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Solutions for Ethers, Epoxides and Thioethers
50 As is often true when explaining the properties of molecules, hydrogen bonding is the key.
HO OH
OH
glycerol
mol. wt. 92 g/mol
b.p. 290 °C
d 1.24 g/mL
O O
O
mol. wt. 309 g/mol
b.p. 180 °C
d 0.88 g/mL
SiSi
Si
 Glycerol has extremely strong intermolecular hydrogen bonding because of the three OH groups per 
molecule. Overcoming these intermolecular forces requires a lot of energy: thus, glycerol has a high 
boiling point despite its fairly low molecular weight, and it flows slowly because hydrogen bonding must be 
overcome in order for molecules to slide past each other. The density is high because these molecules pack 
together tightly to maximize hydrogen bonding.
 In contrast, the TMS (trimethylsilyl) ether of glycerol not only has no hydrogen bonding, but on each 
end of the molecule, there is a nonpolar and essentially spherical group—this is like putting on boxing 
gloves and trying to pick up a dime, or anything! So in spite of the high molecular weight, these molecules 
tend to stay far apart, explaining the ease of flow, the low density, and the relatively low boiling point.
 For a boiling point comparison, look up the structure of "isocetane," which is a highly branched alkane 
of molecular weight 226 g/mol, with boiling point 240 °C. The van der Waals forces in the TMS ether 
above must be even lower.
51 Approaching a good synthesis problem begins with comparing the product to the starting material. If 
new carbons appear in the product, then the synthesis must include a carbon-carbon bond-forming reaction, 
of which there are very few.
HO Br HO
CH2CH3
CH3
OH?
new bond
At first glance, this appears to require a simple Grignard reaction, but then we recall that a Grignard reagent 
cannot coexist with an OH group in the same molecule. Aha! The OH group needs to be protected before 
the Grignard can proceed.
HO Br
HO
CH2CH3
CH3
OH
The new bond is 
shown in bold.
TIPSCl
Et3N
O Br(i-Pr)3Si Mg
ether
This could be isolated.
O MgBr(i-Pr)3Si
Grignard reagents are 
stable only in solution; 
they cannot be isolated.
O
O
CH2CH3
CH3
O– MgBr+
(i-Pr)3SiBu4N+ F–
H2O
Fluoride removes the Si group, 
and water protonates the oxygens.
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