[upitt21]

Hello, I'm confused as to how to go about solving this problem.. from what i've learned thus far, i initially assumed that I would first have to suggest a synthetic route something along the lines of:

Φ-Ch2Br (HO-, H2O)  Φ-Ch2OH (PCC, CH2Cl2)  Φ-C=O, etc.

but it doesn't seem to be working..any ideas? and sorry if this is a dumb question, but i just started learning about syntheses in my OChem 1 lecture

[nj_bartel]

When you say it doesn't seem to be working, do you mean you're actually attempting to run this?

[upitt21]

When you say it doesn't seem to be working, do you mean you're actually attempting to run this?

no, it's just a synthesis problem from a set of practice problems posted by my instructor..all i have to do is show the steps required to make phenylacetaldehyde starting from benzyl bromide

when i said it doesn't seem to be working, i meant that i'm having trouble reaching the final product using methods that i'm used to

[nj_bartel]

Have you learned the grignard reaction?

[upitt21]

Have you learned the grignard reaction?

Yes, I've learned all of the synthetic methods which would be necessary to solve this problem, yet I'm having trouble recognizing their appropriate use for this specific synthesis.  I'm sure the answer will seem obvious once it's determined, but for now I am officially stumped.  Any help would be greatly appreciated.

[orgopete]

In that case, try working backwards. How many ways do you know to synthesize an aldehyde? If it is from an alcohol, how many ways do you know how to synthesize an alcohol? Are Grignard reagents used for any of them?

[upitt21]

Actually, would it be possible to do something like this:

(continued from first post..) Φ-C=O + Ch3MgBr  Φ-CHOHCH3  (PCC, Ch2Cl2) to get phenylacetaldehyde ?

If that doesn't work, then I really have no idea..

[nj_bartel]

That will get you a ketone.  Try forming a grignard from your starting material.

[upitt21]

That will get you a ketone.  Try forming a grignard from your starting material.

Φ-Ch2Br + Mg  [(Ch3Ch2)2O] Φ-Ch2MgBr + HCOH  Φ-Ch2Ch2OH  (PCC, Ch2Cl2) to get phenylacetaldehyde

would that work?

[nj_bartel]

Looks good to me upitt    It's generally a good strategy to build up your carbon framework THEN functionalize.

[upitt21]

Looks good to me upitt    It's generally a good strategy to build up your carbon framework THEN functionalize.

Thanks for your help

[movies]

The Grignard way is perfectly reasonable (although benzylic Grignard reagents are sometimes problematic), but I would like to add an "old school" alternative:

PhCH₂Br + NaCN ---> PhCH₂CN

PhCH₂CN + DIBAL ---> PhCH₂CHO  (after aqueous workup)



You can think of other ways to convert the nitrile to aldehyde as well, this is just the most direct.  The cyanide anion is a very useful tool for one-carbon homologation!

[orgopete]

There is no question that the DIBAL reduction is a good route. The difficulty that the poster creates for anyone giving an answer is our uncertainty about the poster's knowledge. What reactions does the poster know? The DIBAL may be a good route, but was it a reaction covered in the poster's class?

I have been puzzled by students asking a question about how to propose a synthesis for a structure that involved the formation of a carbon-carbon bond when they had only been given a one bond forming reaction. I dare surmise that the poster knew only a very limited number of reactions that result in an aldehyde. It seems plausible to me that one of those methods should serve as a precursor to the product. Then, what would be the precursor's precursor, etc.

[movies]

I agree, organopete, but I still think it is reasonable to show some alternatives.  I know the DIBAL reduction of nitriles might not be covered in all introductory classes, but I know for a fact that it is covered in some.  Even if it were not covered, certainly other common reactivity of nitriles would be.  You could also work out a synthesis where you would hydrolyze the nitrile to the carboxylic acid, reduce with LAH, and oxidize with PCC.

At any rate, I think it is a good exercise for the more advanced members to consider alternative syntheses using all the tools they know while at the same time trying to figure out how to do it with the tools available to a less experienced chemist.  I think it helps you to understand the challenges in efficiency in organic synthesis, and it helps you to realize which problems have been solved and which still remain.