The Mechanism of Free radical substitution

The Mechanism of Free radical substitution Alkanes with Cl2 or Br2 “A photochemical Reaction” Reagent & Condition: Cl2(g) in the presence of ultraviolet light – or sunlight. CH4(g) + Cl2(g) ⎯⎯Sunli⎯ght /⎯U⎯V → CH3Cl + HCl (g) 4 organic products ( CH2Cl2 ;CHCl3 ;CCl4) would be obtained in principle Chain initiation The gaseous mixture of CH4 & Cl2(g) is irradiated with U.V light (400 KJ/mol) which breaks the chlorine molecule [Homolytic fission] into two chlorine radicals. Cl Cl Cl + Cl H = +244 KJmol–1 radicals Chain propagation reactions CH4 + Cl CH3 + H–Cl CH3 + Cl2 Cl + CH3–Cl Chain termination reactions The reactions which remove free radicals as it occurs between any two free radicals thus the reaction stops. They are all exothermic [bond formation] Cl + Cl Cl2 CH3 + Cl CH3–Cl CH3 + CH3 CH3–CH3 which is C2H6 1. Write an equation for the reaction between ethane and a chlorine free radical, and name the type of step in the mechanism where this occurs. Curly arrows are not required. 2. C5H12 reacts with chlorine to form a mixture of products. (i) Classify the type and mechanism of this reaction. (ii) Write the equations for the two propagation steps for this mechanism. Use the molecular formula, C5H12, in your first equation. Curly arrows are not required.

MULTIPLE SUBSTITUTION IN THE CH4 & Cl2 REACTION All the hydrogens in the CH4 can in turn be replaced by chlorine atoms. So you can get dichloromethane CH2Cl2, CHCl3 or tetrachloromethane CCl4 Dichloromethane, CH2Cl2 Chain propagation reactions For Making CH2Cl2 The chlorine radical takes (abstracts) a hydrogen from the chloromethane CH3Cl + Cl CH2Cl + H–Cl The chloromethyl radical formed can then interact with a Cl2 molecule. CH2Cl + Cl2 CH2Cl2 + Cl Tetrachloromethane CCl4 Chain propagation reactions For Making CCl4 The chlorine radical takes (abstracts) a hydrogen from the tri–chloromethane, CHCl3 CHCl3 + Cl CCl3 + H–Cl The trichloromethyl radical formed can then interact with a Cl2 molecule. CCl3 + Cl2 CCl4 + Cl 3. The equations below show some of the processes that occur when methane and chlorine react. A Cl2(g) → 2Cl•(g) B Cl•(g) + CH4(g) → CH3•(g) + HCl(g) C CH3•(g) + CH3•(g) → C2H6(g) D Cl2(g) + CH4(g) → CH3Cl(g) + HCl(g) (a) Which equation shows a propagation step? Answer B (b) Which equation shows an initiation step? Answer A (c) Which equation shows a termination step? Answer C AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 2

Larger Alkanes with Cl2 or Br2 A mixture of substitution products is obtained. e.g propane, C3H8, you could get one of two monobrominated isomers: Br Br 2 Br H = +193 KJmol–1 Chain propagation reactions C3H8 + Br CH2CH2CH3 + H–Br AND CH3CHCH3 which is (CH3)2CH 2o radical CH3CH2CH2 + Br2 CH3CH2CH2–Br + Br 1 – bromopropane CH3CHCH3 + Br2 CH3CHBrCH3 + Br 2–bromopropane AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 3

Chain termination reactions Br2 CH3CH2CH2–Br Br + Br CH3CH2CH2 + Br CH3CH2CH2 + CH2CH2CH3 C6H14 C–C–C C–C–C C–C–C–C–C–C n–hexane C–C–C C C C C–C–C–C C C CH3CH2CH2–CH(CH3)2 CH3CH2CH2 CH(CH3)2 (CH3)2CH CH(CH3)2 2–methylpentane (CH3)2CH–CH(CH3)2 2,3–dimethylbutane AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 4

Electrophilic addition reactions of Alkenes Alkenes are more reactive than alkanes. Most of their reactions are type electrophilic addition in which a molecule adds across the double bond. Explanation ➢ The  electrons are less tightly held than the  (sigma) electrons, since in a  bond, the electron density lies above and below the sigma bond. ➢ The energy released when 2 new () bonds are formed exceeds that needed to break one , so the addition reactions are energetically favourable. 1. Addition of hydrogen (Hydrogenation) • Reactant : Hydrogen gas • Conditions : heated Nickel or platinum catalyst / at 150°C. CH3CH = CHCH3 + H2 ⎯N⎯i /1⎯50⎯o C→ CH3CH2CH2CH3 but–2–ene n – butane 2. Addition of halogens (Halogenation) The addition of halogens produces di-substituted halogenoalkanes. [vicinal dihalides i.e halides on adjacent carbons] CnH2n + X2 ⎯r⎯oom⎯te⎯mp.→ CnH2nX2 Where X2 = {H2 , Cl2 ,Br2} I2 is unreactive • Reactant : liquid bromine Bubble the gaseous alkene into bromine dissolved in CCl4 • Conditions : Electrophilic addition • Type of reaction : H2C = CH2 (g) + Cl2 (g) ⎯r⎯oom⎯te⎯mp.→ ClCH2CH2Cl CH2 = CH2 + Br2(l) ⎯r⎯oom⎯te⎯mp.→ BrCH2CH2Br reddish Brown colourless solution HH H + Br2(l) ⎯r⎯oom⎯te⎯mp.→ HH H    H⎯C⎯C C⎯H H⎯C⎯C⎯C⎯H  H  H Br Br 1,2 –dibromopropane AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 5

