Intramolecular SN2

[SaintJude]

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Kaplan says this rx is really common on MCAT. If I were taken the MCAT today, I would not have realized this is an intramolecular SN2 reactions.
I don't this rx is clearly rationalized, b/c it's an SN2 reaction in the presence of a protic solvent.

3 q:
1. Foraged the SDN forum & found one tip: Halohdyrins (alcohol next to halogen) are commons intramolecular SN2 Reactions. Any other common compounds that commonly undergo intramolecular SN2 reactions?

2. Is there a side SN1 reactions that occurs w/ this as well? Edit: Actually shouldn't b/c it's primary carbon.

3. So deprotonated alcohol = the nucleophile/ & carbon = electrophile ?

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[gettheleadout]

1) Intramolecular attack of a carbonyl carbon by an OH to make a hemi/acetal is the only one I can think of at the moment, but I'm sure there are others. Intramolecular Diels-Alder occurs as well and while this is a single step it's not exactly SN2, though as we saw in a past thread you can assign nucleo/electrophile designations to the diene/dienophile.

3) Yes. The carbon in the C-X bond will be partially positive and thus electrophilic.

 

[SaintJude]

1) Intramolecular attack of a carbonyl carbon by an OH to make a hemi/acetal is the only one I can think of at the moment

Hmm. interesting. you got a pic?

Edit: Waaaait...Are you saying anomerization = intramolecular SN2 rx?

I'm reading that intramolecular nucleophilic acyl substitution (anomerization) is DIFFERENT from SN2. SN2 is 1-step process, while acyl substitution is 2-step process.

 

[MedPR]

Hmm. interesting. you got a pic?

Edit: Waaaait...Are you saying anomerization = intramolecular SN2 rx?

I'm reading that intramolecular nucleophilic acyl substitution (anomerization) is DIFFERENT from SN2. SN2 is 1-step process, while acyl substitution is 2-step process.

I don't think cyclization of glucose (or any other sugar) is an SN2.

 

[pfaction]

Can't be a SN2 because it breaks a carbonyl bond and makes Sp2->Sp3; it's an addition reaction.

 

[indianjatt]

nucleophilic acyl substitution is an addition/elimination reaction, not SN2. The Hemiacatal formation is also simply an addition reaction, not SN2.

 

[SaintJude]

Kaplan says this in the Key concept:

Ring formation is a intramolecular nucleophilic acyl substitution

I don't think it conflicts what you guys are saying about glucose cyclization rx b/c wiki says the nucleophilic acyl substitution is a "basically a two-step nucleophilic addition and elimination reaction."

It just means that nucleophlic acyl substitution ≠ SN2. ?

And so basically, only halohydrins are prominent MCAT compounds that undergo intramolecular SN2.

 

[pfaction]

Also I would have never thought of halohydrins acting as SN2. Good thanks.

 

[MedPR]

Can't be a SN2 because it breaks a carbonyl bond and makes Sp2->Sp3; it's an addition reaction.

nucleophilic acyl substitution is an addition/elimination reaction, not SN2. The Hemiacatal formation is also simply an addition reaction, not SN2.

These are both incorrect. A nucleophilic acyl substitution is not an addition/elimination, otherwise it would be called "nucleophilic acyl addition/elimination. It is a substition reaction and the only sp2 to sp3 conversion is to form a tetrahedral intermediate (as with almost all of the carbonyl reactions). However, in the end, there is no change in hybridization (sp2 prevails). The tetrahedral intermediate forms and, just like in an SN2 reaction, the nucleophile and the leaving group are attached to the central carbon at the same time. Obviously to form this intermediate, the carbonyl oxygen must take on a full negative charge. This isn't ideal, and the oxygen passes the electrons back to the carbon thereby pushing off the leaving group while the nucleophile remains. Thus, a substitution reaction occurs.

This does happen in two steps, however, so it is not an SN2 reaction.

Here is a good overview of the process.

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch20nucleophilic.html

 

[syoung]

These are both incorrect. A nucleophilic acyl substitution is not an addition/elimination, otherwise it would be called "nucleophilic acyl addition/elimination. It is a substition reaction and the only sp2 to sp3 conversion is to form a tetrahedral intermediate (as with almost all of the carbonyl reactions). However, in the end, there is no change in hybridization (sp2 prevails). The tetrahedral intermediate forms and, just like in an SN2 reaction, the nucleophile and the leaving group are attached to the central carbon at the same time. Obviously to form this intermediate, the carbonyl oxygen must take on a full negative charge. This isn't ideal, and the oxygen passes the electrons back to the carbon thereby pushing off the leaving group while the nucleophile remains. Thus, a substitution reaction occurs.

This does happen in two steps, however, so it is not an SN2 reaction.

Here is a good overview of the process.

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch20nucleophilic.html

I like. simple.

 

[gettheleadout]

Hmm. interesting. you got a pic?

Edit: Waaaait...Are you saying anomerization = intramolecular SN2 rx?

I'm reading that intramolecular nucleophilic acyl substitution (anomerization) is DIFFERENT from SN2. SN2 is 1-step process, while acyl substitution is 2-step process.

Whoa sorry, what I was thinking of is definitely an addition reaction. My bad.