chapter 8 - addition reactions of alkenes

hydrologenation

addition of H-X

hydration

addition of H and OH

hydrogenation

addition of H and H

halogenation

addition of X and X

halohydrin formation

addition of OH and X

dihydroxylation

addition of OH and OH

what happens to bonds in addition reactions?

pi bond turned into 2 new sigma bonds

addition reactions are favored by _______ and not favored by ________

favored by enthalpy

not favored by entropy

in hydrohalogenation, what products are produced if the alkene is not symmetrical?

two regioisomers are possible depending on which carbon get the H and the X

what kind of addition is hydrohalogenation?

Markovnikov addition

what is Markovnikov addition?

the halogen is generally installed at the more substiented carbon

what happens when we use peroxides with hydrohalogenation?

anti-markovnikov addition

what is anti-markovnikov addition?

the halogen is added to the less substituted side

can hydrohalogenation products be controlled?

yes, depending if you use peroxide or not (as a solvent)

is hydrohalogenation a two step product?

yes

proton transfer then nucleophilic attack

if there is a chiral center produced from hydrohalogenation, what does this mean?

there are actually two Markovnikov products formed - two enantiomers are formed in equal amounts

would there be carbocation rearrange for hydrohalogenation?

yes, it can shift to a more stable carbocation

how are the H and OH added in acid catalyzed hydration?

markovnikov addition

OH added to the more substituted side

what addition reaction is sulfuric acid commonly used with? (H2SO4)

acid-catalyzed hydration

the more substituted the carbon is, the _______ the reaction

faster

if we are synthesizing an alcohol, we would use ______________ ________________

****for hydration

excess water

for hydration - the nucleophilic attack produces an _________ ion

oxonium

what is deprotonated to afford the alcohol product?

oxonium ion

if hydration reaction produces a chiral center, how many products are producded?

2 products - equal mixture of S and R enantiomer

why does hydration have limited application?

since rearrangements occur, it gives mixed products

what is an alternative to hydration?

oxymercuration - demercuration

why is oxymercuration - demercuration an alternative to hydration?

adds H and OH across the double bond but no rearrangements occur

why don't rearrangements occur for oxymercuration - demercuration reactions?

pi bond attacks the mercuric cation and a stabilized cation is formed (NOT A CARBOCATION which is why it doesn't rearrange)

what does NaBH4 replace in oxymercuration - demercuration reactions?

HgOAc

do oxymercuration - demercuration and hydration reactions have the same products?

Yes

how does Hydroboration-Oxidation add H and OH?

anti - Markovnikov

(OH is added to the less substituted side)

is Hydroboration-Oxidation a two step reaction?

yes

Hydroboration-Oxidation is __________

stereroselective

how does H and OH add in Hydroboration-Oxidation reactions?

syn fashion - SAME SIDE of double bond

geometry of BH3 is analogous to what?

carbocation

why does BH3 form a dimer?

it does not have an octet

how can BH3 dimer (B2H6) be stabilized?

by using an ether solvent which is BH3*THF

what regioselectivity does hydroboration follow?

anti-Markovnikov

how does hydroboration add to double bond stero wise?

syn addition

with hydroboration, if one chiral center is formed, then what does this mean?

pair of enantiomers is formed because it can be added to either face of alkene

with hydroboration, if only 2 chiral centers are formed, what is obtained?

pair of enantiomers

with hydroboration, if there are 2 chiral centers but only 1 is formed, what are obtained?

pair of diasteromers

what does catalytic hydrogenation require?

a metal catalyst

what happens to the alkene in catalytic hydrogenation?

alkene ----> alkane

is catalytic hydrogenation sterospecific?

yes, only syn addition is observed

in catalytic hydrogenation, what are the stereoisomers resulting from?

syn addition

why do we add in syn addition for catalytic hydrogenation?

metal surface binds to one side of the molecule so we can only add to the same other side

in catalytic hydrogenation, will adding H2 to a symmetrical alkene produce a pair of enantiomers?

no - meso compound

if H2 is added to a symmetrical alkene, what will it produce?

meso compounds - so just 1 product

catalyst that does not dissolve in reaction medium, like Pt or Pd?

heterogeneous

catalyst that does dissolve in the reaction medium, accomplished by using ligands with the metal

homogenous

what happens if we use a chiral catalyst in asymmetric hydrogenation?

synthesize only 1 enantiomer

which halogens are used in halogenation?

Cl and Br

NOT I nor F

what is the stereoselectivity with halogenation?

anti addition (added to different sides of the double bond)

is Br2 polarizable?

yes, it will induce a dipole

what type of intermediate do we get with halogenation?

ex for name )- bromunium ion intermediate (we get the little triange thing)

why do we get a bromonium ion intermediate?

anti addition

what is the halohydrin reaction?

when halogenation is conducted in water

in halohydrin reaction, what is the nucleophile?

water - attacks bromonium ion (instead of other Br)

why does H2O attack instead of Br in halohydrin?

because there are so many more of them

what happens to water after it is attacked in halohydrin mechanism?

deprotonated by other water molecules

what is the product for halohydrin mechanism?

halogen and OH added on opposite sides

plus we get the enantiomer

in halohydrin formation, where do the halide and the OH add to?

halide to less sub carbon

OH to the more sub carbon

how do we get regioselectivity in halohydrin formation?

H2O attacks the more sub. carbon- deprotonating it

need to go over anti

syn

oznolysis