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Topic: 2-phenylethanol to benzoic acid  (Read 22344 times)

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Offline THC

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2-phenylethanol to benzoic acid
« on: March 02, 2008, 02:42:09 AM »
Q1: How would you prepare benzoic acid from 2-phenylethanol?
A1: Ar-CH2-CH2OH -> Ar-CH=CH2 (with H2SO4 - elimination) -> Ar-COOH (with KMnO4 - cleavage of double bonds)
Is that feasible?

Q2: Predict the product of the following transformation: Propylene oxide + HBr -> ?
Propylene oxide is: http://en.wikipedia.org/wiki/Propylene_oxide
A2: Is the product 1-brompropan-2-ol?

Thanks.

Offline macman104

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Re: 2-phenylethanol to benzoic acid
« Reply #1 on: March 02, 2008, 03:15:57 AM »
Seems feasible to me, but KMnO4 will also oxidize alkyl groups (including your ethanol) to your benzoic acid.

Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #2 on: March 02, 2008, 03:52:47 AM »
Wouldn't KMnO4 oxidize Ar-CH2CH2OH to Ar-CH2CHO or Ar-CH2COOH?

Also, could you check Q2?

Thank you.

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #3 on: March 02, 2008, 10:33:39 AM »
#2 is wrong.  what are the differences in epoxide-opening mechanisms under acidic or basic conditions?
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #4 on: March 02, 2008, 01:03:30 PM »
#2 is wrong.  what are the differences in epoxide-opening mechanisms under acidic or basic conditions?

I didn't know... but I checked Wikipedia: http://en.wikipedia.org/wiki/Epoxide#Reactions
The product is 2-bromopropan-1-ol, right?
Also, wikipedia says: "Under basic conditions, the nucleophile attacks the least substituted carbon, in accordance with standard SN2 nuclephilic addition reaction process." Isn't R1 the least substituted carbon? If R2 is the most substituted carbon?

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #5 on: March 02, 2008, 01:21:45 PM »
Quote
The product is 2-bromopropan-1-ol, right?

yes.  Wanna discuss stereochemistry?

Quote
Under basic conditions, the nucleophile attacks the least substituted carbon

true.

Quote
Isn't R1 the least substituted carbon?

Well, I don't know what you label as R1 in your starting material.  If we're referring to the wikipedia page, the carbon atom with R2 attached is the most substituted in the acidic mechanism, and the carbon atom with R1 attached is the more substituted in the basic mechanism.

So why do you suspect 2-bromopropanol is the product?  That would imply the nucleophile is attacking the more sterically hindered carbon atom. 
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #6 on: March 02, 2008, 02:26:55 PM »
Thank you :)

Quote
The product is 2-bromopropan-1-ol, right?

yes.  Wanna discuss stereochemistry?

It's a net anti reaction. I think I need more information before I can decide whether it's R or S.

Well, I don't know what you label as R1 in your starting material.  If we're referring to the wikipedia page, the carbon atom with R2 attached is the most substituted in the acidic mechanism, and the carbon atom with R1 attached is the more substituted in the basic mechanism.

So why do you suspect 2-bromopropanol is the product?  That would imply the nucleophile is attacking the more sterically hindered carbon atom. 
Yes, I was referring to the wiki article. I just got a bit confused since R1 != R2, but I think I'm with you now.

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #7 on: March 02, 2008, 02:38:43 PM »
Quote
I think I need more information

Stereochemistry was not assigned in the problem, assume the starting material is a mixture of both enantiomers.
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #8 on: March 02, 2008, 02:45:29 PM »
Quote
I think I need more information

Stereochemistry was not assigned in the problem, assume the starting material is a mixture of both enantiomers.

Then the product is a racemic mixture.

Could you also help me with Q1?

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #9 on: March 02, 2008, 02:53:10 PM »
I think your answer to #1 is fine, but there is one slight technical detail missing from your second step.  The modification of one reaction condition changes the chemical outcome of the product.  Do you know what I'm talking about and what the two different conditions/products are?
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #10 on: March 02, 2008, 03:05:56 PM »
I think your answer to #1 is fine, but there is one slight technical detail missing from your second step.  The modification of one reaction condition changes the chemical outcome of the product.  Do you know what I'm talking about and what the two different conditions/products are?

Ah, of course. It's KMnO4 in acidic solution - if it was in basic solution, the product would be a 1,2-diol. But what about the solution macman104 proposed? That would be a lot easier.

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #11 on: March 02, 2008, 03:20:31 PM »
no... my book says both products occur under basic conditions.  but something else is modulated to provide either the diol or the di-acid.

As for the other proposal, I'm unfamiliar with that reactivity of KMnO4, but that doesn't mean it wouldn't work.  I don't see that specific example in my undergrad organic text, but again that doesn't mean it wouldn't work.
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #12 on: March 02, 2008, 03:35:01 PM »
no... my book says both products occur under basic conditions.  but something else is modulated to provide either the diol or the di-acid.

As for the other proposal, I'm unfamiliar with that reactivity of KMnO4, but that doesn't mean it wouldn't work.  I don't see that specific example in my undergrad organic text, but again that doesn't mean it wouldn't work.

That's weird. "Fundamentals of organic chemistry" (McMurry), pp. 113-114 says that alkene + KMnO4 in basic solution yields a diol, while alkene + KMnO4 in acidic solution cleaves the double bond and 1) if the double bond is tetrasubsituted, the products are ketones 2) if a H is present on the double bond, the product is a carboxylic acid and 3) if two H are present, CO2 is formed.
What does your book says?

Offline azmanam

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Re: 2-phenylethanol to benzoic acid
« Reply #13 on: March 02, 2008, 03:38:12 PM »
Ok, then stick with your text book, as that's the same textbook your instructor will be grading you from ;)

My text says keeping the reaction cold and dilute favors diol, and concentrated and hot favors cleaved products (ketone/acid - depending).

Good work, though.
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Offline THC

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Re: 2-phenylethanol to benzoic acid
« Reply #14 on: March 02, 2008, 03:51:18 PM »
Ok, then stick with your text book, as that's the same textbook your instructor will be grading you from ;)

My text says keeping the reaction cold and dilute favors diol, and concentrated and hot favors cleaved products (ketone/acid - depending).

Good work, though.

I'll ask my instructor, then :)
Thanks.

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