Alkenes and unsaturated hydrocarbons - Basic Organic Chemistry

Alkenes and unsaturated hydrocarbons

Lessons

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 now know of alkanes as saturated hydrocarbons, compounds made of only carbon and hydrogen atoms where all carbon atoms make four bonds to other atoms. However, many organic compounds are unsaturated – their carbon atoms do not all make four bonds to four different atoms because they contain double or triple bonds, to either carbon or another atom.

• Alkenes, like alkanes, are a class of hydrocarbons, and another homologous series of hydrocarbons. The definition of an alkene is an unsaturated hydrocarbon with one or more carbon-carbon double bonds within the molecule.

Alkenes have the general formula: Cn_nH2n_{2n}. This means that in a simple alkene (only one double bond) there are twice as many hydrogen atoms as there are carbon atoms.

Alkenes are very useful for making plastics and other polymers. This is especially true of ethene, the simplest alkene.

• Alkenes are more reactive than alkanes because their double bond(s) can be ‘opened up’ by chemical reactions. This is why alkenes are more reactive than alkanes – alkanes do not have a double bond but alkenes do, and they can make new bonds with other atoms using this ‘un-opened’ double bond.
→ Reactions where alkenes open up their double bond are called addition reactions.
→ This is also why alkenes are unsaturated and alkanes are saturated – like a sponge being saturated by water, an alkane is saturated from bond-forming – it can’t form any more bonds, but alkenes can.

• 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.

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 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+_2 \;+\; C2_2H4_4 \; \; C2_2H4_4Br2_2
→ This is an important test for a double bond because alkanes do not have a double bond and this reaction with bromine water does not occur with them.

• Using the basic naming system given in the “Organic Chemistry – introduction” lesson, we can name simple alkenes. See the table below for the first four alkenes. Remember, there

Carbon chain length

Suffix

Alkene name

Molecular formula

2

Eth-

Ethene

C2H4

3

Prop-

Propene

C3H­6

4

But-

Butene

C4H8

5

Pent-

Pentene

C5H10



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.

Systematically, alkenes and their double bonds are named in the same way that branches in an alkane are named:
→ 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:
\circ Two double bonds in the molecule: -diene
\circ Three double bonds in the molecule: -triene
\circ 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:

• 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_nH2n2_{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”.
  • Intro Lesson
    Alkenes: introduction.
    • a)
      C10_{10} H22_{22}
    • b)
      C6_6 H6_6
    • c)
      C5_5 H10_{10}
    • d)
      C7_7 H14_{14}
    • e)
      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.
  • 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.
  • 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.
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Alkenes and unsaturated hydrocarbons

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