1. Home
  2. >Chemistry
  3. >Basic Organic Chemistry
Help

Alkenes and unsaturated hydrocarbons

Get the most by viewing this topic in your current grade. Pick your course now.

?
Intros
Lessons
  1. Alkenes: introduction.
  2. Definitions, properties and uses of alkenes.
  3. Reactions and testing of alkenes
  4. Naming alkenes.
  5. Geometric (E-Z) isomers of alkenes.
  6. Alkynes: definition and properties.
?
Examples
Lessons
  1. Recall the general formula of alkenes.
    Study the following chemical formulae and identify which fit the general formulae of an alkene.
    1. C10_{10} H22_{22}
    2. C6_6 H6_6
    3. C5_5 H10_{10}
    4. C7_7 H14_{14}
    5. C2_2 H2_2
  2. Recall the test to distinguish alkenes and alkanes.
    A student has solutions of two simple hydrocarbons, chemical A and B, and adds them separately to two identical test tubes containing a solution of bromine water.
    The student observed no change in the test tube with chemical A and bromine water.
    In the test tube with chemical B and bromine water, there was a noticeable change in the solution.
    1. What was observed when chemical B was mixed with bromine water?
    2. Explain why this change was seen with chemical B and not with chemical A.
    3. Chemical B is a simple hydrocarbon with three carbon atoms in a straight chain. Suggest the name of chemical B.
  3. Apply organic nomenclature to draw structural formula of alkenes.
    Draw the structural or skeletal formula for the following compounds using their IUPAC systematic names.
    1. 2-methylbut-2-ene
    2. 3-ethylhex-3-ene
    3. 4-methylpent-2-ene
    4. Methylpropene
  4. Interpret IUPAC systematic names of chemicals by applying organic nomenclature.
    Study the following compound names. Identify the mistake in each one, and correct it to give the correct IUPAC systematic name.
    1. 3-methylbut-2-ene.
    2. 2-ethylhex-4-ene.
    3. 2-methylprop-2-ene.
Topic Notes
?

Introduction to Alkenes and Unsaturated Hydrocarbons

Alkenes are a crucial class of unsaturated hydrocarbons, characterized by the presence of at least one carbon-carbon double bond. These compounds play a significant role in organic chemistry and various industrial applications. The introduction video provides a comprehensive overview of alkenes, offering valuable insights into their structure, properties, and reactions. Understanding unsaturated hydrocarbons is essential for grasping the fundamentals of organic chemistry. Unlike saturated hydrocarbons, which contain only single bonds between carbon atoms, unsaturated hydrocarbons have at least one double or triple bond. This key difference results in distinct chemical and physical properties. Saturated hydrocarbons, such as alkanes, are generally less reactive, while unsaturated hydrocarbons like alkenes are more reactive due to their double bonds. This reactivity makes alkenes versatile building blocks in organic synthesis and important precursors in the production of plastics, fuels, and other industrial chemicals. The video serves as an excellent starting point for exploring the fascinating world of unsaturated hydrocarbons and their applications.

Definition and Structure of Alkenes

Alkenes are a fundamental class of organic chemistry of alkenes that play a crucial role in chemistry and various industrial applications. These unsaturated hydrocarbons are characterized by the presence of at least one carbon-carbon double bond in their molecular structure. This distinctive feature sets alkenes apart from their saturated counterparts, alkanes, and gives them unique chemical properties and reactivity.

The general structure of alkenes consists of a chain or ring of carbon atoms with at least one double bond between two adjacent carbon atoms. This double bond is formed by the sharing of four electrons between two carbon atoms, creating a stronger and shorter bond compared to the single bonds found in alkanes. The presence of this double bond is what defines alkenes as unsaturated hydrocarbons.

When comparing alkenes to alkanes, the most significant difference lies in their bonding structure. While alkanes contain only single bonds between carbon atoms, alkenes feature at least one double bond. This structural difference leads to distinct physical and chemical properties. For instance, alkenes are generally more reactive than alkanes due to the presence of the electron-rich double bond, which can participate in various chemical reactions.

