Sign In
Try for Free
 flag-uk

Mastering Alkenes and Unsaturated Hydrocarbons
Dive into the world of alkenes and unsaturated hydrocarbons. Understand their unique structure, reactivity, and industrial applications. Enhance your organic chemistry knowledge with our comprehensive lessons.

  1. Intros0/5 watched
  2. Examples0/11 watched
  1. 0/5
  2. 0/11
Now Playing:Alkenes and unsaturated hydrocarbons – Example 0a
Intros
0/5 watched
  1. Alkenes: introduction.
  2. Alkenes: introduction.
    Definitions, properties and uses of alkenes.
  3. Alkenes: introduction.
    Reactions and testing of alkenes
Examples
0/11 watched
  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

Alkenes and unsaturated hydrocarbons
Jump to:NotesConceptFAQsPrerequisites
Notes
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".
Concept

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.

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.

Prerequisites

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.