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Chapter 8: Reactions of Alkanes; Radicals Page REACTIONS OF ALKANES: CONTENTS Reactivity Considerations Chlorination and Bromination of Alkanes Reactivity–Selectivity Principle Radical Substitution of Benzylic and Allylic Hydrogens Stereochemistry of Radical Substitution No Radical Reactions in Biological Systems 2 RADICALS Sources include: Hydrogen peroxide . Alkyl peroxides . Light causes homolysis of the weak O-O bond . 3 HETEROLYSIS & HOMOLYSIS Heterolytic bond cleavage or heterolysis H Br H+ + Br Homolytic bond cleavage or homolysis H Br H + Br Homolysis produces radicals, which are very reactive species 4 RADICAL CHAIN REACTIONS Initiation turns stable species into radicals by breaking a bond. 5 RADICAL CHAIN REACTIONS Propagation causes products to form without resulting in net consumption of radicals. (production = consumption) 6 RADICAL CHAIN REACTIONS Termination results in net radical destruction. Generally occurs when reactants are used up. 7 CHLORINATION AND BROMINATION OF ALKANES C H A P T E R 12 8 PRODUCT DISTRIBUTION 9 CHLORINATION RATES 10 CHLORINATION PRODUCT DISTRIBUTION 11 BROMINATION RATES 12 REACTIVITY–SELECTIVITY PRINCIPLE The very reactive chlorine atom will have lower selectivity and attack pretty much any hydrogen available on an alkane The less reactive bromine atom will be more selective and tends to react preferentially with the easy targets, i.e. benzylic/allylic > 3° > 2° > 1° 13 RADICALS Stability of alkyl radicals is similar to stability of carbocations 14 CRUDE RADICAL STABILITY INDEX Add 1 for each attached carbon. Add 3 for adjacent double bond or phenyl ring. Radical equally stable on double bond carbon as on single bonded carbon Profile similar to carbocations but resonance contributes more to radical stability, and radical is OK on C=C. 15 ADDITION OF RADICALS Markovnikov Addition Anti-Markovnikov Addition Source of confusion regarding addition of HBr for years; can’t do ionic product analysis with radicals!!! 16 RADICAL ADDITION IS UNIQUE TO HYDROGEN BROMIDE HBr and peroxide goes anti-Markovnikov! 17 HYDROGEN HALIDE ADDITION IN THE PRESENCE OF PEROXIDE Hydrogen peroxide has same effect on hydrogen bromide addition to an alkyne as to an alkene (reversed regioselectivity). C H A P T E R 12 18 BENZYLIC/ALLYLIC RADICALS Benzylic and allylic radicals are even more stable than tertiary alkyl radicals It should be easy for a halogen radical to abstract a benzylic or allylic hydrogen 19 BENZYLIC/ALLYLIC RADICALS Problem in making allylic radical is the greater likelihood that HBr or Br2 will add electrophilically to the double bond rather than making the allylic radical. Br Br2 Br + HBr Br 20 BENZYLIC/ALLYLIC RADICALS Problem solved with N-bromosuccinimide (NBS) is a good reagent for supplying low concentrations of bromine radical NBS doesn’t produce much Br2 or HBr so it only replaces benzylic/allylic H’s 21 BENZYLIC AND ALLYLIC RADICALS Allylic or benzylic radical stabilized by resonance. Only H which leads to most stable set of resonance structures is removed (like C+). Unlike