Organic Chemistry: Alkenes and Alkynes

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alkenes: Unsaturated hydrocarbons containing a carbon-carbon double bond ($C=C$). General formula: $C_nH_{2n}$ (for one double bond). They are unsaturated because the double bond contains fewer than the maximum number of hydrogen atoms.

Alkynes: Unsaturated hydrocarbons containing a carbon-carbon triple bond ($C\equiv C$). General formula: $C_nH_{2n-2}$ (for one triple bond).

Key Members:

• Ethene ($C_2H_4$): $CH_2=CH_2$ — plant hormone (ripening fruit), made industrially by cracking alkanes
• Propene ($C_3H_6$): $CH_3CH=CH_2$
• Ethyne (acetylene, $C_2H_2$): $CH\equiv CH$ — used in oxy-acetylene welding

Nomenclature:

• Alkenes: suffix -ene (ethene, propene, but-1-ene, but-2-ene)
• Alkynes: suffix -yne (ethyne, propyne)
• Number the chain from the end nearest the multiple bond to give the lowest locant
• But-2-ene exists as geometric (cis-trans) isomers due to restricted rotation about the double bond

⚡ WAEC Exam Tip: The bromine water test is the key distinction between alkanes and alkenes. Alkenes decolourise bromine water (orange to colourless) due to addition across the double bond. Alkanes produce no visible change. This is a guaranteed WAEC question.

$$CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br \text{ (1,2-dibromoethane — colourless)}$$

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Bonding in Alkenes ($sp^2$ hybridisation): Each carbon in a double bond is $sp^2$ hybridised — three hybrid orbitals form sigma bonds in a trigonal planar arrangement (120° bond angles), and one unhybridised $p$-orbital forms the pi ($\pi$) bond by sideways overlap. The $\pi$-bond is weaker than the $\sigma$-bond and is the site of reaction in alkenes.

Bonding in Alkynes ($sp$ hybridisation): Each carbon in a triple bond is $sp$ hybridised — two hybrid orbitals form sigma bonds (linear geometry, 180° bond angles), and two unhybridised $p$-orbitals each form one $\pi$-bond. The triple bond consists of one $\sigma$-bond and two $\pi$-bonds.

Addition Reactions of Alkenes:

Hydrogenation: $CH_2=CH_2 + H_2 \xrightarrow{Ni, 150°C} CH_3CH_3$ (alkane) Halogenation: $CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$ (colourless product — decolourises bromine water) Hydration (Markovnikov addition): $CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$ (ethanol) — acid catalyst required

⚡ WAEC Exam Tip: Markovnikov’s Rule states: in the addition of H–X to an alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already present. For $CH_3CH=CH_2 + HCl$: H attaches to $CH_3CH$– (2 H atoms) and Cl attaches to the other carbon — giving $CH_3CHClCH_3$ (2-chloropropane), NOT 1-chloropropane.

Preparation of Alkynes:

• Calcium carbide method: $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$ (ethyne)
• Laboratory: Dehydrohalogenation of vicinal dihalides (remove two molecules of HX using alcoholic KOH)

Addition Reactions of Alkynes:

• One molecule of $Br_2$: $CH\equiv CH + Br_2 \rightarrow CHBr=CHBr$ (1,2-dibromoethene)
• Excess $Br_2$: $CH\equiv CH + 2Br_2 \rightarrow CHBr_2CHBr_2$ (1,1,2,2-tetrabromoethane)
• Hydration: $CH\equiv CH + H_2O \xrightarrow{Hg^{2+}, H^+} CH_3CHO$ (acetaldehyde — ethyne gives ethanal; other alkynes give ketones)

Oxidation of Alkes:

• Cold dilute $KMnO_4$: $3CH_2=CH_2 + 2KMnO_4 + 4H_2O \rightarrow 3CH_2(OH)CH_2(OH) + 2MnO_2 + 2KOH$ (diol, purple $KMnO_4$ turns brown $MnO_2$)
• Hot concentrated $KMnO_4$: Cleaves the double bond — gives carboxylic acids or ketones depending on substitution pattern

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Cis-Trans (Geometric) Isomerism in Alkenes: But-2-ene exhibits cis-trans isomerism because rotation about the C=C bond is restricted (would require breaking the $\pi$-bond). Conditions for geometric isomerism:

1. Restricted rotation about a bond (C=C)
2. Two different groups on each carbon of the double bond

Isomer	Structure	Physical Properties
cis-but-2-ene	Both methyl groups on same side	BP = 3.7°C; more polar
trans-but-2-ene	Methyl groups on opposite sides	BP = 0.9°C; less polar

trans-but-2-ene is more stable because the bulky methyl groups are further apart, minimising steric strain. The cis-isomer has higher boiling point due to slight molecular polarity.