The concept of unsaturation in hydrocarbons refers to the presence of multiple bonds (double or triple) between carbon atoms. Alkenes are considered unsaturated because they have the capacity to add more hydrogen atoms to their structure, specifically at the site of the double bond. This property makes alkenes more reactive and allows them to undergo addition reactions, where other atoms or molecules can be added across the double bond.

The general formula for alkenes is CnH2n, where n represents the number of carbon atoms in the molecule. This formula differs from that of alkanes (CnH2n+2) by having two fewer hydrogen atoms. The reason for this difference is the presence of the double bond in alkenes, which requires two fewer hydrogen atoms to maintain the tetravalency of carbon. For example, the simplest alkene, ethene (C2H4), has two carbon atoms and four hydrogen atoms, following the CnH2n formula.

Understanding the structure and properties of alkenes is essential in organic chemistry of alkenes and various industrial processes. Their unsaturated nature and the presence of the double bond make them versatile starting materials for the synthesis of many important compounds. From the production of plastics to the manufacture of pharmaceuticals, alkenes serve as key building blocks in numerous chemical reactions and transformations.

In summary, alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This structural feature distinguishes them from alkanes and gives them unique chemical properties. The concept of unsaturation in alkenes refers to their ability to add more hydrogen atoms or other molecules across the double bond. With the general formula CnH2n, alkenes play a vital role in organic chemistry and industrial applications, making them an important subject of study in the field of chemistry.

Properties and Reactions of Alkenes

Alkenes are a class of unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This double bond is the key feature that defines the properties and reactivity of alkenes, setting them apart from their saturated counterparts, the alkanes. The presence of the double bond makes alkenes more reactive than alkanes, leading to a rich array of chemical reactions that are both interesting and industrially important.

The carbon-carbon double bond in alkenes consists of one sigma (σ) bond and one pi (π) bond. The σ bond is formed by the head-on overlap of atomic orbitals, while the π bond results from the side-by-side overlap of p orbitals. This π bond is weaker than the σ bond and is responsible for the high reactivity of alkenes. When alkenes react, they do so by breaking the π bond, leaving the stronger σ bond intact. This process of breaking the π bond is the fundamental mechanism by which alkenes react by breaking the bond.

One of the most characteristic reactions of alkenes is the addition reaction. In these reactions, new atoms or groups are added to the carbons of the double bond, converting the alkene into a saturated compound. Addition reactions are possible because the π bond in alkenes is relatively weak and can be easily broken. Common examples of addition reactions include hydrogenation (addition of hydrogen), halogenation (addition of halogens), and hydration (addition of water).

A classic test for the presence of a carbon-carbon double bond is the decolorization of bromine water. When an alkene is added to a solution of bromine in water, which has a reddish-brown color, the solution quickly becomes colorless. This occurs because the bromine molecules add across the double bond of the alkene in a process called bromination. The disappearance of the bromine color indicates that the double bond has reacted, confirming the presence of an alkene. This test is a simple yet effective method for distinguishing between saturated and unsaturated hydrocarbons.

The reactivity of alkenes stands in stark contrast to that of alkanes. While alkanes are relatively inert and undergo substitution reactions under harsh conditions, alkenes readily participate in addition reactions under milder conditions. This difference in reactivity can be attributed to the presence of the π bond in alkenes, which provides a site of high electron density that can be easily attacked by electrophiles (electron-seeking species).

Unsaturated hydrocarbon examples include ethene (C2H4), propene (C3H6), and butene (C4H8). These compounds serve as important starting materials in the chemical industry for the production of plastics, fuels, and other valuable products. The double bond in these molecules allows for a variety of transformations that are not possible with alkanes.

In addition to simple addition reactions, alkenes can undergo more complex transformations such as polymerization, where many alkene molecules join together to form long chains or networks. This process is the basis for the production of many common plastics like polyethylene and polypropylene. Alkenes can also participate in oxidation reactions, leading to the formation of alcohols, aldehydes, or ketones depending on the conditions and reagents used.

