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Unformatted text preview: 1 4-47 The fonnula CgHgO has five elements of unsaturation (enough (4) for a benzene ring). The IR i s usefu l for what is does n ot s how . There i s neither OH nor C=O, so the oxygen must be an ether functional group. The NMR s hows a 5H signal at 8 7 .2, a monosubstituted benzene. No peaks in the 8 4. 5 -6.0 range indicate the absence of an alkene, so the remaining element of un saturation must be a ring. The three protons are non-equi valent, with complex splitting. These pieces can be assembled in only one manner consistent with the data. <> + 2C + 0
ether + 3 H + r ing (Note that the CH2 hydrogen s are not equi valent (one is cis and one is t rans t o the phenyl) and therefore have distinct chemical shifts . ) 1 4-48 The key concept is that reagents always go to the less hindered side o f a molecule first. I n this c ase, t he " underneath" side is less hindered; the " top" side has a CH3 hovering over the double bond and approach from the top w i l l be much more difficult, and therefore slower, than approach from underneath . �
H H H M CPB A MCPBA approaches from the bottom; the epoxide is fanned from MCPB A w i thout the participation of any other reagent. The epoxide fonns on the less hindered side. B less hindered side J
B r2 I o n the less hindered side fonnation of the bromohydrin begins with fonnation of the bromonium ion 2 ,6-lutidine w ater m ust attack from the side opposite the bromonium ion , from the TOP Br
o c E poxides B and C are di astereomers ; they will have different chemical and physical properties. Nucle� philes can react with C m uch faster than with B for precisely the same reason that explai ned thei r fonnatlOn: approach from underneath i s less hindered and is faster than approach from t h e top.
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