“Organic Chemistry: Hydrocarbons”

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Organic Chemistry: Hydrocarbons — Key Facts for JAMB

Alkanes ($C_nH_{2n+2}$): Saturated hydrocarbons. $sp^3$ hybridisation throughout. Boiling point increases with molecular weight. Straight chain < branched (for same MW, branched has lower BP). Methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$), butane ($C_4H_{10}$).

Alkenes ($C_nH_{2n}$): Unsaturated with at least one C=C double bond. $sp^2$ hybridisation. Ethene ($C_2H_4$), propene ($C_3H_6$). Addition reactions: hydrogenation ($Ni, 300°C$), halogenation, hydration ($H^+, H_2O$), polymerisation.

Alkynes ($C_nH_{2n-2}$): Unsaturated with at least one C≡C triple bond. $sp$ hybridisation. Ethyne (acetylene, $C_2H_2$), propyne ($C_3H_4$). Triple bond is linear (180°). Terminal alkynes can be deprotonated with $NaNH_2$ to form acetylides.

Isomerism: Structural: chain (straight vs branched), position (location of functional group/double bond), functional group. Stereoisomerism: geometric (E/Z for alkenes) and optical (R/S for chiral centres).

⚡ Exam tip: Alkenes decolourise bromine water (reddish-brown to colourless) and $KMnO_4$ (purple to colourless). This distinguishes them from alkanes which do not react with these reagents.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Organic Chemistry: Hydrocarbons — JAMB UTME Study Guide

Nomenclature (IUPAC):

1. Find longest carbon chain
2. Number from end that gives lowest locant to functional group or multiple bond
3. Name substituents alphabetically with numerical prefixes (di-, tri-, etc. don’t count for alphabetical order)
4. For alkenes: -ene; for alkynes: -yne
5. Cycloalkanes: prefix “cyclo-”

Examples:

• $CH_3CH_2CH_2CH_3$: butane
• $CH_3CH(CH_3)CH_2CH_3$: 2-methylbutane
• $CH_3CH=CHCH_3$: but-2-ene
• $CH_3C≡CH$: propyne
• Cyclohexane: $C_6H_{12}$ (ring)

Reactions of Alkanes:

• Combustion: $CH_4 + 2O_2 → CO_2 + 2H_2O$; complete combustion (excess $O_2$) vs incomplete (limited $O_2$ → $CO$, $C$)
• Halogenation: $CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$. Free radical mechanism: initiation → propagation → termination.
• Cracking: breaking large alkanes into smaller ones (thermal cracking at high $T$; catalytic cracking with $Al_2O_3/SiO_2$)
• Reforming: converting straight-chain alkanes to branched or aromatic compounds for better petrol

Reactions of Alkenes:

• Addition of $H_2$: hydrogenation ($Ni, 300°C$) → alkane
• Addition of $X_2$ (Br₂/Cl₂): dibromide/dichloride product (bromine test — decolourises $Br_2$ in $CCl_4$)
• Addition of $HX$: Markovnikov’s rule — H adds to carbon with more H atoms, X adds to carbon with fewer H atoms. For $CH_3CH=CH_2 + HCl$: product is $CH_3CHClCH_3$ (2-chloropropane).
• Addition of $H_2O$: hydration ($H_2SO_4$ catalyst) → alcohol. Markovnikov’s addition.
• Oxidation: cold dilute $KMnO_4$ → diol ($cis$ product); hot concentrated $KMnO_4$ → $CO_2$ or carboxylic acid.

