanes are saturated hydrocarbons and contain only C-H and C-C single bonds, whereas alkenes are unsaturated and also contain one or more C=C double bonds.

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