The study of alkene reactions is crucial in organic chemistry and has significant applications in industrial processes. Understanding how alkenes react by breaking the bond allows chemists to design and control reactions to produce desired products efficiently. From the production of pharmaceuticals to the synthesis of advanced materials, the chemistry of alkenes plays a vital role in modern technology and industry.

In conclusion, the properties and reactions of alkenes are fundamentally governed by the presence of the carbon-carbon double bond. This feature makes alkenes more reactive than alkanes and enables a wide range of addition reactions. The decolorization of bromine water serves as a simple yet effective test for the presence of alkenes. As unsaturated hydrocarbons, alkenes are versatile building blocks in organic synthesis and industrial applications, showcasing the importance of understanding their unique reactivity and properties in the field of chemistry.

Naming Alkenes: IUPAC Nomenclature

IUPAC nomenclature provides a systematic approach to naming organic compounds, including alkenes. Understanding these rules is crucial for chemists and students alike. Alkenes are hydrocarbons containing at least one carbon-carbon double bond, and their naming follows specific guidelines that differ slightly from those used for alkanes.

The first step in naming alkenes is to identify the longest carbon chain that contains the double bond. This becomes the parent chain and determines the base name of the alkene. Unlike alkanes, which use the suffix "-ane," alkenes use the suffix "-ene." For example, a two-carbon alkene is called "ethene," while a three-carbon alkene is "propene."

Numbering the carbon chain is a critical aspect of naming alkenes. The chain is numbered to give the carbon atoms of the double bond the lowest possible numbers. This numbering is essential for indicating the position of the double bond in the name. For instance, in CH-CH=CH-CH-CH, the double bond is between the second and third carbons, so it's named "2-pentene."

When naming branched alkenes, the same principles apply as with alkanes, but with the added consideration of the double bond position. Substituents are named and numbered based on their position on the parent chain. For example, (CH)CH-CH=CH-CH would be named "4-methyl-2-pentene." The "4-methyl" indicates a methyl group on the fourth carbon, while "2-pentene" shows the double bond starts at the second carbon of a five-carbon chain.

In cases where there are multiple double bonds, the suffix changes to reflect this: "-diene" for two double bonds, "-triene" for three, and so on. The positions of all double bonds must be indicated in the name. For example, CH=CH-CH=CH-CH would be named "1,3-pentadiene."

It's important to note that when naming alkenes, the double bond takes precedence over single bonds when numbering. This means that even if a substituent could get a lower number, the double bond position determines the numbering direction. For instance, CH-CH(CH)-CH=CH-CH is named "4-methyl-2-pentene," not "2-methyl-3-pentene," because the double bond must start from carbon 2.

Compared to alkanes, the key difference in naming alkenes lies in the necessity of indicating the double bond position. While alkanes simply use the "-ane" suffix and number substituents, alkenes require both the "-ene" suffix and a number to show where the double bond begins. This additional information is crucial because the position of the double bond can significantly affect the compound's properties and reactivity.

In more complex cases, such as cyclic alkenes or those with multiple functional groups, additional rules come into play. For cyclic alkenes, the prefix "cyclo-" is added, and numbering starts at one of the carbons involved in the double bond. When other functional groups are present that take naming priority over alkenes (like alcohols or carboxylic acids), the double bond is treated as a substituent and given the name "alkenyl" (e.g., ethenyl, propenyl).

Mastering IUPAC nomenclature for alkenes is essential for clear communication in organic chemistry. It allows chemists to unambiguously describe the structure of compounds, facilitating research, synthesis, and discussion of chemical properties. By following these rules and practicing with various examples, students and professionals can become proficient in naming even the most complex alkene structures.

Geometric Isomerism in Alkenes

Geometric isomerism is a fascinating concept in organic chemistry, particularly prevalent in alkenes. This type of structural isomerism occurs when molecules have the same molecular formula and bonding sequence but differ in the spatial arrangement of their atoms. In alkenes, geometric isomerism arises due to the restricted rotation around the carbon-carbon double bond, leading to distinct spatial configurations of substituents.

