ch 8- alkenes and alkynes ii. addition reactions a characteristic reaction of compounds with...
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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 trans-dihaloalkene
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 inter-related aspects:1) Construction of the Carbon Skeleton2) Functional Group interconversions3) Control of regiochemistry4) 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
ConversionsIs Essential toSynthesis!!!!
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.