Lesson Video: Properties of Alkenes Chemistry

In this video, we will learn how to write and interpret the names and formulas of alkenes and describe trends in properties, such as melting points.

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Video Transcript

In this video, we will learn how to write and interpret the names and formulas of alkenes and describe trends and properties, such as melting points. Alkenes are unsaturated hydrocarbons that contain at least one carbon–carbon double bond. A hydrocarbon is a compound that only contains carbon and hydrogen atoms. And an unsaturated hydrocarbon is a compound that contains at least one double or triple bond. The general formula for an alkene that contains only one carbon–carbon double bond is C𝑛H2𝑛, where 𝑛 represents the number of carbon atoms in the molecule. If an alkene has three carbon atoms and only contains one double bond, then it must contain two times three hydrogen atoms and have the molecular formula C3H6.

Now let’s take a look at the simplest alkenes, one containing two carbon atoms and one containing three carbon atoms. We can give each alkene a name to differentiate between the two molecules, following the rules established by the International Union of Pure and Applied Chemistry, or IUPAC for short. To name an alkene, we begin by naming the longest continuous chain of carbon atoms that contains the carbon–carbon double bond. This name consists of a prefix that indicates the number of carbon atoms that are in the continuous chain, followed by the suffix -ene to indicate that the molecule is an alkene. The first 10 prefixes are shown in the table. Then, we name any substituents that are attached to the base chain by placing the substituent name in front of the base chain name.

Following these rules, the two-carbon alkene is given the name ethene, eth- meaning two carbon atoms and -ene for alkene. And the three-carbon alkene is given the name propene, prop- meaning three carbon atoms and -ene for alkene. Naming alkenes becomes slightly more complicated when there are four or more carbon atoms in the continuous chain. Let’s consider these displayed formulas. Following the naming rules for naming alkenes, we get the name butene for both of these structures, although they are clearly different. To differentiate between the two butene molecules, we will need to indicate the location of the double bond in the name. To do this, we number the carbon atoms of the chain so that the double bond has the lowest possible position number.

Looking at the top butene molecule, we could number the carbon atoms of the chain from left to right or from right to left. If we number the chain from left to right, the double bond starts with carbon atom number one. And if we number the chain from right to left, the double bond starts with carbon atom number three. Therefore, we should number this carbon chain from left to right to give the double bond the lowest possible position number. Looking at the bottom butene molecule, the double bond starts with carbon atom number two, regardless of the direction in which we count the carbon chain. Next, we will add the position number to the name in between the prefix and suffix, separated by dashes. This means that we will name the two structures of butene but-1-ene and but-2-ene.

But-1-ene and but-2-ene are positional isomers, molecules with the same molecular formula and functional group, in this case an alkene, but the functional group has a different position on the carbon chain. As the number of carbon atoms in a molecule increases, so too does the number of possible straight-chain positional isomers. As such, it is important to distinguish between these molecules by using displayed formulas, skeletal formulas, structural formulas, condensed formulas, or the IUPAC name.

Now that we can recognize and name alkenes, let’s take a look at how alkenes are produced. Alkenes are often obtained through the process of cracking. Cracking is a type of decomposition reaction where larger organic molecules are broken down into smaller molecules. The large molecules used in this process are often long, undesirable hydrocarbons distilled from crude oil. In catalytic cracking, the hydrocarbons are vaporized and heated with a catalyst. This causes the hydrocarbon to break apart or crack into smaller alkanes and alkenes, which are then separated. In this example, decane was shown to crack into ethene and octane, although many other products are possible depending on where the carbon chain breaks.

Cracking is an industrial method for producing alkenes. But alkenes can also be prepared in the laboratory by dehydrating an alcohol with sulfuric acid. Dehydration is a chemical reaction in which a water molecule is eliminated from the reactant. In the dehydration of an alcohol, the hydroxy group and a hydrogen atom bonded adjacent to the position of the hydroxy group will be eliminated. And a new double bond will form between the two carbon atoms that each lost a substituent. Depending on the hydroxy group position, this reaction may be in equilibrium with the hydration of an alkene and may require the temperature of the reaction to be closely monitored.

Now that we’ve examined how to produce alkenes via cracking or dehydration, let’s take a look at some of the properties of alkenes. Many of the properties that alkenes exhibit can be explained by examining the electrostatic attractions between alkene molecules. Let’s consider propene and hex-1-ene. There exists a week electrostatic force of attraction between two propene molecules, called dispersion force. Dispersion forces are due to the random motion of electrons in the molecules and are covered in more detail in another video.

There are also dispersion forces between hex-1-ene molecules. The dispersion forces between hex-1-ene molecules are greater than the dispersion forces between propene molecules. This is because hex-1-ene molecules have more electrons than propene molecules. And there is more surface area contact between adjacent hex-1-ene molecules than adjacent propene molecules. In general, as the length of the alkene carbon chain increases, so too does the strength of the dispersion force between the molecules.

