Ch 9 - Addition Reactions of Alkenes Flashcards
addition reactions
common reactions of alkenes characterized by the addition of two groups across a double bond
- a pie bond is broken in the process
many times addition is simply the reverse of
an elimination reaction
delta G = delta H + (-Tdelta S)
delta G = delta H + (-Tdelta S)
sigma bonds are
stronger than pie bonds
hydrohalogenation
the treatment of alkenes with HX(X = Cl, Br, I) results in an addition reaction in which H and X are added across the pie bond
H is generally placed at the vinylic position already bearing the
larger number of hydrogen atoms
the halogen is generally placed at the
more substituted position
Markovnikov Addition
the vinylic position bearing more alkyl groups is more substituted and that is where the halogen is placed
- the regiochemical preference - also observed in HCl and HI - regioselective
anti-Markovnikov Addition
when the alkyl group which is less substituted is where the halogen gets placed
- HBr does this sometimes - impurity of peroxides(ROOH) even in trace amounts cause this
purity of reagents is the critical feature of Markovnikov addition
- pure reagents go by Markovnikov addition
- impure reagents sometimes go by anti-Markovnikov addition
- ROOH(peroxides) lead to anti-Markovnikov addition
mechanism for hydrohalogenation
- step 1
- the piece bond of the alkene is protonated, generating a carbocation intermediate
- step 2
- the intermediate is attack by a bromide ion(NA)
regioselectivity of an ionic addition reaction is determined by the preference for the reaction to proceed through the more stable carbocation intermediate
- tertiary is the most stable carbocation
- lower Ea and total energy needed and therefore a faster reaction
hydrohalogenation often involved the formation of a chirality center
the two enantiomers are produced in equal amounts(racemic mixture)
if the intermediate of a hydrohalogenation can undergo carbocation rearrangement it will
the result is in an unequal mixture of products of rearranged and nonrearranged products
when carbocation rearrangements can occur,
they do occur
hydration
the methods for adding water(H and OH) across a double bond
acid-catalyzed hydration
addition of water across a double bond in the presence of an acid
for most simple alkenes the acid-catalyzed hydration reaction proceeds
via Markovnikov addition
H3O+ is representative of
H2O and an acid source(H+)
brackets around the proton source above a reaction arrow indicate the proton source is not consumed in the reaction
- it’s a catalyst
- acid-catalyzed hydration
with each additional alkyl group the reaction rate will increase by orders of magnitude
the OH is placed at the more substituted position
three step for acid-catalyzed hydration
- the alkene is protonated to generate a carbocation intermediate
- then attacked by the nucleophile
- results in an oxonium ion(exhibits an oxygen atom with a +)
- bc the NA started neutral a proton transfer is necessary at the end
acid-catalyzed hydration is an equilibrium process
the reverse is an E1 acid-catalyzed dehydration reaction(alcohol to an alkene)
the equilibrium of an acid-catalyzed reactions are sensitive to temperature and concentration of water
- can use dilute or concentrated acids to control the amount of water
- LeChateliers principle(the system responds to minimize stress)
- dilute acid increases water is used to convert alkene into an alcohol
- concentrated acid will reduce water and is used to favor the formation of the alkene
- remove water = favor alkene
an acid-catalyzed hydration will have an intermediate carbocation which can be NA from either side
When a chirality center is formed the mixture will be
a racemic mixture of enantiomers
if protonation of the alkene leads to carbocation rearrangement then acid-catalyzed hydration is
an inefficient method for adding water across the alkene
oxymercuration-demercuration
a hybrid of two resonancy structures
- an intermediate which instead of an ordinary carboncation is forms a bridge with its electrons to the carbocation lcoation
a mercurium ion has some character of a carbocation and some character of a bridge three member ring
reduces carbocation rearrangement while still being subject to a NA
Hydroboration-oxidation uses
anti-Markovnikov addition of water
hydroboration-oxidation
places the OH group at the less substituted carbon
syn addition
when two new chirality centers are formed the addition of water(H and OH) occurs in a way that places the H and OH on the same face of the pie bond
- stereospecific because only two of the four possible stereoisomers are formed
syn addition will not form stereoisomers with H and OH on
opposite sides of a pie bond
hydroboration-oxidation must explain both:
- the regioselectivity(anti-Markovnikov addition)
- stereospecificity(syn addition)
BH3 structure is similar to a carbocation without the charge
- has an empty p orbital
- lacks an octet = very reactive
diborane
when two borane interact forming a dimeric structure
- possesses a special type of bonding - H atoms actually break their bond with the borane to resonate between the two boranes
three-center, two-electron bonds
resonance structure in which a H atom from 2 borane is partially bonded to two borane atoms with a total of two electrons between the three entities
mechanism for hydroboration-oxidation
Regioselectivity
stereospecificity of Hydroboration-Oxidation
mechanism for hydroboration-oxidation
Regioselectivity
- electronic consideration
- attack of the pie bond triggers a simultaneous hydride shift
- pie bond attacks the empty p orbital of boron and one of the vinylic positions can begin to develop a partial positive charge leading to the hydride shift
- steric considerations
- since BH2 is bigger than H the BH2 will be placed in the less substituted carbon atom(less steric hindrance and lower energy)
mechanism for hydroboration-oxidation
stereospecificity
- syn addition(same face of the double bond) of the H and BH2 groups across the face of the alkene
- the number of chirality centers formed is key:
- 0 chirality centers = no stereospecificity
