CH11: Addition Reactions with Alkenes Flashcards by Lauren Hillin

Regiochemistry of a symmetrical alkene with 2 different groups added

Irrelevant. Groups can be added to either side.

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Regiochemistry of an unsymmetrical alkene with 2 of the same groups added

Irrelevant. Since they are the group it doesn’t matter.

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Regiochemistry of an unsymmetrical alkene with 2 different groups added

Relevant. Regiochemistry depends on Markovnikov or Anti-Markovnikov mechanism.

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Where do groups go in a markovnikov reaction?

More nucleophilic atom (more negative, think halides) goes to the more substituted side

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What is the more substituted side?

Side with more R-groups attached to it

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Where do the groups go in an anti-markovnikov reaction?

The more nucleophilic atom goes to the LESS substituted side.

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What is a syn addition?

Both groups add to the same side.
(both will be wedges or both will be dashes)

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What is an anti addition?

Each group adds to a different side (one will be wedge, one will be dash)

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What is hydrogenation?

Addition of 2 hydrogens on an alkene.

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Hydrogenation:
What type of stereochemistry addition mechanism? Why?
What type of regiochemistry addition mechanism? Why?

Syn Addition - alkene takes Hs from Pd/C from one side
Neither Mark/Anti-Mark due to adding same group

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Hydrogenation:
What are the reagents?

H2
——->
Pd/C
(Paladium on Carbon)

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H-X (Ionic Mechanism):
What type of stereochemistry addition mechanism? Why?
What type of regiochemistry addition mechanism? Why?

Neither anti or syn addition - carbocation intermediate.
Markovnikov - Halide added to more substituted side due to carbocation intermediate’s stability

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H-X (Ionic Mechanism):
What is the mechanism?

1st step is Proton transfer. Alkene attacks H.
H gets added to the less substituted side, as this will form a more stable carbocation intermediate.
2nd step is Nucleophillic Attack. Negatively charged halide attacks carbocation.

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H-X (Ionic Mechanism)
What are the reagents?

H-X
X = halides (I, Br, Cl)

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H-X (Radical Mechanism)
What type of stereochemistry addition mechanism? Why?
What type of regiochemistry addition mechanism? Why?

No stereochemistry due to carbocation intermediate.
Anti-markovnikov. Bromine is added first, so it is added to the less substituted side.

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H-X (radical mechanism)
What are the reagents?

H-X
——–>
ROOR + hv
ROOR = peroxide,
hv = light

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H-X (Radical Mechanism)
What is the mechanism? (name steps)

1) Initiation
2) Propagation
3) Termination

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Explain the steps in a radical mechanism

1) Initiation: Peroxide is cleaved to form a radical.
2) Propagation: Formation of products. Radical is regenerated at end to continue a cycle of formation.
3) Termination: 2 radicals combine to stop the reaction.

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Explain the steps in propagation.

Radical formed in step 1 “steals” H, leaving behind 1 electron and forming a halide radical.
This halide radical then attacks the pi bond on the alkene, breaking the bond and forming a radical intermediate.
The halide bonds to the less substituted side to form a more stable radical intermediate
The radical intermediate attacks the H on H-X, stealing the H and forming the same halide radical to continue the cycle.

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What is hydration?

Adding HOH across an alkene in a markovnikov addition

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Hydration:
What type of stereochemistry addition mechanism? Why?
What type of regiochemistry addition mechanism? Why?

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No stereochemistry due to carbocation formation.
Markovnikov mechanism. H2O group added to the more substituted side.

Hydration:
What are the reagents?

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H2O
————>
H2SO4 (cat)

*Can also just be H3O+

Hydration:
What is the mechanism?

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H2O deprotonates H2SO4, becoming H3O+
The pi bond on the alkene “steals” the hydrogen from H3O+, and the hydrogen goes to the less substituted side of the alkene
*Carbocat formed
H2O deprotonates the positive H2O group, regenerating the H3O+ acid

What is hydroboration?

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The addition of H and BH2 across an alkene in a single step, then replacing BH2 with OH.

Hydroboration: What type of stereochemistry addition mechanism? Why? What type of regiochemistry addition mechanism? Why?

Syn addition. Due to BH3 adding at once. Anti-Markovnikov due to Boron needing more space, so it goes to the less hindered site on the alkene.

Hydroboration: What are the reagents?

1) BH3 * THF --------------------> 2) H2O2 , NaOH 1) THF is a solvent that BH3 will be dissolved in.

