Topic 5E Reactions of alkenes and alkynes

Reaction Mechanisms 52 In an organic reaction: we break bonds and form bonds, and these bonds are covalent; electron pairs are involved A mechanism describes the sequence in which bond breaking and bond formation occurs as well as how the energy of the system changes during a process Use arrow notation to depict movement of electrons and energy profile diagrams to depict energy changes during that process.

Reaction Energy Profiles 53 Exothermic reaction pathway:

Reaction Energy Profiles 53 Endothermic reaction pathway:

A two-step reaction 54 Intermediates lie in shallow energy wells Rate-determining step— step with highest E A

Addition reactions summary 55 RCHCH 2 RCHCH 2 XH HXRCHCH 2 HOH H + /HOH RCHCH 2 XX X 2 RCHCH 2 HH H 2

Addition reactions summary 37 RCHCH 2 RCHCH 2 XH HX

Addition of Hydrogen Halides 55 Hydrogen halides react with alkenes Alkyl halides are formed CH 2 2 HBrCHCH 3 CH 2 Br Ethyl bromide

Addition of Hydrogen Halides 56 Two products are possible Only 2-chloropropane is formed H always adds to side of double bond with most hydrogens — the Markovnikov rule Cl CH 3 CHCH 3 CH 3 –CH=CH 2 +HCl CH 3 CH 2 2 Cl Not formed

Mechanism 57 Acids are sources of electrophilic H + H + is attacked by the  electon pair of  bond leaving a carbocation a positive carbon (Double headed arrows for electron pairs)

Mechanism animation 57 Addition of HCl to 2-methylpropene Ocol, Bcorgchm CD ROM McMurry 2.0

Mechanism in detail 57 Two electrons from  bond form  bond with H +, an electrophile This leaves sp 2 carbon with five electrons, a carbocation CH 3 H H H 3 H H H H + vacant p z orbital sp 3

Mechanism in detail 58 Finally Cl –, a nucleophile,donates a pair of electrons to carbocation forming a C—Cl  bond Both carbons are sp 3 hybridised. H—Cl has added across the double bond CH 3 H H H H H H HCl H + – sp CH 3

Mechanism in detail 58 CH CHCH 3 (not CH 3 CHCH )H + secondary carbocation primary carbocation CCH 2 + H CCH C 2 H not tertiary carbocation primary carbocation Why?

Classification of carbocations 58 Three Groups attached to cation centre CH 3 3 H H CH 3 C 3 3 C Primary SecondaryTertiary Two Groups attached to cation centre One Group attached to cation centre

Stability of carbocations 59 Alkyl groups push electrons through sigma bonds, they are electron donating They are positively INDUCTIVE (+I) and charge is stabilised through delocalisation or dispersion The more alkyl groups the greater the stability Inductive effects operate over only one to two bonds CH 3 3 H H CH 3 C 3 3 C ++ ++ ++ ++ ++ ++ Primary SecondaryTertiary RCH 2 + << R 2 CH + < R 3 C + increasing carbocation stability

Mechanism in detail 59 CH CHCH 3 (not CH 3 CHCH )H + secondary carbocation primary carbocation CCH 2 + H CCH C 2 H not tertiary carbocation primary carbocation Why? RCH 2 + << R 2 CH + < R 3 C + increasing carbocation stability

Stability of carbocations 59 More stable carbocation is formed more easily It is a lower activation energy process Energy Reaction Coordinate) CH 3 CH=CH 2 + H + CH not formed E a (1°) CH 3 3 observed reaction E a (2°)

Other additions of HX 60 An addition of HBr: CC CH H CHCH not + 2  carbocation CCHCH 2 CH  carbocation + H + C CH Br CH 3 Br – 2-bromo-2-methylbutane

Other additions of HX 61 An addition of HI: CCH CCHCH I CHCH CCH 2 CH 3 I – + H + 3  carbocation (not 1  ) 3-iodo-3-methylpentane An addition of HCl: CH 3 3 H 3 Cl – + H + 3  carbocation 1-Chloro-1-methylcyclohexane

Summary 61 Positive part adds to the carbon of the double bond which has the greater number of hydrogens attached to it Today this is better stated that addition of an electrophile gives the most stable carbocation This is a general rule for addition to alkenes All alkenes can be expected to react in this manner Markovnikov's Rule:

Alkene addition reactions 61 RCHCH 2 RCHCH 2 XH HXRCHCH 2 HOH H + /HOH

Hydration of alkenes 61 An example of Markovnikov addition of water

Hydration of alkenes 61 Mechanism:

Alkene addition reactions 63 RCHCH 2 RCHCH 2 XH HXRCHCH 2 HOH H + /HOH RCHCH 2 XX X 2

Addition of halogen molecules 63 RCHCH 2 RCHCH 2 XX X 2

General mechanism 63 CH 2 Br H H H H + CH 2 2 Br + Br – H H H H Br + but then Br – X CH 2 A "bridged bromonium ion" is formed Addition is trans.

