Ch 8- Alkenes and Alkynes II
Addition Reactions
• A characteristic reaction of compounds with
carbon-carbon double and triple bonds is an
addition reaction
• Generic example:
• We have already covered the addition of
hydrogen in chapter 7
Two Characteristics
• There are two characteristics of double bonds
that help explain why these reactions occur:
1) Converting 1 pi bond and 1 sigma bond to 2
new sigma bonds is energetically favorable
2) The electrons of the pi bond are exposed and
therefore can react as a nucleophile
A. Addition of Hydrogen Halides to
Alkenes
• Hydrogen Halides- HI, HBr, HCl, HF
• Order of Reactivity:
HI > HBr > HCl >> HF
• Examples:
Markovnikov’s Rule
• Unsymmetrical alkenes follow Markovnikov’s
Rule
• Markovnikov’s Rule- in the addition of HX and
water to an alkene, the hydrogen atom will
add to the carbon of the double bond that
already has the greater number of hydrogen
atoms
• example
Markovnikov’s Rule
• Markovnikov actually stated that the reaction will
proceed through the most stable carbocation
intermediate.
• Mechanism:
• Both versions typically give the same results,
however, focusing on the carbocation is always
right, whereas the focusing on the hydrogens will
only be right about 95% of the time.
Markovnikov’s Rule
• When an addition follows Markovnikov’s rule, it is
said to be a Markovnikov addition
• This is an example of a Regioselective reaction
• Regioselective Reaction- When a reaction that can
potentially yield two or more constitutional
isomers actually produces only one, or a
preponderance of one.
• There are ways to do an Anti-Markovniknov
addition, which we will cover later.
Stereochemistry of Ionic Additions to
an Alkene
• When HX is added to an alkene, a stereogenic
carbon is possibly formed.
• Since the reaction proceeds through an achiral
carbocation, the result is a racemic mixture,
equal amounts of R and S forms.
• This is the same conclusion and reasoning we
had with Sn1 reactions.
Addition of Hydrogen Halides to
Alkynes
• Alkynes react with hydrogen chloride and
hydrogen bromide to form haloalkenes or
geminal dihalides depending on whether one
or two molar equivalents of the hydrogen
halide is used
• Both additions are regioselective and follow
Markovnikov’s Rule
• examples
Addition of Hydrogen Halides to
Alkynes
• By using acetyl Bromide with alumina, we can
selectively form the haloalkene
• This is also Markovnikov addition.
• If we add HCl or HBr with peroxides present,
we can get the anti-markovnikov haloalkene
Addition of Sulfuric Acid to Alkenes
• The mechanism is the same as adding HX, but
we are adding cold H2SO4
• Markovnikov addition
• Mech:
• The alkyl hydrogen sulfate product can easily
hydrolyze to an alcohol by heating with water
• Ex.
Addition of Sulfuric Acid to Alkenes
• The overall result of the addition of Sulfuric
Acid to an alkene followed by hydrolysis is
Markovnikov addition of water!
Addition of Water to Alkenes
• Acid Catalyzed Hydration
• Mech:
• Markovnikov addition
• Via carbocation, watch for rearrangements!
Addition of Water to Alkenes
• Oxymercuration-Demercuration
• Advantage: Avoids carbocation and
rearrangements
• Markovnikov addition
Addition of Water to Alkenes
• Hydroboration-Oxidation
• Advantages:
– Avoids Carbocation
– Anti-Markovnikov addition
– Syn-addition
• Ex
Addition of Water to Alkenes
• Hydroboration-Oxidation, cont.
– Hydroboration step can be done with diborane, B2H6,
but it is a gas and hard to work with
– It is easier to use a Borane/THF complex
• Mechanism for Hydroboration:
– Concerted step = syn-addition
– Borane bonds to least substituted carbon due to steric
factors
Addition of Water to Alkenes
• Hydroboration-Oxidation, cont
• Mechanism for Oxidation step:
– The oxidation and hydrolysis takes place with retention
of configuration at the carbon initially bearing the boron
and ultimately bearing the –OH group
– In the end, the hydroxyl group ends up in the exact
space the boron was in!
