Lesson Explainer: Properties of Alkenes Chemistry

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

Alkenes are unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond. Alkenes and alkanes have similar boiling points and melting points, but alkenes tend to be much more chemically reactive. Alkenes readily react with halogens, and they can also be reacted with other unsaturated compounds to make desirable plastics such as polyethylene and synthetic rubber polymers.

Definition: Alkene

Alkene molecules are unsaturated hydrocarbon compounds that have at least one carbon-to-carbon double bond (CC).

Ethene (ethylene) is the simplest alkene, and it contains just two carbon atoms and four hydrogen atoms. Ethene molecules contain four carbon-to-hydrogen single bonds (CH) and one carbon-to-carbon double bond (CC). Ethene is an important chemical feedstock, and it is used to make industrial solvents like ethane-1,2-diol and desirable plastics like polyvinyl chloride.

Ethene is the first member of the monounsaturated straight-chain alkene homologous series, and propene and butene are the second and third members. Propene (propylene) contains three carbon atoms and six hydrogen atoms. Propene molecules have one carbon-to-carbon single bond (CC) and one carbon-to-carbon double bond (CC).

Definition: Monounsaturated Alkenes

Monounsaturated alkene compounds are hydrocarbon molecules that contain one carbon-to-carbon double bond (CC).

There are many monounsaturated alkenes that are larger than ethene or propene, and some of these longer-chain alkenes are shown in the following table. You will notice that the naming system for the monounsaturated alkene molecules is similar to the naming system for the fully saturated alkane molecules. Alkanes are called hexane when they have six main-chain carbon atoms, and alkenes are called hexene when they have six main-chain carbon atoms. Alkanes are called heptane when they have seven main-chain carbon atoms, and alkenes are called heptene when they have seven main-chain carbon atoms. The alkane and alkene molecules are named with the same set of base (root) terms, and these conserved base terms were themselves derived from the Greek and Latin languages.

Alkene NameChemical BondingChemical Formula
EtheneCHCH22CH24
PropeneCHCHCH23CH36
ButeneCHCHCHCH223CH48
PenteneCHCHCHCHCH2223CH510
HexeneCHCHCHCHCHCH22223CH612
HepteneCHCHCHCHCHCHCH222223CH714
OcteneCHCHCHCHCHCHCHCH2222223CH816
NoneneCHCHCHCHCHCHCHCHCH22222223CH918
DeceneCHCHCHCHCHCHCHCHCHCH222222223CH1020

Definition: Saturated Alkanes

Fully saturated alkane compounds are hydrocarbon molecules that do not contain any double or triple bonds.

The CH2 formula can be used to determine the empirical formula for most alkene molecules. It can be used to determine how many hydrogen atoms (2𝑛) there are in lots of alkenes that have some known number of carbon atoms (𝑛). For example, we can determine that propene has six hydrogen atoms because the term prop- suggests that propene has three carbon atoms and 2𝑛=6 when 𝑛=3.

Example 1: Remembering the General Algebraic Formula for the Homologous Series of Monounsaturated Straight-Chain Alkene Molecules

What is the general formula for the group of the compounds to which the following displayed formula is related?

CHHHCCHHHCHCHCHHHH
  1. CH2
  2. CH222
  3. CH22
  4. CH2+2
  5. CH

Answer

General formulas are simple mathematical formulas that describe the abundance of different element atoms in a compound. General formulas exist for groups of hydrocarbons such as straight-chain alkane compounds and straight-chain alkene compounds. The figure shows a monounsaturated straight-chain alkene that has six carbon atoms. The CH2 formula can be used to determine the empirical formula for any monounsaturated straight-chain alkene. We can use these statements to determine that option A is the correct answer for this question.

It is not always easy to name alkenes because we sometimes have to specify the position of the CC double bond. The following figure shows that but-1-ene has its CC double bond between the first and second carbon atoms and but-2-ene has its CC double bond between the second and third carbon atoms.