Longer alkenes tend to have a higher melting point and boiling point, as more energy is necessary to disrupt the stronger dispersion forces between molecules. The increasing strength of the dispersion force with increasing chain length can be used to explain why at room temperature small alkenes, such as ethene, propene, and butene, exist as gases. Alkenes containing between five and 15 carbon atoms tend to be in the liquid state. And alkenes consisting of more than 15 carbon atoms are in the solid state. Density and volatility are also affected by the chain length. The density of an alkene tends to increase as the chain length increases, while volatility tends to decrease with increasing chain length.

We’ve examined how alkenes compare with one another. But how do alkenes compare with alkanes, single-bonded hydrocarbons? Alkanes and alkenes have similar melting and boiling points. Both are nonpolar and insoluble in water and other polar solvents. But alkanes and alkenes readily dissolve in nonpolar solvents, such as hexane or benzene. Alkanes participate in very few reactions, but alkenes readily react with a number of reagents. Alkenes are more reactive than alkanes because alkenes contain an electron-rich carbon–carbon double bond. We can use this difference in reactivity to determine if an unknown hydrocarbon contains an alkane or an alkene by performing a bromination reaction or the Baeyer test.

Bromine water, a solution of diatomic bromine in water, has a characteristic brownish-orange color. It reacts with both alkanes and alkenes. However, the reaction with an alkane requires ultraviolet light or heat, while the reaction with an alkene rapidly occurs without any additional supply of energy. When bromine water is added to an alkane, the resulting solution is orange in color, as without any additional supply of energy no noticeable reaction between the alkane and bromine will occur. When bromine water is added to an alkene, the resulting solution is colorless. This is because the alkene readily reacts with the bromine to produce a dibromine, which is colorless in solution. Therefore, if the bromine water is decolorized, the sample may contain an alkene.

It’s worth noting here that bromine also reacts with alkynes, phenols, and anilines and can indicate the presence of these functional groups as well. As an alternative to bromination, we can perform the Baeyer test. To perform the Baeyer test, cold alkaline potassium permanganate solution, which is purple in color, is added to the sample. If the sample contains an alkane, no reaction will occur and the resulting solution will be purple in color. If the sample contains an alkene, a reaction occurs to produce a diol, which is colorless in solution, and manganese dioxide, a brown precipitate. It is worth noting here that potassium permanganate solution will be decolorized and a brown precipitate will be formed when reacted with alkenes as well as alkynes and aldehydes.

After all of this discussion of alkenes, we may be wondering what alkenes are used for. The most common use of an alkene is as a reagent to produce other important organic molecules. For example, ethene is used to manufacture ethanol, a solvent, gasoline additive, and disinfectant; ethane-1,2-diol, also called antifreeze, used as an engine coolant in countries with cold climates; and polyvinyl chloride, or PVC, a plastic commonly used for making water supply pipes, waterproof clothing, and blood collection and IV bags. Ethene is also used to produce polystyrene, another plastic that is the main component of disposable cups, packing materials, and home insulation, as well as ethylene-propylene rubber found in many garden hoses and automotive parts, like drive belts.

Before we summarize what we’ve learned about alkenes, let’s take a look at a question.

Which of the following molecules is but-2-ene?

The prefix but- means that the molecule contains a chain of four carbon atoms. As such, we can eliminate answer choice (B) as this molecule only contains a three-carbon-atom chain. We can also eliminate answer choice (C) as this molecule contains a five-carbon-atom chain. The suffix -ene indicates that the molecule is an alkene, an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. This means that we can eliminate answer choice (D), as this molecule contains a triple bond, called an alkyne, instead of a double bond.

The number two is the position number of the alkene in the carbon atom chain. This means that the double bond begins with the second carbon atom of the chain and connects carbon atoms two and three. The remaining answer choice with a double bond between the second and third carbon atoms of the chain is answer choice (E). The displayed formula that correctly represents but-2-ene is answer choice (E).

Now let’s summarize what we’ve learned with the key points. Alkenes are unsaturated hydrocarbons that contain at least one carbon–carbon double bond. The general formula of an alkene with one double bond is C𝑛H2𝑛. We can name alkenes following the IUPAC rules. The name of an alkene will end in -ene. Alkenes can be produced by cracking larger hydrocarbons or by dehydrating alcohols. Longer-chain alkenes compared to shorter-chain alkenes tend to have stronger dispersion forces and have higher melting and boiling points. They also tend to be more dense and less volatile.

Alkenes have similar melting and boiling points as alkanes but are more reactive. We can use this difference in reactivity to test for alkenes. Bromine water will be decolorized by an alkene. And cold alkaline potassium permanganate solution will be decolorized and produce a brown precipitate when combined with an alkene. Alkenes are used to produce larger molecules. For example, ethene is an important reagent for many compounds, particularly plastics like PVC.

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