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 H 2 SO 4 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, B 2 H 6, 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 KMnO 4 examples: O 3 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 Skeleton 2)Functional Group interconversions 3)Control of regiochemistry 4)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.