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CHAPTER 8 275 
 
withdrawing. Therefore, the  bonds in these cases are 
less electron-rich than in compounds 1 or 2. The acetoxy 
group of alkene 5 is expected to be a stronger electron-
withdrawing group than the methoxy group of 3 since 
the carbonyl group in the former compound will enhance 
the electron-withdrawing ability of the oxygen atom via 
resonance. Compound 4 is interesting in that the acetoxy 
oxygen atom is expected to be an electron-donating 
group with respect to the alkene  bond (via resonance), 
much like the oxygen atom in compound 1. From the 
relative reactivity data, however, the lone pairs on the 
acetoxy oxygen are apparently less efficient at donating 
to the carbon-carbon  bond. This can be explained by 
recognizing that the lone pairs on the acetoxy oxygen are 
already partially delocalized into the carbonyl group: 
 
 
 
That is, the lone pairs on the acetoxy oxygen are less 
available to donate electron density to the  bond. 
Therefore, the acetoxy group of 4 influences the 
reactivity primarily through its inductive and steric 
effects, which will be much like that of the substituents 
in compounds 3 and 5. 
 
(b) Compound 6 is apparently able to impose the 
electron-withdrawing effect due to induction of the –CN 
group on the alkene  bond via the shorter  bond (due 
to the sp3-sp orbital overlap) between the CH2 and the 
CN groups, and thus the closer proximity of the partial 
positive charge to the  electrons of the alkene. This is 
apparent when comparing the relative rates between 
compounds 5 and 6. Here, though the oxygen atom in 
compound 5 is more electronegative than carbon, the 
carbon atom of the cyano group apparently possesses a 
very large partial positive change that effectively renders 
this carbon atom more electronegative than that of the 
acetoxy oxygen atom of 5. The low reactivity associated 
with compound 7 may be due to both induction and 
steric effects, since the inductive withdrawal of electron 
density by the chlorine atom is not expected to exceed 
the substituent in compound 5. 
 
(c) Compounds 2, 8 and 9 illustrate the consequences of 
steric effects on the hydroboration reaction. Compounds 
8 and 9 are the only disubstituted alkenes among the 
series, and it is not surprising that they have the lowest 
relative reactivity, due to increased steric effects. They 
are more than 100 times lower in reactivity than 
compound 2, the only monosubstituted alkene with no 
significant electronic effects (i.e. resonance and 
induction). In compound 9, the electron-withdrawing 
effect of the chlorine atom, which is superimposed upon 
the increased steric effect, further lowers the reactivity. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
8.95. When approaching this problem, it would be advisable to first label the carbon side chain coming off the benzene 
ring so that you can determine what new connections have been made. Your numbering system does not need to 
conform to IUPAC rules for assigning locants. Rather, it is OK to use an arbitrary numbering system, because the goal 
of the numbering system is to track the fate of all atoms during the transformation: 
 
 
 
With this numbering system, the benzene ring is attached to C2 of the chain and the phenolic oxygen is attached to C6 
of the chain. 
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