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820 CHAPTER 20 
 
Now let’s draw the forward scheme. Hydrolysis of acetonitrile gives acetic acid which is subsequently reduced to 
ethanol upon treatment with excess LiAlH4, followed by water work-up. Upon treatment with PBr3, ethanol is 
converted to ethyl bromide which is then converted to ethyl magnesium bromide (a Grignard reagent). A Grignard 
reaction with carbon dioxide followed by protonation with H3O+ gives propanoic acid. Conversion to the acid chloride, 
followed by reaction with excess ethyl magnesium bromide (prepared as described above) produces the desired tertiary 
alcohol, 3-ethyl-3-pentanol. 
 
 
 
Br
PBr3
OH
Mg
1) xs LiAlH4
O
OH
O
Cl SOCl2
OH
OH3O+
OH
CH3CN
2) H2O
1) Mg
2) CO2
3) H3O+
1) xs EtMgBr
2) H2O
heat
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
20.72. The mechanism shown below has 12 steps: 1) proton transfer, 2) loss of a leaving group, 3) nucleophilic attack, 
4) proton transfer, 5) proton transfer, 6) loss of a leaving group, 7) proton transfer, 8) nucleophilic attack, 9) proton 
transfer, 10) proton transfer, 11) loss of a leaving group and 12) proton transfer. In step 1 (proton transfer), one of the 
acetal oxygen atoms is protonated by H3O+, activating it as a leaving group (either oxygen atom can be protonated, 
which will ultimately lead to the same product). In step 2, an alcohol group leaves as a leaving group. Water then 
attacks the activated carbonyl group in step 3 (nucleophilic attack). In step 4 (proton transfer), water deprotonates the 
cationic oxygen atom, and in step 5 (proton transfer), the other oxygen atom in the hemiacetal is protonated by H3O+, 
producing a cationic intermediate. In step 6, an alcohol serves as a leaving group. Protonation of the carbonyl group 
(step 7, proton transfer) activates this carbonyl group for intramolecular attack by one of the tethered alcohol groups 
(step 8, nucleophilic attack), thus forming a ring (a five-membered ring is more likely formed than a more strained, 
four-membered ring). Deprotonation by water (step 9, proton transfer) followed by protonation by H3O+ (step 10, 
proton transfer) produces a cationic intermediate with an activated leaving group (water). In step 11, water serves as a 
leaving group. In step 12 (proton transfer), water serves as a weak base to deprotonate the cationic oxygen atom, 
resulting in the formation of the final product (a lactone). 
 
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