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Chapter 8. Alkenes and Alkynes II: Addition Reactions. Addition Reactions of Alkenes. p -bond. s -bond. 2 s -bonds. 1A. How To Understand Additions to Alkenes. This is an addition reaction: E – Nu added across the double bond. Bonds broken. Bonds formed.

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chapter 8

Chapter 8

Alkenes and Alkynes II:

Addition Reactions

1a how to understand additions to alkenes
p-bond

s-bond

2 s-bonds

1A. How To Understand Additions to Alkenes
  • This is an addition reaction: E–Nu added across the double bond.

Bonds broken

Bonds formed

slide4
Since p bonds are formed from the overlapping of p orbitals, p electron clouds are above and below the plane of the double bond.

p electron clouds

slide5
Electrophilic
    • electron seeking
    • C=C and C≡C p bonds are particularly susceptible to electrophilic reagents (electrophiles)
  • Common electrophile
    • H+, X+ (X = Cl, Br, I), Hg2+, etc.
slide6
In an electrophilic addition, the p electrons seek an electrophile, breaking the p bond, forming a s bond and leaving a positive charge on the vacant p orbital on the adjacent carbon. Addition of B– to form a s bond provides an addition product.
slide8
Electrophilic Addition ofHydrogen Halides to Alkenes: Mechanism and Markovnikov’sRule
  • Mechanism:
slide9
Mechanism
    • Sometimes do not go through a “free carbocation”, may go via:
slide10
Markovnikov’s Rule
    • For symmetrical substrates, there is no regiochemistry problem.
slide11
Markovnikov’s Rule
    • But for unsymmetrical substrates, two regioisomers are possible.
slide12
Markovnikov’s Rule
    • In the electrophilic addition of an unsymmetrical electrophile across a double bond of an alkene, the more highly substituted and more stabilized carbocation is formed as the intermediate in preference to the less highly substituted and less stable one.
slide13
Markovnikov’s Rule
    • Thus:

Note: carbocation stability  3o > 2o > 1o

slide14
Addition of Hydrogen Halides
    • Addition of HCl, HBr and HI across a C=C bond.
    • H+ is the electrophile.
2a theoretical explanation of markovnikov s rule
2A. Theoretical Explanation ofMarkovnikov’s Rule
  • One way to state Markovnikov’s rule is to say that in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.
slide17

Step 1

Step 2

2b modern statement of markovnikov s rule
2B. Modern Statement of Markovnikov’s Rule
  • In the ionic addition of an unsymmetrical reagent to an unsymmetrical double bond, the positive portion of the added reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.
2c regioselective reactions
2C. Regioselective Reactions
  • When a reaction that can potentially yield two or more constitutional isomers actually produces only one (or a predominance of one), the reaction is regioselective.
2d an exception to markovnikov s rule
2D. An Exception to Markovnikov’s Rule
  • Only with HBr in presence of peroxide.
  • Via a radical mechanism (see Chapter 10).
  • This anti-Markovnikov addition does not take place with HI, HCl, and HF, even when peroxides are present.
slide25
Stereochemistry of the IonicAddition to an Alkene

attack from top

racemate

achiral

trigonal planar

carbocation

attack from bottom

slide26
4. Addition of Sulfuric Acid to Alkenes

more stable

3o cation

less stable

1o cation

  • Addition of H–OSO3H across a C=C bond.
4a alcohols from alkyl hydrogen sulfates
4A. Alcohols from Alkyl HydrogenSulfates
  • The overall result of the addition of sulfuric acid to an alkene followed by hydrolysis is the Markovnikov addition of H– and –OH.
slide28
Addition of Water to Alkenes:Acid-Catalyzed Hydration
  • Overall process:
    • Addition of H–OH across a C=C bond.
    • H+ is the electrophile.
    • Follow Markovnikov’s rule.
5b rearrangements
5B. Rearrangements
  • Rearrangement can occur with certain carbocations.
slide31
Alcohols from Alkenes throughOxymercuration–Demercuration:Follows Markovnikov Addition.
  • Step 1: Oxymercuration
  • Step 2: Demercuration
6a regioselectivity of oxymercura tion demercuration
6A. Regioselectivity of Oxymercura-tion–Demercuration
  • Oxymercuration–demercuration is also highly regioselective and follows Markovnikov’s rule.
6b rearrangements rarely occur in oxymercuration demercuration
6B. Rearrangements Rarely Occur inOxymercuration–Demercuration
  • Recall: acid-catalyzed hydration of some alkenes leads to rearrangement products.
slide35
Rearrangements of the carbon skeleton rarely occur in oxymercuration–demercuration.

no rearrangement

6c mechanism of oxymercuration
6C. Mechanism of Oxymercuration

There is no “free carbocation”.

slide37
Stereochemistry
    • Usually anti-addition.
slide38
Although attack by water on the bridged mercurinium ion leads to anti addition of the hydroxyl and mercury groups, the reaction that replaces mercury with hydrogen (step 2) is not stereocontrolled. This step scrambles the overall stereochemistry.
  • The net result of oxymercuration–demercuration is a mixture of syn and anti addition of –H and –OH to the alkene.
slide39
Solvomercuration-Demercuration

Note:

Step 1 with THF-HOH gives an alcohol.

