Synthesis of trans-9-(2-phenylethenyl) Anthracene: A Wittig Reaction

OCHEM II Lab Final

Chemical reactions involving organic molecules can be classified into three categories?

• molecular rearrangement

• elimination

• addition

Alkenes are commonly formed through?

elimination reactions

Wittig reaction

• preferred as a method of synthesizing alkenes b/c of its high level of regioselectivity

• addition reaction

• generates an alkene from the reaction of a carbonyl compound with a carbon containing phosphorous reagent (ylide) which is made from a phosphonium halide

• Ph3P = good nucleophile + weak base —> so no elimination reaction

Regioselectivity

• the tendency of a reaction to form predominantly one isomer from a single reactant

The product resulting from the reaction of the phosphonium halide with a strong base is called?

• ylide

• neural molecule, which among its atoms, has two adjacent atoms that have opposite charges

Controversy lies in whether the oxaphosphentane intermediate is formed by one step concerted process or a two step process. What forms oxaphosphetane?

• betaine reacts to form the oxaphosphetane, stable at -78 C

• at room temp, it decomposes to yield the alkene and triphenyl-phosphine oxide

What is the driving force for the decomposition of oxaphosphetane?

• formation of the strong-phosphorous oxygen bond of the phosphine oxide, a bond strength estimated to be at least 540 kj/mol

Wittig reaction forms?

• carbon-carbon double bond between the carbonyl carbon and the carbon adjacent to the P atom in the ylide

What are we doing in this experiment?

• 9-anthraldehyde + ylide ( from triphenylbenzylphosphonium chloride) → trans -9-(2-phenylethenyl)anthracene

Reactions

Mechanism

Formation of ylide

What safety precautions must be observed when using dichloromethane?

• fume hood

What safety precautions must be observed when using 50% aq. sodium hydroxide?

• gloves + goggles, prevent skin contact

What safety precautions must be observed when using toluene?

• don’t use it near flames or other heat sources

• use fume hood

Briefly explain the advantage of a Wittig synthesis over the more common dehydrohalogenation reaction.

• high level of regioselectivity

• choose precise location of new bond

Limiting reactant

Theoretical Yield

Carbonyl compound + ylide?

Compare the IR spectra of 9-anthraldehyde + your product.

• The IR for 9-anthraldehyde shows a very strong peak at around the 1600-1700 cm-1 mark

• This represents an aldehyde peak. However, this peak is not present in the trans-9-(2-phenylethenyl) anthracene product, as there is carbonyl shown.

• Additionally, a trans alkene peak is present at the 960 cm-1 mark in the product, which is not shown in the IR for 9-anthraldehyde.  

What evidence supports the synthesis of the trans isomer rather than the cis isomer?

• A trans isomer showcases a peak at around 960 cm-1, which is present in the IR

• a trans isomer has a higher amount of de-shielding protons compared to the cis isomer

What are the steps for this experiment?

1. benzyltriphenylphosphonium chloride + 9-anthraldehyde added to a test tube

2. add dichloromethane (DCM) and stir vigorously

3. add 50% NaOH dropwise slowly while stirring

4. stopper tube loosely and stir for 30 min

5. move mixture to centrifuge tube, rinse reaction tube with DCM + H2O

6. shake + vent centrifuge tube and let layers separate

7. transfer bottom DCM layer to clean test tube and repeat extraction with DCM

8. add CaCl2 to DCM layers and let sit for 2 min

9. decant DCM into RBF and rinse solid with DCM

10. rotovap

11. recrystallize w/ 2-propanol

TLC Plate?

• 9-anthraladehyde has lowest Rf value

• trans-9-(2-phenylethenyl)anthracene has higher Rf value

• used toluene as eluent + solvent

Why do we use DCM as the solvent?

• can dissolve organic molecules and immiscible with water

Why do mix 50% NaOH slowly and stir for 30 minutes?

• base deprotonates the phosphonium salt to create ylide

• wittig reaction can be exothermic, and adding the base slowly prevents the reaction from overheating

• stirring helps the two liquids react

Why do we add water?

• water is added to dissolve inorganic salts and pull them away from organic product

• cacl2 is the drying agent and absorbs water