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Brominationas a tool for structure
determinationExperimental:
• Dissolve the sample in DCM (1 mL)• Add dropwise a 1% bromine solution of DCM• Positive test, if red color disappears
Conclusion:The sample contains a multiple bond, which undergoes bromination
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Hypothesis:The sample is an alkene
if the red color disappears
Br2 Br
Br
Br2 consumedred color disappears
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Sample 2 also an
alkene
H3CO
OCH3
O
O
O O
CH3H3C OO
dimethyl fumarate dimethyl maleate
O O
CH3H3C OO Br2red color persists
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O O
CH3H3C OO Br2red color persists
No Reaction?
Why?
Let’s take a look at the mechanism of alkene bromination
Electrophilic Additon
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Br BrBr
Electrophilic Additon
Alkene is the NucleophileThe stronger the nucleophile, the faster is
the reactionAn electron-rich alkene
makes a stronger nucleophile,
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What makes an electron rich alkene?
O O
CH3H3C OO
Is a maleate an electron rich alkene?
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O O
CH3H3C OO
Let’s take a look at the substituents
O
O
Electron-Withdrawing• Reduces the electron
density of the double bond
• Makes the double bond less nucleophilic
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Br Br
O
does not happen
Maleate is not electron-rich enough(even though it is an alkene)
Conclusion:“False” Negative
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What if we “helped” maleate to go over the activation energy
barrier?by heating it
Drawbacks:We might evaporate the whole
solventO O
CH3H3C OO
Br Br
expected producta White
Solid
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Sample 3 not an alkene
OCH3
anisole
an aromatic compoundaromatic compounds do not undergo electrophilic addition reactions
BUT OCH3
Br2red color disappears
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OCH3
Br2
Is anisole undergoing an electrophilic addition?
Or could it be an aromatic electrophilic substitution (EAS)?(the first step of which is an electrophilic addition anyway)
Can this happen in the absence of a Lewis Acid?
Check the Substituent
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OCH3 Methoxy group,• a strong
activatorwill enable anisole undergo bromination (EAS)in the absence of a Lewis Acid
OCH3
Br Br
OCH3H
Br
BrOCH3
Br
+ HBr
can HBr be detected?
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Conclusion:“False” Negative
OCH3
Br2
OCH3
Br
+ HBr
OCH3
Br
+
red color disappears
Further Conclusions (if HBr is detected):Sample is an aromatic compoundwith a strongly activating
substituent