Addition Reactions of Carbon-Carbon Pi Bonds - Part 1 Reactions of Carbon-Carbon ... Hydrogen Halide...

Addition Reactions of Carbon-CarbonPi Bonds - Part 1

C CH

H H

H

C

H3C

H3C

CH2H Brδ+ δ−

C

H3C

H3C

CH2

Br H

+ C

H3C

H3C

CH2

H Br

What Is an Addition Reaction?Addition reaction: Atoms or groups are added to opposite ends of a pi bond.

Industrial production of ethanolC

H

H

C

H

H

H3PO4

H OHC C

H

H

H

H

H

OH

Partial hydrogenation ofpolyunsaturated fatH

H

H H

H HPt H

H

HH H

H

Why should I study this topic?•Addition reactions common in organic, industrial, and biological chemistry.•Examples:

Addition not limited to just alkenes.

C C C C

X Y

Addition Reactions of Carbon-Carbon Pi Bonds Lecture Supplement Part 1 --

Alkene Pi BondsDetails of addition controlled by structure of functional groups

How does an alkene appear to the outside world?

Alkene: Two sp2 carbons

C CH

H H

H

Explore alkene pi bond shape with molecular models

Side view Top view End viewOne pi bond (two lobes)

How does an alkene react?

Pi electron cloud

Pi electrons NucleophilicReacts withelectrophiles

Therefore ThereforeCC

H H

HH

Nuc

Elec

C CH

H H

H

Alkyne Pi BondsDetails of addition controlled by structure of functional groups

How does an alkyne appear to the outside world?

Alkyne: Two sp carbons

Explore alkyne pi bond shape with molecular models

Two perpendicular pi bonds

How does an alkyne react? Similar to alkene...

Pi electron cloud

Pi electrons NucleophilicReacts withelectrophiles

Therefore Therefore

Side view Top view

CCNuc

Elec

HH

C CH H

pz

pyC CH H

pz

pyC CH H

pz

py

End view


Generic Pi Bond Addition Mechanism

•Similar process for alkynes

•Overall process: Electrophilic addition

•Not every pi bond addition reaction follows this mechanism

Elec has +, δ+, or open octet

Lost: Pi bond Gained: C-Elec bond

Lost: No bonds Gained: C-Nuc bond

rds?

Carbocation fates!

•Capture a nucleophile → addition

C C

rds?

Elec

C

Elec

C

Nuc

C

Elec

C

Nuc

Carbocation Captures Nuc Not Always SN1An important caveat.

Conclusion: Not all reactions in which a carbocation captures a nucleophile are SN1.

Avoid this common misconception: Carbocation captures a nucleophile is always SN1.

Generic electrophilic addition to an alkene mechanism:

C C

Elec

C

Elec

C

Nuc

C

Elec

C

Nuc Capture a nucleads to addition.

Capture a nucleads to substitution.

R C

R

R

LG R C

R

R

Nuc R C

R

R

Nuc

Generic SN1 mechanism:


Hydrogen Halide Addition to an Alkene

X = F, Cl, Br, I

Why is H-X electrophilic?

H Xδ+ δ−

Mechanism:C CH2

H

H3C

CH3

Br-

C CH2H3C

CH3

HBr-

C CH2

H

H3C

CH3

Br

C CH2

Br

H3C

CH3

H

C

H3C

H3C

CH2

δ+ δ−H Br

Overall reaction: C CH X

C

H

C

X

HigherΔG‡

LowerΔG‡

C

H3C

H3C

CH2

H

Br δ−

δ+

C

H3C

H3C

CH2

H

Brδ−

δ+

C

H3C

H3C

CH2H Brδ+ δ−

C

H3C

H3C

CH2

Br H

+ C

H3C

H3C

CH2

H Br

Markovnikov's Rule

Major Minor

3o 1o

Markovnikov's Rule: In the addition of a protic acid HX to an alkene, the acid hydrogen(H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X)group becomes attached to the carbon with more alkyl substituents.

Vladimir MarkovnikovPhD thesis 1870

•"Them that has, gets" "The (hydrogen) rich get (hydrogen) richer"

•Orientation due to stability of carbocation intermediate

•Similar for HF, HCl, HBr, HI, H-OH2+, etc.


Hydrogen Halide Addition to an Alkyne

Observation:•Alkene has one pi bond

•Alkene adds one molecule HBr

Questions: Alkyne has two pi bonds.

