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59 Cards in this Set

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General Addition Reaction of Alkenes:
What are the two characteristics of the double bond that helps us understand how addition reactions occur?
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.
Electrophiles:
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.
Nuclophile:
Molecules/ ions that can furnish an e- pair (ex: lewis bases)
General hydrogen halide addition to the double bond of a alkene:
How the reaction of an addition of a hydrogen halide to an alkene occurs:
- 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.
The order of reactivity of hydrogen halides:
HI > HBr > HCl > HF

- unless alkene is highly subsituted, HCl reacts so slowly that it is hardly ever used.
Markovnikov's Rule:
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
A mechanism for the reaction: Addition of a H-X to an alkene:
Energy diagram for the addition of a HX to an alkene reaction:
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.
A mechanism for the reaction: Addition of HBr to 2-Methylpropene
The Modern statement of Markovnikov's Rule:
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]
Regioselective:
When a reaction that can potentially yield 2/ more constitutional isomers actually produces only one (or a predominance of one)
Anti-Markovnikov addition:
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)
The Stereochemistry of the reaction: Ionic Addition to an Alkene:
The addition of Sulfuric acid to an alkene:
Alkenes dissolve b/c they react by electrophilic additon to form alkyl hydrogen sulfates.
- is regioselective
- follows Markovnikov's Rule
Creating alcohols from Alkyl Hydrogen Sulfates:
Alkyl hydrogen sulfates can be easily hydrolyzed to alochols by heating them w/ water.

- Markovnikov addition of H- and -OH
A Mechanism for the reaction: Acid-Catalyzed Hydration of an Alkene:
- reaction follows markovnikov's rule
Oxymercuration- Demercuration:
- 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.
Does rearrangements occur in oxymercuration-demercuration?
is not prone to hydride or alkanide rearrangements.
A Mechanism for the reaction: Oxymercuration:
Alchols from alkenes through hydroboration-oxidation: Anti-Markovnikov Syn Hydration:
A Mechanism for the Reaction: Hydroboration:
- 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)
The stereochemistry of Hydroboration:
A Mechanism for the Reaction: Oxidation of Trialkylboranes (oxidation and hydrolysis of alkylboranes)
- 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.
Protonolysis of Alkylboranes:
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.
Bromine test detecting alkenes, and alkynes:
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.
Mechanism for the Reaction: Addition of Bromine to an Alkene:
- an SN2 reaction
What occurs when a "cyclo" is attacked with bromine?
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.
How inversion occurs in the reaction of cyclohexan with bromine:
Stereospecific reaction:
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.
A Stereochemistry of the reaction: Addition of Bromine to cis- and trans-2-butene:
A Mechanism for the reaction: Halohydrin Formation from an Alkene:
If the alkene is unsymmetrical, the halogen ends up on the carbon atom with greater # of H atoms.
Carbenes:
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.
Methylene:
- 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
7,7-Dichlorobicyclo[4.1.0]heptane (59%)
1,2-Dihydroxylation:
Mechansim for syn dihydroxylation of alkenes:
- 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.
Cleavage w/ hot basic Potassium Permanganate
Using Potassium Permanganate (KMnO4) to test for alkenes/ alkynes:
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.
Ozonolysis:
- 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)
ozonolysis:
A Mechanism for the reaction: Ozonolysis of an Alkene
- 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.
Addition of hydrogen halides to alkynes in general:
- regioselective
- folow markovnikov's rule
- follows Markovnikov's rlue
Anti-Markovnikov addition
cleavage at the Carbon carbon triple bond.
cleavage at the carbon-carbon triple bond.
When planning a synthesis what must be considered (4 interrelated aspects):
1. construction of the carbon skeleton

2. functional group interconversions

3. control of regiochemistry

4. control of stereochemistry.
Stereoselective Reaction:
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)
Stereospecific Reaction:
one that produces predominantly/ exclusively one stereoisomer of the product when a specific stereoisomeric form of the reactant is used.
Summary and Review Tools: Mechansim Review: Summary of Alkene Addition Reactions
Summary and Review Tools: Sunthetic Connections of Alkynes and Alkenes: II