Barbier Reaction

What is the Barbier Reaction?

Addition of an Organometallic to an Aldehyde to prepare an Alcohol

How are organometallic reagents normally prepared?

Normally, organicmetallic reagents are prepared with a carbonyl compounded added to it

How is the reaction referred to as an “in situ” reaction?

1. The precursor to the organometallic compound and the carbonyl start out in solution together.

2. The organometallic compound is then generated in the solution in the presence of the carbonyl compound.

3. An “in situ” generation, distinguishing the Barbier reaction.

What are Organolithiums

• PYROPHORIC: spontaneously ignites on exposure to air.

• Super basic, pKb ~ -35

• Requires hydrocarbon solvents (generally).

• Extremely moisture sensitive

What are Organomagnesium halides (Grignard Reagent)

• Generally, not pyrophoric.

• Extremely basic.

• Requires ether solvents.

• Extremely moisture sensitive.

Describe the Activated (Stabilized) Organozinc Reaction

1. An organozinc reagent will be generated, in situ, under Barbier conditions.

→Activated organozinc complexes generated in the presence of ammonium has a much slower reaction kinetics for water than for addition to an aldehyde or ketone.
→No need for inert atmosphere or dry conditions.
→Reaction is very fast and high yield.

Draw the Barbier Reaction

What are some cautions for Allyl bromide?

-It’s a strong lachrymator: that is it acts like tear-gas.
-Keep in a fume-hood, and bottle closed when not transferring reagent.
-DO NOT GET IT ON YOUR SKIN!

What are some cautions for Saturated NH₄Cl

-It’s acidic and irritating.
-Wash skin with cold water for ~15 min

What are the 6 reagents? Draw them

1. Benzaldehyde

2. THF

3. Allyl Bromide

4. Ammonium Chloride

5. Zinc

6. Ether

Describe the procedure (7 steps)

1. Into a clean 125 mL Erlenmeyer flask, add 20 mL of NH₄Cl, 5 mL of THF, 20 mmole of benzaldehyde, and 34 mmole of allyl bromide.

2. Add 24mmole of zinc to flask and swirl for 45 min (notice heat & gas evolution)

3. Transfer mixture to a separatory funnel using 10 mL of ether to rinse reaction and add 15mL more of ether to the funnel.

4. Extract the organics into the ether, wash ether layers with 10mL of water and 5mL of 5% NaHCO_3. Extract aqueous layer with 5mL of ether.

5. Dry ether layer over anhydrous sodium sulfate. Filter into a pre-weighed 100mL beaker and evaporate on a hot plate (low)

6. Periodically, swirl the contents of the beaker. When the residue stops reducing, cool to room temp and weigh.

7. Put back on hot plate (swirl occasionally), cool & weigh. If the weight differs by less than 0.5g then use 2nd weighing to determine yield and % yield.