Question

Wittig Reaction:  synthesis an alkene ( olefin) from the reaction of an aldehyde or a ketone with a phosphorous ylide ( phosphorane ). The ylide is produced from phoshonium salt with a base ( NaOH ). The alkene synthesized is trans-9-(2-phenylethenyl)anthracene.

1.) The final product is an E isomer, although a Z isomer could potentially be formed. What is the definition of E and/or Z isomer of alkenes? How the group priority is established?

2.) Speculate why the E isomer is preferentially formed (tip: consider the stabilization of your ylide and the effect on the stereochemistry of product

3.) The product for this reaction is colored. Why?

4.) Why the product precipitates with 1-propanol/water solution?

5.) What is an ylide? In this experiment, that was provided to you. How can an ylide be synthesized?

What is the function of your NaOH solution in this reaction?

Answer 1

(1) The groups attached double bond are ranked based on their atomic numbers.

If same priority groups are at same side of double bond the isomer is assigned 'Z'

If same priority groups are at opposite side of double bond the isomer is assigned 'E'.

(2) Because in E isomer the same priority groups are at far apart from oneantoher which makes less repulsions which inturn causes more stability,

But in Z iosmer the same priority groups are at same side, so more repulsions exist causing less stability.

SO, in general E isomer is foremd more

(3) It absorbs visible light.

(4) Because the product is more non polar in nature, so it is insoluble in propanol or water.

(5) The main role of NaOH is to produce ylide from phosphonium salt.

AN ylide is a dipolar molecule containing a formal negatively charged atom directly attached to hetero atom with formal positive charge.