WWU -- Chemistry Alkenes and Alkynes I. Addition Reactions Chapter Eight

Types of Additions =>

WWU -- Chemistry Sect 8.1: Addition Reactions

WWU -- Chemistry Addition of H-X

WWU -- Chemistry Addition of Water

WWU -- Chemistry Sect. 8.2: Introduction to Mechanisms mechanism two step rate determining step (i.e. slow step) energy diagram transition states intermediates

WWU -- Chemistry MECHANISM STEP-BY-STEP ACCOUNT OF WHAT HAPPENS CC E + CC E + CC E X step 2 step 1 intermediates are formed during a reaction but are not products :X-:X- Intermediate

WWU -- Chemistry product HH intermediate TS 2 TS 1 ENERGY PROFILE two step reaction ENERGYENERGY step 1step 2 CC E + X - + CC CC E X E

WWU -- Chemistry ACTIVATED COMPLEXES correspond to transition states for each step ACTIVATED COMPLEXES CC E X X - + CC E E + CC CC E CC E X intermediate show bonds in the process of breaking or forming (bonds are half formed or half broken) ++ ++ -- ++

WWU -- Chemistry Sect 8.3: Electrophilic Addition to a Double Bond

WWU -- Chemistry Hyperconjugation

CARBOCATION STABILITY HYPERCONJUGATION CC H H H + R R electrons in an adjacent C-H  bond help to stabilize the positive charge of the carbocation by proximity (overlap) tertiarysecondaryprimary >> Most stable Least stable C R R R + RCHR + RCH 2 +..

WWU -- Chemistry Can you recognize the following carbocations? 1 o, 2 o, 3 o 1 o 2 o 3 o 2o2o 2 o

WWU -- Chemistry Sect 8.4: Addition of Hydrogen Halides: Markovnikov’s rule

WWU -- Chemistry Markovnikov’s Rule In the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom which already holds the greater number of hydrogens. –“Them that has, gets!” –“The richer get richer!” (V. W. Markovnikov )

WWU -- Chemistry Markovnikov

MARKOVNIKOV RULE CH 2 + HCl CH 3 Cl When adding HX to a double bond the hydrogen of HX goes to the carbon which already has the most hydrogens..... conversely, the anion X adds to the most highly substituted carbon ( the carbon with most alkyl groups attached).

WWU -- Chemistry REGIOSELECTIVE REACTION CCH 3 CH 3 CH 2 CCH 3 CH 3 CH 3 Cl + CH CH 3 CH2H2 Cl CH 3 HCl majorminor one of the possible products is formed in larger amounts than the other one Compare REGIOSPECIFIC only one of the possible products is formed (100%).

WWU -- Chemistry Mechanism (Markovnikov) Secondary C+ Major product

WWU -- Chemistry Mechanism (anti-Markovnikov) Minor! Primary carbocation

WWU -- Chemistry COMPETING PATHWAYS lower energy intermediate higher energy intermediate rate-determininng (slow) step rate-determining step faster slower 1 o 2 o

WWU -- Chemistry Markovnikov Addition to an Alkene

WWU -- Chemistry Mechanism 3 o C+

WWU -- Chemistry SOME ADDITIONAL EXAMPLES CH 3 + HCl CH 3 Cl CH 2 CH 3 Cl CHCH 2 CH CH 3 Cl only major product is shown

WWU -- Chemistry Sect 8.5 Addition of Sulfuric Acid to an Alkene

WWU -- Chemistry HOSO O O H CC CC H + CC H OSO 3 H OSO O O H alkyl hydrogen sulfate cold - ALKYL HYDROGEN SULFATES water room temp FOLLOWS MARKOVNIKOFF RULE SLOW

WWU -- Chemistry Addition of Water to an Alkene

WWU -- Chemistry Mechanism of Hydration

WWU -- Chemistry Sect 8.6 Addition of Bromine to an Alkene

WWU -- Chemistry ADDITION OF BROMINE CC Br Br + CC Br CC BrBr Br CCl 4 Br Br : ++ -- alkene polarizes bromine SLOW

WWU -- Chemistry H H Br Br H H Br H Br H Br H H Br Br Br cis compoundtrans compound NOT OBSERVED THE REACTION IS STEREOSPECIFIC ACTUAL PRODUCT ANTI ADDITION anti syn anti syn + “open“ carbocation would give both cis and trans -

WWU -- Chemistry WHAT WOULD EXPLAIN FORMATION OF ONLY THE trans PRODUCT ? OF ONLY THE trans PRODUCT ?..... A BRIDGED OR CYCLIC INTERMEDIATE

WWU -- Chemistry CYCLIC BROMONIUM ION note size of bromine +

WWU -- Chemistry Br + BRIDGED BROMONIUM ION bridging blocks approach from this side

WWU -- Chemistry ENANTIOMERS symmetric intermediate + FORMATION OF ENANTIOMERS Addition could also start from the top with bromide attacking the bottom.