- 1 chirality center = both enantiomers are obtains because syn addition can take place from either side equally
- 2 chirality centers = the requirement of syn addition determines which pair of enantiomers are obtains while the other 2 possible enantiomers are not obtained
catalytic hydrogenation
the addition of a molecular hydrogen(H2) across a double bond in the presence of a metal catalyst
- the net result of the process is to reduce an alkene to an alkane
when two chirality centers are present only the pairs with
syn addition are formed
catalytic hydrogenation is accomplishing by
treating an alkene with H2 gas and a metal catalyst
- often at high pressures
in any given case the stereochemical outcome is dependent on the number of chirality centers formed in the process
must be careful with symmetrical alkenes as they will produce a meso compound instead of a pair of enantiomers
heterogeneous catalysts
catalyst which do not dissolve in the reaction medium
homogeneous catalysts
catalysts which are soluble in the reaction medium
the most common homogeneous catalyst for hydrogenation is
the Wilkinson’s catalyst
a syn addition is observed in
homogeneous catalysts
asymmetric hydrogenation
creating only one enantiomer instead of a pair
- achieved by using a chiral Wilkinson’s catalyst which can change the Ea of one enantiomer more than the other allowing the creation of only one(or significantly high ee)
halogenation
involved the addition of X2(either Br2 or Cl2) across an alkene
- only practical for chlorine or bromine - fluorine is too violent and iodine produces very low yields
anti addition
addition of halogenation occurs in a way where the halogen atoms end up on opposite sides of the pie bond
for most simple alkenes halogenation appears to proceed primarily via
an anti addition
bromonium ion
bridged intermediate
- similar to the mercurinium ion
halogenation mechanism
NA + LG create a bromonium atom followed by a second NA by a Br
the stereochemical outcome for halogenation reactions is dependent on the configuration of the starting alkene
can be cis, trans, meso
bromination occurring in a non-nucleophilic solvent results in the addition of Br2 across the pie bond
the addition of Br2 across the pie bond
bromination in the presence of water can allow the bromonium ion that is initially formed to be captured by the water molecule instead of the bromide resulting in the formation of Halohydrin instead
- halohydrin formation
- bromohydrin
- if a chlorine is used then chlorohydrin
halohydrin formation is typically regioselective
the OH generally positioned at the more substituted position
a partial positive charge passes through the carbon atom allowing the transition state for a halohydrin formation to bear partial carbocationic character
allows the water molecule to attack the more substituted carbon(the carbon can better stabilize the partial positive charge in transition)
dihydroxylation
the addition of OH and OH across an alkene
epoxide
three membered cyclic ether
peroxy acids are strong oxidizing agents
capable of delivering an oxygen atom to an alkene in a single step
a protonated epoxide can also be attacked by water from
the back side
when an alkene is treated with osmium tetroxide(OsO4) a cyclic osmate ester is produced
- concerted process where both oxygen atoms attach to the alkene simultaneously
- same face of alkene so syn addition
OsO4 acts as a catalyst to produce large quantities of
diol
Syn Dihydroxylation creates a product with
two OH groups on the same face via syn addition
ozonolysis
add across an alkene and completely cleaving the C=C bond
- after the split there are now two separate C=O double bonds
stereochemistry and regiochemistry become irrelevant
irrelevant for oxidative cleavage
For oxidative cleavage, ozone(O3) wil react with an alkene to produce an initial primary ozonide(or molozonide) which
undergoes rearrangement to produce a more stable ozonide
three questions to consider to predict the products of an addition reaction
- What are the identities of the groups being added across the double bond?
- what is the expected regioselectivity(Markovnikov or anti-Markovnikov addition)?
- What is the expected stereospecificity(syn or anti addition)?
it is absolutely essential to recognize reagents
must understand the proposed mechanisms for each reaction type
a proposed mechanism must explain the experimental observations
the mechanism for each reaction can serve as the key to remembering the three pieces of information
elimination reactions can be used to convert
alkyl halides or alcohols into alkenes
addition reactions are characterized by
two groups adding across a double bond
be familiar with all reagents employed for each type of reaction
be familiar with all reagents employed for each type of reaction
it is possible to combine elimination and addition reactions
- two step sequence(elimination then addition to change to position of a leaving group)
- in the elimination reaction the product can be either to Zaitsev or Hofmann product
- a careful choice of reagents makes it possible to control the regiochemical outcome
- strong base to more substituted position
- sterically hindered base to less substituted position
when changing the location of a leaving group make sure to remember that OH is a terrible leaving group
- this is critical because you may think you can add acid to protonate the group BUT remember if its an E1 process the regiochemical can not be controlled
- it is not possible to protonate with a strong acid and then use a strong base to achieve an E2 reaction because the strong acid and strong base will neutralize each other
- a better avenue is to convert the OH group into tosylate which is a much better leaving group which can then used anti-Markovnikov addition of H-OH
2 step process to change the position of a pie bond(addition then elimination)
- allows for the regioselectivity of each step to be carefully controlled
- uses Markovnikov addition with HBr and anti-Markovnikov addition si achieved by using HBr with ROOR(peroxides)
- the elimination step can be accomplished by using a strong base for Zaitsev products and the Hofmann can use a strong, sterically hindered base
