vanillin reduction reaction

Why is the reduction of aldehydes and ketones to alcohols important?

It's one of the most widely used reactions in organic chemistry for functional group interconversion

What makes NaBH₄ and LiAlH₄ effective reducing agents?

They contain metal-hydrogen bonds polarized toward hydrogen (B and Al are less electronegative than H), making them sources of hydride (H⁻) ion

Why is hydride (H⁻) effective for carbonyl reduction?

Hydride is a good nucleophile that attacks the electrophilic carbonyl carbon, initiating addition across the C=O bond

What is the overall transformation in carbonyl reduction with metal hydrides?

Addition of H₂ across the C=O bond: the carbonyl oxygen becomes OH and the carbonyl carbon gains H

Why do both NaBH₄ and LiAlH₄ contain four equivalents of hydride?

Each can reduce four carbonyl groups before being consumed; they release hydride ions sequentially

What is the key difference in reactivity between NaBH₄ and LiAlH₄?

LiAlH₄ is extremely reactive (reacts violently with water, reduces most functional groups); NaBH₄ is milder (reacts slowly with water, only reduces aldehydes/ketones)

Why can NaBH₄ be used in aqueous NaOH solution?

It reacts slowly enough with water under basic conditions to allow the reduction to occur before significant decomposition

Why does NaBH₄ react less rapidly with water under basic vs acidic conditions?

Under basic conditions, there are fewer protons available to react with the hydride; acidic conditions protonate NaBH₄ rapidly, causing decomposition

Why is LiAlH₄ never used in aqueous solutions?

It reacts violently with water as a strong base, rapidly producing H₂ gas (fire/explosion hazard)

Why is vanillin dissolved in 1M aqueous NaOH rather than neutral water?

Basic conditions slow the decomposition of NaBH₄ with water, allowing it to selectively reduce the aldehyde

Why is the flask placed in an ice bath before adding NaBH₄?

The reduction is exothermic; cooling controls the reaction rate and prevents side reactions

Why is NaBH₄ added slowly over 4 minutes in two portions?

To control the exothermic reaction and prevent violent bubbling or loss of material; gradual addition ensures complete dissolution

Why is the flask removed from ice bath and stirred for 30 minutes?

To allow the reduction reaction to go to completion at a controlled rate after the initial cooling period

Why is the flask placed back in ice bath before adding HCl?

To control the exothermic reaction between excess NaBH₄ and acid, which generates H₂ gas

Why is 3M HCl added slowly with a Pasteur pipette?

The reaction between NaBH₄ and HCl generates H₂ gas; slow addition prevents violent bubbling and loss of product

What is the purpose of adding HCl to the reaction mixture?

1) Protonate the alkoxide intermediate to form the alcohol product; 2) Destroy excess NaBH₄; 3) Acidify to precipitate the product

Why does vanillyl alcohol precipitate when HCl is added?

The alcohol is less soluble in acidic aqueous solution than the alkoxide salt; changing pH causes crystallization

What is produced when excess NaBH₄ reacts with HCl?

Hydrogen gas (H₂) and boric acid (H₃BO₃)

Why should the HCl not be added too rapidly?

Rapid addition can cause violent H₂ evolution and may prevent proper crystallization of the product

Why is the solution weakly acidic (pH ~3) after HCl addition?

Enough acid to protonate the alkoxide and destroy excess NaBH₄, but not so acidic as to cause unwanted side reactions

Why is the product rinsed with cold water during vacuum filtration?

Cold water removes soluble impurities (salts, unreacted starting material) while minimizing product loss (cold reduces solubility)

Why is vacuum left on for 10 minutes after rinsing?

To thoroughly dry the product by removing as much water as possible from the crystals

Why should bigger solid clumps be broken down on the filter paper?

To increase surface area for better drying and more accurate mass determination

Why is wet mass recorded during lab but dry mass measured later?

Wet crystals contain variable amounts of water; dry mass (measured after complete drying) gives accurate yield

Why is the product dissolved in acetone for TLC?

Acetone is a good solvent for vanillyl alcohol and evaporates quickly for spotting; it's also compatible with the TLC solvent system

Why are three lanes used: vanillin standard, vanillyl alcohol standard, and your product?

To compare Rf values and confirm identity; vanillin standard shows if any starting material remains; alcohol standard confirms product identity

Why is a more concentrated spot (multiple applications) used in lane 4?

To detect trace impurities that might not be visible in a single-spot application

Why is 4:6 ethyl acetate:hexanes used as the TLC mobile phase?

This ratio provides good separation between vanillin and vanillyl alcohol based on their polarity differences

What does the Rf value tell you about your product purity?

If product spot matches vanillyl alcohol standard and no vanillin spot appears, the reduction was complete and product is pure

Why is vanillin more polar than vanillyl alcohol on TLC?

Vanillin has an aldehyde group (C=O) which is more polar than the alcohol group (C-OH) in vanillyl alcohol. Actually, BOTH have OH groups on the ring, but the aldehyde vs. alcohol functional group changes polarity slightly

Why does vanillyl alcohol have potential as a renewable source material?

It's derived from vanillin (from vanilla/lignin), making it bio-based rather than petroleum-based

Why must NaBH₄ be handled carefully despite being "mild"?

It's an irritant and can react violently with water to produce flammable hydrogen gas under certain conditions

Why are aqueous NaOH and HCl handled in the hood with PPE?

Both are corrosive; NaOH causes chemical burns, HCl produces irritating fumes and is corrosive

Why is all glassware rinsed only with acetone (no water) after this lab?

Next week is the Grignard reaction, which requires anhydrous conditions; water would contaminate the glassware