Synthesis of Trans-9-(2-Phenylethenyl)Anthracene: A Wittig Reaction Lab 7

why is the Wittig the preferred method of synthesizing alkenes

it has high regioselectivity.

What kind of reaction is the Wittig reaction

it is an addition reaction. It creates an alkene from a carbonyl and an ylide.

how to create an ylide

An ylide is crated by using a primary or secondary alkyl halide and PPh3. There will be nucleophilic substitution and halide will leave and then the Ph3P-CH2R will be created which is the phosphnium salt. But then the base will come in an deprotonate the CH2 so there will be a + and - charge which will be the ylide. It will resonate btwn an alkene and two charges.

mechanism for the reaction

the carbon from the carbonyl will attach to the negatic C from the ylide and then the electrons will get pushed up to the oxygen, making the oxygen negative. Then the oxygen will attack the P and then they will make the box thing and then break off.

what is the controversy with the Wittig reaction

they havent figured out whether the square intermediate (oxophosphotane) is formed in one or two steps. If it is two steps then the conformation of just the C bonded to the carbonyl C and then there being a negative O and positive P would be betaine. Remeber js betaine turns into oxophosphetane.

oxophosphetane is stable at

-78 Celsius but will decompose to form the alkene at room temp. The strong Oxygen Phosphorous bond is what drives the reaction.

mechanism of the reaction from this lab

the benzyltriphenylphosphonium chloride will be deprotonated by NaOH to form benzyltriphenylphosphnium ylide. Then it will be added to 9-anthraldehyde.

why was the product seperated with DCM and water

The athracene would remain in the DCM layer as they are both hydrocarbons and hydrophobic. The remaining Cl and triphenylphosphine oxide will attach to the water and be washed out. Then anhydrous CaCl2 was added to dry out any remaining water.

IR for trans alkene and IR for cis alkene

Trans: 966 Cis: 675-730

how to distinguish between cis and trans in proton NMR

trans is more shielded meaning the peak would be smaller and if it was cis the peak would be more deshielded and the peak would be larger.