19.2 Aliphatic Hydrocarbons: Alkanes, Alkenes, Alkynes & Addition Reactions

Aliphatic Hydrocarbons: Alkanes, Alkenes, Alkynes & Addition Reactions

In the Intermediate Chemistry Part II syllabus and the UET ECAT entry test, aliphatic chemistry constitutes a core testing domain. ECAT questions focus heavily on reaction mechanisms: the multi-step radical chain pathways of alkane halogenation, the regiochemical nuances of Markovnikov versus anti-Markovnikov alkene additions, ozonolysis degradation for double-bond elucidation, and the unique chemical tests distinguishing terminal alkynes from internal alkynes and alkenes.

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1. Alkanes (Paraffins): Structural Stability & Synthetic Methods

Alkanes are saturated aliphatic hydrocarbons having the general molecular formula $\text{C}_n\text{H}_{2n+2}$. Every carbon is $sp^3$ hybridized, forming four strong $\sigma$-bonds with tetrahedral bond angles ($109.5^\circ$). Alkanes are historically termed paraffins (from Latin parum affinis, meaning "little affinity") because they are extraordinarily unreactive under standard conditions toward strong mineral acids, strong bases, oxidizing agents like $\text{KMnO}_4$, and reducing agents. This chemical inertness stems from two structural attributes:

1. Non-polarity of Bonds: The electronegativity difference between carbon ($2.55$) and hydrogen ($2.20$) is tiny ($0.35$). The bonding electrons are almost symmetrically shared, making the $\text{C}-\text{H}$ bond virtually nonpolar. The $\text{C}-\text{C}$ bond is purely nonpolar. Consequently, electrophiles and nucleophiles find no localized partial charges to initiate attack.
2. High Bond Energies: The $\sigma$-bonds possess exceptionally high dissociation energies ($E_{\text{C-H}} \approx 413\text{ kJ/mol}$, $E_{\text{C-C}} \approx 348\text{ kJ/mol}$). Reactions therefore require severe energetic conditions (elevated temperatures or ultraviolet radiation) to trigger homolytic fission.

Primary Preparative Methods for Alkanes

1. Sabatier-Senderens Reaction (Catalytic Hydrogenation)

Unsaturated hydrocarbons (alkenes or alkynes) react with molecular hydrogen in the presence of finely divided Raney nickel at $200\text{ to }300^\circ\text{C}$ (or platinum/palladium at room temperature and atmospheric pressure):

$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}, 250^\circ\text{C}} \text{CH}_3-\text{CH}_3$

$\text{HC}\equiv\text{CH} + 2\text{H}_2 \xrightarrow{\text{Ni}, 250^\circ\text{C}} \text{CH}_3-\text{CH}_3$

2. Wurtz Reaction

Two moles of an alkyl halide react with metallic sodium in dry, alcohol-free diethyl ether to yield a symmetrical alkane containing twice the number of carbon atoms:

$2\text{R}-\text{X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R}-\text{R} + 2\text{NaX}$

$2\text{CH}_3\text{CH}_2\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 + 2\text{NaBr}$

ECAT Exam Alert: The Wurtz reaction cannot be utilized to synthesize methane ($\text{CH}_4$) because the minimum product formed contains at least two carbon atoms. If two different alkyl halides are coupled (e.g., $\text{CH}_3\text{I} + \text{C}_2\text{H}_5\text{I}$), a mixture of three distinct alkanes (ethane, propane, and butane) is obtained, which have closely spaced boiling points and are exceedingly difficult to separate fractionally.

3. Kolbe's Electrolytic Decarboxylation

Electrolysis of a concentrated aqueous solution of the sodium or potassium salt of a monocarboxylic acid produces a symmetrical alkane and carbon dioxide at the anode, while hydrogen gas and alkali hydroxide accumulate at the cathode:

$\text{Overall:} \quad 2\text{RCOOK} + 2\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} \text{R}-\text{R} + 2\text{CO}_2 \uparrow + 2\text{KOH} + \text{H}_2 \uparrow$