C1HC2HHC3HC4HHHHC1HHC2C3C4HHHHHH

The location of the CC double bond needs to be specified when it is not immediately apparent where it is in the main chain of carbon atoms. We do not have to specify the position of the CC double in ethene because it has to be between the first and second carbon atoms, but we always have to specify the position of the CC double bond in larger alkenes, such as butene or pentene.

Example 2: Identifying the Displayed Formula for But-2-ene Molecules

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

A.

CHCHCHCHHHHH

B.

CHCHCHHHH

C.

CHCHHHHCHCCHHHH

D.

CCCHHHCHHH

E.

CHHHCHCHCHHH

Answer

The combination of a number with base and suffix terms can be used to determine the displayed formula of an alkene. We are presented with the but-2-ene word and asked to determine the associated displayed formula. The but-2-ene word contains the -2- number. It contains the but- base and -ene suffix as well.

The term but- indicates that there are four carbon atoms and the suffix 2-ene indicates that there is one CC double bond at the second main-chain carbon atom (2). Figure E shows the structure of a monounsaturated straight-chain alkene that has four covalently bonded carbon atoms and one CC double at the second main-chain carbon atom (2). We can use these statements to determine that option E is the correct answer for this question.

Chemists have studied both short- and long-chain alkenes, and they have come to the realization that many alkene properties scale with alkene chain length. Longer alkenes usually have higher melting points and boiling points, and they also tend to be much more dense and less volatile.

Straight-Chain AlkeneChemical FormulaBoiling point (C)
EtheneCH24104
PropeneCH3648
But-1-eneCH486
Pent-1-eneCH51030
Hex-1-eneCH61263
Hept-1-eneCH71494
Oct-1-eneCH816121
Non-1-eneCH918147
Dec-1-eneCH1020172

The relationship between the physical properties of alkenes and their chain length can be understood if we think about chain–chain dispersion forces. Dispersion forces are the electrostatic attractions set up between the slightly positive end of one molecule and the slightly negative end of another molecule.

Definition: Dispersion Forces

Dispersion forces are the electrostatic attractions set up between the slightly positive end of one molecule and the slightly negative end of another molecule.

Dispersion forces are strong when there is a lot of surface area for contact between adjacent hydrocarbons. There is a lot of surface area for contact between long alkenes. There is much less surface area for contact between shorter alkenes that have fewer carbon atoms. Longer alkenes are able to generate the stronger intermolecular forces, and this explains why longer alkenes have higher melting points and boiling points. It also explains why longer alkenes have greater densities and lower volatility values.

Flammability values quantify how easy it is to burn or ignite a chemical substance and induce a combustion reaction. Larger alkene molecules usually have lower flammability values because they contain more carbon atoms and they are harder to ignite.

Example 3: Understanding the Relationship Between Alkene Chain Length and Boiling Point

Which of the following alkenes has the lowest boiling point?

  1. Hexene
  2. Pentene
  3. Propene
  4. Butene
  5. Ethene

Answer

Boiling points quantify the temperature that is needed to convert a liquid into a gas. The boiling point of monounsaturated alkenes scales with alkene chain length. The smallest alkenes have the lowest boiling points, and the largest alkenes have the highest boiling points. Ethene is the smallest monounsaturated straight-chain alkene from the list, so it must have the lowest boiling point. We can use these statements to determine that option E has to be the correct answer for this question.

Alkenes are usually obtained through the process of thermal catalytic cracking. Cracking uses heat and a catalyst to break down long and undesirable petroleum hydrocarbons into smaller and more desirable short-chain products. The short-chain products can include both medium-length alkanes such as octane (CH818) and short- or medium-length alkenes such as ethene (CH24) and octene (CH816). Cracking is usually performed by industrial chemists in large petrochemical plants, but it can also be performed by teachers and students with relatively simple chemical equipment.

Example 4: Remembering What Process Can Be Used to Produce Short-Chain Alkene Molecules From Longer-Chain Alkane Molecules

Which industrial process produces alkenes from long-chain alkanes?

