The Chemistry of Alkenes: Structure, Naming, Uses and Reactions
What are Alkenes?
Alkenes are one of the most important, useful molecular families in all of organic chemistry. They're characterised by a covalent carbon-carbon double bond. The nature of this bond, which will be discussed in more detail later on, makes it much more reactive than a normal single covalent bond and because of this, alkenes can undergo many reactions that saturated hydrocarbons (compounds containing only single carbon bonds, like alkanes) can't. This article explores the structure of alkenes, the general formula used to describe them, how they're named, their uses, and some of the most common reactions that they undergo.
What's the Structure of Alkenes?
As mentioned before, alkenes are hydrocarbons. This means that they're comprised of a chain of carbon atoms bonded together, with each carbon atom bonded to hydrogen atoms to make a total of four bonds per carbon. What differentiates alkenes from the standard family of hydrocarbons, alkanes, is that they contain one or more carbon-carbon double bonds.
Single covalent bonds are also known as sigma bonds. When an extra bond is added, forming a double bond, the second bond is known as a pi bond. The pi bond is much weaker than the sigma bond and breaks quite easily, which is why alkenes are much more reactive than their fellow hydrocarbons.
Another important feature of a double bond is that it doesn't allow for free rotation. Single covalent bonds can twist and flip, but double bonds are rigid. This means that alkenes can exhibit cis/trans isomerism, where the bulkiest group attached to each carbon atom participating in the double bond can either be on the same side (a cis isomer) or on opposite sides (a trans isomer).
What's the General Formula of Alkenes?
Hydrocarbon families can be described by general formulas, which dictate how many hydrogen atoms are present for each carbon atom. For mono-unsaturated alkenes, which have only one double bond, the general formula is CnH2n. In other words, the amount of hydrogen atoms is equal to twice the number of carbon atoms.
This rule can be proven by looking at the structures of common mono-unsaturated alkenes, such as ethene (C2H4) and propene (C3H6) which have twice the number of hydrogens as they do carbons. For polyunsaturated alkenes, that have more than one double bond, the general formula becomes more complicated. For each extra double bond, two hydrogens must be subtracted. For example:
- Two Double Bonds: CnH2n-2
- Three Double Bonds: CnH2n-4
- Four Double Bonds: CnH2n-6
These formulas can also be used to figure out the number of double bonds in a given alkene molecule from its molecular formula. For example, if you're given an alkene with the molecular formula C5H10, it's clear that only one double bond is present as the number of atoms follows the rule of monounsaturated alkenes, CnH2n. However, if your alkene has the formula C5H8, you can deduce that two double bonds are present as the ratio of carbons to hydrogens follows the CnH2n-2 rule.
Manipulating the alkene general formula like this can take a bit of practice, but once you understand it's a useful skill to have.
How Does the Naming of Alkenes Work?
Organic chemistry nomenclature, the rules used to name chemical compounds, can be complicated and confusing. Thankfully, the rules set out to name alkenes are fairly straight forward and can be arranged into five key steps.
Count the longest unbroken carbon chain that you can find. Just like with alkanes, the number of carbons dictates the prefix used in naming the molecule:
Number of Carbons
Count the number of double bonds. If the molecule has one double bond, then the suffix -ene is used. If there's two, -diene is used. For three, it's -triene, and so on.
Look for any substituents on the carbon chain. A substituent is any group coming off the chain that isn't a hydrogen. For example, there might be a CH3 group attached to the chain. In this case, the word methyl- would be put in front of the name of the parent alkene. A C2H5 group is named as ethyl and a C3H7 group is called a propyl group. Other common substituents include halogens (group 17 elements). If a fluorine atom is attached, the word fluro- is used. If it's chlorine, it's chloro-, if it's bromine, it's bromo-, and if it's iodine it's iodo-. Of course, there are hundreds of potential substituents that could be attached to a carbon chain, but in naming basic alkenes these are the most common.
Determine the numbering of the carbon chain. This is done by assigning the end of the chain closest to the double bond as carbonone and then numbering down the chain from there. In other words, the double-bonded carbons must have the lowest number possible. Once you've numbered each carbon you can assign a number to any substituent, for example 2-methyl or 4-chloro, and number the double bond. If the double bond was on the third carbon from the end of a seven carbon chain you would name it hept-3-ene or 3-heptene (either are acceptable).
Focusing on the double bond, determine whether the molecule might exhibit cis/trans isomerism To do this, check to see whether each of the carbon atoms participating in the bond has two different groups attached to it. For example, ethene doesn't give cis/trans isomers because both carbon atoms only have hydrogens in them. 2-Butene, however, does have the possibility of isomerism, because the doubly bonded carbons both have a methyl group and a hydrogen group attached. If no isomerism is possible, you're finished!
If cis/trans isomerism is possible, look carefully at the groups on either side of the double bond. If the highest priority groups are on the same side, the prefix cis- should be added. If they're on the opposite sides, trans- should be used. To determine the highest priority group, look at the atomic numbers of the atoms bonded directly to each carbon. The atom with the higher atomic number is the higher priority; for example, in the case of 2-butene, the methyl group is higher priority that the hydrogen group because carbon has a higher atomic number than hydrogen. If both atoms are the same, then continue down the chain until there's a point of difference. If there's more than one double bond, this process should be repeated and the molecule will be named either cis,cis , trans,trans, cis,trans, or trans,cis.