• Anodic Mechanism (Oxidation): Carboxylate anions discharge at the positive anode by losing two electrons to generate carboxyl radicals, which immediately decarboxylate to alkyl radicals. The alkyl radicals dimerize: $2\text{RCOO}^- \xrightarrow{-2e^-} 2\text{RCOO}^\bullet \rightarrow 2\text{R}^\bullet + 2\text{CO}_2 \uparrow$ $\text{R}^\bullet + \text{R}^\bullet \rightarrow \text{R}-\text{R}$
• Cathodic Mechanism (Reduction): Water is reduced at the cathode, generating hydroxide ions and hydrogen gas, causing the pH of the electrolytic bath to rise progressively ($\text{pH} > 7$): $2\text{H}_2\text{O} + 2e^- \rightarrow 2\text{OH}^- + \text{H}_2 \uparrow$

4. Decarboxylation of Monocarboxylic Salts with Soda Lime

Heating the sodium salt of a fatty acid with dry soda lime (a solid mixture of $3\text{ parts } \text{NaOH} + 1\text{ part } \text{CaO}$) produces an alkane with one fewer carbon atom than the parent acid:

$\text{CH}_3\text{COONa} + \text{NaOH} \xrightarrow{\text{CaO}, \Delta} \text{CH}_4 \uparrow + \text{Na}_2\text{CO}_3$

Calcium oxide ($\text{CaO}$) keeps the sodium hydroxide dry, prevents the mixture from melting and attacking the glass vessel, and lowers the reaction temperature.

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2. Alkane Reactions: Free-Radical Halogenation Mechanism

Under direct sunlight or high temperatures ($250\text{-}400^\circ\text{C}$), alkanes react with halogens via a three-stage free-radical substitution mechanism:

1. Chain Initiation (Homolytic Cleavage): Ultraviolet light ($h\nu$) supplies sufficient photon energy to dissociate the relatively weak halogen-halogen bond homolytically without cleaving the stronger hydrocarbon $\text{C}-\text{H}$ bonds: $\text{Cl}-\text{Cl} \xrightarrow{h\nu} 2\text{Cl}^\bullet \quad (\Delta H = +242\text{ kJ/mol})$
2. Chain Propagation (Self-Sustaining Cycle):
   • A reactive chlorine free radical abstracts a hydrogen atom from methane, producing hydrogen chloride and an energetic methyl radical: $\text{Cl}^\bullet + \text{CH}_4 \rightarrow \text{HCl} + {}^\bullet\text{CH}_3$
   • The methyl radical then attacks a chlorine molecule, extracting a chlorine atom to form chloromethane and regenerating a chlorine free radical: ${}^\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet$ This regenerative cycle repeats thousands of times, sustaining the reaction.
3. Chain Termination (Radical Recombination): The chain stops when two free radicals collide and combine, eliminating active intermediates: $\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2$ ${}^\bullet\text{CH}_3 + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl}$ ${}^\bullet\text{CH}_3 + {}^\bullet\text{CH}_3 \rightarrow \text{CH}_3-\text{CH}_3 \quad (\text{Traces of ethane confirmed})$

Reactivity of Halogens:

$\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$

• Fluorine: Reacts explosively even in the dark, cleaving $\text{C}-\text{C}$ bonds (fluorination requires inert nitrogen dilution).
• Chlorine: Rapid in diffused sunlight, yielding a mixture of substitution products ($\text{CH}_3\text{Cl}, \text{CH}_2\text{Cl}_2, \text{CHCl}_3, \text{CCl}_4$).
• Bromine: Slower and highly regioselective, abstracting hydrogen preferentially from tertiary ($3^\circ > 2^\circ > 1^\circ$) carbons due to tertiary radical stability.
• Iodine: Endothermic and reversible; the $\text{HI}$ byproduct is a strong reducing agent that reduces alkyl iodides back to alkanes. Iodination succeeds only in the presence of strong oxidizing agents ($\text{HNO}_3, \text{HIO}_3$) that destroy $\text{HI}$ as it forms ($5\text{HI} + \text{HIO}_3 \rightarrow 3\text{I}_2 + 3\text{H}_2\text{O}$).

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3. Alkenes (Olefins): Preparation & Electrophilic Additions

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($\text{C}=\text{C}$, general formula $\text{C}_n\text{H}_{2n}$). The double bond consists of one strong $\sigma$-bond ($348\text{ kJ/mol}$) and one weaker $\pi$-bond ($263\text{ kJ/mol}$). The $\pi$-electrons reside in diffuse electron lobes above and below the molecular plane, acting as an electron source (Lewis base/nucleophile). Consequently, electrophilic addition reactions dominate alkene chemistry.

