Chapter 8 Alkenes and Alkynes II

12 Pages·2008·0.54 MB·English

Additions to Alkenes
Chapter 8
Alkenes and Alkynes II:
Generally the reaction is exothermic because one πand one σ
Addition Reactions bond are converted to two σbonds
Alkenes are electron rich (cid:141) The carbocation produced is an electrophile
(cid:122) It can react with a nucleophile such as a halide
The πelectrons of the double bond are loosely held and
are a source of electron density
Alkenes are nucleophilicand react with electrophiles
such as H+from a hydrogen halide to form a In addition reactions the alkene is a nucleophile in the first
carbocation step and an electrophilein the second
X-H X H
< +
C C
+ X-
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Addition of Hydrogen Halides to Alkenes: Markovnikov’sRule Mechanism for hydrogen halide addition to an alkene
Addition of HBrto propeneoccurs to give 2-bromopropane (cid:141) The reaction has a highly endothermic first step (rate determining) and a
highly exothermic second step
1.
(cid:122) Markovnikov’sRule (Original): addition of HX to an 2.
alkeneproceeds so that the hydrogen atom adds to the
carbon that already has the most hydrogen atoms
Theoretical Basis of Markovnikov’sRule Addition of HBrto 2-methylpropene gives only tert-butyl bromide
Product with the more stable carbocationintermediate predominates
Transition state for the rate determining step (first step) resembles a
carbocationand is stabilized by factors which stabilize carbocations
(cid:141) Modern Statement of Markovnikov’sRule: 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 carbocationas an intermediate
(cid:141) RegioselectiveReaction: When a reaction that can potentially yield two
or more constitutional isomers actually produces only one or a
predominance of one isomer
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Stereochemistry of the Ionic Addition to an Alkene Addition of Sulfuric Acid to Alkenes
If the addition of HBr to buteneyields a chiral Addition of concentrated sulfuric acid to an alkene leads to
molecule, a racemicmixture is produced because an alkyl hydrogen sulfate.
the intermediate carbocation is achiral. The addition follows Markovnikov’srule
(cid:141) The sulfate can be hydrolyzed by heating with water
(cid:122) The net result is Markovnikov additionof water to the
alkene
Addition of Water to Alkenes: Acid-Catalyzed Hydration
The reaction of alkenes with dilute aqueous acid leads
to Markovnikovaddition of water
The mechanism is the reverse of that for dehydration of an alcohol (cid:141) The hydration of alkenes and the dehydration of
(cid:206)The first step in which a carbocationis formed is rate determining alcohols are simply reverse reactions of one other
(cid:122) The reaction is governed by how you push the equilibria.
(cid:122) Hydration is favoredby addition a large amount of water and a low
concentration of acid.
(cid:122) Dehydration is favoredby concentrated acid with the removal of
water.
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Hydration of Alkenes Through Oxymercuration-Demercuration
Carbocation rearrangements are possible
A third, alternative way to add water to an alkene:
•Uses Hg+2as the initial electrophile
•Removal of mercury with sodium borohydride(a reducing agent)
CH3 OH CH3
H2O
CH3 C CH CH2 H+ CH3 C CH CH3
CH3 CH3
3,3-Dimethyl-1-butene 2,3-Dimethyl-2-butanol
Hydration of Alkenes Through Oxymercuration-Demercuration Mechanism involves formation of a bridged mercuriniumion
•High yields
•No carbocationrearrangements
•Follows Markovnikovaddition
Why do we say it involves a bridged ion?
No rearrangements = no free carbocation
BUT Markownikovaddition means it must
be a partial carbocation.
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Mechanism involves formation of a bridged mercuriniumion After reduction with NaBH , net result is the addition of water
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•High yields
Water then attacks the bridged ion at the Markownikovcarbon
•No carbocation rearrangements
•Follows Markovnikov addition
A Fourth Way to Make Alcohols from Alkenes Step 1 : Hydroboration
Hydroboration: Synthesis of Alkylboranes
Hydroboration-Oxidation (cid:122) In this addition reaction, one is adding boron (B) and
hydrogen (H)
(cid:206)BH is an electrophile. It actually exists as its dimerBH.
An important method because it is: 3 2 6
1. an anti-Markovnikov addition H B H H H H
B B
H
2. a synaddition H H H
Borane
Diborane
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Mechanism of hydroboration Mechanism of Hydroboration
(cid:141) In reality, each boranemolecule adds successively to three
molecules of alkene.
R δ δ H
C C
H H
δ
H B H
δ
H
1. The B and the H add in a concerted fashion.
Boron becomes attached to less substituted carbon of double bond
(cid:122) Bulkier boron group approaches the less hindered carbon more easily
2. The B is δ+; the H is δ-
(cid:122) This orientation also allows a δ+ charge in the transition state to
reside at the most substituted carbon
(cid:141) The boron and hydride add with synstereochemistry
Mechanism of hydroboration Mechanism of hydroboration
Recall that boron compounds are Lewis acids, and in this case readily
complexes with the electron pair of the alkene
Transfer of B and H must be concerted to account for the synaddition
Process repeats to form a trialkylborane
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Step 2: Oxidation and Hydrolysis of the Alkylborane
Hydroboration: oxidationgives the anti-Markovnikov
Once the alkylboraneis formed by the addition reaction, product with synaddition of the elements of water
oxidation to the alcohol takes place with retention of
stereochemistry at the carbon bonded to boron.
