Chapter 12: Reactions of Alkenes

alkenes usually react by

breaking the pi bond and forming two new sigma bonds

hydrogenation

reaction of an alkene with H2 and a catalyst (Pd or Pt) to form a saturated compound with two new C-H bonds

the catalyst for hydrogenation is

required

• helps to break the strong H-H bond

• the catalyst attacks one H atom, and the e- from the bond go back to the catalyst, so both H atoms are bound to the catalyst, then one H atom attacks the C, and the pi bond in C=C attacks the catalyst, and when the C has been bonded to the catalyst, it attacks the other H atom bonded to the catalyst to form the two new C-H bonds

• the catalyst is not consumed in the reaction

hydrogenation is

stereospecific

• the resulting product is cis only, where both H atoms come from the same face of the C-C bond

syn addition

new groups (H,H) are added to the same face of the C=C bond in the alkene

there is no

facial selectivity in an achiral substrate

• the products can be racemic, but cis only

  • enantiomeric products formed in equal amounts

for a chiral substrate,

sterics can lead to selectivity in which face is attacked

• still syn addition, but only one diastereomer is formed based on attack of the less hindered side by the 2 H atoms

the pi e- cloud in alkenes is

polarizable

• can act as a nucleophile/base

electrophilic addition of H-X

mechanism - the pi bond abstracts the H atom, and the X is cleaved from H-X to form a carbocation on one C atom from the double bond, which is then attacked by the X- to form the product R-X

in unsymmetrical alkenes, the reaction is

regioselective for the more substituted C atom in the double bond

• why - the degree of substitution of the carbocation relates to stability (faster formation if more stable)

  • formation of the carbocation is the rate determining step, thus the more substituted C in the C=C bond ends up bonded to X since formation of the carbocation here is more stable

Markovnikov addition of H-X across a double bond gives the product with

X at the more substituted C in the C=C bond

electrophilic hydration via addition of H, OH

mechanism - the pi bond abstracts the H atom from the acid which then forms the conjugate base of the acid and a carbocation on the other C of the double bond, and then OH2 attacks the carbocation and the H atom is then abstracted by the conjugate base of the acid to form the ROH

• this is also a Markovnikov addition mechanism at the more substituted C atom

• this mechanism requires the acid for hydration since the alkenes are stable in H2O

addition/elimination of H2O is

reversible

• it is an equilibrium process between adding the acid and water to form the addition product, and adding high heat to form the elimination product

  • high temperature favors elimination due to entropic considerations

  • low temperature favors addition

carbocation intermediates mean

rearrangements are possible via 1,2 hydride or 1,2 alkyl shifts

• mechanism - the pi bond abstracts the H atom from H-X to then form the carbocation, then rearrangement occurs to form the more stable carbocation, then the X- attacks the carbocation to form R-X

electrophilic addition of X2

this produces only the trans stereoisomer and can be racemic by adding two X atoms across the C-C bond

anti addition

X groups add to the opposite faces of the C=C bond

mechanism of electrophilic addition of X2

in a concerted step, the pi bond attacks one X atom in the bond, and the lone pairs on that X atom attack the C atom in the C=C bond, and the other X atom is cleaved from the X-X bond to form the bromonium ion with a positive charge on X, and the X- that was cleaved reattacks the C atom to open the ring to form the product

• there is inversion at the reacting C atom which is attacked by X-

• enantiomers can be formed in the products

the bromonium intermediate can be opened by

H2O, ROH

• net addition of X, OH or X, OR

• this reaction is regioselective to attack the bromonium ion at the more hindered side

• mechanism - the pi bond attacks one X atom, which then attacks the C atom and the other X atom is cleaved to form the bromonium ion, and H2O or ROH will then attack the more hindered side to open the ring, and is then deprotonated to form the product with inversion

oxymercuration-demurcation

Markovnikov addition of H,OH without a carbocation intermediate

• mechanism - the bond between OAc and HgOAC breaks to form +HgOAc as the electrophile, and the pi bond then attacks the +HgOAc and the +HgOAc binds to the less hindered side to form the mercurinium ion, and then OH2 attacks the more hindered side and is then deprotonated to form the OH bond, and then HgOAc is removed and replaced by H by NaBH4 (not stereospecific)

• H2O can also be replaced with ROH (addition of H, OR)

hydroboration-oxidation

anti-Markovnikov addition of H,OH

• mechanism - the pi bond attacks the B atom of BH3, and the H atom attacks the more hindered side of the alkene to form the BH2 bond on the less substituted C atom, which is then turned into OH by H2O2 and -OH with a retention of stereochemistry

