Synthesis Problems
In organic chemistry synthesis problems are very common. The key to these problems is the
identification of several reactions necessary to transform the given
molecule to the given product.
My suggested strategy for this kind of problems has these steps:
1.
Does the product have
the same number of carbons as the reactant?
-
If yes, you don’t have
to worry about increasing the number of carbons
-
If no, then consider
turning your product at some point into a terminal alkyne, have it react with
NaH and the appropriate Alkyl Halide
2.
What reaction(s) can
produce our product?
-
Determine a reaction,
and a reactant A
3.
What reaction(s) can
produce reactant A?
-
Determine a reaction,
and a reactant B
Continue like this until
you get the compound on the left of the reaction. This whole process
is called retro-synthesis and is represented by this arrow =>.
Some suggestions for this type of problems:
-
Memorize all the reactions! (This is obviously the key to this problems)
-
Stare at the compounds for a while and brainstorm some ideas
-
Still find it impossible? Follow the steps above
-
Be careful! Stereochemistry matters
Problem 35 from Chapter 10 on Sapling Learning is actually asking us to choose the best reagents
from the list provided to produce the given product as shown:
 |
| Problem 35, Chapter 10 on Sapling Learning |
1. After counting the number of carbons you will be able to see that an addition
of carbons is needed. Therefore propyne is converted into acetolyne using the
strong base NaH, and alkylated via alkyl bromide (bromoethane).
 |
| Part of the Reaction Drawn Using ChemDraw |
2.
3. What do we need for an intramolecular SN2 reaction? A bromide
and hydroxide group.
4. Does stereochemistry matter? YES! Notice that the products are
enantiomers.
5.How can we get a bromide and a hydroxide in an alkane? By adding Br2
and H2O to an alkene. At this point we are really close to the
solution.
6. In step one we produced an alkyne. How can we get an alkene from an
alkyne? By addition of H2 using Lindlar’s catalyst (Be careful: cis product).
7. Use 1 through 7 to write forward reaction:
7. Use 1 through 7 to write forward reaction:
 |
| Forward Reaction Drawn Using ChemDraw |
Using this forward reaction you can now give an answer to the problem:  |
| Solution to Problem 35, Chapter 10 on Sapling Learning |
Your post is very detailed and easy to follow. It helped me to understand that problem a little bit better, so thank you.
ReplyDeleteKiante,
DeleteThank you very much for your feedback. I am glad it helped!
Eni,
ReplyDeleteThanks for posting both your strategy for solving these types of problems and the work (using Chem Draw) to this particular problem. In particular, I found your explanations of the stereochemistry in this process to be helpful (especially going from A-> A' and B-> B' in the last step).
Hart,
DeleteThank you very much for your comment. I found Chem Draw to be really helpful for this post, and I'm glad it helped!
This problem took me like 33 tries and you make it so simple and easy, but I was not doing the problem in order. When I first saw the problem, the only thing I recognized immediately was the stereochemistry and assumed that it was Sn2.
ReplyDeleteJuan,
DeleteThanks for your comment! When you are having difficulties, the suggestions on Sapling help too. You can also try to work on both the left side and the right side at the same time, and hope that they meet at some point!
Sincerely,
Eni