Make sense yet? It can be more than a little confusing the first time you learn nomenclature, so here's an example to better illustrate the steps you need to go through.
In the case of this compound, going through the steps would look like this:
- There are six carbons in the longest chain. Therefore, the prefix is hex-
- There is only one double bond, so the suffix to be used is -ene. This means that the basic alkene unit is hexene.
- There is a substituent on one of the carbons. It's a CH3 group, which is also known as a methyl group. Therefore, our name has expanded to methylhexene.
- The lowest number that the doubly bonded carbon can have is 2. Therefore, we should start numbering from the right of the molecule. The methyl group is on carbon three, giving us 3-methylhex-2-ene.
- Cis/trans isomerism is possible in this molecule. The second carbon is bonded to a CH3 and a hydrogen. The third carbon is bonded to a CH3 and a CH2CH2CH3.
- For the second carbon, the highest priority group is CH3, because carbon has a higher atomic number than hydrogen. This group is pointing above the molecule. For the third carbon, CH2CH2CH3 has the higher priority. Even though both of the atoms bonded directly to the doubly bonded carbon are the same, as you continue down the chain of each group it's clear that CH2CH2CH3 wins out. This group is pointing below the molecule. Therefore, the molecule is trans.
Putting together all the clues we've figured out from going through each step, we can finally name our alkene as trans-3-methylhex-2-ene!
How are Alkenes Made?
Alkenes can be synthesised from a number of different chemical compounds, such as haloalkanes. However, the most common way to obtain them is through fractional distillation. In this process, natural gas or oil is heated to extremely high temperatures. This causes the splitting, or fractioning, of the oil into its constituent components, based on their boiling points. These fractions are then collected and, through a process called cracking, split into a mixture of alkenes and alkanes. Burning oil and natural gas releases greenhouse gases, which are destructive to the environment, but despite this fractional distillation is still the most convenient way to obtain alkenes.
What are Some Uses of Alkanes?
Alkenes are extremely useful products. In regards to science, they can be used in the synthesis of many more complicated products, such as in industrial-grade chemicals and in pharmaceuticals. They can be used to make alcohols and many kinds of plastic, including polystyrene and PVC. Alkenes are found in important natural substances too, such as vitamin A and natural rubber. Even ethene, the simplest alkene, has an important role in the ripening of fruit.
Is Benzene an Alkene?
A common question asked by people starting to learn about alkene chemistry is whether benzene, which is an unsaturated ring-structure with six carbons bonded to each other, is an alkene. While it might look like it contains carbon-carbon double bonds, the real structure of benzene is slightly more complicated. Instead of having fixed pi bonds the electrons in a benzene ring are shared between each of the atoms. This means that, though it's sometimes represented in a way that could be mistaken for an alkene, as shown below, it doesn't actually fit into the alkene family. The figure below shows that, while the structure on the left implies that benzene contains double bonds, the structure on the right shows that the electrons are actually distributed across all of the carbons.
Common Reactions of Alkenes:
There are hundreds of organic chemistry reactions, and many of the most commonly used reactions in labs all over the world involve alkenes. As mentioned before, the double covalent bond that makes alkenes what they are is highly reactive. This means that alkenes most often undergo addition reactions, where the pi bond breaks and two extra atoms add to the molecule.
- Hydrogenation of Alkenes
The hydrogenation reaction is the most commonly used way to turn alkenes back into alkanes. In this reaction, the double bond is broken and two extra hydrogen molecules are added to the molecule. H2 gas is used to achieve this, with a nickel catalyst that helps lower the activation energy of the reaction.
- Halogenation of Alkenes:
As in the hydrogenation reaction, in the halogenation reaction the double bond of the alkene is broken. However, instead of two molecules of hydrogen being added, a halogen substituent is bonded to the carbon atom. For example, hydrochloric acid (HCl) and ethene react together to form chloroethane as the double bond breaks, hydrogen is added to one carbon, and chlorine is added to the other.
- Hydration of Alkenes:
The hydration reaction is what turns alkenes into alcohols. Sulphuric acid and water are mixed with an alkene to form the corresponding alcohol. For example, the reaction below shows the conversion of ethene to ethanol.
- Polymerisation of Alkenes:
Polymerisation reactions are one of the most commercially used reactions of alkenes and are how all plastics are made. The most basic example of this reaction occurs between molecules of ethene. The carbon-carbon double bond is broken and the molecules attach to each other; that is, the lefthand carbon of one molecule attaches itself to the righthand carbon of another, forming a chain. Under the right conditions, more and more units of ethene continue joining together until a string of the plastic polyethylene is formed.
The ozonolysis is the most complicated of the reactions listed here, but is also one of the most useful. Ozone gas, which is an important part of the earth's atmosphere, is added to an alkene. The result is that the alkene is split at the double bond into two molecules that have a carbon compound double bonded to an oxygen, also known as a carbonyl compound. Carbonyls are another family of compounds that are extremely useful in both laboratory and real-world settings, so this reaction is a great way to convert a sample reactant to a slightly more complex product.
Alkenes are a critical molecular family in the study of organic chemistry. Their structure is defined by a reactive carbon-carbon double bond, they have a general formula of CnH2n, they can be named by following a series of simple steps, they have many uses in nature as well as in industrial and laboratory settings, and some of their most common reactions include hydrogenation (alkene to alkane), halogenation (alkene to haloalkane), hydration (alkene to alcohol), polymerisation, and ozonolysis.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2019 K S Lane