WWU -- Chemistry CH 3 HH CH 3 CH 3 HCH 3 H CH 3 CHCHCH 3 + Br 2 CH 3 CHCHCH 3 BrBr ADDITION OF BROMINE TO 2-BUTENE ** cis-2-butenetrans-2-butene WILL THESE STEREOISOMERS GIVE THE SAME PRODUCTS? 2 n possible stereoisomers

WWU -- Chemistry enantiomersmeso NO ! THEY GIVE DIFFERENT PRODUCTS These results can only be explained by stereospecific anti addition. cistrans

WWU -- Chemistry C H ROTATE ENANTIOMERS ADD TO RIGHT ADD TO LEFT cis - -

WWU -- Chemistry ROTATE ADD TO RIGHT ADD TO LEFT MESO IDENTICAL (also meso) trans _ _ +

WWU -- Chemistry Bromination of an Unsymmetrical Alkene

WWU -- Chemistry Stereochemistry of Bromination of Alkenes Simple alkenes: Addition of bromine or chlorine goes exclusively anti, with the formation of a bridged ion If a resonance-stabilized open-chain carbocation is possible, there may be a mixture of mechanisms, with some molecules reacting via a bridged ion and some molecules reacting via an open- chain carbocation

WWU -- Chemistry Stereochemistry of Bromination of Alkenes --Part Two In cases where a resonance-stabilized carbocation is possible, if the solvent is made more polar (acetic acid or nitromethane), the proportion of molecules reacting via an open-chain carbocation increases. For simple alkenes, changing solvents has little or no effect on stereochemistry.

WWU -- Chemistry Sect 8.7 Halohydrin Formation Sect 8.7 Halohydrin Formation Br 2 + H 2 OHO-Br+ HBr

WWU -- Chemistry Mechanism

Sect. 8.8: Carbocation Rearrangements

WWU -- Chemistry Sect 8.10 Free Radical Addition of HBr to Alkenes (anti-Markovnikov!)

WWU -- Chemistry ADDITION OF HBr Markovnikov Addition Anti-Markovnikov Addition Oxygen

WWU -- Chemistry

STABILITY OF CARBON RADICAL INTERMEDIATES Radicals are electron-deficient just like carbocations and have the same stability order. lowest energy highest energy tertiarysecondaryprimarymethyl and they are stabilized by resonance and / or hyperconjugation. etc. CH 2 ( ) ()

WWU -- Chemistry Sect and 8.12: Hydrogenation of Alkenes and alkynes

WWU -- Chemistry Hydrogenation of Alkenes Also PtO 2 -- sometimes Ru, Rh, or Re

WWU -- Chemistry HYDROGEN ADSORBS ON THE SURFACE OF THE CATALYST n H 2 + Pt Pt(H ) 2n HH H. Hydrogen Adsorbed on Catalyst Surface.. finely-divided particles dispersed in solution Pt. H. H. H.

WWU -- Chemistry MECHANISM OF HYDROGENATION. H. H. H. H. H. H. H HH CATALYST

WWU -- Chemistry. H. H. H. H. H. H. H. H. H HYDROGEN ADSORBS MECHANISM OF HYDROGENATION

WWU -- Chemistry MECHANISM OF HYDROGENATION. H. H. H. H. H. H. H. H R R R R... H ALKENE APPROACHES

WWU -- Chemistry MECHANISM OF HYDROGENATION. H. H. H. H. H. H. H. H. H R R R R.. ALKENE PICKS UP TWO HYDROGENS

WWU -- Chemistry MECHANISM OF HYDROGENATION. H. H. H. H. H. H. H R R RR.. H.. H ALKANE IS FORMED

WWU -- Chemistry BOTH HYDROGENS ADD TO THE SAME SIDE OF THE DOUBLE BOND CH 3 H H H H syn addition anti addition X not observed

WWU -- Chemistry EXAMPLES EXAMPLES

WWU -- Chemistry Sect. 8.14: Addition Polymers

WWU -- Chemistry Polymerization of propene

Indirect Hydration Oxymercuration-Demercuration –Markovnikov product formed –Anti addition of H-OH –No rearrangements Hydroboration –Anti-Markovnikov product formed –Syn addition of H-OH

Hydroboration Borane, BH 3, adds a hydrogen to the most substituted carbon in the double bond. The alkylborane is then oxidized to the alcohol which is the anti-Mark product.

Borane Reagent Borane exists as a dimer, B 2 H 6, in equilibrium with its monomer. Borane is a toxic, flammable, explosive gas. Safe when complexed with tetrahydrofuran. THF THF. BH 3

Predict the Product Predict the product when the given alkene reacts with borane in THF, followed by oxidation with basic hydrogen peroxide. syn addition

Epoxidation Alkene reacts with a peroxyacid to form an epoxide (also called oxirane). Usual reagent is peroxybenzoic acid.

One-Step Reaction To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid. The reaction is stereospecific.

Chapter 8 68 Syn Hydroxylation of Alkenes Alkene is converted to a cis-1,2-diol, Two reagents: –Osmium tetroxide (expensive!), followed by hydrogen peroxide or –Cold, dilute aqueous potassium permanganate, followed by hydrolysis with base =>

WWU -- Chemistry