Chemistry · Organic chemistry · Year 3

Radical and electrophilic mechanisms

Why does ethene add bromine at once, while methane reacts with chlorine only in light?

The situation

At the Shurtan gas-chemical complex in Kashkadarya, ethane separated from natural gas is heated and turned into ethene, from which polyethylene is made. In the plant laboratory a chemist shows two simple tests: ethene or cyclohexene decolourises bromine water at once, while hexane does nothing; methane does not react with chlorine in the dark, but in light it is chlorinated quickly. Why are saturated and unsaturated hydrocarbons so different? The answer lies in the reaction mechanism: how bonds break, what particles (radicals, carbocations) form, and why one product is plentiful and another scarce.

Topic

Reactions of hydrocarbons: radical and electrophilic mechanisms. Shurtan gas-chemical complex: ethene and polyethylene from natural gas. In the laboratory, bromine water is decolourised, methane reacts with chlorine in light, acetylene…

Key ideas

Radical chain and selectivity
Carbocation, Markovnikov's rule
Kinetic and thermodynamic control

Formulas

Cl_{2} \overset{\overset{}{hν}}{\to} 2 Cl \cdot, Cl \cdot + CH_{4} \overset{}{\to} HCl + CH_{3} \cdot, CH_{3} \cdot + Cl_{2} \overset{}{\to} CH_{3} Cl + Cl \cdot

Radical chain mechanism: initiation and the two propagation steps (the chain).

%_{i} = \frac{n_{H, i} r_{i}}{\sum_{j} n_{H, j} r_{j}} \cdot 100 %

Share of halogenation products: number of hydrogens × relative reactivity (Cl, 25 °C: 1 : 3.8 : 5; Br: ≈ 1 : 82 : 1600).

CH_{3} - CH = CH_{2} + HBr \overset{}{\to} CH_{3} - CHBr - CH_{3} CH_{3} - CH = CH_{2} + HBr \overset{\overset{}{ROOR}}{\to} CH_{3} - CH_{2} - CH_{2} Br

Markovnikov's rule (via a carbocation) and the Kharasch effect (radical mechanism in the presence of peroxide).

K = \frac{[1.4]}{[1.2]} = e^{- Δ G^{\circ} / R T}, \frac{[1.2]}{[1.4]} |_{kinetic} = e^{Δ Δ G^{‡} / R T}

Addition to dienes: at low temperature the rate decides (kinetic control), at high temperature stability decides (thermodynamic control).

HC \equiv CH + H_{2} O \overset{\overset{}{HgSO_{4}, H_{2} SO_{4}}}{\to} CH_{3} CHO

Kucherov reaction: water adds to an alkyne, and the enol passes into the keto form (aldehyde or ketone).

Theory

Reaction mechanism: radicals, ions and curly arrows

A covalent bond can break in two ways: homolytic — the electron pair is shared out equally and two radicals (particles with an unpaired electron) form (light, high temperature, peroxides); heterolytic — the pair goes to one atom, giving a carbocation (C⁺) and an anion, or a carbanion (C⁻). A particle that needs electrons is an electrophile (H⁺, Br⁺, Hg²⁺, a carbocation); one that donates an electron pair is a nucleophile (Br⁻, H₂O, NH₂⁻). In a mechanism the movement of electrons is shown with a curly arrow: a full-headed arrow means an electron pair, a half-headed (“fishhook”) arrow means a single electron; the arrow starts where the electrons come from (a bond, a lone pair, a π cloud). Every step passes through a transition state (an energy peak); the intermediate (radical, cation) is the dip between two peaks; the step with the highest barrier sets the rate. The classification of organic reactions and electronic effects are covered in the “Organic chemistry: structure” experiment.