The most common form of geometric isomerism in alkenes is cis-trans isomerism. In this system, "cis" refers to isomers where similar groups are on the same side of the double bond, while "trans" indicates that they are on opposite sides. For example, in 2-butene, the methyl groups can be on the same side (cis-2-butene) or on opposite sides (trans-2-butene) of the double bond.

The restricted rotation around the double bond is key to understanding geometric isomerism. Unlike single bonds, which allow free rotation, the pi bond in a double bond prevents rotation. This restriction locks the molecule into specific configurations, giving rise to distinct isomers with different physical properties of isomers and chemical properties.

While cis-trans nomenclature is widely used, it has limitations, especially for more complex molecules. To address this, chemists developed the E-Z notation system. In this system, "E" (from the German "entgegen," meaning opposite) corresponds to groups of higher priority on opposite sides of the double bond, while "Z" (from "zusammen," meaning together) indicates higher priority groups on the same side. Priority is determined using the Cahn-Ingold-Prelog (CIP) rules, which rank substituents based on atomic number.

Geometric isomers often exhibit markedly different properties despite having the same molecular formula. For instance, cis-2-butene has a boiling point of 3.7°C, while trans-2-butene boils at 0.8°C. This difference is due to the more compact shape of the cis isomer, allowing for stronger intermolecular forces. Similarly, in biological systems, the cis and trans isomers of retinal play crucial roles in the visual cycle, with only the cis form able to interact effectively with opsin to initiate the visual response.

Another striking example is found in unsaturated fatty acids. Oleic acid, a cis fatty acid, is liquid at room temperature and is considered healthier in diets. Its trans isomer, elaidic acid, is solid at room temperature and is associated with negative health effects when consumed in large quantities. This demonstrates how geometric isomerism can significantly impact both physical properties of isomers and biological activity.

The concept of geometric isomerism extends beyond simple alkenes. It's also observed in cyclic compounds and in coordination complexes of transition metals. In these cases, the restricted rotation principle still applies, but the spatial arrangements can be more complex, leading to a wider variety of isomeric forms.

Understanding geometric isomerism is crucial in many fields, including pharmaceuticals, where the spatial arrangement of atoms can dramatically affect a drug's efficacy and safety. It's also important in materials science, where the properties of polymers and other materials can be influenced by the geometric configuration of their constituent molecules.

In conclusion, geometric isomerism in alkenes, characterized by cis-trans isomerism and described using E-Z notation, is a fundamental concept in organic chemistry. It arises from the restricted rotation around double bonds and results in isomers with distinct spatial arrangements and properties. This phenomenon plays a significant role in various scientific and practical applications, underscoring the importance of spatial arrangement in molecular behavior and interactions.

Alkynes: Another Class of Unsaturated Hydrocarbons

Alkynes represent another important class of unsaturated hydrocarbons in organic chemistry, alongside alkenes. These compounds are characterized by the presence of at least one carbon-carbon triple bond, which sets them apart from both alkanes and alkenes. The general formula for alkynes is CnH2n-2, where n is the number of carbon atoms in the molecule. This formula reflects the higher degree of unsaturation in alkynes compared to alkenes (CnH2n) and alkanes (CnH2n+2).

The structure of alkynes is unique due to the triple bond, which consists of one sigma (σ) bond and two pi (π) bonds between two carbon atoms. This triple bond is linear, with bond angles of 180 degrees, creating a straight-line geometry around the bonded carbon atoms. This linearity is a key structural feature that distinguishes alkynes from the bent structure of alkenes and the tetrahedral arrangement in alkanes.

Naming alkynes follows similar rules to those used for alkanes and alkenes, with the suffix "-yne" indicating the presence of a triple bond. For example, the simplest alkyne is ethyne (C2H2), also known as acetylene. As the carbon chain lengthens, we encounter propyne (C3H4), butyne (C4H6), and so on. When there are multiple triple bonds or when the triple bond is not at the end of the carbon chain, numbering is used to specify its position.