• Mech:
Addition of Water to Alkenes
SUMMARY
Protonolysis of Alkyl Boranes
• Instead of oxidation, a carboxylic acid with
heat can be used to protonate the alkyl
borane
– Happens with retention
– Used to label with D or T
– The H, D, or T takes exact place of Boron
– Example:
Addition of Bromine and Chlorine to
Alkenes
•
•
•
•
Reacts to form vicinal dihalides
Saw this in ch. 7
Ex.
Mech:
Stereochemistry of Halogen Addition
• Because the Bromonium ion forces opposite
side attack, this is anti-addition
• Either carbon of the bromonium ion can be
attacked which leads to a mix of enantiomers
• Addition of halogens can also be
stereospecific reaction
Stereospecific Reaction
• Stereospecific Reaction- when a particular
stereoisomeric form of the starting material
reacts in such a way that it gives a specific
stereoisomeric form of the product.
• Stereoselective and stereospecific are
different!
• Consider cis-2-butene vs trans-2-butene
Addition of Bromine and Chlorine to
Alkynes
• Reacts the same way alkenes do
• May react once or twice, depending on the molar
equivalents of bromine/chlorine added
• Example
• It is usually possible to prepare dihaloalkenes by
simply adding one molar equivalent of the halogen
• Most additions are anti, and give the transdihaloalkene
Halohydrin formation
• If a halogenation of an alkene is done in water
instead of an organic solvent, you do not get a
vicinal dihalide
• Instead you get a halo alcohol, called a
halohydrin
• Ex.
Halohydrin formation
• The mechanism starts like the halogenation
but instead of the second halide attacking,
water does.
• If the alkene is unsymmetrical, the halogen
ends up on the carbon with the most
hydrogens because the water will attack the
most substituted carbon.
Oxidation of Alkenes
• Syn 1,2-dihydroxylation
– Alkenes can undergo a number of reactions in
which the carbon-carbon double bond is oxidized
– One reaction is with Osmium tetraoxide to
produce 1,2-diols (also called glycols)
– This is syn-addition
• Ex.
• Mech.
Oxidative Cleavage of Alkenes
• Alkenes can be oxidatively cleaved using
either Potassium Permanganate or Ozone
• Potassium Permanganate is stronger and will
result in ketones and carboxylic acid groups
• Ozone is milder and will result in ketones and
aldehydes
Oxidative Cleavage of Alkenes
• KMnO4 examples:
• O3 examples:
Oxidative Cleavage of Alkynes
• Treatment of Alkynes with either Ozone or
Potassium permanganate will lead to
carboxylic acids
• examples
Synthetic Strategies, Revisited
• In planning a synthesis, we consider four interrelated aspects:
1)
2)
3)
4)
Construction of the Carbon Skeleton
Functional Group interconversions
Control of regiochemistry
Control of stereochemistry
Construction of Carbon Skeleton
• This is making carbon-carbon bonds.
• We only know two ways:
1) Using cyanide as nucleophile in substitution reaction
-Not really useful right now because we don’t know how to do
anything with the cyano group
2) Alkylation of an alkynide ion
-This is very useful! We can add multiple carbons at one time and
we know how to make alkynes as well as reactions of alkynes
2,3, and 4
• Aspects 2, 3, and 4 all have to do with one
another
• As we convert functional groups into each
other, we have to keep in mind regioselectivity
and stereoselectivity
Example
• If we wanted to make an alcohol from an alkene, we
know 3 ways:
1) Acid-Catalyzed Hydration
-no chance of rearrangement or want rearrangement?
-Markovnikov addition
2) Oxymercuration-Demercuration
-prevents rearrangements
-Markovnikov addition
3) Hydroboration-Oxidation
-anti-markovnikov addition
Knowing your
Functional Group
Conversions
Is Essential to
Synthesis!!!!
What we know so far…
• (there may be more!!!)
• Once you know these conversions, you just
take it one step at a time!!
Sample Problem
• Outline a synthesis of 2-bromobutane from
compounds of two carbon atoms or fewer.