TBR test 2 biological sciences q 99

[2010premed]

When the silyl protecting group shown in Figure 1 is removed using tetrabutyl ammonium fluoride, the stereochemistry of the alcohol remains the same. This implies that the:

A. fluoride attacks the chiral carbon and removes the silyl group.
B. fluoride attacks the silicon and kicks out an alkoxy group, which eventually gets protonated to form the alcohol.
C. oxygen of the silyl group attacks one of the butyl groups on nitrogen before undergoing hydrolysis.
D. nitrogen of the tetrabutyl ammonium attacks the silicon and kicks out the alkoxy group.
Answer: B

In the explanation, they say C is wrong b/c the O is neutral and therefore cannot donate any e's. How do you know the O is neutral? I found this question overall challenging

---

Members don't see this ad.

 

[crazyasian]

Here's my take on it. This is how I reasoned through the question. Oxygen when bound to 2 other atoms has 2 lone pairs. On a carbonyl, where it is able to delocalize these e- and create a - charge it makes for a strong nucleophile. In this case there's no pi bond to delocalize e-, therefore it cannot create a - charge and it is not very nucleophilic. Therefore Oxygen wouldn't do the attacking. Flouride in this case is the only thing that can generate a - charge here so it has to be the nucleophile. a bulky Nitrogen with a + charged and electron deficient is an extremely poor nucleophile therefore it won't attack.

 

[2010premed]

Thx =)