The reactivity of alkynes is largely influenced by the triple bond, which is a region of high electron density. This makes alkynes more reactive than alkanes but generally less reactive than alkenes. Alkynes can undergo addition reactions, where molecules add across the triple bond, reducing it to a double or single bond. Common reactions include hydrogenation (addition of hydrogen), halogenation (addition of halogens), and hydration (addition of water).

The significance of the triple bond in organic chemistry extends beyond its reactivity. The linear geometry of alkynes makes them valuable building blocks in synthetic organic chemistry. They can be used to create complex molecular structures and are particularly useful in the synthesis of polymers and other materials. Additionally, the acidity of terminal alkynes (those with the triple bond at the end of the chain) allows for unique reactions not possible with alkenes or alkanes.

In industrial applications, alkynes, particularly acetylene, play crucial roles. Acetylene is used in welding and cutting metals due to the high temperatures achieved when it burns with oxygen. It's also a starting material for the production of various chemicals and plastics. The study of alkynes is essential for understanding more complex organic molecules and reactions, making it a fundamental topic in organic chemistry curricula.

The unsaturated nature of alkynes also contributes to their importance in biochemistry and natural product chemistry. Many naturally occurring compounds contain triple bonds, and understanding the properties of alkynes helps in the study and synthesis of these molecules. Furthermore, the ability of alkynes to participate in click chemistry reactions has made them valuable tools in fields such as drug discovery and materials science.

In conclusion, alkynes represent a fascinating and important class of unsaturated hydrocarbons. Their unique structure, characterized by the carbon-carbon triple bond, gives them distinct properties and reactivity. From their naming conventions to their industrial applications and role in organic synthesis, alkynes are an integral part of organic chemistry. Understanding alkynes is crucial for grasping more advanced concepts in organic chemistry and appreciating the diversity and complexity of carbon-based compounds.

Conclusion: Importance of Alkenes in Organic Chemistry

Alkenes in organic chemistry, a crucial class of unsaturated hydrocarbons, play a pivotal role in organic chemistry. These compounds, characterized by their carbon-carbon double bonds, exhibit unique reactivity and structural properties. Understanding alkenes in organic chemistry is essential for grasping fundamental concepts in organic synthesis, petrochemistry, and biochemistry. Their ability to undergo addition reactions, such as hydrogenation and halogenation, makes them versatile building blocks for creating more complex molecules. Alkenes are ubiquitous in nature, found in plant oils and pheromones, and are vital in the production of plastics, pharmaceuticals, and fuels. Mastering the chemistry of alkenes provides a solid foundation for exploring more advanced topics in organic chemistry, including aromatic compounds, polymers, and natural products. As you delve deeper into organic chemistry, remember that alkenes are just the beginning. Continue your journey by investigating related topics such as alkynes, cycloalkenes, and the fascinating world of stereochemistry. The study of unsaturated hydrocarbons opens doors to endless possibilities in chemical research and industrial applications.

Alkenes: Introduction

Alkenes are a fundamental class of hydrocarbons known for their unique properties and wide range of applications. This guide will walk you through the definitions, properties, and uses of alkenes, providing a comprehensive understanding of these unsaturated hydrocarbons.

Step 1: Definition of Alkenes

Alkenes are unsaturated hydrocarbons, which means they contain at least one double bond between carbon atoms. This double bond is the defining feature of alkenes and distinguishes them from alkanes, which are saturated hydrocarbons with only single bonds. The general formula for alkenes is CnH2n, where 'n' represents the number of carbon atoms.

Step 2: Difference Between Alkenes and Alkanes

While alkanes are saturated hydrocarbons with single bonds, alkenes are unsaturated due to the presence of one or more double bonds. This double bond allows alkenes to participate in a variety of chemical reactions that alkanes cannot. For example, the double bond in alkenes can open up to form new bonds with other atoms, making them more reactive than alkanes.

