Alkenes and Addition Reactions

• General formula: $C_nH_{2n}$
• Contain a C=C double bond (one σ bond + one π bond)
• The π bond is a region of high electron density above and below the plane of the molecule
• This electron-rich area attracts electrophiles

E/Z Isomerism (Geometric Isomerism)

Occurs when:

1. There is a C=C double bond (no rotation)
2. Each carbon of the double bond has two different groups

Naming uses Cahn-Ingold-Prelog priority rules (based on atomic number):

• Z (zusammen = together): higher priority groups on the same side
• E (entgegen = opposite): higher priority groups on opposite sides

What Is an Electrophile?

An electrophile is an electron-pair acceptor — it's attracted to electron-rich regions (like the C=C π bond).

General Mechanism

1. The electrophile is attracted to the π bond electron density
2. The π bond breaks and one pair of electrons forms a new bond to the electrophile
3. A carbocation intermediate forms
4. A nucleophile attacks the carbocation to complete the addition

2.1 Addition of Hydrogen Halides (HBr)

$\text{CH}_2\text{=CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br}$

Mechanism:

1. The H-Br bond is polar ($\text{H}^{\delta+}$, $\text{Br}^{\delta-}$)
2. $\text{H}^{\delta+}$ is attracted to the electron-rich C=C
3. A curly arrow from the C=C π bond to the H
4. The H-Br bond breaks heterolytically → H bonds to one carbon, Br⁻ is released
5. A carbocation forms on the other carbon
6. Br⁻ attacks the carbocation (curly arrow from Br⁻ to C⁺)

2.2 Markovnikov's Rule

With unsymmetrical alkenes (like propene), two products are possible:

$\text{CH}_3\text{CH=CH}_2 + \text{HBr} \rightarrow \begin{cases} \text{CH}_3\text{CHBrCH}_3 & \text{(major)} \\ \text{CH}_3\text{CH}_2\text{CH}_2\text{Br} & \text{(minor)} \end{cases}$

Markovnikov's rule: The hydrogen adds to the carbon with more existing H atoms (or equivalently, the halide goes to the more substituted carbon).

Why? The secondary carbocation ($\text{CH}_3\text{C}^+\text{HCH}_3$) is more stable than the primary carbocation ($\text{CH}_3\text{CH}_2\text{C}^+\text{H}_2$) due to the electron-donating (inductive) effect of alkyl groups.

2.3 Addition of Bromine (Br₂)

$\text{CH}_2\text{=CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br}$

Bromine is non-polar, but the electron-rich C=C induces a dipole in Br₂. This is the test for alkenes (bromine water decolourises from orange to colourless).

2.4 Addition of Steam (Hydration)

$\text{CH}_2\text{=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}_3\text{PO}_4, 300°\text{C}} \text{CH}_3\text{CH}_2\text{OH}$

Produces ethanol. Used industrially.

2.5 Hydrogenation

$\text{CH}_2\text{=CH}_2 + \text{H}_2 \xrightarrow{\text{Ni catalyst}} \text{CH}_3\text{CH}_3$

Used to harden vegetable oils into margarine.

Many alkene monomers join to form a long-chain polymer: $n\text{CH}_2\text{=CH}_2 \rightarrow (-\text{CH}_2\text{-CH}_2-)_n$

Drawing Polymers

• Open the C=C double bond to show single bonds extending from the repeat unit
• Enclose in brackets with $n$ subscript
• For substituted alkenes (e.g. propene, chloroethene), show the substituent in the repeat unit

• Always show curly arrows clearly in mechanisms (from electron-rich to electron-poor)
• For Markovnikov's, remember: H goes to the C with more H's
• E/Z isomerism: use priority rules based on atomic number
• The test for alkenes: bromine water decolourises (orange → colourless)

• Alkenes: $C_nH_{2n}$, C=C double bond, unsaturated
• E/Z isomerism due to restricted rotation around C=C
• Electrophilic addition: HBr, Br₂, H₂O, H₂ add across C=C
• Markovnikov's rule: major product has more stable carbocation intermediate
• Addition polymerisation: many monomers → long polymer chain