Step 1 with THF-ROH gives an ether.

slide40
Alcohols from Alkenes throughHydroboration–Oxidation:Anti-Markovnikov Syn Hydration
  • BH3 is called borane.
  • Addition of H–BH2 across a C=C bond.
slide42
An anti-markovnikov product results:

syn addition

Anti-Markovnikov addition

of “H” & “OH”

slide43
Compare with oxymercuration-demercuration:

anti addition

Markovnikov addition

of “H” & “OH”

slide48
Oxidation and Hydrolysis ofAlkylboranes

B always ends

up on the least

hindered carbon

slide52
Overall synthetic process of hydroboration-oxidation-hydrolysis.
  • Overall: anti-Markovnikov addition of H–OH across a C=C bond.
  • Forms regioisomer opposite to oxymercuration-demercuration.
slide53
anti-Markovnikov

syn addition

  • Example

This oxidation step occurs with retention of configuration

slide55
Examples

With rearrangement

Markovnikov addition of H2O, no rearrangement

anti-Markovnikov, syn addition of H2O

No rearrangement

slide56
Protonolysis of Alkylboranes
  • Protonolysis of an alkylborane takes place with retention of configuration; hydrogen replaces boron where it stands in the alkylborane.
  • Overall stereochemistry of hydroboration–protonolysis: syn.
slide58
Electrophilic Addition of Bromine and Chlorine to Alkenes
  • Addition of X–X (X = Cl, Br) across a C=C bond.
12a mechanism of halogen addition
12A. Mechanism of Halogen Addition

Br–Br bond becomes polarized when close to alkene.

(vincinal

Dibromide)

(cyclic bromonium ion)

slide61
Stereochemistry
    • Anti addition
slide62
Stereospecific Reactions
  • A reaction is stereospecific when a particular stereoisomeric form of the starting material reacts by a mechanism that gives a specific stereoisomeric form of the product.
slide63
Reaction 1
  • Reaction 2
slide66
Halohydrin Formation
  • Addition of –OH and –X (X = Cl, Br) across a C=C bond.
  • X+ is the electrophile.
  • Follow Markovnikov’s rule.
slide68
Other variation
    • If H2O is replaced by ROH, RÖH will be the nucleophile.
slide69
Divalent Carbon Compounds:Carbenes

15A. Structure and Reactions ofMethylene

slide71
15B. Reactions of Other Carbenes:Dihalocarbenes
  • :CX2 (e.g. :CCl2 = dichlorocarbene).
  • Generation by a-elimination of chloroform.
  • Note: E1 & E2 are β-eliminations.
slide74
15C. Carbenoids: The Simmons-SmithCyclopropane Synthesis
  • I-CH2-ZnI is the Simmons-Smith reagent.
slide76
Oxidation of Alkenes:Syn 1,2-Dihydroxylation
  • Overall: addition of 2 OH groups across a C=C bond giving a vicinal diol.
  • Reagents: dilute KMnO4 / OH⊖ / H2O / cold or OsO4, pyridine then NaHSO3, H2O.
slide79
Comparison of the two reagents
    • KMnO4: usually lower yield and possibly side products due to over-oxidation.

(oxidative cleavage of C=C)

  • OsO4: usually much higher yield but OsO4 is extremely toxic.
slide80
Oxidative Cleavage of Alkenes

17A. Cleavage with Hot BasicPotassium Permanganate

slide86
Addition of Hydrogen Halidesto Alkynes
  • Regioselectivity
    • Follow Markovnikov’s rule.
slide88
As with akenes, Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture.
slide90
How to Plan a Synthesis:Some Approaches & Examples
  • In planning a synthesis we often have to consider four interrelated aspects:
    • Construction of the carbon skeleton.
    • Functional group interconversions.
    • Control of regiochemistry.
    • Control of stereochemistry.
slide91
21A. Retrosynthetic Analysis
  • How to synthesize ?
  • Retrosynthetic analysis
slide92
Markovnikov addition

of H2O

  • Synthesis
slide93
How to synthesize ?
  • Retrosynthetic analysis
  • Synthesis

anti-Markovnikov addition of H2O

slide94
21B. Disconnections, Synthons, andSynthetic Equivalents
  • One approach to retrosynthetic analysis is to consider a retrosynthetic step as a “disconnection” of one of the bonds.
  • In general, we call the fragments of a hypothetical retrosynthetic disconnection Synthons.
slide95
Example
  • Retrosynthetic analysis

(gem-dibromide came from addition of HBr across a C≡C bond.)

slide96
Retrosynthetic analysis

synthons

disconnection

synthetic equivalent

slide99
Retrosynthetic analysis
    • The precursor of a vicinal dibromide is usually an alkene.
    • Bromination of alkenes are anti addition.

(rotate 180o)

(anti addition of Br2)

(anti addition of H2)

slide100
Synthesis

(anti addition of H2)

(anti addition of Br2)

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