Step 3: Properties of Alkenes

Alkenes exhibit several unique properties due to their double bonds. These properties include:

  • Reactivity: The double bond in alkenes makes them more reactive than alkanes. They can undergo addition reactions where the double bond is broken and new atoms are added to the carbon atoms.
  • Combustion: Like alkanes, alkenes can combust, but they also participate in other types of reactions due to their double bonds.
  • Physical State: Alkenes can be gases, liquids, or solids at room temperature, depending on the number of carbon atoms in the molecule.

Step 4: Testing for Alkenes

One common method to test for the presence of alkenes is the bromine water test. When bromine water, which is brown, is added to an alkene, the solution becomes colorless. This reaction occurs because the double bond in the alkene reacts with the bromine, breaking the double bond and forming a dibromo compound.

Step 5: Naming Alkenes Using IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic method for naming alkenes. The name of an alkene is derived from the name of the corresponding alkane by replacing the '-ane' suffix with '-ene'. For example, ethane (C2H6) becomes ethene (C2H4). The position of the double bond is indicated by a number placed before the name of the alkene.

Step 6: Uses of Alkenes

Alkenes have a wide range of applications in various industries. Some of the common uses include:

  • Polymer Production: Alkenes are used as monomers to produce polymers such as polyethylene and polypropylene, which are essential materials in the plastic industry.
  • Chemical Synthesis: Alkenes serve as starting materials for the synthesis of various chemicals, including alcohols, aldehydes, and carboxylic acids.
  • Fuel: Some alkenes are used as fuels or fuel additives due to their high reactivity and energy content.

Step 7: Summary

In summary, alkenes are a crucial class of unsaturated hydrocarbons characterized by the presence of at least one double bond. Their unique properties, such as increased reactivity and the ability to undergo addition reactions, make them valuable in various industrial applications. Understanding the definitions, properties, and uses of alkenes provides a solid foundation for further study and practical applications in organic chemistry.

FAQs

Here are some frequently asked questions about alkenes and unsaturated hydrocarbons:

  1. What makes alkenes unsaturated hydrocarbons?

    Alkenes are classified as unsaturated hydrocarbons due to the presence of at least one carbon-carbon double bond in their molecular structure. This double bond allows alkenes to add more hydrogen atoms or other molecules, making them "unsaturated" compared to alkanes, which have only single bonds between carbon atoms.

  2. How are alkenes and alkynes named?

    Alkenes and alkynes are named using IUPAC nomenclature. For alkenes, the suffix "-ene" is used, and the position of the double bond is indicated by a number. For example, CH-CH=CH-CH is named "2-butene". Alkynes use the suffix "-yne", as in "propyne" for CH-CCH. The longest carbon chain containing the multiple bond determines the base name.

  3. What is the difference between saturated and unsaturated hydrocarbons?

    Saturated hydrocarbons (like alkanes) contain only single bonds between carbon atoms and have the maximum number of hydrogen atoms possible. Unsaturated hydrocarbons (like alkenes and alkynes) contain at least one double or triple bond between carbon atoms, allowing them to add more hydrogen atoms or other groups.

  4. How do alkenes react?

    Alkenes primarily undergo addition reactions due to the reactivity of their double bond. Common reactions include hydrogenation (addition of hydrogen), halogenation (addition of halogens), hydration (addition of water), and polymerization. These reactions typically involve breaking the π bond of the double bond while maintaining the σ bond.

  5. What is the general formula for unsaturated alkenes?

    The general formula for alkenes is CnH2n, where n is the number of carbon atoms. This formula reflects that alkenes have two fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms, due to the presence of one double bond.

Prerequisite Topics

Understanding the fundamental concepts in organic chemistry is crucial when delving into the study of alkenes and unsaturated hydrocarbons. One of the most essential prerequisite topics for this subject is arrow pushing (curly arrows) in organic chemistry. This concept is vital because it forms the basis for comprehending the mechanisms of various reactions involving alkenes and other unsaturated hydrocarbons.