HR C C HH H2HO22O/HO HR C C HH + B(OH)3
H B H OH
the alkylborane the alcohol boric acid
MECHANISM:
Addition of Bromine and Chlorine to Alkenes
Comparison of alkene Hydration Procedures
(cid:141) Alkenes readily accept Br or Cl to form vicinal dihalides
2 2
1. Acid-catalyzed hydrolysis: Markovnikov
addition, reversible, possible rearrangements
2. Oxymercuration/Demercuration: Markovnikov Note: antiaddition
addition, no rearrangement (cid:141) Used as a test for alkenes because the red color of the bromine
disappears when an alkene (or alkyne) is present.
(cid:122) Alkanesdo not react with bromine in the dark
3. Hydroboration-Oxidation: anti-Markovnikov and
synaddition, no rearrangement
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Mechanism of Halogen Addition
(cid:122) Stereochemistry of the addition of Halogens to Alkenes
(cid:206)The net result is antiaddition because of S 2 attack on the
(cid:141) Mechanism must explain the exclusive anti-addition bromoniumion intermediate. N
(cid:141) A bromoniumion intermediate holds the geometry (cid:206)For example, when cyclopentene reacts, the product is a racemic
mixture of trans-1,2-dibromocyclopentane enantiomers.
Br
H
b
Br b H
H Br
Br2 H a
H
Br Br
a Br
H
trans-1,2-Dibromocyclopentane
(enantiomers)
Halogenation of double bonds is stereospecific HalohydrinFormation
(cid:122) A reaction is said to be stereospecificif a particular If halogenationis carried out in aqueous solvent, the water
stereoisomer of the starting material reacts in such a way that
molecule can act as a nucleophileto open the haloniumion
it gives a specific stereoisomer of the product
(cid:122) Example: cis-and trans-2-butene give stereoisomericproducts
when halogenated
Reaction 1 H3C C H Br2 H3C CH Br
H C CH3 CCl4 Br CH CH3 Same first step
((2R,3S)-2,3-Dibromobutane
(a meso compound)
Water competes
as nucleophile
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Reaction shows regioselectivity Divalent Carbon Compounds: Carbenes
(cid:141) In unsymmetrical alkenes, the bromoniumion will have some of its δ+ (cid:141) Carbeneshave a divalent, neutral carbon
charge density on the more substituted of the two carbons (cid:141) Carbenesare highly reactive C
(cid:122) The most substituted carbon can best accommodate δ+ charge
(cid:141) Structure and Reaction of Methylene (CH)
(cid:141) The water nucleophile will tend to react at the carbon with the most δ+ 2
charge (cid:206)Methylenecan be made by heat or light initiated decomposition of
diazomethane
(cid:43) Loss of a molecule of the stable gas nitrogen drives this reaction
(cid:206)Methylenereacts with alkenes to form cyclopropanes
R R R
R R R
Br Br Br
s i g nciofnictrainbtu rteosronance m i ncoorn rterisbountaonrce
Reactions of Other Carbenes: Dihalocarbenes
Carbenoids: the Simmon-Smith reaction
Carbenesadd to double bonds in a stereospecific manner
A carbene-like species is formed which then reacts with alkenes
transalkene = transcyclopropane
For example:
Dihalocarbenesformed by αelimination of compounds such as chloroform
H CCl3 + CH3 CCCHH33O K CCl3 + CH3 CCCHH33OH dichlCoCrolc2arb+e n Cel CZnH(2CI2u)
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Oxidations of Alkenes Mechanism for SynHydroxylation of Alkenes
1. Syn1,2-Dihydroxylation
(cid:122) Either OsO4 or KMnO4will give 1,2 diols(glycols) A cyclic intermediate results when an alkenereacts with MnO4-
A synadditionof the oxygensoccurs resulting in a cis-diol
after the oxygen-metal bonds are cleaved with the base.
Oxidation 2 of Alkenes: Oxidative Cleavage Useful to identify location of double bonds
(cid:141) Reaction of an alkenewith hot KMnO4results in cleavage of the (cid:122) Example:
double bondand formation of highly oxidized carbons. (cid:206)An unknown alkenewith formula CH yields only the following
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(cid:122) Unsubstitutedcarbons become CO2, monosubstitutedcarbons product on oxidation with hot KMnO4:
become carboxylic acids, and disubstitutedcarbons become
ketones.
(cid:206)Answer: Since no carbons are missing and the product contains
two carbonyl groups, the original alkenemust be a ring.
(cid:141) This reaction can be used as a chemical test for alkenesin which the
purple color of the KMnO disappears and forms brown MnO residue if
4 2
an alkene(or alkyne) is present.
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