HS Chemistry - Essential Functional Groups

Alkenes

Overview of The Page

This page will cover:

• What are alkenes?
• How are alkenes named?
• What reactions do alkenes participate in?

Alkenes are another kind of organic molecule, made only of Hydrogen atoms and Carbon atoms. However, unlike alkanes, alkenes are defined by the presence of a double bond between at least one pair of Carbon atoms. This page will cover some important information about alkenes.

Straight-chained & Branched Alkenes

Alkenes contain double bonds, which means that they do not contain the maximum number of Hydrogen atoms possible, as there is at least 1 pair of Carbon atoms with more than one bond between them. Therefore, alkenes are said to be unsaturated.

All alkenes that are part of the 1-alkene homologous series $all alkenes that only contain one double bond between a pair of Carbon atoms$ have the general formula C_nH_2n. Before continuing further, we'll introduce ethene, the simplest alkene:

Its molecular formula is C₂H₄. It consists of two Carbon atoms connected to each other by a double bond, and 2 Hydrogens bonded to each Carbon atom. It is considered a straight-chained alkene, as it has no branches.

Its structural formula would be:

H₂C=CH₂

And its skeletal formula would be:

Notice how in both the structural and skeletal formulae, we noted the position of the double bond. The skeletal formula, with only two Carbon atoms, has only two end points, and a double line is used to represent the double bond. The rest of the Hydrogens are implied - to complete a total of 4 bonds per Carbon atom, there will be 2 Hydrogen atoms bonded to each Carbon atom.

In the structural formula, we used a double line between the two Carbon atoms to show where the double bond exists. Notice that we've written our molecule so that the two Carbon atoms with double bonds are next to each other. This allows us to show that the double bond exists between the Carbon atoms. We could also have shown the structural formula as CH₂=CH₂, but it's important to remember that the double bond exists between the Carbon atoms, not the Hydrogen atoms.

Another example of an alkene is:

This molecule contains both single bonds between Carbon atoms and a double bond between Carbon atoms. Moreover, it contains more single bonds between Carbon atoms than double bonds between Carbon atoms. Is it an alkane or an alkene?

This molecule would be classified as an alkene. An alkane can be thought of as the default functional group, when no other identifiable functional group is present. But the moment there is another identifiable functional group, that functional group becomes the functional group of the molecule.

That means that when numbering the Carbon atoms, we start from the side that will give us the smallest number for the double bond. In this case, that's from right to left:

The double bond is at position 1.

Before we name this compound, we have to determine whether it is a straight or branched alkene. Fortunately, this is simple. Given how there is a single line connecting all the non-Hydrogen atoms, we can conclude that this is a straight-chained compound. If that weren't the case, then we would have a branched molecule.

When we do have a branched molecule, though, there is a rule for identifying the main chain. Since double-bond functional groups take precedence over single-bond functional groups, the main chain will always be the longest chain that includes the most double bonds, even if another chain that includes less Carbon-Carbon double bonds is longer.

Back to our molecule:

The double bond is at position 1, and we know our functional group is an alkene, and that our main chain has 4 Carbon atoms in it. Since our molecule has no branches, we don't need to worry about prefixes. Our parent name is "but-" and our suffix is "-ene". When we need to note the location of a functional group, we add the number location to the suffix $although we can also choose to add it to the prefix, that is completely fine as well$.

Thus, our molecule's name is but-1-ene. It can also be written as 1-butene. The structural formula of the molecule would be:

CH₂=CHCH₂CH₃

The structural formula could also be written as:

H₂C=CHCH₂CH₃

And its skeletal formula would be:

Now that we've covered some basic information about alkenes, we can cover their reactions.

Alkene Reactions

Alkenes, because of their double bond, are more reactive than alkanes. There are a few common reactions with alkanes to know about:

• When H₂ gas and an alkene are passed over a Nickel (Ni) catalyst at 140 °C, both of the Hydrogen atoms from the H₂ are added to the alkene, making it an alkane.

• When H₂O and an alkene are reacted at a temperature of 330 °C and a very high pressure using a catalyst $concentrated phosphoric acid, H~3~PO~4~, is commonly used$, an OH^- ion and a Hydrogen atom are added to the alkene, making it an alcohol. The H₃PO₄ catalyst is necessary because it donates an H⁺ ion to initiate the reaction; at the end, the H⁺ ion returns to the H₃PO₄ catalyst, leaving it unchanged and not a part of the reaction.

• When an alkene is bubbled through a solution of either Bromine or Chlorine, or through a concentrated solution of any Hydrogen halide, both of the atoms in the halogen molecule will be added to the alkene, producing a halogenoalkane. The double bond in the alkene, being an area of high electron density, repels the electrons in the halogen/Hydrogen halide away from it. This induces a dipole in the halogen/Hydrogen halide, making one atom slightly electron-deficient and giving the other atom a slightly higher concentration of electrons. The atom that is slightly electron-deficient then attacks the double bond, breaking it and bonding with one of the Carbon atoms in an electrophilic addition. The other atom, with a slight excess of electrons, bonds with the other Carbon atom in a nucleophilic addition.

  Since the first step in the total addition process was an electrophilic addition, the entire process is called an electrophilic addition, even though the second part was a nucleophilic addition.

• Alkenes can be oxidized by acidified potassium permanganate $KMnO~4~$ solution. The product of the oxidation reaction depends on the temperature and concentration of the solution:

  • If it is dilute and cold or at room temperature, the alkene is turned into a diol $an organic compound with two alcohol functional groups$, as the Carbon-Carbon double bond is not fully broken.

  • If it is hot and concentrated:

    • If one of the Carbons in the double bond is bonded to two Hydrogen atoms, then CO₂ will be formed.
    • If one of the Carbons in the double bond is bonded to one Hydrogen atom and one other group, an aldehyde will be formed $the aldehyde will then be oxidized into a carboxylic acid if the reaction is continued$.
    • If neither one of the Carbons in the double bond is bonded to a Hydrogen atom, a ketone will be formed.

  • Using hot and dilute solution or cold and concentrated solution will give mixed results, so it is not recommended.

There is one final thing to mention in regards to alkenes. The presence of the alkene functional group in a compound can be tested by dissolving it in Bromine or Chlorine water. If the water changes color and becomes colorless, alkenes are present.