C+ case no thermo or kinetic product exists; Br found on all radical-bearing carbons 22 STEREOCHEMISTRY OF RADICAL SUBSTITUTION Radicals give both enantiomers when they make products like carbocations 23 REACTIONS OF ALKENES - CATALYTIC HYDROGENATION Pt, Pd + H2 H C C H or Ni Heat of hydrogenation - the heat liberated during this reaction. H is ~125 kJ/mol for each double bond in the compound. MECHANISM OF ALKENE HYDROGENATION H2 H H catalyst H2 adsorbed at surface H H H H complexation of hydrogen insertion alkene to catalyst + catalyst H H alkane MECHANISM OF ALKENE HYDROGENATION Hydrogenation is stereospecific. The two hydrogens add to the same side of the double bond - a syn addition. REACTIONS OF ALKENES C C The electrons are less tightly held than the electrons. The double bond therefore acts as a source of electrons - a base – a nucleophile. It reacts with electron deficient compounds - acids - electrophiles. ELECTROPHILIC ADDITION Electron seeking reagents are called electrophilic reagents. The typical reaction of alkenes is one of electrophilic addition - an acid - base reaction. C C + YZ C C Y Z Don’t forget that free radicals are electron deficient. They undergo addition reactions with alkenes. ADDITION OF HYDROGEN HALIDES + HX H C C X HX = HCl, HBr, HI ADDITION OF HYDROGEN HALIDES H2C=CH2 + HCl CH3CH2Cl only one product is possible, chloroethane but.... CH3-CH=CH2 ? CH3-CH=CH2 CH3CH2CH2Cl H-Cl Only 2-chloropropane is formed MARKOVNIKOV’S RULE CH H CH3 HCl 3 H3C C CH3 H CH3 Cl In 1869, Markovnikov proposed that in the addition of an acid to an alkene, the hydrogen of the acid bonds to the carbon which is already bonded to the greater number of hydrogens. MARKOVNIKOV’S RULE CH3CH2CH=CHCH 3 + HI CH3CH2CHICH2CH3 + CH3CH2CH2CHICH3 Each carbon of the double bond is bonded to one H therefore both isomers are formed. MARKOVNIKOV ADDITION - A REGIOSELECTIVE REACTION These reactions are said to be regioselective because only one of the two possible directions of addition occurs. Regioselectivity - the preferential formation of one isomer in those situations where a choice is possible. HBR - THE PEROXIDE EFFECT 1933, Kharasch and Mayo absence of CH3 H3C C CH3 peroxides Br CH3 HBr (-O-O-) CH3-C=CH2 presence of CH3 H C C CH Br peroxides 3 2 H ADDITION OF SULFURIC ACID cold H O H 2SO4 CH3CHCH3 2 CH3CH=CH2 CH3CHCH3 80% OSO3H OH HYDRATION + H CH3 H H CH3 + H2O H C C CH3 H CH3 H OH a Markovnikov addition THE MECHANISM OF THE ADDITION 1. C C + X- H + H X - 2. C C + X C C + H H X + HX = HCl, HBr, HI, H2SO4, H3O AN EXAMPLE H3C H H H 1. H C C C H + Cl- 3 + H H H H Cl H H H H 2. H C C C H + Cl- H C C C H 3 + 3 H Cl H ORIENTATION Cl + Cl- CH3CHCH3 CH3CHCH3 HCl CH3CH=CH2 + X CH3CH2CH2 ORIENTATION CH CH3 3 Cl- CH CCH CH3CCH3 CH 3 3 3 HCl + Cl CH C=CH 3 2 CH 3 + X CH3CHCH2 CH CH 3 - 3 CH CH CCH Cl CH CH CCH 3 2+ 3 3 2 3 CH3 HCl Cl CH CH=C 3 CH CH3 + 3 X CH3CHCCH3 H A MORE GENERAL “RULE” Electrophilic addition to a carbon - carbon double bond involves the intermediate formation of the most stable carbocation. Why? Let’s look at the transition state:- + + H C C C C H + + + H