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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: a13b13 on January 12, 2013, 04:11:49 PM

Title: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 12, 2013, 04:11:49 PM
I have to fill in boxes A and B, and C is filled in.
So box A is first then above the arrow to box B is "Mg, ether" and from box B to C is "1. acetaldehyde, 2. H2O". and in box C is 2-pentanol. 
I really don't know how to approach this because I don't know what acetaldehyde does, does it combine with water and make pentane or something? Thanks
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 12, 2013, 04:47:42 PM
Firstly what compounds react with Mg in ether and what do they give? This will be boxes A and B
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 12, 2013, 06:08:36 PM
Well I figure I need to work backwards, I just don't know what acetaldehyde does. Going back from that though I guess I'll end up with some sort of alkyl halide.
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 12, 2013, 06:34:48 PM
OK an alkyl halide. You have 5 carbons in your product, acetaldehyde has 2, so what is your alkyl halide?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: Raphael on January 12, 2013, 08:52:59 PM
Going to need more info!
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 01:42:17 AM
Quote from: Raphael on January 12, 2013, 08:52:59 PM
Going to need more info!

No we don't really need more info. All we need is in the OP.
Title: Re: Synthesis using Mg then acetaldehyde
Post by: Raphael on January 13, 2013, 11:36:24 AM
Quote from: discodermolide on January 13, 2013, 01:42:17 AM
Quote from: Raphael on January 12, 2013, 08:52:59 PM
Going to need more info!

No we don't really need more info. All we need is in the OP.


oops. you're right. forget I said anything. Just my foggy mind misreading what was written.
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 01:23:42 PM
Quote from: discodermolide on January 12, 2013, 06:34:48 PM
OK an alkyl halide. You have 5 carbons in your product, acetaldehyde has 2, so what is your alkyl halide?

So if the halogen is bromine (does it matter?) then my alkyl halide is 1-bromopropane? Which Mg is added to, and inserts between the carbon and bromine?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 01:49:25 PM
OK, propyl bromide is fine. This gives the Grignard reagent propylmagnesium bromide.
How, now does this react with acetaldehyde to give the final product?
What do you know about the reactions of aldehydes?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 02:02:06 PM
The problem is I haven't even learned aldehyde reactions but for some reason it's on my homework, but I looked it up and I think the aldehyde adds a C-O bond and the H2O and acid create the OH and pop off the MgBr?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 02:07:28 PM
Not quite.
The C=O bond in the aldehyde is polarised by the electronegative oxygen atom, making the carbon of the C=O slightly positive in nature. Now if you have a Grignard reagent which has a carbon which carries some negative charge what will happen?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 02:19:02 PM
If the carbon has a negative charge then the Grignard would act as a nucleophile, I think.
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 02:19:59 PM
That is correct, so what is the electrophile?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 02:37:21 PM
So the aldehyde is the electrophile? And the Grignard attacks the aldehyde
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 02:40:13 PM
That's right it attacks the carbon of the C=O to give…..what?
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 03:01:09 PM
So that would give 2-pentanol. Thank you so much for the help, I really appreciate it!
Title: Re: Synthesis using Mg then acetaldehyde
Post by: discodermolide on January 13, 2013, 03:05:15 PM
No problem.
Here is the reaction.
Title: Re: Synthesis using Mg then acetaldehyde
Post by: a13b13 on January 13, 2013, 03:14:25 PM
Awesome, thanks!