Lab 2: Reduction of Vanillin to Vanillyl Alcohol

What are the objectives of this experiment?

- reduce the aldehyde group in vanillin to a primary alcohol (reduction of vanillin to vanillyl alcohol)

- monitor the reaction using TLC analysis

What was used as the reducing agent?

Sodium borohydride (NaBH4) in NaOH

Why are aldehydes more reactive than ketones?

they're more reactive than ketones because of the C-H bond.

An aldehyde reduction and oxidation give what products?

Redution = primary alcohol

Oxidation = carboxylic acid

Define reduction and in what ways it can be accomplished.

When you increase carbon electron density

(1) - you can form a bond between the carbon to a less electronegative atom such as hydrogen

(2) - you can break a bond between carbon and an electronegative atom such as oxygen

In what ways do NaBH4 and LiAlH4 differ?

NaBH4:

- reduces aldehydes and ketones only

- easy to handle

- does not react violently with water

- can be exposed to an open atmosphere

LiAlH4:

- reduces A and K but also esters and carboxylic acids

- need to be handled with care

- reacts violently with water and hydroxylic groups

- cannot be exposed to moisture from the atmosphere

What is the purpose of using NaOH rather than water as the solvent?

(1) - It is necessary to keep the pH of the solution high so that the borohydride is not lost because it reacted with protons. The proton concentration must be low to slow the reaction between water and borohydride. The NaOH contributes to maintaining a high pH.

(2) - Vanillin is insoluble in water and needs to be deprotonated to become soluble. NaOH allows for this deprotonation. After the reduction is complete, the acid will re-protonate making it insoluble in water once again, which will cause precipitate to form. The product is easier to collect this way.

What are the main differences in IR spectrums of both molecules?

The alcohol will no longer have a c=o, c-H (aldehyde) stretch, and the alcohol intensity shape would be as broad.

Why is the solvent used in the TLC analysis appropriate and of proper polarity?

Hexane: will seperate any non polar compounds

Ethy acetate: more polar and will elute the aldehyde

Acetic acid: alcohol is very polar and will need a more polar solvent to elute it which is what acetic acid allows for.

What is the procedure of this reaction?

- dissolve vanillin in NaOH (spot TLC plate)

- add NaBH4 to vanillin while swirling constantly (spot TLC plate once when you added all of it and then 10 mins after)

- check for the presence of vanillin on the TLC plate

- add acid dropwise until pH = 1-2

- cool and then isolate precipitate by vacuum filtration

What is the limiting and excess reagent?

LR = vanillin

ER = sodium borohydride