Chem 240: Ch#08- Alkenes And Alkynes Ii: Addition Reactions

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General Addition Reaction of Alkenes:

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What are the two characteristics of the double bond that helps us understand how addition reactions occur?

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1. An addition reaction results in the conversion of one pi bond and one sigma bond into two sigma bonds. (usually energetically favorable b/c pi bonds are weaker... usually exothermic)

2. e-s of the pi bond are exposed. B/c the pi bond results from overlapping p orbitals, the pi e-s lie above and below the plane of the double bond.

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Electrophiles:

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Electron-seeking reagents.

--- include:
- (+) reagents such as H+
- neutral reagents such as bromine (b/c it can be polarized so one end is (+))
- lewis acids such as BH3, BF3, AlCl3, metal ions that contain vacant orbitals... etc.

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Nuclophile:

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Molecules/ ions that can furnish an e- pair (ex: lewis bases)

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General hydrogen halide addition to the double bond of a alkene:

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How the reaction of an addition of a hydrogen halide to an alkene occurs:

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- by dissolving the H-X in a solvent such as acetic acid, or CH2Cl2

- by bubbling the gaseous H-X directly into the alkene and using the alkene itself as the solvent.

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The order of reactivity of hydrogen halides:

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HI > HBr > HCl > HF

- unless alkene is highly subsituted, HCl reacts so slowly that it is hardly ever used.

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Markovnikov's Rule:

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in the addition of HX to an alkene, the H atom adds to the C atom of the double bond that already has the greatest # of H atoms

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A mechanism for the reaction: Addition of a H-X to an alkene:

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Energy diagram for the addition of a HX to an alkene reaction:

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Step 1:
- rate determining step
- alkene donates a pair of e-s to the proton of HX & forms a carbocation
- highly endergonic
- has high free energy of activation
- takes place slowly
step 2:
- highly reactive carbocation stabilizes itself by combining with halide ion
- exergonic
- low free energy of activation
- takes place quickly.

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A mechanism for the reaction: Addition of HBr to 2-Methylpropene

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The Modern statement of Markovnikov's Rule:

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In the ionic additon 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 stale carbocation as an intermediate [determines the overall orientation of the reaction]

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Regioselective:

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When a reaction that can potentially yield 2/ more constitutional isomers actually produces only one (or a predominance of one)

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Anti-Markovnikov addition:

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When the H atom becomes attached to the C atom w/ the fewer H atoms.

ex: The addition of HBr to alkenes in the presence of periodixes (ROOR)

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The Stereochemistry of the reaction: Ionic Addition to an Alkene:

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The addition of Sulfuric acid to an alkene:

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Alkenes dissolve b/c they react by electrophilic additon to form alkyl hydrogen sulfates.
- is regioselective
- follows Markovnikov's Rule

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Creating alcohols from Alkyl Hydrogen Sulfates:

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Alkyl hydrogen sulfates can be easily hydrolyzed to alochols by heating them w/ water.

- Markovnikov addition of H- and -OH

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A Mechanism for the reaction: Acid-Catalyzed Hydration of an Alkene:

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- reaction follows markovnikov's rule

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Oxymercuration- Demercuration:

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- synthesizing alcohols from alkenes that avoids rearrangements
- is hignly regioselective, follows Markovnikov's rule

2-step method:
step 1:
OXYMERCURATION: water and mercuric acetate add to the double bond
step 2:
DEMERCURATION: sodium borohydride reduces the acetoxymercury group & replaces it w/ H.

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Does rearrangements occur in oxymercuration-demercuration?

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is not prone to hydride or alkanide rearrangements.

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A Mechanism for the reaction: Oxymercuration:

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Alchols from alkenes through hydroboration-oxidation: Anti-Markovnikov Syn Hydration:

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A Mechanism for the Reaction: Hydroboration:

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- in each addition step the boron atom becomes attached to the less sub. C atom of the double bond, and H atom is transferred from the boron atom to the other C atom of the double bond

- due to electronic and steric factors (the bulky boron-containing group can approach the less sub. C atom more easily)

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The stereochemistry of Hydroboration:

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A Mechanism for the Reaction: Oxidation of Trialkylboranes (oxidation and hydrolysis of alkylboranes)

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- regioselective
- net reslt is anti-Markovnikov addition of water
- gives us a method for preparation of alcohols that cannot normally be obtained through the acid-catalyzed hydration of alkenes or by oxymercuration-demercuration.
- the hydroxyl group replaces the B atom where it stands in the alkylboron compound.
- net results is the syn addition of -H and -OH.