Arrow pushing, also known as electron pushing, is a technique used to illustrate the movement of electrons during chemical reactions. When studying alkenes and unsaturated hydrocarbons, this skill becomes indispensable. Alkenes are characterized by their carbon-carbon double bonds, which are electron-rich and prone to various addition reactions. By mastering arrow pushing, students can better visualize and predict how these reactions occur.

For instance, when exploring addition reactions in organic chemistry, such as the addition of halogens or hydrogen halides to alkenes, arrow pushing helps elucidate the step-by-step process. It shows how the pi bond in the alkene acts as a nucleophile, attacking the electrophilic species, and how electrons subsequently move to form new bonds. This understanding is crucial for grasping concepts like Markovnikov's rule and anti-Markovnikov addition.

Moreover, arrow pushing is essential for comprehending more complex reactions involving unsaturated hydrocarbons, such as electrophilic aromatic substitution in benzene and other aromatic compounds. It aids in explaining the formation of resonance-stabilized intermediates and the overall reaction mechanisms.

By mastering arrow pushing techniques, students develop a deeper understanding of electron behavior in organic molecules. This knowledge not only aids in studying alkenes and unsaturated hydrocarbons but also serves as a foundation for more advanced topics in organic chemistry, such as substitution and elimination reactions, and the chemistry of carbonyl compounds.

In conclusion, a solid grasp of arrow pushing in organic chemistry is paramount for anyone looking to excel in understanding alkenes and unsaturated hydrocarbons. It provides the tools necessary to interpret and predict reaction mechanisms, making it easier to tackle more complex concepts in this field. Students who invest time in mastering this prerequisite topic will find themselves better equipped to handle the intricacies of organic chemistry, particularly when dealing with the reactive and versatile world of unsaturated hydrocarbons.

In this lesson, we will learn:
  • The definition of an alkene and their general formula.
  • The major uses and properties of alkenes.
  • How to test for alkenes in a chemical reaction.
  • How to name alkenes using IUPAC organic nomenclature.

Notes:
  • We saw in Alkanes that alkanes are saturated hydrocarbons, compounds made of only carbon and hydrogen atoms where carbon makes only single bonds. However, many organic compounds are unsaturated. This means that not all the bonds made by carbon are single bonds, they also contain double or triple bonds, to either carbon or another atom.
    Like alkanes, alkenes are another homologous series of hydrocarbons. Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. This C=C double bond is the functional group that defines a molecule as an alkene.
    Alkenes have the general formula: CnH2n. This means that in a simple alkene (only one double bond) there are twice as many hydrogen atoms as there are carbon atoms. If you compare to an alkane with the formula CnH2n+2 an alkene has lost the +2 because of the C=C double bond now present.

  • Alkenes are more reactive than alkanes because their double bond(s) can be opened up by chemical reactions; it is a more reactive bond than a single bond.
    This means alkenes are very useful for making polymers, which are very long chains of hydrocarbons made by a repeating unit. This is especially true of ethene, the smallest alkene, which is the monomer unit of the important plastic polymer (poly)ethene.
    Alkenes can make two new bonds with other atoms by opening up this double bond.
    • We call reactions that open up the C=C bond addition reactions.
    • This is where the terms saturated and unsaturated come from. Like a sponge saturated by water, an alkane is saturated by bonds; it cant form any more bonds, but alkenes can so it is unsaturated./li>

  • Just like alkanes, alkenes are flammable, reacting with oxygen in combustion reactions. Alkenes produce more soot when burning than alkanes do, which have a cleaner flame.
    Practically, this just means it requires more oxygen to burn cleanly because there are more C-C bonds (with the double bond present) to have to break up.

  • Alkenes can react with hydrogen halides in an addition reaction:

    • (CH3)2C=C(CH3)2 + HCl   \, →  \, (CH3)2(H)CC(Cl)(CH3)2

    This is another example of an addition reaction of alkenes where the HCl molecule adds across the C=C double bonds.