Reactions of Alkynes:

• Addition of $H_2$ (2 steps): (1) $H_2/Pb, BaSO_4$ (Lindlar’s catalyst) → cis-alkene; (2) $H_2/Na, NH_3$ → trans-alkene
• Addition of halogens (2 steps): forms tetrahalide
• Addition of $HX$ (2 steps): Markovnikov’s addition at each step
• Addition of $H_2O$: hydration requires $HgSO_4/H_2SO_4$ → ketone (tautomerises from enol)
• Acidic hydrogen: terminal alkynes ($R-C≡CH$) react with $Na$ or $NaNH_2$ to form acetylides

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Organic Chemistry: Hydrocarbons — Comprehensive Chemistry Notes

Free Radical Halogenation — Mechanism:

Initiation: $Cl_2 \xrightarrow{UV} 2Cl^\cdot$ (chlorine radicals)

Propagation:

1. $Cl^\cdot + CH_4 → HCl + \cdot CH_3$ (methyl radical)
2. $\cdot CH_3 + Cl_2 → CH_3Cl + Cl^\cdot$ (regenerates Cl radical)

Termination:

1. $\cdot Cl + \cdot Cl → Cl_2$
2. $\cdot CH_3 + \cdot CH_3 → C_2H_6$
3. $\cdot CH_3 + \cdot Cl → CH_3Cl$

For propane $C_3H_8$: two types of H — primary ($6 \times$) and secondary ($2 \times$). Relative reactivity: secondary H : primary H = 3.8 : 1. So chlorination gives more 2-chloropropane than expected from just counting H atoms.

Markovnikov’s Rule — Explained: The addition of $HX$ to an alkene goes through a carbocation intermediate. More substituted carbocations are more stable (tertiary > secondary > primary). So in $CH_3CH=CH_2 + HCl$: the secondary carbocation ($CH_3\stackrel{+}{C}HCH_3$) is more stable than the primary one ($CH_3CH_2\stackrel{+}{C}H_2$), so $Cl$ attaches to the more substituted carbon.

Carbocation Rearrangements: During addition reactions, carbocations can rearrange (hydride shift or methyl shift) to form more stable carbocations. This leads to unexpected products. Example: $CH_3CH_2C(CH_3)=CH_2 + HCl$ can give a rearranged tertiary chloride via a 1,2-methyl shift.

Geometric Isomerism (E/Z): For an alkene $C_a=C_b$ where each carbon has two different substituents:

• Z (zusammen = together): highest priority groups on same side
• E (entgegen = opposite): highest priority groups on opposite sides
• Priority by CIP rules: higher atomic number of directly attached atom

Example: But-2-ene: $CH_3CH=CHCH_3$. E-but-2-ene (trans): methyl groups opposite. Z-but-2-ene (cis): methyl groups same side.

Polymerisation:

• Alkenes undergo addition polymerisation via free radical or ionic mechanisms
• Polyethene (PE): $(-CH_2-CH_2-)_n$: from ethene with peroxide or Ziegler-Natta catalyst
• Polypropene (PP): $(-CH_2-CH(CH_3)-)_n$: from propene
• PTFE (Teflon): $(-CF_2-CF_2-)_n$: from tetrafluoroethene
• Natural rubber: cis-polyisoprene from latex of Hevea brasiliensis; vulcanised with sulfur (cross-links for elasticity)

Aromatic Hydrocarbons (Benzene): Benzene ($C_6H_6$): planar, $sp^2$ hybridised, 6 $\pi$ electrons delocalised in a ring. Unreactivity relative to alkenes: electrophilic substitution rather than addition. Reactions:

• Halogenation: $Br_2/FeBr_3$ → bromobenzene
• Nitration: $HNO_3/H_2SO_4$ → nitrobenzene
• Sulfonation: $H_2SO_4$ → benzenesulfonic acid
• Friedel-Crafts alkylation: $RCl/AlCl_3$ → alkylbenzene
• Combustion: $2C_6H_6 + 15O_2 → 12CO_2 + 6H_2O$ (sooty flame due to high C:H ratio)

JAMB Pattern Analysis: JAMB questions frequently test: (1) Distinguishing alkanes from alkenes using $Br_2$ water or $KMnO_4$, (2) Markovnikov addition products, (3) IUPAC naming, (4) Polymerisation reactions, (5) Isomerism in alkenes. JAMB 2023: “Which of the following decolourises bromine water: (a) ethane (b) ethene (c) benzene?” Answer: ethene (bromine adds across the C=C double bond). Note: benzene does NOT decolourise bromine water under normal conditions (it requires a catalyst for substitution).

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