A few reflections on the material from Monday morning’s lecture.
Epoxide ring-opening. First, epoxides can react with strong nucleophiles without adding acid. The strained ring makes the epoxide reactive. These reactions look like SN2 reactions.
Second, epoxides can react with much weaker nucleophiles by adding acid. Examples of weak nucleophiles include water, alcohols, carboxylic acids. The mechanisms of these reactions are a little strange. If one epoxide carbon is tertiary, the nucleophile adds there, but regardless of where the nucleophile adds, a backside attack occurs. So a little SN1 and a little SN2-like behavior all at the same time.
The strange change in epoxide behavior parallels that of alkyl bromides and cyclic bromonium ions. The former contains a good leaving group (Br-), but the latter contains an even better leaving group (Br) and gets another boost from relief of angle strain. Take another look at section 5.2, p. 181-185, and you will be surprised by the similarity between cyclic bromonium ions and protonated epoxides.
Organometallic reagents. When you draw these reagents as synthetic reagents, please draw:
- RMgX – Grignard
- RLi – organolithium
- R2CuLi – lithium dialkylcuprate (FYI – you don’t need to learn the more complicated higher-order cuprates that Loudon mentions. Life is complicated enough already. Just assume that all cuprates can do the same chemistry.)
However, when you draw these reagents as part of a reaction mechanism, i.e., with curved arrows to show electron flow, its OK to draw them as ionic compounds, R- M+ (include whatever metal is appropriate).
Glycols. I don’t think many students reached the model that described the preparation of cis-glycols using OsO4. Your book’s discussion of this reagent gets pretty complicated. Although I included one amine oxide in the ChemActivity, you don’t need to learn these reagents – simply draw “OsO4” whenever you want to turn an alkene into a cis-glycol. The reagent may be toxic and expensive, but we’re just drawing plans on paper and that’s very safe and (nearly) free.
Your book also mentions the use of KMnO4 to make glycols. This is not a very satisfactory reaction. Skip it.
Finally, your book mentions the “oxidative cleavage” of glycols with periodic acid. This is an important reaction, especially in the chemistry of carbohydrates (section 24.8). However, we have to draw the line somewhere and I’m going to draw it here. Formation of glycols – learn that. Oxidative cleavage of glycols – skip it (until Pat says go back and learn it).