• syn addition of H,OH

• the transition state involves the formation of the H bond to the more substituted C, the formation of the BH2 bond to the less substituted C, and the breaking of the BH2-H bond

summary of 3 methods for hydration of C=C (adding OH,H)

• acid-catalyzed

  • regiochemistry - Markovnikov - regioselective

  • stereochemistry - not selective

• oxymercuration-demercuration

  • regiochemistry - Markovnikov - regioselective

  • stereochemistry - non-stereospecific

  • no carbocation intermediate (no rearrangement, but OH formation at the more substituted C atom)

• hydroboration-oxidation 

  • regiochemistry - anti-Markovnikov - not regioselective

  • stereochemistry - syn addition

  • no carbocation intermediate (no rearrangement, but OH formation at the less substituted C atom)

acid-catalyzed hydration and oxymercuration-demercuration addition differ in the

carbocation intermediate

• in the acid-catalyzed reaction, there is rearrangement of the carbocation possible

• in the oxymercuration-demercuration reaction, there is no carbocation formed, thus there is no rearrangement possible but the OH is added to the more substitued C atom

alkene to cyclopropane

occurs via the carbene

• diazomethane - CH2N2

  • loses N2 gas to generate an intermediate carbene that is reactive

    • carbene. -H2C with a lone pair of e- on C

carbene is both

nucleophilic and electrophilic

• it is sp2

• there is an empty p orbital which can accept e-

• there is a p orbital that contains two e- that can donate e-

reaction with C=C can form a

cyclopropane

• the pi bond attacks the C atom of the carbene, and the lone pair of e- can then attack the other C atom to form the ring

• syn addition

synthesis of epoxides can occur in 

multiple ways

• one way is that H2C=CH2 can react with Cl2 and H2O to form OH-CH2-CH2-Cl, which can then react with NaOH to form the epoxide

peroxycarboxylic acids

R-C=O-O-O-H (common name - MCPBA)

• can form the epoxide in a single step

• syn addition

• mechanism - the pi bond attacks the O attached to the H atom, and the e- from the O-O bond are pushed between the O-C bond, and the C=O bond is broken, and that O abstracts the H atom in a single step to form the epoxide and carboxylic acid (R-C=O-OH)

the more e- density rich C=C is

more reactive, and the epoxide forms there

epoxides can be opened by a variety of nucleophiles

variety of nucleophiles

• there is a net anti addition of OH, OH to the C=C bond

OsO4 enables 

syn addition of OH, OH

• mechanism - the pi bond of the alkene attacks one O atom of the attached O on OsO4, and the e- from that pi bond are pushed onto Os, and the e- from another pi bond to O attack the other C atom of the alkene, which then furnishes a cyclic intermediate in which both O atoms are on the same face, and the H2S delivers 2 H atoms to the O molecules to furnish the two OH bonds on the same side of the alkane

  • OsO4 delivers both O atoms in a single step, thus they are on the same face

syn addition vs. anti addition of OH,OH

• anti addition occurs with MCPBA and -OH, H2O via the formation of an epoxide intermediate (racemic)

• syn addition occurs with OsO4 and H2S (achiral, meso)

ozonolysis

oxidative cleavage of the C=C bond (pi and sigma bond)

• the C=C bond is cleaved through reaction with O3 and Zn, H2O to form C=O compounds

radical addition of HBr

• reactions with HBr on its own give the Markovnikov addition product via the formation of a carbocation to the more substituted C atom

• addition of HBr with peroxide (ROOR) gives the anti-Markovnikov product with reversed regioselectivity on the less substituted C atom

peroxide changes the mechanism to a

radical process

• initiation - the RO-OR bond breaks homolytically to form two RO radicals, and one of them reacts with the H atom of H-Br which also breaks homolytically to form ROH and the Br radical for the radical chain process

• radical chain - the pi bond of the alkene breaks homolytically where one e- reacts with the Br radical, and the other e- is delivered to the more substituted C atom to form the C-Br bond with the unpaired e- on the more substituted C to form the more stable radical due to huperconjugation, and then the H-Br bond again is broken homolytically to give one H atom to the radical C atom, and to form another Br radical to furnish the anti-Markovnikov product

addition of RSH with ROOR also goes by

anti-Markovnikov addition and regioselectivity to furnish the SR bond on the less substituted C atom

spotting retrosynthetic disconnections with

organometallics

• based on whether the OH group is on the reacting C atom (linear) or away from the reacting C atom (cyclic)