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Protonolysis of Alkylboranes:

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heating an alkylborane w/ acetic acid causes cleavage of the C-B bond and replacement w/ H

- H replaces B where it stands in the alkylborane. The stereochemistry of this reaction therefore is SYN.

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Bromine test detecting alkenes, and alkynes:

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If bromine is added to an alkene (or alkyne) the red-brown color of the bromine disappears almost instantly as long as the alkene (or alkyne) is present in excess.

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Mechanism for the Reaction: Addition of Bromine to an Alkene:

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- an SN2 reaction

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What occurs when a "cyclo" is attacked with bromine?

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anti-addition occurs and product is a racemic mixture. A bromide ion attacks a C atom of hte ring from the side opposte that of the bromonium ion.

- SN2 reaction

- The nucleophilic attack causes inversion of the congiguraiton of the carbon being attacked.

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How inversion occurs in the reaction of cyclohexan with bromine:

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Stereospecific reaction:

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a reaction is stereospecific when a particular stereoisomeric form of the starting material reacts in such a way that it gives a specific stereoisomeric form of the product.

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A Stereochemistry of the reaction: Addition of Bromine to cis- and trans-2-butene:

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A Mechanism for the reaction: Halohydrin Formation from an Alkene:

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If the alkene is unsymmetrical, the halogen ends up on the carbon atom with greater # of H atoms.

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Carbenes:

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group of compounds in which carbon forms only two bonds.

- most are highly unstable

- soon after they are formed, they usually react w/ another molecule.
---- reactions usually show a remarkable degree of stereospecificity.

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Methylene:

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- the simplest carbene
- can be prepared by decomposition of diasomethane (CH2N2) a very posionous yellow gas.
----- can be accomplished by...
1. heating (thermolysis)
2. irradiating it with light of wavelengths that can absorb (photolysis)

METHYLENE REACTS W/ ALKENES BY ADDING TO THE DOUBLE BOND TO FORM CYLOPROPANES

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7,7-Dichlorobicyclo[4.1.0]heptane (59%)

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1,2-Dihydroxylation:

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Mechansim for syn dihydroxylation of alkenes:

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- involves a cyclic intermidate that results in SYN ADDITION of the O atom

- cleavage @ the oxygen-metal bonds take place w/o altering the stereochemistry of the 2 new C-O bonds.

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Cleavage w/ hot basic Potassium Permanganate

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Using Potassium Permanganate (KMnO4) to test for alkenes/ alkynes:

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it is a chemical test for the presence of unsaturations in an unknown compound. Solutions of potassium permanganate are purple... if an alkene / alkyne is present the purple color is discharge and a brown precipitate of manganese dioxide (MnO2) forms as the oxidation takes place.

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Ozonolysis:

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- the most useful method for cleaving alkenes.

- consits of bubbling ozone into a very cold (-71 degree C) soln. of the alkene in CH2Cl2, followed by treatment of the soln w/ zinc and acetic acid (or dimethyl sulfide)

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ozonolysis:

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A Mechanism for the reaction: Ozonolysis of an Alkene

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- the addition may occur once or twice, depending on the # of molar equivalents of halogen that are employed

- Most additions of Cl & Br to alkynes are anti additions and yield trans-dihaloalkenes.

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Addition of hydrogen halides to alkynes in general:

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- regioselective
- folow markovnikov's rule

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- follows Markovnikov's rlue

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Anti-Markovnikov addition

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cleavage at the Carbon carbon triple bond.

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cleavage at the carbon-carbon triple bond.

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When planning a synthesis what must be considered (4 interrelated aspects):

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1. construction of the carbon skeleton

2. functional group interconversions

3. control of regiochemistry

4. control of stereochemistry.

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Stereoselective Reaction:

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one in which the reactant is not necessarily chrial but in which the reaction produces predominantly or exclusively one stereoisomeric form of the product (or a certain subset of stereoisomers from among all those that are possible)

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Stereospecific Reaction:

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one that produces predominantly/ exclusively one stereoisomer of the product when a specific stereoisomeric form of the reactant is used.

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Summary and Review Tools: Mechansim Review: Summary of Alkene Addition Reactions

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Summary and Review Tools: Sunthetic Connections of Alkynes and Alkenes: II

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