Alkanes: radical chain substitution and selectivity

Alkanes react only by the radical route: initiation (hν: $Cl_{2} \overset{}{\to} 2 Cl \cdot$ ), propagation ( $Cl \cdot + RH \overset{}{\to} HCl + R \cdot$ , $R \cdot + Cl_{2} \overset{}{\to} RCl + Cl \cdot$ — a single photon makes thousands of molecules react) and termination (two radicals combine). Stability of radicals: tertiary > secondary > primary > CH₃• (hyperconjugation); the C–H bond energies are correspondingly 404 < 411 < 421 < 439 kJ/mol. That is why the relative reactivity per hydrogen is 1 : 3.8 : 5 for chlorine (25 °C) and ≈ 1 : 82 : 1600 for bromine: the reaction with Cl• is exothermic — the transition state is “early”, resembles the reagents and is hardly “aware” of the stability of the radical; with Br• it is endothermic — the transition state is “late”, resembles the radical (the Hammond postulate) — so bromine is much more selective. Share of products = number of hydrogens × reactivity. Other radical reactions: sulfochlorination ( $RH + SO_{2} + Cl_{2} \overset{\overset{}{hν}}{\to} RSO_{2} Cl + HCl$ — a raw material for detergents), sulfoxidation (SO₂ + O₂), nitration (Konovalov: dilute HNO₃, ≈ 140 °C, R–NO₂) and oxidation (to alcohols, ketones and acids). Joining carbon skeletons: the Corey–House reaction ( $R_{2} CuLi + R^{'} X \overset{}{\to} R - R^{'} + RCu + LiX$ ) and palladium-catalysed cross-coupling. In strong superacids alkanes also undergo electrophilic substitution ( $CH_{4} + H^{+} \overset{}{\to} CH_{5}^{+}$ ; G. Olah, Nobel Prize 1994).

Alkenes: electrophilic addition, Markovnikov's rule and stereochemistry

The π cloud of the double bond attracts an electrophile: π complex → carbocation (or a cyclic bromonium ion) → the nucleophile adds. Markovnikov's rule — H⁺ goes to the carbon with more hydrogens, because this gives the more stable (more alkylated) carbocation: 3° > 2° > 1°. Carbocations can rearrange: an H or CH₃ on the neighbouring atom shifts with its electron pair (a 1,2-shift) and gives a more stable cation — 3,3-dimethylbut-1-ene + HCl gives roughly equal amounts of the ordinary and the rearranged chloride. Stereochemistry: Br₂ adds anti through the bromonium ion: cis-but-2-ene gives racemic (2R,3R)/(2S,3S)-2,3-dibromobutane, trans-but-2-ene gives the meso form (stereochemistry — in the “Stereochemistry and conformations” experiment). Syn addition: heterogeneous hydrogenation (on the surface of Ni, Pt, Pd) and homogeneous hydrogenation (Wilkinson's catalyst — an Rh complex in solution), cold dilute KMnO₄ (the Wagner reaction — a cis-diol; on heating the double bond is cleaved), OsO₄. The Kharasch effect (1933): in the presence of peroxide HBr adds by a radical mechanism — Br• goes to the terminal carbon, and the product is anti-Markovnikov (only for HBr). At high temperature Cl₂ substitutes in the allylic position ( $CH_{2} = CH - CH_{3} + Cl_{2} \overset{\overset{}{500 ° C}}{\to} CH_{2} = CH - CH_{2} Cl + HCl$ ). The palladium-catalysed Heck reaction (aryl halide + alkene; Nobel Prize 2010) and olefin metathesis (the C=C fragments swap partners; Grubbs and Schrock catalysts; Nobel Prize 2005) are modern tools of synthesis. School level — “Alkenes and alkadienes”.

Alkadienes: conjugation, kinetic and thermodynamic control

Double bonds can be cumulated (allene $CH_{2} = C = CH_{2}$ , cumulenes), conjugated (buta-1,3-diene) or isolated. In a conjugated diene four p orbitals form a single π system (s-trans and s-cis conformers), and the C2–C3 bond is shortened (1.47 Å). Heats of hydrogenation: but-1-ene −126.8, buta-1,3-diene −236.7 kJ/mol — the conjugation energy is ≈ 2·126.8 − 236.7 ≈ 17 kJ/mol. 1,3-Dienes are made by the Lebedev method (from ethanol) and by dehydrogenating butane. When HBr adds, an allyl cation forms — the positive charge is on C2 and C4; Br⁻ goes to C2 (the 1,2-product) or to C4 (the 1,4-product). The barrier of the 1,2 route is low — it is the kinetic product; the 1,4-product (an internal, more alkylated double bond) is more stable — the thermodynamic product. At −80 °C the reaction is irreversible: ≈ 80 % 1,2; at 40 °C the products convert into each other through the cation and an equilibrium is established: ≈ 85 % 1,4. The model in the experiment reproduces both figures with two parameters.

