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