A MORE GENERAL “RULE” Ea > Ea E + CH3CH2CH2 CH3CHCH3 + + CH3CH=CH2 + H CARBOCATION REARRANGEMENTS CH3 HCl CH3 + H3C CH=CH2 H3C CH-CH3 CH3 CH3 CH3 CH3 H H C +CH-CH H C CH 3 3 3 + 3 CH3 CH3 CH3 H CH3 H H C CH H C CH 3 + 3 3 3 CH3 Cl CH3 Cl- OXYMERCURATION OXYMERCURATION An anti addition via a mercurinium ion: + Dissociation: Hg(OAc) - 2 CH3CO2 + HgOCOCH3 + HgOCOCH3 Electrophilic + CH HgOCOCH attack: 3 3 CH3 + HgOCOCH3 CH3 Nucleophilic OH Hg opening: CH3 OCOCH3 H H-O-H OXYMERCURATION Why do we observe Markovnikov addition? + HgOAc HgOAc In the mercurinium ion, the positive charge is shared between the more substituted carbon and the mercury atom. Only a small portion of the charge resides on this carbon but it is sufficient to account for the orientation of the addition but is insufficient to allow a rearrangement to occur. HYDROBORATION H.C. Brown and G. Zweifel, J. Am. Chem. Soc., 83, 2544 (1961) H2O2 + (BH3)2 + B(OH)3 OH- diborane H B H OH Brown was co-winner of the 1979 Nobel Prize in Chemistry. HYDROBORATION 1. (BH ) 3 2 3HC H syn - addition H3C 2. H2O2/OH H OH trans-2-methyl- cyclopentanol (CH3)3CCH=CH2 (CH3)3CCH2CH2OH no rearrangement no carbocation! HYDROBORATION H C=CH H C=CH 2 2 2 2 (CH CH ) BH (BH3)2 CH3CH2BH2 3 2 2 H2C=CH2 H2O2 3C2H5OH + B(OH)3 (CH3CH2)3B OH- HYDROBORATION - THE MECHANISM 1. (BH3)2 CH3CH=CH2 - CH3CH2CH2OH 2. H2O2/OH + HX CH CH CH - CH3CH=CH2 3 2 CH3CHCH 3 + X H X HYDROBORATION - THE MECHANISM 1. (BH ) 3 2 CH3CH=CH2 - CH3CH2CH2OH CH2.3 > HCH2O2/OHCH2 H B H H CH3 > CH CH2 H B H H HYDROBORATION - THE MECHANISM R R - R B O-OH R -B O OH R R R R R -B O OH B OR + HO- R R RO - HO 3- B OR 3ROH + BO3 RO ADDITION OF HALOGENS + X2 X C C X X 2 = Cl 2, Br 2 usually iodine does not react Br2 in CH3CH=CH2 CH3CHBrCH2Br CCl4 1,2-dibromopropane MECHANISM OF X2 ADDITION + X-X C C + X- + X - C C + X C C + X X X + - X X polarisation MECHANISM OF X2 ADDITION H H Br - 2 + Br CH2BrCH2 CH2BrCH2Br H H NaCl NaCl CH BrCH Cl no reaction 2 2 STEREOSPECIFIC REACTIONS Br2 * * CH3CH=CHCH3 CH3CHBrCHBrCH3 (Z)-2-butene gives racemic 2,3-dibromobutane and no meso compound is formed. (E)-2-butene gives only meso-2,3-dibromobutane. A reaction is stereospecific if a particular stereoisomer of the reactant produces a specific stereoisomer of the product. SYN AND ANTI ADDITION Y syn Z Y-Z anti Y Z BROMINE ADDITION - AN ANTI ADDITION I. Roberts and G.E. Kimball, J. Am. Chem. Soc., 59, 947 (1937) + Br 1. Br Br Br- + C C bromonium + ion Br Br 2. C C + Br- C C Br THE BROMONIUM ION Br Br-Br H H H + CH3 H CH3 CH3 CH3 Br H Br + (S,S) H H H Br- Br Br H H + Br (R,R) H Br H Br- HALOHYDRIN FORMATION + X2 + H2O C C + HX X OH X2 = Cl2, Br2 HALOHYDRIN FORMATION greater + here + Br2 Br CH3-CH=CH2 H3CHC CH2 + Br H3CHC CH2 CH3CH-CH2Br O + H2O H H + CH3CH-CH2Br -H 1-bromo-2-propanol O + H H UNSYMMETRIC ELECTROPHILES In the electrophilic addition of an unsymmetric reagent, the electrophilic part adds to the less substituted carbon of the alkene unit: CH3CH=CH2 + A-B CH3CH-CH2 B A FREE RADICAL ADDITION REACTIONS peroxides + Y-Z c C or h Y Z 1.
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