Alkynes: sp carbon, the Kucherov reaction, acidity

In alkynes the carbon is sp-hybridised: linear (180°), C≡C length 120 pm, one σ bond and two mutually perpendicular π bonds (two pairs of π clouds in the scene). Naming: the suffix “-yne” (propyne, but-1-yne, but-2-yne); isomerism — chain, position of the triple bond, and between classes with alkadienes. Ethyne is made from calcium carbide ( $CaC_{2} + 2 H_{2} O \overset{}{\to} C_{2} H_{2} + Ca (OH)_{2}$ ) and by pyrolysis of methane at 1500 °C; its homologues by removing HX from dihaloalkanes. Reactions: hydrogenation (over Lindlar catalyst it stops at the cis-alkene), addition of halogens and HX in two steps (Markovnikov: a geminal dihalide), the Kucherov reaction (1881, Hg²⁺/H⁺: water adds → enol → keto–enol tautomerism gives an aldehyde or ketone; propyne gives acetone), alcohols (vinyl ethers), ethanoic acid (vinyl acetate), HCN ( $HC \equiv CH + HCN \overset{}{\to} CH_{2} = CH - CN$ , acrylonitrile — nucleophilic addition), hydroboration → vinylboranes → on oxidation an aldehyde (anti-Markovnikov). The s character of an sp carbon is 50 % — the electrons are close to the nucleus, so a terminal alkyne is an acid (pKa 25; ethene 44, ethane ≈ 60): with NaNH₂ it forms acetylides, and in ammoniacal $Ag^{+}$ and $Cu^{+}$ solutions a precipitate forms (the qualitative test for a terminal alkyne). Acetylides lengthen the chain with alkyl halides; Pd/Cu-catalysed cross-coupling (Sonogashira). School level — “Alkynes, benzene and styrene”.

Laboratory (Organic L6), safety and the model

L6: cyclohexene is made from cyclohexanol (dehydration with H₃PO₄ or H₂SO₄, the alkene formed is distilled off), hept-1-ene, 2-methylbut-2-ene and pent-2-ene from the corresponding alcohols; the double bond is detected with bromine water: the yellow-orange colour disappears (hexane — the colour stays). A more alkylated double bond reacts faster with electrophilic Br₂. Quantitative version: m mg of alkene is titrated, $n (Br_{2}) = c V = n (alkene)$ , $M = m / n$ (for accurate quantitative work a bromide–bromate mixture or a solution of bromine in ethanoic acid is used — the alkene dissolves poorly in water). Safety: bromine and chlorine are toxic and burn the skin — work only in a fume cupboard, wearing gloves and goggles; mercury salts (the Kucherov catalyst) are very toxic — not used in a school laboratory; dry silver and copper acetylides explode on impact — destroy them while still wet; do not look at the UV lamp. Model: the halogenation selectivity is carried from the 25 °C values to other temperatures with the Arrhenius law (same pre-exponential factor); the diene model is fitted with two energies; the mechanism geometries are schematic (1 Å ≈ 5 cm), and the decolourisation times show only the order; the concentration of bromine water is taken as 0.050 mol/L.

Did you know?

That peroxides reverse the direction of HBr addition was discovered in 1933 by M. Kharasch and F. Mayo: before that, different laboratories had got different products from the same reaction, causing confusion — the cause was peroxides formed in the air in old samples of alkenes.
A factory in Minamata, Japan, produced acetaldehyde in 1932–1968 by hydrating ethyne over a mercury catalyst (the Kucherov reaction); methylmercury in the waste water built up in fish and poisoned thousands of people — “Minamata disease”.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Stereochemistry and conformations — Year 2
SN1, SN2 and elimination — Year 3
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Carbonyl compounds and synthesis — Year 3
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