⚡ WAEC Exam Tip: E/Z nomenclature replaced E/S (old) and cis/trans for more complex cases. Priority rules: compare atomic numbers of atoms directly attached to each double-bonded carbon. The letter Z (zusammen = together) is assigned when the two higher-priority groups are on the same side.

Mechanism of Electrophilic Addition to Alkenes: This is the core mechanism WAEC expects at this level:

1. The $\pi$-bond is electron-rich and attacks the partially positive end of the polar reagent (electrophile)
2. The $\pi$-electrons move to form a new sigma bond with the electrophile
3. A carbocation intermediate is formed (unstable, $sp^2 \rightarrow sp^3$)
4. The nucleophile attacks the carbocation to complete addition

For HBr addition to ethene: $H^\delta+ \rightarrow Br^\delta-$ (polarised molecule) Step 1: $CH_2=CH_2 + H^+ \rightarrow \overset{+}{C}H_2CH_3$ (ethyl carbocation) Step 2: $\overset{+}{C}H_2CH_3 + Br^- \rightarrow CH_3CH_2Br$ (bromoethane)

Carbocation stability: tertiary > secondary > primary. This explains why Markovnikov’s addition gives the more substituted product (secondary or tertiary carbocation is more stable than primary).

Polymerisation: Alkenes undergo addition polymerisation — many monomer units join via the double bond without loss of small molecules.

• Polyethene (polythene): $n(CH_2=CH_2) \rightarrow (CH_2-CH_2)_n$ — plastic bags, bottles
• Polypropene: $n(CH_3CH=CH_2) \rightarrow$ polypropylene — packaging, textiles

The double bond opens at the $\pi$-bond, and each carbon forms two new single bonds to adjacent monomers. No atoms are lost — all atoms from the monomer become part of the polymer.

Comparative Study — Alkenes vs Alkynes:

Property	Alkenes ($C=C$)	Alkynes ($C\equiv C$)
Hybridisation	$sp^2$	$sp$
Bond angle	~120°	180°
$\sigma$-bonds in C–C bond	1	1
$\pi$-bonds in C–C bond	1	2
Reaction type	Electrophilic addition	Electrophilic/nucleophilic addition
Bromine water test	Decolourises (1 mole $Br_2$)	Decolourises (2 moles $Br_2$)
$KMnO_4$ (cold dilute)	Diol (purple→brown)	Diol (purple→brown)
$KMnO_4$ (hot conc.)	Carboxylic acids + $CO_2$	Carboxylic acids + $CO_2$
Shape around C=C carbon	Trigonal planar	Linear

Distinguishing Tests — Summary:

Test	Observation with Alkene	Observation with Alkyne
Bromine water ($Br_2(aq)$)	Colour disappears rapidly	Colour disappears (needs 2× more $Br_2$)
Acidified $KMnO_4$	Brown $MnO_2$ precipitate	Brown $MnO_2$ precipitate
Ammoniacal $AgNO_3$	No change	White precipitate of silver acetylide (for terminal alkynes)
Ammoniacal $Cu_2Cl$	No change	Red-brown precipitate of copper acetylide (for terminal alkynes)

⚡ WAEC Exam Tip: Terminal alkynes ($R-C\equiv CH$) give precipitates with ammoniacal silver nitrate and copper(I) chloride. Internal alkynes ($R-C\equiv C-R’$) do not give these precipitates. This is a reliable distinguishing test in WASSCE Paper 2.

WAEC Past Question Patterns:

• Drawing structural and displayed formulas of ethene, propene, but-1-ene, but-2-ene
• Writing equations for addition of $H_2$, $Cl_2$, $H_2O$, $HCl$, $HBr$ to alkenes
• Applying Markovnikov’s rule to predict products
• Identifying cis-trans isomers of but-2-ene
• Explaining why but-1-ene does NOT exhibit geometric isomerism
• Writing polymerisation equations (e.g., formation of polyethene from ethene)
• Describing the addition of bromine to alkynes (stage 1 and stage 2)
• Writing the reaction between ethyne and ammoniacal silver nitrate

⚡ WAEC Exam Tip: In Paper 2 equations, state the conditions precisely. For hydrogenation write “Ni catalyst at 150°C”; for hydration of ethene write “steam with $H_3PO_4$ catalyst”; for ethyne hydration write “$Hg^{2+}$ and dilute $H_2SO_4$.” Vague conditions lose marks.