Presentation on theme: "Reactions of Alkenes 1: Miniscale Synthesis Bromination of Fumaric Acid Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch March 8 & 10, 2010."— Presentation transcript:
Reactions of Alkenes 1: Miniscale Synthesis Bromination of Fumaric Acid Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch March 8 & 10, 2010
Addition Reactions Today: Conduct an addition reaction –One of the fundamental reactions of alkenes What you will learn/observe: Review/introduce the bromination of alkenes Bromination fumaric acid –Fumaric acid is alkenyl dicarboxylic acid Look at the products formed –stereoisomers possible See which product will predominate –racemic and meso forms possible –MP determination may differentiate
Addition Reactions Let’s Look at: –Addition reaction: a fundamental organic reaction Two species are added to a substrate Variety of organic families have this reaction Alkenes have many addition reactions Result is an entirely new compound Bromination of alkenes –Addition of Br 2 to an alkene –multiple step process –Proceeds through a bromonium ion intermediate –Both carbons of the original C=C end up with a Br
Some Notes on this Reaction Halogen atoms are electrophilic –and nucleophiles can attack them Examples:
More Notes An alkene can also be viewed as a nucleophile –can attack a halogen atom Intermediate is called a bromonium ion –note the 3-membered ring –positive charge on the bromine 3-membered ring has considerable ring strain –combined with positive charge on bromine –Ring opening reaction likely –form a stable product
Stereochem of this Reaction Called anti addition Because the 2 bromines “end up” in an anti configuration Why? because of the mechanism –must be a back-side attack of the nuclophile (Br - )
Mechanism: Bromination of Fumaric acid
Why Meso Product Bromide can attack either carbon of the bromonium ion
Procedure Use a 10 ml RB flask Add 200 mg of fumaric acid to this flask Now, add 2.5 ml of bromine stock solution swirl to mix, add magnetic stirrer Attach a condenser No need to hook up water lines Start stirring and heating (low setting…5) should turn orange (~15 min) then a precipitate should form, solution turns yellow If it turns clear, add a few more drops (2-3) of bromine solution Continue heating for 15 more minutes Remove solution, put in ice bath
Procedure II Collect product by vacuum filtration use a Hirsh funnel Wash crystals with ice-cold water Recrystallize from water Air-dry until next week Next week: weigh crystals check MP of crystals MP should be 255°-256° C but a racemic compound could form!!! has a MP of 166°-167° C If it doesn’t melt by 175°C, assume you have the meso compound
For Your Report Formal lab report required. Conclusion: –State amount of crystals produced –State % yield –Report MP –Analyze: did you get the racemic mixture or the meso compound?
Additional Question to Answer It appears that the racemic compound could be formed in this reaction. Could you come up with a mechanism by which this could happen? (HINT: it’s a similar mechanism but something else is different)
Your Report Formal Report Required Make sure to include the mechanism of the reaction in your report (separate section) Results Section: –Make sure to state all results Prove that you made the alkenes –Include GC chromatogram (annotated) –Determine % of your alkenes –IR spectrum, annotated (if completed) Conclusions section: –Was the experiment successful? How much of the alkenes are produced? Any leftover 3,3-dimethyl-2-butanol? –If unsuccessful, any ideas why?