Preparative Routes to Alkenes

1. Dehydrohalogenation of Alkyl Halides ($\beta$-Elimination): Heating an alkyl halide with concentrated alcoholic potassium hydroxide ($\text{KOH}$ dissolved in ethanol) removes a hydrogen halide molecule: $\text{CH}_3-\text{CH}_2-\text{Br} + \text{alc. KOH} \xrightarrow{\Delta} \text{CH}_2=\text{CH}_2 + \text{KBr} + \text{H}_2\text{O}$ (Note: Aqueous $\text{KOH}$ promotes substitution to alcohols, whereas alcoholic $\text{KOH}$ acts as a strong, non-solvated base favoring elimination!)
2. Dehydration of Alcohols: Heating alcohols with concentrated sulfuric acid: $\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{conc. } \text{H}_2\text{SO}_4, 180^\circ\text{C}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$ (Critical Temperature Distinction: At $140^\circ\text{C}$, excess ethanol reacts with conc. $\text{H}_2\text{SO}_4$ via intermolecular condensation to produce diethyl ether, $\text{C}_2\text{H}_5\text{OC}_2\text{H}_5$!) Dehydration can also be effected by passing alcohol vapors over alumina ($\text{Al}_2\text{O}_3$) at $350^\circ\text{C}$.
3. Dehalogenation of Vicinal Dihalides: Heating vicinal dihalides with zinc dust in methanol: $\text{Br}-\text{CH}_2-\text{CH}_2-\text{Br} + \text{Zn} \xrightarrow{\text{CH}_3\text{OH}, \Delta} \text{CH}_2=\text{CH}_2 + \text{ZnBr}_2$

Electrophilic Addition Mechanisms

1. Halogenation (Bromine Water Test)

Addition of bromine dissolved in carbon tetrachloride ($\text{Br}_2 / \text{CCl}_4$) across an alkene discharges the reddish-brown color rapidly in the cold without evolving gas. This serves as the classical diagnostic test for unsaturation. The mechanism involves the attack of electrophilic bromine to form a cyclic bromonium ion intermediate, which forces the second bromide ion to attack from the opposite side (stereospecific anti-addition):

$\text{CH}_2=\text{CH}_2 + \text{Br}_2 \xrightarrow{\text{CCl}_4} \underset{\text{1,2-dibromoethane (colorless)}}{\text{Br}-\text{CH}_2-\text{CH}_2-\text{Br}}$

2. Hydrohalogenation & Markovnikov's Rule

When an unsymmetrical reagent (e.g., $\text{HX}$, $\text{H}_2\text{O}$) adds to an unsymmetrical alkene (e.g., propene), Vladimir Markovnikov formulated the empirical rule in 1869:

Markovnikov's Rule: In the electrophilic addition of an unsymmetrical reagent to an unsymmetrical alkene, the positive part (electrophile, typically $\text{H}^+$) attaches to that double-bonded carbon atom which already carries the greater number of hydrogen atoms.

Mechanistic Basis: Addition of $\text{H}^+$ is the rate-determining step. In propene ($\text{CH}_3-\text{CH}=\text{CH}_2$):

• If $\text{H}^+$ adds to $\text{C}1$ (which has two hydrogens), it creates a secondary carbocation ($\text{CH}_3-\overset{+}{\text{C}}\text{H}-\text{CH}_3$).
• If $\text{H}^+$ adds to $\text{C}2$ (one hydrogen), it creates a primary carbocation ($\text{CH}_3-\text{CH}_2-\overset{+}{\text{C}}\text{H}_2$).

