Introduction to Alkenes

Welcome to Topic 6C: Alkenes! While alkanes (which you met in Topic 6B) are relatively "boring" and unreactive, alkenes are the stars of the show in organic chemistry. They are unsaturated hydrocarbons, meaning they contain at least one carbon-to-carbon double bond (\(C=C\)).

This double bond is a "hotspot" of high electron density, making alkenes much more reactive than alkanes. In this chapter, we will explore why that bond is so special, how alkenes react with different chemicals, and how they are used to make the plastics we use every day.

1. The Structure of the Double Bond: Sigma (\(\sigma\)) and Pi (\(\pi\)) Bonds

In an alkene, the double bond isn't just "two lines." It is actually made of two different types of covalent bonds:

The Sigma (\(\sigma\)) Bond

This is the first bond that forms. It is created by the head-on overlap of atomic orbitals. It is a very strong bond because the electron density is concentrated directly between the two carbon nuclei. All single bonds in organic chemistry are \(\sigma\) bonds.

The Pi (\(\pi\)) Bond

The second bond in the double bond is the \(\pi\) bond. This is formed by the sideways overlap of two "p-orbitals" (one from each carbon). These p-orbitals stick out above and below the plane of the carbon atoms. Because the overlap is sideways and "looser" than a \(\sigma\) bond, the \(\pi\) bond is weaker and easier to break.

Did you know? Because the \(\pi\) bond sits above and below the carbon atoms, it acts like a "cloud" of negative charge. This attracts electrophiles (electron-loving species), which is why alkenes react the way they do!

Key Takeaway: A \(C=C\) double bond consists of one strong \(\sigma\) bond and one weaker \(\pi\) bond. The \(\pi\) bond makes alkenes reactive.

2. Addition Reactions of Alkenes

Most reactions of alkenes are addition reactions. This is where the \(\pi\) bond breaks, and two new atoms (or groups of atoms) are added onto the carbon atoms, turning the "unsaturated" double bond into a "saturated" single bond.

A. Hydrogenation (Addition of \(H_2\))

Alkenes react with hydrogen gas to form alkanes.

  • Conditions: A nickel (Ni) catalyst and a temperature of about \(150^\circ C\).
  • Example: \(C_2H_4 + H_2 \xrightarrow{Ni} C_2H_6\) (Ethene to Ethane).
  • Real-world link: This process is used to turn liquid vegetable oils into solid margarines!

B. Halogenation (Addition of \(Cl_2, Br_2, I_2\))

Alkenes react quickly with halogens at room temperature. This is the basis for the test for unsaturation.

  • The Bromine Water Test: When you add orange/brown bromine water to an alkene, it turns colourless. This happens because the \(Br_2\) adds across the double bond to form a colourless dibromoalkane.
  • Example: \(C_2H_4 + Br_2 \rightarrow CH_2BrCH_2Br\) (1,2-dibromoethane).

C. Addition of Hydrogen Halides (\(HX\))

Alkenes react with gaseous hydrogen halides like \(HBr\) or \(HCl\) to form halogenoalkanes.

  • Example: \(CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br\) (Bromoethane).
  • If the alkene is unsymmetrical (like propene), you can get two different products. We decide which is the "major" product using carbocation stability (see the Mechanism section below).

D. Hydration (Addition of Steam)

This is how we manufacture alcohols on an industrial scale.

  • Conditions: Steam (\(H_2O(g)\)), an acid catalyst (usually phosphoric acid, \(H_3PO_4\)), high temperature, and high pressure.
  • Example: \(CH_2=CH_2 + H_2O(g) \rightleftharpoons CH_3CH_2OH\) (Ethanol).

E. Oxidation with Acidified Manganate(VII)

Alkenes can be oxidized by potassium manganate(VII) (\(KMnO_4\)) in acidic conditions.

  • Observation: The purple solution turns colourless (or very pale pink).
  • Product: A diol is formed (a molecule with two \(-OH\) groups).
  • Example: Ethene + \([O]\) (from \(KMnO_4\)) + \(H_2O \rightarrow CH_2(OH)CH_2(OH)\) (Ethane-1,2-diol).

Key Takeaway: Addition reactions "open up" the double bond to add new groups. The bromine water test is the standard lab test for alkenes.

3. The Mechanism: Electrophilic Addition

For your exam, you must be able to draw the electrophilic addition mechanism using curly arrows. A curly arrow always starts from a lone pair of electrons or a covalent bond and shows where the electrons are moving.

Step-by-Step Mechanism (e.g., Ethene + \(HBr\)):

  1. The Attack: The electron-rich \(\pi\) bond of the alkene attacks the slightly positive (\(\delta+\)) Hydrogen atom in \(H-Br\). The \(H-Br\) bond breaks (heterolytic fission), and the electrons move to the Bromine.
  2. The Intermediate: A carbocation is formed. This is a molecule where one carbon atom has a positive charge (\(C^+\)) because it is missing a bond. A Bromide ion (\(Br^-\)) is also produced.
  3. The Final Bond: The \(Br^-\) ion uses its lone pair to attack the positive carbocation, forming a new \(C-Br\) bond.

Carbocation Stability (The "Major Product" Rule)

When reacting \(HBr\) with an unsymmetrical alkene (like propene), the Hydrogen atom will preferentially attach to the carbon that creates the most stable carbocation.

  • Primary carbocation: \(C^+\) is attached to 1 alkyl group. (Least stable)
  • Secondary carbocation: \(C^+\) is attached to 2 alkyl groups.
  • Tertiary carbocation: \(C^+\) is attached to 3 alkyl groups. (Most stable)

Why? Alkyl groups (like \(-CH_3\)) "push" electrons toward the positive carbon, spreading the charge out and making the intermediate more stable. The more stable the intermediate, the more likely that product is to form!

Key Takeaway: Curly arrows go from electron-rich (bond/lone pair) to electron-poor areas. Tertiary carbocations are the most stable.

4. Addition Polymerisation

Alkenes can join together in long chains to form polymers (plastics). This is called addition polymerisation because the monomers (the alkenes) simply add to one another with no other products formed.

  • The Process: The \(\pi\) bond in each alkene monomer breaks, allowing the carbons to bond to the carbons of neighboring molecules.
  • The Repeat Unit: When drawing a polymer, we draw the repeat unit in square brackets with an "\(n\)" outside. The brackets must go through the horizontal bonds.
  • Example: Many ethene molecules join to form poly(ethene).

Disposal of Polymers

Because addition polymers are made of strong \(C-C\) and \(C-H\) bonds, they are chemically inert (unreactive) and do not biodegrade. This leads to environmental issues.

Ways to manage polymer waste:

  • Recycling: Sorting and melting plastics to make new products.
  • Incineration for Energy: Burning plastics to produce heat/electricity. Warning: Burning chlorinated plastics like PVC can release toxic \(HCl\) gas, which must be removed (scrubbed).
  • Feedstock Recycling: Breaking polymers back down into small organic molecules (monomers) to be used again in chemical plants.

Key Takeaway: Alkenes make polymers. Because they are unreactive, we must find clever ways to recycle or dispose of them safely.

Quick Review: Common Mistakes to Avoid

  • Arrow Direction: Always start your curly arrow at the bond or a lone pair. Never start it at a positive charge or a bare atom.
  • Carbocation Charges: Don't forget to put the \(+\) sign on the carbon atom in your mechanism intermediate.
  • Nickel vs. Phosphoric Acid: Remember: Nickel is for adding \(H_2\); Acid is for adding \(H_2O\). Don't mix them up!
  • Polymer Brackets: Make sure the bonds extend through the square brackets to show the chain continues.