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.