Since most organic compounds are somewhat non-polar a non.
DRAWING A WITTIG REAGENT IN CHEMDOODLE FREE
Most organic reactions are done in solution where the molecules are free to move around,Ĭollide, and ultimately react. In this experiment however the Wittig reagent will beįormed under phase-transfer conditions in which the phosphonium salt functions as a phase. Inert atmosphere and at low temperatures. Such as a dry-ice cooling bath and a dry nitrogen line is needed to conduct the reaction under an The use of butyllithium is generally restricted to a research laboratory since special equipment Stability (thermodynamic control) but by kinetics in formation of the oxaphosphetane (kinetic The stereochemical outcome of the reaction is therefore not governed by product The stereochemistry around the new carbon-carbon double bond is predominantly the Z isomer Phosphorous affording a cyclic intermediate (oxaphosphetane) that breaks down to yield theĪlkene product and the byproduct, triphenylphosphine oxide. The Wittig reagent forms a bond with the carbonyl carbon. Structures may be drawn for the phosphorous ylide since phorphorus can expand its valence shellĪfter preparing the Wittig reagent an aldehyde or ketone is added. An ylide is a neutral compound that contains twoĪdjacent atoms one with a positive charge and the other with a negative charge. The Wittig reagent in an example of an ylide. R CH 2 P(Ph) 3 Br + Li R CH P(Ph) 3 + LiBr + The molecule HBr is eliminated so this is an acid-base reaction. To prepare the Wittig reagent the phosphonium halide is reacted with a strong base such aīutyllithium. Shown below, to form an alkyltriphenylphosphonium halide. This is done by reacting triphenylphosphine with an alkyl halide in an SN2 reaction, as Preparation of the Wittig Reagentīefore reacting an aldehyde or ketone with a Wittig reagent it is necessary to prepare the Wittig The stereochemistry of the alkene can also beĬontrolled by selecting different reaction conditions. Would form alkene 3 as the major product. The major product along with 4 and other cis/trans isomers. With the Grignard reagent, methylmagnesium iodide, followed by dehydration, would form 2 as For example, the synthesis of 3 by the reaction of 1 The reason for this is that a carbon-Ĭarbon double bond can be formed at a specified location in the molecule without obtaining a Variations are especially important in organic synthesis. Of the many reactions that form carbon-carbon double bonds the Wittig reaction and its Modifications like the Horner-Wadsworth-Emmons and Schlosser preferentially form the E-alkene.Experiment 16 The Wittig Reaction: The Synthesis of (E)-Stilbene Introduction It yields both the E-alkene and Z-alkene without preference. The Wittig doesn’t have selectivity for any particular stereochemistry without modification.
The oxide attacks the cationic phosphorus to form an oxaphosphetane, which undergoes rearrangement to produce an alkene and phosphine oxide. The carbonyl acts as an electrophile as the anionic carbon attacks it to form a betaine (pronounced beta-ene). The ylide’s carbon is a pretty good nucleophile, and it can participate in nucleophilic addition. Great, now all that’s left is to react the ylide with the carbonyl. Notice below that the ylide is zwitterionic that is, it’s got adjacent opposite charges. Now we’ve got that triphenylphosphonium ion, we’re one step away from forming our ylide! All we need to do is add a strong base to form a carbanion. The best way to do it is to use a primary alkyl bromide (or other alkyl halide) but secondary will do. The first thing we need to do is get preparation going on the ylide. The box-out method is great and all, but there’s nothing like a good mechanism to help understand exactly what’s going on. From there, imagine joining the two double bonds together through a double bond. There is a complicated mechanism, but let’s see how we can skip it to just figure out what that product is:īox-out methodBasically, you can just draw a box around the carbonyl oxygen and the triphenylphosphine (Ph 3P). On the reactant side, we’ve got an aldehyde on the left and an ylide on the right on the product side, we’ve got an alkene. If you see an aldehyde or ketone and an ylide, you can actually use something called the box-out method to predict the product. The Wittig reaction, also known as Wittig olefination, is a great way to turn aldehydes and ketones into alkenes.īefore we get into the mechanism, let’s look at a really quick way to get the right answer on an exam. Multi-Functionalized Carbonyl Nomenclauture Physical Properties of Ketones and Aldehydes Overview of Nucleophilic Addition of Solvents
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