Because hyperconjugation and inductive donation stabilize carbocations in the order $3^\circ > 2^\circ > 1^\circ > \text{methyl}$, the secondary carbocation forms faster. Subsequent nucleophilic attack by halide produces 2-bromopropane as the major product ($> 95\%$):

$\text{CH}_3-\text{CH}=\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3-\text{CH(Br)}-\text{CH}_3 \quad (\text{Major})$

3. The Peroxide Effect (Kharasch Anti-Markovnikov Addition)

In 1933, Morris Kharasch discovered that when propene reacts with $\text{HBr}$ in the presence of organic peroxides ($\text{R}-\text{O}-\text{O}-\text{R}$), the regiochemical outcome inverts, yielding 1-bromopropane:

$\text{CH}_3-\text{CH}=\text{CH}_2 + \text{HBr} \xrightarrow{\text{Peroxide}} \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{Br} \quad (\text{Anti-Markovnikov})$

Mechanism: Peroxides decompose homolytically to alkoxy radicals ($\text{RO}^\bullet$), which abstract hydrogen from $\text{HBr}$ to form a bromine free radical ($\text{Br}^\bullet$). The bromine radical adds first to the double bond. It attacks $\text{C}1$ to generate the more stable secondary carbon radical ($\text{CH}_3-\dot{\text{C}}\text{H}-\text{CH}_2\text{Br}$), which abstracts hydrogen from another $\text{HBr}$ molecule to yield 1-bromopropane.

ECAT Crucial Exception: The peroxide effect is strictly observed for $\text{HBr}$ only! It fails completely for $\text{HCl}$ and $\text{HI}$:

• For $\text{HCl}$: The $\text{H}-\text{Cl}$ bond is exceptionally strong ($431\text{ kJ/mol}$). In the propagation cycle, the carbon radical formed after $\text{Cl}^\bullet$ adds to the alkene must pull a hydrogen atom off $\text{H}-\text{Cl}$; that step is endothermic and too slow, so the chain does not run.
• For $\text{HI}$: The $\text{H}-\text{I}$ bond is weak, but the addition of iodine radical to the double bond is endothermic. Furthermore, iodine radicals dimerize rapidly to molecular $\text{I}_2$ ($2\text{I}^\bullet \rightarrow \text{I}_2$).

4. Ozonolysis (Locating Double Bonds)

Ozone ($\text{O}_3$) cleaves carbon-carbon double bonds cleanly. The alkene dissolves in an inert solvent ($\text{CH}_2\text{Cl}_2$) and absorbs $\text{O}_3$ at $-78^\circ\text{C}$ to form an unstable molozonide, which rearranges into a stable cyclic ozonide. Reductive workup with zinc dust and water ($\text{Zn} / \text{H}_2\text{O}$) cleaves the ozonide into carbonyl compounds:

$\text{R}_2\text{C}=\text{CHR}' + \text{O}_3 \xrightarrow{\text{Zn} / \text{H}_2\text{O}} \text{R}_2\text{C}=\text{O} + \text{R}'\text{CHO} + \text{H}_2\text{O}_2$

Zinc dust is essential because it reduces hydrogen peroxide ($\text{H}_2\text{O}_2$) to zinc oxide and water, preventing aldehydes from being oxidized into carboxylic acids.

5. Baeyer's Test (Hydroxylation)

Treating alkenes with cold, dilute, neutral or alkaline potassium permanganate solution ($\text{KMnO}_4$, Baeyer's reagent) oxidizes them to vicinal 1,2-diols (glycols). The purple $\text{KMnO}_4$ solution is decolorized with the precipitation of a dark brown sludge of manganese dioxide ($\text{MnO}_2$):

$3\text{CH}_2=\text{CH}_2 + 2\text{KMnO}_4 + 4\text{H}_2\text{O} \rightarrow 3\underset{\text{Ethylene glycol}}{\text{CH}_2(\text{OH})-\text{CH}_2(\text{OH})} + 2\text{MnO}_2 \downarrow + 2\text{KOH}$

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4. Alkynes: Electronic Structure, Terminal Acidity & Reactions

Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond ($\text{C}\equiv\text{C}$, general formula $\text{C}_n\text{H}_{2n-2}$). The triple bond consists of one strong $\sigma$-bond and two perpendicular $\pi$-bonds formed by $sp$-hybridized carbons with linear ($180^\circ$) geometry.

The Acidity of Terminal Alkynes

Terminal alkynes (molecules having the group $-\text{C}\equiv\text{C}-\text{H}$, such as ethyne or propyne) possess weakly acidic hydrogens ($pK_a \approx 25$). By comparison, alkenes ($pK_a \approx 44$) and alkanes ($pK_a \approx 50$) are non-acidic.