Bromination of bacon fat 63 Bacon fat contains unsaturated fats which add bromine Movie from Saunders General Chemistry CD-ROM

Addition of bromine, Br 2 63 Addition is trans.

Br 2 addition to a ring 63 A "bridged bromonium ion" is formed Addition gives the trans product by anti addition HH Br H H trans-1,2-dibromocyclohexane – Br +

Anti-addition 63 A "bridged bromonium ion" explains why only trans is formed since only anti addition is possible Stepwise addition would give the cis and trans product Br H H H H trans cis and trans-1,2-dibromo- cyclohexane Br H H + – – HH – +

Addition of chlorine 63 A bridged chloronium ion is formed Cl – attacks to give the trans addition product The reaction is general for alkenes with halogens

Addition reactions summary 65 RCHCH 2 RCHCH 2 XH HXRCHCH 2 HOH H + /HOH RCHCH 2 XX X 2

Cis addition to alkenes 64 A cyclic osmic ester is first formed Water converts this to the cis diol cis-1,2-diol Cis diol (glycol) formation with osmium tetroxide:

Diol (glycol) formation 65 A cyclic manganese ester is formed Water converts this to the cis diol Brown MnO 2 is generated (purple colour of KMnO 4 lost) cis-1,2-diol Permanganate reacts similarly:

Diol formation — cyclic alkenes 65 Syn-addition to cyclic alkenes affords hydroxyl groups on the same face Cyclic alkenes afford the cis-1,2-diol H OH H H O H O MnO 2 – cis-1,2-cyclohexanediol

Addition reactions summary 65 RCHCH 2 RCHCH 2 XH HXRCHCH 2 HOH H + /HOH RCHCH 2 XX X 2 RCHCH 2 HH H 2

Addition of hydrogen, H 2 65 Addition of hydrogen is exothermic by 120kJmol –1 Addition has a very high activation energy though With catalysts, addition occurs via a low energy path Pt, Pd, Ni, Rh and Ru CC R'' R''' R R' CC R'' R''' R R' HH + H 2 High E A

Catalytic action 66 Catalytic pathways may be multistep Overall activation energy is lower Energy Progress of reaction uncatalysed high E A Catalysed (multistep, low E A each step)

Schematic of catalytic hydrogenation 66 Hydrogen is absorbed onto the surface H—H bond weakened and hydrogens become atom-like HHHH metal surface metal surface

Schematic of catalytic hydrogenation 66 Hydrogens react with  -bond (stepwise) Hydrogens attach on same face (syn addition) to give cis product. metal surface metal surface HH RR R R R R R R H Hcis-addition H RR R R R R R R H H

Cis addition to cyclic alkenes 67 Hydrogens add to one face of the alkene The result is cis addition CH H H H 2 /Pt cis-1,2-dimethylcyclohexane

Reaction of alkynes Very similar to alkenes: –They add halogens twice, X 2 –They add hydrogen halides twice, HX –They add hydrogen twice, H 2 –They add water with acid (hydration) –Markovnikov's rule applies They are slightly acidic and react with strong bases

Addition of bromine Two molecules of bromine add successively HCCH H CC Br H + Br 2 CC Br H H 2 CHCHBr 2 1,1,2,2-tetrabromoethane + Br 2

Addition of HBr Two molecules of HBr add successively Markovnikov addition in both steps CH 3 C 3 C 2 Br CH 3 CCH 3 Br HBr 2,2-dibromopropane

Addition of H 2 Normal catalysis leads to double addition Less active catalysts allow syn addition of one molecule RCCR + H 2 CC R H R H R CHCH 2 CHCH 2 R' special catalyst normal catalyst

Reactions as an acid Very strong base required RCCHRCC+ – NH 3 – 2 Strong baseAlkynide ion Liq. NH 3

Alkynide formation Anions (electron pairs) in sp hybrid orbitals are closer to the carbon nucleus More stable than anions in sp 2 or sp 3 orbitals

Index of hydrogen deficiency (Double bond equivalents) 67 Hexane C 6 H 14 Hydrogen deficiency is TWO 2H is equivalent to either a double bond or a ring C 6 H 12

Index of hydrogen deficiency 68 TWO double bond or ONE ring and ONE double bond or TWO rings Equivalent to either

Combustion analysis 68 Microanalysis gives the percentages of carbon and hydrogen These, when divided by the atomic numbers, give the relative proportion of each element C: 88.16% H: 11.84% ThenC = = H = = 11.75

Combustion analysis 69 This the empirical formula Compare to the weight of the compound If the same, this is the molecular formula If not the molecular formula will be a factor of the empirical formula Divide by lowest number: CH 1.6 x 2 =C 2 H 3.2 x 3 C 3 H 4.8 x 4C 4 H 6.4 x 5C 5 H 8 molar mass 68