  1. Catalytic cracking
  2. Fractional distillation
  3. Contact process
  4. Haber process
  5. Water–gas shift reaction

Answer

Alkenes are usually obtained through the process of thermal catalytic cracking. The thermal catalytic cracking process uses heat and a catalyst to break down long and undesirable petroleum hydrocarbons into smaller and more desirable short-chain products. We can use these statements to determine that option A is the correct answer for this question.

Ethene is an important chemical feedstock and it is used to make some of the most in demand industrial solvents such as ethanol and ethane-1,2-diol. Ethene can also be reacted with oxygen in the presence of a silver catalyst to make the desirable epoxyethane (ethylene oxide) compound that can be processed into different polyols and ethanolamines.

Ethene can also be reacted with other unsaturated molecules to make some of the most in demand plastics such as polyethene and polyvinyl chloride. Ethene can even be converted into 1,2-dichloroethane and then copolymerized with sodium polysulfide to make some unusual synthetic rubber polymers that are highly resistant to swelling and abrasion.

Ethene and the larger and medium-sized alkenes can also be used as a fuel source because they completely combust and produce energy when they react with oxygen. Most people, however, prefer to use short-chain alkanes as a fuel source because they are cheaper and they produce a less sooty flame when they burn.

Example 5: Understanding the Different Uses and Applications of Alkene Molecules

Which of the following is not a use of alkenes?

  1. Manufacturing plastics
  2. Manufacturing rubber
  3. Cracking
  4. Producing industrial chemicals
  5. Burning as a fuel

Answer

Alkenes can be polymerized to produce plastics such as polystyrene and polytetrafluoroethane. They can also be polymerized to make synthetic rubbers such as styrene–butadiene rubber. Alkanes are an important chemical feedstock and ethene is regularly used by industrial chemists to make ethanol and ethane. Alkenes can be completely combusted with oxygen to produce carbon dioxide and water. The complete combustion of alkenes does generate energy, but we generally prefer to use alkanes for fuel because alkane fuels are more cost-effective and produce a less sooty flame when they burn. Chemists do not use catalytic cracking processes to convert alkenes into other molecules, but they do use catalytic cracking processes to make alkene molecules from larger hydrocarbons. We can use these statements to determine that option C is the correct answer for this question.

Chemists are sometimes uncertain if a short-chain hydrocarbon has at least one CC double bond, and they have to use the bromination reaction or the Baeyer test to determine if the molecule is an alkene or an alkane.

The bromination reaction is the simplest qualitative analysis technique that can be used to distinguish between saturated (alkane) hydrocarbons and unsaturated (alkene) hydrocarbons. Chemists add bromine water to an unknown hydrocarbon and then they wait to see if the bromine water retains its characteristic orange color or if it is decolorized. The unknown molecule can be classed as an alkene if the bromine water loses its orange color, and it can be classed as an alkane if it retains its characteristic orange color.

Chemists can also use the Baeyer test to distinguish between alkanes and alkenes. Chemists add a cold alkaline potassium manganate(VII) solution to an unknown hydrocarbon and then they wait to see if the solution changes color. The unknown compound can be classed as an alkene if the solution loses its purple color and produces a dark brown precipitate. The unknown compound can be classed as an alkane if the solution retains its characteristic purple color and does not produce any dark brown precipitate whatsoever.

Key Points

  • Alkenes contain at least one carbon-to-carbon double bond (CC).
  • The IUPAC naming system specifies how chemists should classify alkenes.
  • The strength of alkene chain–chain dispersion forces increases with alkene chain length.
  • Longer-chain alkenes are less volatile and they have higher boiling points and melting points.
  • Longer-chain alkenes have higher density and viscosity values.
  • Catalytic cracking can be used to produce short-chain alkenes from less desirable long-chain alkanes.
  • Both bromine water and a cold alkaline potassium manganate(VII) solution can be used to test for the presence of carbon-to-carbon double bonds (CC).

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