  • You can use bromine water to test for alkenes:
    • When an alkene solution is added to bromine water, the brown color of the bromine solution will go colorless. We say that alkenes decolorize bromine water.
    • The brown color caused by bromine water disappears because bromine (Br2) is being reacted away. The double bond in the alkene molecule reacts with a bromine molecule and opens up in an addition reaction, using both reactant molecules up. A colorless dibromoalkane product forms in their place. With ethene, this reaction has the equation:

      • Br2 + C2H4   \, →  \, C2H4Br2

    • This is an important test for a double bond because alkanes do not have a double bond so bromine does not react with it.

  • Using the nomenclature in Organic chemistry introduction, we can name simple alkenes. See the table below for the first five alkenes.

    • Carbon chain length

    Suffix

    Alkene name

    Molecular formula

    2

    Eth-

    Ethene

    C2H4

    3

    Prop-

    Propene

    C3H-6

    4

    But-

    Butene

    C4H8

    5

    Pent-

    Pentene

    C5H10



  • Alkene double bonds are named similarly to branches in an alkane:
    • Count the carbon chain length to find the base of the compound's name.
    • Identify which carbon in the chain the alkene begins at, and use this number with '–ene' as the suffix. In simpler compounds, you can also add the number before the root for the carbon chain length, so but-1-ene could be 1-butene. See the examples below:

    • Remember that some alkenes have implicit numbering. Propene can only have the double bond between carbons 1 and 2. If it was between 2 and 3, the numbering would reverse. So prop-1-ene is just propene. See the example:

    • Compounds with more than one double bond have the '-ene' suffix changed to show which carbon atoms in the chain the double bonds are found at, and a prefix to say how many double bonds there are. Remember that alkenes with more than one double bond won't have the same general formula as simple alkanes! Just like with branches in alkanes, the naming of such alkanes is done systematically:
      • Two double bonds in the molecule: -diene
      • Three double bonds in the molecule: -triene
      • Four double bonds in the molecule: -tetraene
    See the example below:


  • In more complicated compounds that have branched alkyl chains and double bonds, numbering your carbon chain should be done to give the alkene double bond the lowest numbering possible. This is because an alkene is a higher order functional group (more on this later) than alkyl chains, so the carbon chain 'starts' with the alkene. See the example:

  • Having a double bond changes the geometry of carbon atoms in a few ways:
    • Carbon atoms with a double bond only bond to three atoms in total – two of its valence of four is used in the double bond, so only two other bonds are made.
    • This makes the molecule around the double bond flat. The bond angle around a carbon atom with a double bond will be about 120° like in a trigonal planar structure, since there are only three adjacent atoms. This double bond cannot freely rotate, unlike single covalent bonds which can.
    • This part of the molecule is flat and locked in position, there is no rotation of the double bond like there is for single bonds.

    This restricted rotation leads to

  • Another type of hydrocarbon that is unsaturated are alkynes. The definition of an alkyne is an unsaturated hydrocarbon containing at least one carbon-carbon triple bond. Alkynes have the general formula Cn_nH2n−2_{2n-2}.

    • Carbon chain length

    Alkyne name

    Molecular formula

    2

    Ethyne (Acetylene)

    C2H2

    3

    Propyne

    C3H4

    4

    Butyne

    C4H6




  • Alkynes are more reactive than alkenes and much more reactive than alkanes because their triple bond(s) can be 'opened up' by chemical reactions, just like double bonds can be but even more easily because the triple bond is weaker than the double bond.

  • The geometry of an alkyne is also different to that of the alkane – in an alkyne a triple bond means the alkyne carbons can only bond to two atoms in total. These atoms position themselves 180° apart, and cause a linear shape around the molecule at the triple bond.

  • Alkynes are systematically named with –yne as the suffix instead of –ene like alkenes or –ane like alkanes. The triple bond in alkynes are named in the same way that double bonds in alkenes or branches in an alkane are named.

  • Alkynes are a lower priority functional group than alkenes. This means that when numbering the carbon chain you should prioritize the alkene double bond above alkyne triple bonds. However, the –ene is named first! When naming compounds with double and triple bonds in them:
    • Prioritize numbering the alkene first.
    • Name the alkene first – these compounds are called "enynes".