Slide Number 1Slide Number 2What is an Addition Reaction?The regio- and stereochemistry of addition reactions of alkenes Slide Number 5Slide Number 6Slide Number 7Markovnikov’s RuleSlide Number 9Processes that can add molecules of water, halogens, carbon, and other functionalities across alkenes Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Exception to Markovnikov’s RuleSlide Number 21Slide Number 22Slide Number 23MechanismMechanismCarbocation RearrangementsSlide Number 27Alcohols from Alkenes Through Oxymercuration-Demercuration: Markovnikov Addition Mechanism of OxymercurationRearrangements of the carbon skeleton seldom occur in oxymercuration–demercuration StereochemistrySlide Number 32Slide Number 33Mechanism of HydroborationAn orbital view of HydroborationMechanism of HydroborationSlide Number 37Alcohols from Alkenes: A ComparisonSlide Number 40Summary of Alkene Hydration MethodsAddition of Bromine and Chlorine to Alkenes Mechanism of Halogen AdditionStereochemistryExamplesSlide Number 46Stereochemistry: Addition of Br2 to Trans-2-ButeneStereochemistry: Addition of Br2 to Cis-2-ButeneHalohydrin FormationMechanismDivalent Carbon Compounds: Carbenes Reactions of Other Carbenes: DihalocarbenesCarbenoid: The Simmons-Smith Cyclopropane SynthesisSlide Number 54Slide Number 55Events that cleave double bonds and provide more highly oxidized compounds Oxidations of Alkenes: Syn 1,2-Dihydroxylation MechanismOxidative Cleavage of AlkenesExamplesSlide Number 61Ozonolysis of AlkenesMechanismSlide Number 64Alkyne reactions that are analogous to alkene reactions Addition of Bromine and Chlorine to Alkynes Addition of Hydrogen Halides to AlkynesMechanismSlide Number 69Slide Number 70Slide Number 71Slide Number 72How to Plan a Synthesis?Slide Number 74Slide Number 75Synthons and Synthetic EquivalentsSlide Number 78SynthesisSlide Number 80Slide Number 81SynthesisAddition Reactions to Alkenes/AlkynesReviewSlide Number 85Slide Number 86Slide Number 87Slide Number 88Slide Number 89Slide Number 90Slide Number 91Slide Number 92Slide Number 93Slide Number 94Slide Number 95Chapter 8 Alkenes and Alkynes II: Addition Reactions 1In this chapter : 1. The regio- and stereochemistry of addition reactions of alkenes 2. Processes that can add molecules of water, halogens, carbon, and other functionalities across alkenes 3. Events that cleave double bonds and provide more highly oxidized compounds 4. Alkyne reactions that are analogous to alkene reactions 2What is an Addition Reaction? C CE NuEC CNu+Generally the reaction is exothermic because one π and one σ bond are converted to two σ bonds. 3THE REGIO- AND STEREOCHEMISTRY OF ADDITION REACTIONS OF ALKENES 4• First step: Alkenes react with electrophiles such as H+ from a hydrogen halide to form a carbocation. • Second step: The carbocation produced is an electrophile. It can react with a nucleophile such as a halide Additions to Alkenes The π electrons of the double bond are loosely held and are a source of electron density, i.e. they are nucleophilic. 5Overall, in addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second. • First step: • Second step: 6C CECCECCNuNuENuδ+δ−Electrophilic Addition to Alkene: Mechanism Note: Sometimes the reaction does not go through a “ free carbocation” but instead: 7C CHHHHE NuECC HHHHEC CHHHHEC CHHHHNuECC HHHHNuNusame assame asMarkovnikov’s Rule • For symmetrical substrate regiochemistry is not a concern 8E NuC CHH3CHHECC HHCH3HEC CCH3HHHEC CCH3HHHNu ECC HHCH3HNuNuNuordifferent fromMarkovnikov’s Rule • For unsymmetrical substrate two regio isomers are possible 9PROCESSES THAT CAN ADD MOLECULES OF WATER, HALOGENS, CARBON, AND OTHER FUNCTIONALITIES ACROSS ALKENES 10Addition of Hydrogen Halides to Alkenes: Markovnikov’s Rule Addition of HBr to propene occurs to give 2-bromopropane as the major product. Markovnikov’s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. 1112 In Class problem: What is the products from the following addition reactions? C CH3CH3CH2CHCH3HBrICl(A) (B)Mechanism for Hydrogen Halide Addition to an Alkene BrBrfastslowr.d.sδ+δ−H Br+The reaction has a highly endergonic first step (rate determining) and a highly exergonic second step. 13H XC CHH3CHHδ+δ−Theoretical Explanation of Markovnikov’s Rule The product with the more stable carbocation intermediate predominates. The most stable carbocation is formed fastest because it has a lower ∆G‡. The transition state for the rate determining step (first step) resembles a carbocation and is stabilized by factors which stabilize carbocations. HCCHHCH3HHC CCH3HHHor2o carbocation(more stable)1o carbocation(less stable)step 1(slowr.d.s.)HCCCH3HHBrHHCCHHCH3BrHBrBrfastfast(minor)(major)14Theoretical Explanation of Markovnikov’s Rule 15Example: Addition of HBr to 2-methylpropene gives only tert-butyl bromide: Modern Statement of Markovnikov’s Rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate. Regioselective Reaction: When a reaction that can potentially yield two or more constitutional isomers actually produces only one or a predominance of one isomer. 1617 In Class Problem: (a) Write all the products of these reactions. (b) which one you predict to be the major product?Exception to Markovnikov’s Rule 1. Via a radical mechanism (see Chapter 10) 2. Be careful: This anti-Markovnikov addition does not take place with HI, HCl, and HF, even when peroxides are present H BrBr(anti-Markovnikovproduct)ROORheatH19CCHHBuHHXC CH2HHBuXXachiral trigonal planar carbocation C CH3HXBu(S)-2-Halohexane(50%)C CH3HXBu(R)-2-Halohexane(50%)attack from top attack from below racemate Stereochemistry of the Ionic Addition to an Alkene Addition of HBr to 1-butene yields a chiral molecule. A racemic mixture is produced because the intermediate carbocation intermediate is achiral. 21Addition of Sulfuric Acid to Alkenes Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid. The addition follows Markovnikov’s rule. The sulfate can be hydrolyzed by