Organic Chemistry Practice Problems and Problem Sets
epoxides
Show how to prepare each compound starting from propylene oxide.
(Propylene oxide image below courtesy of Wikipedia.)
Epoxides can open up in two different ways.
To add a nucleophile to the less substituted side of an epoxide, use basic conditions. This is done in #2 below.
To add a nucleophile to the more substituted side of an epoxide, use acidic conditions. This is done is #1 below.
Why do the conditions matter? Epoxides have two electrophilic carbons. Normally nucleophiles will preferentially attack the less substituted carbon, as they do in SN2 reactions. Recall that SN2 reactions usually happen with strong nucleophiles- that is, negative charges (basic conditions).
When an epoxide reacts under acidic conditions, the transition state has carbocation character, and so it's sort of like an SN1 reaction. That is, instead of less substituted carbons being favored due to less steric bulk, more substituted carbons are favored do to a more stable carbocation. So acidic conditions cause an epoxide to open up on the more substituted side.
MendelSet practice problem # 702 submitted by Matt on July 21, 2011.
Show how each compound can be prepared from the indicated starting material.
All carbon sources must contain three carbons or less.
a) When you see 2 carbons and 1 oxygen, that the tell-tale sign that you're adding ethylene oxide (the simplest epoxide).
But that would only leave you with an alcohol. How do you get to the ether? Using the Williamson ether synthesis.
b) As I mentioned in problem 673, when you see an alcohol you are also looking at a carbonyl, because you can interconvert the two (alcohol to aldehyde/ketone using PCC, aldehyde/ketone to alcohol using NaBH4).
To add the methyl group, convert the alcohol to a ketone (which is an electrophile), and then add methyl Grignard (a nucleophile). But once again, you are only left with an alcohol. How to convert it to an ether? The Williamson ether synthesis.
When you see an ether in a synthesis problem, remember the Williamson ether synthesis. It will come in handy.
MendelSet practice problem # 699 submitted by Matt on July 21, 2011.
Show how each compound can be prepared from an alkene containing 3 carbons (or less).
Each answer will involve the reaction of a Grignard with either a carbonyl or epoxide.
Note: epoxides are prepared from alkenes using a peroxy acid (epoxidation) such as mCPBA.
The trick to synthesis problems in second semester organic chemistry to recognize that alcohols ARE ketones ARE carboxylic acids. What do I mean? Alcohols, ketones/aldehydes, and carboxylic acids can all be easily converted using PCC or Jones Reagent (NaCr2O7/H2SO4).
For example, for a), the product is a ketone, but it may as well be an alcohol, because alcohols can be converted to ketones with PCC.
b) is similar, except the position of the alcohol (one away from the "bond cut", instead of directly connected to the cut as in a) ) indicates the starting material was an epoxide and not a carbonyl.
c) is just like b), except instead of PCC, use Jones Reagent to oxidize the alcohol all the way to a carboxylic acid.
MendelSet practice problem # 673 submitted by Matt on July 19, 2011.