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Transcript 
Protecting Groups: How They Work
OH
Target Molecule:
Na
Reagents:
and
Br
OH
We can't simply mix the reagents here: deprotonation occurs faster
than alkylation (acid base reaction, think about pKa differences)
Na
+ Br
+ Br
OH
ONa
So, we protect the alcohol as an ether first, and we can successfully alkylate:
Br
1. H2SO4
OH 2. CH =C(CH )
2
3 2
OtBu
H3O+/H2O
Na
Br
OtBu
OH
+
t
BuOH
Protecting Groups: Alcohol Alkylation
OH
+
H2SO4
• Alcohol Groups do not “Survive” Many Organic Reactions
• Alkylation (Ether Formation) Protects OH’s During Synthesis
• Can Remove the Protecting Group w/ Dilute Aqueous Acid
• Generally Dissolve Alcohol in Acid, THEN add Isobutylene
• Addition in this Manner Minimizes Isobutylene Dimerization
• Let’s See Why We Might Want to Use a Protecting Group
O
Protecting Groups: Silyl Ethers
Me
Me
OH + Cl
Si
t
Bu
Pyridine
O
Si
t
Bu
DMF
Me
Me
tert-Butylchlorodimethylsilane
O-TBDMS
• Silyl Ethers Stable Over a 4-12 pH Range (Acidic and Basic)
• Can Survive Conditions of Many Organic Reactions
• Typically Removed w/ Flouride Source (NBu4F; aka TBAF)
• Silyl Ethers More Volatile Than Alcohols (GC Applications)
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