3. Alkenes with cold , dilute manganate(VII)  Oxidation of Alkenes CH2 = CH2 cold CH2 ⎯ CH2  + H2O + [O] OH OH Oxygen from the ethan -1,2 -diol Oxidising agent CH3CH=CHCH3 + H2O + [O] CH3CHOH-CHOHCH3 butan–2,3–diol • Reactant : KMnO4(aq) cold & dilute solutions • Conditions : oxidation ( & addition) • Type of reaction : If acidified i.e MnO4¯ / H+(aq) solution is used then the purple solution goes almost colourless (decolourised) by an alkene . 4. Addition of hydrogen halides (Halogenation) CnH2n + HX ⎯r⎯oom⎯te⎯mp.→ CnH2n+1X Where HX = {HCl , HBr} The reactivity of these compounds, due to bond strengths, which is as follows H – F  H – Cl  H – Br  H – I Weakest Bond o Reagents : Hydrogen halide gas HX e.g HBr(g) , HCl(g) o Conditions: reactions occur at room temperatures. o Type of reaction : Electrophilic Addition Symmetrical alkenes : When groups are the same on either side of the C=C double bond.  You always get the same product: because Whichever way round, the electrophile H+ adds onto the double bond , so ONLY ONE carbocation is formed as an intermediate. CH3CH = CHCH3 + HBr (g) ⎯⎯→ CH3 CHCH2CH3  but–2–ene Br Br 2-bromobutane AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 6

Unsymmetrical alkenes  TWO different isomeric products? Electrophile H+ HHH HH H HH H H C C C H + HBr(g) ⎯r⎯oom⎯te⎯mp.→ H C C C H + H C C C H H H Br H H H Br 2–bromopropane 1– bromopropane Major Minor As the electrophile H+ adds onto the double bond ,TWO different carbocation are formed as an intermediate. + – CH3CH CH3 + CH3CH2 CH2 H Br 2o carbocation 1o carbocation CH3CH CH2 Br CH3CH CH3 CH3CHBr CH3 2– bromopropane Lear Br n Major product The major product is formed via a mechanism which involves the formation of the MOST stable carbocation as an intermediate. 4. Pent –1–ene reacts with hydrogen bromide to produce 2– bromopentane as the major product. (a) Outline the mechanism for this reaction (b) Explain why 2– bromopentane is the major product. (c) Identify the minor product formed in this reaction. AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 7

The electrophilic addition of bromine water Electrophile Br+ HHH HH H HH H H C C C H + BrOH ⎯r⎯oom⎯te⎯mp.→ H C C C H + H C C C H H H OH Br H Br OH 1-bromopropan-2-ol 2-bromopropan-1-ol Major product As the electrophile Br+ adds onto the double bond ,TWO different carbocation are formed as an intermediate. + – CH3CH CH2Br + CH3CHBr CH2 Br OH CH3CH CH2 2o carbocation 1o carbocation OH CH3CHOH CH2Br Br 1– bromopropan-2-ol OH CH3CH CH2Br Major product 5. But-2-ene polymerizes to form poly(but-2-ene). Draw a section of this polymer, showing two repeat units. 6. But-1-ene is an alkene with properties similar to ethene. Write an equation, using skeletal formulae for the organic compounds, showing the conversion of but-1-ene to butane. State the essential condition needed. 7. Alkenes react with hydrogen gas in the presence of a nickel catalyst. 0.2 mol of an alkene reacted completely with 19.2 dm3 of hydrogen gas at room temperature and pressure. How many C = C bonds are there in a molecule of this alkene? [The molar volume of a gas is 24.0 dm3 mol–1 at room temperature and pressure] AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 8

8. Alkenes are able to exhibit stereo (E-Z) isomerism, whilst alkanes do not exhibit stereo isomerism. Explain why this is the case as fully as possible. You should incorporate ideas about bonding using pent-2-ene as an example, illustrating your answer with diagrams where appropriate. HH H H H H3C C2H5 H CC C C CH HH cis– pent–2–ene H HH No rotation about this double bond (stereocenter) Each sp2 carbon ( of the C = C ) is bonded to 2 different groups HH HH H C2H5 H3C H H CC C C C H H HHH trans– pent–2–ene • Stereoisomers: compounds have the same structural formulae (i.e. the same atoms connected by the same bonds) but different spatial arrangements of the atoms. Stereoisomerism arises when two different groups are attached to either end of a C = C double bond ( or ring) about which there is restricted rotation. AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 9