Origin of Acidity: In an $sp$-hybridized alkyne carbon, the hybrid orbital has $50\%\, s$-character (compared to $33.3\%$ in $sp^2$ and $25\%$ in $sp^3$). Because $s$-orbitals are spherically symmetrical and penetrate closer to the positive nucleus, electrons in an $sp$ orbital experience a stronger effective nuclear charge. The $sp$-carbon is significantly more electronegative, polarizing the $\text{C}-\text{H}$ bond and stabilizing the conjugate base (acetylide carbanion, $\text{R}-\text{C}\equiv\text{C}:^-$) upon proton departure.

Terminal alkynes are weaker acids than water ($pK_a = 15.7$) and alcohols ($pK_a \approx 16\text{-}18$), but stronger acids than ammonia ($pK_a \approx 38$). Thus, sodamide ($\text{NaNH}_2$) deprotonates terminal alkynes:

$\text{CH}_3-\text{C}\equiv\text{C}-\text{H} + \text{NaNH}_2 \xrightarrow{\text{liquid } \text{NH}_3} \text{CH}_3-\text{C}\equiv\text{C}^-\text{Na}^+ + \text{NH}_3$

Diagnostic Heavy Metal Acetylide Tests

These precipitation reactions uniquely distinguish terminal alkynes from internal alkynes and all alkenes:

1. Tollens' Test (Ammoniacal Silver Nitrate): Passing ethyne or a terminal alkyne through ammoniacal silver nitrate solution ($\text{AgNO}_3 + \text{NH}_4\text{OH}$) produces a white precipitate of silver acetylide: $\text{HC}\equiv\text{CH} + 2[\text{Ag}(\text{NH}_3)_2]\text{NO}_3 \rightarrow \underset{\text{Disilver acetylide (white ppt)}}{\text{Ag}-\text{C}\equiv\text{C}-\text{Ag} \downarrow} + 2\text{NH}_4\text{NO}_3 + 2\text{NH}_3$
2. Ammoniacal Cuprous Chloride Test: Passing ethyne through ammoniacal cuprous chloride ($\text{Cu}_2\text{Cl}_2 + \text{NH}_4\text{OH}$) produces a reddish-brown precipitate of dicopper acetylide: $\text{HC}\equiv\text{CH} + \text{Cu}_2\text{Cl}_2 + 2\text{NH}_4\text{OH} \rightarrow \underset{\text{Dicopper acetylide (red ppt)}}{\text{Cu}-\text{C}\equiv\text{C}-\text{Cu} \downarrow} + 2\text{NH}_4\text{Cl} + 2\text{H}_2\text{O}$

Internal alkynes (e.g., but-2-yne, $\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_3$) lack a terminal acidic hydrogen and give negative tests (no precipitate) with both reagents.

Crucial Alkyne Addition & Polymerization Reactions

1. Catalytic Hydration of Alkynes: Alkynes add one molecule of water in the presence of $10\%\, \text{H}_2\text{SO}_4$ and $1\%\, \text{HgSO}_4$ catalyst at $75\text{-}80^\circ\text{C}$. The initial enol product rapidly tautomerizes:
   • Ethyne yields Acetaldehyde: $\text{HC}\equiv\text{CH} + \text{H}_2\text{O} \xrightarrow{\text{Hg}^{2+} / \text{H}^+} [\text{CH}_2=\text{CH}-\text{OH}] \rightleftharpoons \text{CH}_3-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{H}$
   • All other alkynes yield Ketones (Markovnikov orientation): $\text{CH}_3-\text{C}\equiv\text{CH} + \text{H}_2\text{O} \xrightarrow{\text{Hg}^{2+} / \text{H}^+} [\text{CH}_3-\text{C(OH)}=\text{CH}_2] \rightleftharpoons \text{CH}_3-\overset{\overset{\text{O}}{\parallel}}{\text{C}}-\text{CH}_3$
2. Cyclic Polymerization (Trimerization to Benzene): When ethyne gas is passed through a red-hot copper or iron tube at $300^\circ\text{C}$, three ethyne molecules undergo cyclic trimerization to form benzene: $3\text{HC}\equiv\text{CH} \xrightarrow{\text{Cu tube}, 300^\circ\text{C}} \text{C}_6\text{H}_6$

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5. Summary of Aliphatic Transformations & Mechanisms