Alkenes

Making alkenes:

• Elimination from a halogenoalkane: ethanolic NaOH, heat — e.g. $\text{CH}_3\text{CH}_2\text{Br} \rightarrow \text{CH}_2\text{=CH}_2 + \text{HBr}$.
• Dehydration of an alcohol: heated Al₂O₃ catalyst, or concentrated H₂SO₄/H₃PO₄ — e.g. $\text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O}$.

Addition reactions of the C=C (electrophilic addition):

Reagent / conditions	Product
H₂, Ni/Pt catalyst, heat	alkane (hydrogenation)
Steam H₂O(g), H₃PO₄ catalyst	alcohol (hydration)
HX(g), room temperature	halogenoalkane
X₂ (e.g. Br₂)	dihalogenoalkane

Bromine test for C=C: shaking an alkene with aqueous bromine turns it from orange/brown to colourless (decolourised) — a positive test for unsaturation.

The C=C double bond is electron-rich (the π electrons), so it attracts electrophiles. Mechanism for ethene + Br₂:

1. As Br₂ approaches the C=C, the π electrons induce a dipole in Br₂ (Br$^{\delta+}$–Br$^{\delta-}$).
2. The π electrons attack the δ+ bromine (curly arrow from the C=C to Br); the Br–Br bond breaks heterolytically (curly arrow to the other Br), forming a carbocation and a Br⁻ ion.
3. The Br⁻ ion (nucleophile) then bonds to the carbocation, giving 1,2-dibromoethane.

For HBr + propene, the same mechanism applies: H$^{\delta+}$ is the electrophile, and the H adds first, then Br⁻ adds to the carbocation. Show curly arrows starting at the C=C and at the H–Br bond.

When HX adds to an unsymmetrical alkene (e.g. propene), two carbocations are possible. The reaction goes mainly via the more stable carbocation, giving the Markovnikov product.

Carbocation stability: tertiary > secondary > primary. Alkyl groups are electron-donating (positive inductive effect) — they push electron density towards the positive carbon, spreading out and stabilising the charge. More alkyl groups attached = more stable cation.

Worked — HBr + propene ($\text{CH}_3\text{CH=CH}_2$):

• Possible cations: a secondary cation ($\text{CH}_3\overset{+}{\text{C}}\text{HCH}_3$) or a primary cation ($\text{CH}_3\text{CH}_2\overset{+}{\text{C}}\text{H}_2$).
• The secondary cation is more stable → the major product is 2-bromopropane ($\text{CH}_3\text{CHBrCH}_3$).

Markovnikov's rule (a shortcut): the H of HX adds to the carbon that already has more H atoms.

Oxidation by acidified potassium manganate(VII):

• Cold, dilute KMnO₄ → a diol (two –OH groups added across the C=C). The purple KMnO₄ is decolourised — another test for C=C. e.g. ethene → ethane-1,2-diol.
• Hot, concentrated KMnO₄ → the C=C is ruptured (broken), giving carboxylic acids, ketones and/or CO₂ depending on the groups on each double-bond carbon. The products reveal the position of the double bond in a larger molecule.

Addition polymerisation: many alkene molecules (monomers) join by addition to form a long-chain polymer with no other product: $n\,\text{CH}_2\text{=CH}_2 \rightarrow {-}(\text{CH}_2\text{CH}_2)_n{-}$ giving poly(ethene); propene gives poly(propene). (Developed further in Topic 20.)