• Geometric isomers: the two common atoms or groups can either be on the same side or opposite sides of a double bond or ( stereo-centre) cis-1,2-dichlorocyclohexane Trans-1,2-dichlorocyclohexane Conditions for geometrical isomerism 1. Restricted free rotation due to the presence of double bond or presence of cyclic structure. 2. Each sp2 carbon atom (the double bonded or stereo-centre) is bonded to two different groups or atoms. Z – isomer A stereoisomer in which the two higher priority groups are on same sides of the double bond.    Cl Br H3C Cl   CC  C  C  HF  CH3 H E – isomer A stereoisomer in which the two higher priority groups are on opposite sides of the double bond. AS –Chemistry 2022-2023  H  It has two CH3 groups on the  Cl same side of the double bond. Cl F C  This is a cis- isomer   C CC H3C  BUT   CH3 H Br two higher priority groups are on opposite sides of the double  bond, and so this is an E –isomer. [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 10

Homolytic fission: The bonding pair of electrons split equally so that each atom retains (takes) ONE electron of the bonded electron pair. This bond breaking results in the formation of radicals. Cl Cl Cl + Cl radicals Heterolytic fission: The bonding pair of electrons split unequally so that ONE atom retains(takes) TWO electron of the bonded electron pair. This bond breaking results in the formation of cations & anions. H Br H + Br Anion 9. Why are alkenes not used as fuels?[ Both alkanes & alkenes have very close enthalpy of combustion value ! Alkenes are too valuable as synthetic intermediates / used to make polymers 10. What do you understand by the term : (a) homologous series is series of organic compounds with the same functional group OR /similar chemical properties and exhibit trends in their physical properties. Members of homologous series have the same general formula (each member differs from the previous or successive one by one –CH2 unit) (b) Functional group: is an atom or group of atoms in an organic compound which shows similar reactions regardless of the structure to which it is bonded. e.g. –OH (alcohol) Structural isomers: Compounds that have the same molecular formula but different structural formulae. i.e. structural isomers contain the same number of each type of atom, connected together with different bonding patterns AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 11

• Sigma (σ) bond A region of electron density for which the maximum density lies along the line joining the centres of two atoms. • Pi (π) bond Due to end-on (axial) overlap between atomic orbitals. Dative bond • Free radical A region of electron density for which the maximum • Electrophile density lies above and below the line joining the centres • Nucleophile of two atoms • Addition Reaction Due to sideways (lateral) overlap between two p orbitals A covalent bond in which both electrons come from the same (a single) atom. “ Co-ordinate bond” A chemical species with an unpaired electron. e.g Cl A chemical species that accepts a pair of electrons. A chemical species that donates a pair of electrons. It forms a dative covalent bond with a positive (electron deficient) centres. A type of chemical reaction in which two or more chemical species react to form a single product. • Substitution H2C = CH2 + HBr ⎯⎯→ C2H5Br A type of chemical reaction in which one atom or group of atoms in a molecule is replaced by another atom or group e.g. CH4(g) + Cl2(g) ⎯U⎯.V → CH3Cl(liq) + HCl(g) • Addition A chemical reaction in which many small molecules (monomers) Polymerisation join together to form a long chain molecule (a polymer). Carbocation: A positively charged ion that contains carbon resulted when an organic molecule loses an atom or group of atoms. e.g +CH3 Hydrocarbon An organic compound of hydrogen and carbon ONLY Saturated Hydrocarbon: A compound that contains only (C—C) single bonds. Cannot undergo addition (reactions) and has maximum number of hydrogen atoms . Empirical formula: is a formula that gives ONLY simplest whole-number ratio of the atoms of each element in a compound. e.g. The empirical formula of butane C4H10 is C2H5 AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 12

Molecular formula: is a formula that gives the exact (actual) number of atoms of each element in ONE molecule of a compound. Cracking: is the breaking of a long chain hydrocarbon into smaller hydrocarbon one of which is an alkene. It involves the breaking of C – C bonds in alkanes. Economic reasons for Cracking: 1. Cracking satisfies the high demand for smaller products like petrol and unsaturated hydrocarbons (e.g ethene H2C = CH2) used in making plastics. 2. Cracking preserves the oil stock Reforming: converting straight chain to a branched chain or ring/ or to aromatic(arenes) compound Reforming (Isomerisation) n – hexane 2,3 – dimethyl butane Reforming (cyclisation) + H2 cyclohexane Economic reasons for Reforming: 1. Reforming produces branched– chain alkanes which are better fuels because they burn smoothly & efficiently. [So that maximum amount of energy is transferred to the pistons] 2. Reforming produces cycloalkanes / arenes for the chemical industry. AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 13

Polarising power A measure of the ability of a cation to distort the electron cloud Polarisability in neighbouring anion. Periodicity (of an anion) A measure of the ease with which the electron cloud of an anion may be distorted by a neighbouring cation. “Repeating trend (properties) in each period” OR The repeating variation of physical & chemical properties of the elements in a similar pattern with atomic number. AS –Chemistry 2022-2023 [Topic 5_ ORGANIC _Mechanisms] AOUDI A. 14