Hydroboration: What is the mechanism?

1) Boron is neutral with 6 electrons, so the pi bond on the alkene attacks the boron. 2) A bond with a hydrogen on the boron breaks, and the hydrogen attaches to the more substituted side while the boron attaches to the LESS substituted side. 3) MAGIC: BH2 is replaced by OH

What is Bromination/Chlorination?

The addition of 2 halides. Bromination is addition of 2 bromines Chlorination is addition of 2 chlorines

Bromination: What type of stereochemistry mechanism? why? What type of regiochemistry mechanism? Why?

Anti-addition. A SN2 mechanism occurs, resulting in the second Halide group being added in an anti addition. No regiochemistry - same groups being added.

Bromination: What are the reagents?

Br2 ------->

Bromination: What is the mechanism?

1) Pi bond attacks a bromine. Bromine attaches to both ends of the alkene, forming a bromonium ion. 2) Second bromine (now negative), performs an SN2 reaction on the more substituted carbon, kicking off the 1st bromine from that side. 3) the 1st bromine remains connected to the other carbon, resulting in an anti-addition + its enantiomer.

What is halohydrin formation?

The addition of a halide and an OH group (BrOH)

Halohydrin formation: What type of stereochemistry mechanism? why? What type of regiochemistry mechanism? Why?

Anti-addition. An SN2 mechanism results in the OH group being added in an anti-addition. Markovnikov. THe OH group is added to the more substituted carbon due to it having a larger partial positive charge.

Halohydrin formation: What are the reagents?

Br2 --------> H2O *Instead of H2O it could also be MeOH

Halohydrin formation: What is the mechanism?

1) Pi bond attacks a bromine. Bromine attaches to both ends of the alkene, forming a bromonium ion. 2) An H2O group performs an SN2 reaction on the more substituted carbon, kicking off the bromine from that side. 3) Another H2O deprotonates the positive H2O group on the molecule. 4) anti addition + enantiomer formed. negatively charged Bromine is also formed.

OH-OH (peroxy): What type of stereochemistry mechanism? Why? What type of regiochemistry mechanism? Why?

Anti-addition due to SN2 reaction occurring. No regiochemistry - same groups being added

OH-OH (Anti-addition): What are the reagents?

1) RCO3H -----------------------> 2) H3O+ *RCO3H is a peroxy acid.

OH-OH (peroxy) What is the mechanism?

1) MAGIC: an oxygen is attached to both ends of the alkene using MCPBA/RCO3H. This forms an epoxide. 2) The attached oxygen deprotonates H3O+ , causing it to become positive. 3) H2O attacks a carbon attached to the OH in an SN2 reaction. The OH is kicked off, and H2O is added in an anti-addition from the OH. 4) H2O deprotonates the H2O on the molecule.

OH-OH (no mechanism) What type of stereochemistry mechanism? Why? What type of regiochemistry mechanism? Why?

Syn addition. No regiochemistry - same groups being added

OH-OH (syn); What are the reagents? The mechanism?

1) OsO4 -----------------> 2) H2O2 No mechanism given.

What is ozonolysis?

The use of Ozone (O3) to cleave an alkene and add an oxygen on each end of the cut.

Ozonolysis: What are the reagents?

1) O3 -----------> 2) DMS

Ozonolysis: What is the mechanism?

1) label/number the carbons 2) "cut" across the alkene. 3) add 2 oxygens to each side of the cut (these oxygens are NOT bonded together) 4) flatten out the molecule - if needed.

What is the most common peroxy acid and what mechanism is it used for?

MCPBA (meta-chloroperoxy benzoic acid) Used for OH-OH Anti-additions.

Why does a carbocation intermediate get rid of stereochemistry additions?

A carbocation intermediate is unstable, so the group being added just adds wherever it can. This results in an even mix of products (in regards to stereochem)

What 3 mechanisms have a carbocation intermediate?

Hydration H-X (Markovnikov / Ionic) H-X (Anti-Markovnikov / Radical)

What 3 mechanisms have a syn addition?

OH + OH (OsO4) Hydroboration Hydrogenation

What 3 mechanisms have an anti addition?

OH + OH (mCPBA / RC3OH) Bromination Halohydrin Formation

What 2 mechanisms are anti-markovnikov?

H-BR (Radical) Hydroboration

What 2 mechanisms are markovnikov?

Hydration H-Br (Ionic)

CH11: Addition Reactions with Alkenes Flashcards

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