C

H3C

H3C

CH2H Brδ+ δ−

C

H3C

H3C

CH2

Br H

C CH3C HH Brδ+ δ−

???

Explore via mechanism...

•Does alkyne add two molecules HBr?

•Does alkyne + HBr follow Markovnikov's

rule?

Hydrogen Halide Addition to an AlkyneMechanism...

•Alkyne is nucleophile (just like alkene)

•Protonate pi bond to form more stable carbocation (just like alkene)H3C C C H

H Brδ+ δ−

or C

H3C

CH

H

H3C C CH2

Br

CH3C CH2

Br

•Vinyl carbocation: Has positive charge on C=C carbon

•Carbocation fates – be deprotonated?

•Rearrange? Vinyl carbocations do not rearrange

•Capture a nucleophile? Br -

•Are we done yet? Product = alkene – can be protonated...


H3C C

Br

H

CH2


•Alkene is nucleophile

•Protonate pi bond to form more stable carbocation

or

•Carbocation fates – be deprotonated?

•Rearrange?

•Capture a nucleophile? Br -

CH3C CH2

Br

CH3C CH2

Br H Brδ+ δ−

H3C C

Br

CH3

H3C C

Br

Br

C

H

H

H

•Which carbocation is favored?

•Product is a dibromide


Overall mechanism: Addition follows Markovnikov's rule? Yes No

H3C C C H

H Brδ+ δ−

H3C C CH2

Br

CH3C CH2

Br H Brδ+ δ−

Br

H3C C

Br

CH3 H3C C

Br

Br

C

H

H

H

Markovnikov addition oftwo molecules HBr

Alkyne has twopi bonds

Therefore


C

H3C

H3C

CH2H OHδ+

C

H3C

H3C

CH2

OH Hδ−

Addition of Water to a Pi Bond (Hydration)

Observation: H Brδ+ δ-

Question: Do other molecules add to carbon-carbon pi bonds?

Example:

H Xδ+ δ-

H OHδ+

C

H3C

H3C

CH2H Brδ+ δ−

C

H3C

H3C

CH2

Br H

•Prediction: Markovnikov addition of water

•But... H2O (pKa 15.7) is not sufficiently acidic to protonate pi bond

•Strong acid + H2O → H3O+ (pKa -1.8) Strong acid = H2SO4, HI, HBr, HCl, H3PO4

Why?

δ−

Addition of Water to a Pi Bond (Hydration)Does alkene + H3O+ mechanism parallel alkene + HBr mechanism?

•Protonate pi bond; form more stable carbocation

•Carbocation fates: Be deprotonated?

CH3C CH3

H3C

or CH3C CH2

HH3COH2

•Oxonium ion: Oxygen has three bonds and +1 formal charge

CH3C CH3

OH3C H

H

OH2

CH3C CH3

OHH3C

+ H3O+ •The product is an alcohol

Rearrange? Capture nucleophile? Most prevelant nuc = H2O

CH3C CH2

H3C H OH2

•Deprotonation driven by Le Chatelier's principle•Use strongest base present = H2O


Addition of Water to a Pi Bond (Hydration)

Overall mechanism:

•Net reaction: Markovnikov addition of water

•Reaction is reversible:

CH3C CH2

H3CH OH2

CH3C CH3

H3C OH2

CH3C CH3

OH3C H

H

CH3C CH3

OHH3COH2

+ H3O+

CH3C CH2

H3C

+ H2OH3O+ Hydration

H3O+ E1 dehydrationC

H3C CH3

OHH3C

•Equilibrium position controlled by Le Chatelier's principle...

Excess water favors alcohol

Removing water favors alkene

•H3O+ is a _______________________

Ph C C HH OH2δ+

H2O

CPh

HO

CH

H

H OH2δ+

H2OPh C

HO

HO

C

H

H

H

Alkyne Hydration

Observation:

•Alkene has one pi bond.

•Alkene adds one molecule of H2O.

•Addition obeys Markovnikov's rule.

Questions: Alkyne has two pi bonds.

Explore via mechanism...

•Does alkyne add H2O?

•Does this addition follow Markovnikov's rule?

•Does alkyne add two molecules H2O?

C

H3C

H3C

CH2H OH2δ+

C

H3C

H3C

CH2

OH H

? ?


Addition Reactions of Carbon-Carbon Pi Bonds - Part 1 Reactions of Carbon-Carbon ... Hydrogen Halide...

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