Organic Chemistry Lab 318

Spring, 2012

DUE DATES• Today

– At end of lab -- copy of laboratory notebook pages for today's experiment

• Next Week– At beginning of lab – Grignard Reaction

Report

Today

• Please look over your graded Unknown ID report.

• Most of the problems were with an incomplete logical analysis.

• Take more care with the integration of the H-NMR.

• All important information from the IR and NMR spectra need to be explained and incorporated in the written report.

The Experiment

• Last week you made the Grignard reagent

• And then reacted it with O=C=O

• You then hydrolyzed the benzoate salt

Br

Mg0

Mg-Brether

Mg-BrO C O

C

O

OMgBr

benzoate salt

C

O

OMgBr

HClH2O C

O

OH

The Experiment

• Today, you will isolate the benzoic acid product from mixture of product, by-products, etc.

• Remember what you started with: PhBr, Mgº, diethyl ether, maybe I2, CO2 (all sublimed by now); and added: HCl/H2O; and possible by-product: Ph-Ph and “MgBrCl”. The PhCO2H must be separated from all of these.

• The Separation Scheme begins here.

The Experiment

• Since the ether and aq. HCl have substantially evaporated since last week, you’ll begin by adding new quantities of them: 20 ml of ether and 20 ml of 6M HCl. Stir to dissolve most of the solid (it may take some work).

• Use a glass funnel to pour the mixture into the separatory funnel.

• You should see two layers in the separatory funnel. Gently shake to dissolve any solids.

Experiment

• Follow the instructions in Pavia, pp. 287-288 to isolate your benzoic acid product.

• The instructions are a bit difficult to follow because they are all jammed into a paragraph.

• Follow along with the illustrations in the Manual.

Experiment Notes

• This is the overall logic

Hydrolysis of Grignard addition product with HCl (in beaker) 

PhCO2− (MgBr)+ + HCl(aq) PhCO2H + “MgClBr”

 

benzoate benzoic acid Mg salts

insoluble in ether soluble in ether insol. in ether

soluble in water insoluble in water sol. in water

 

 

 

Experiment NotesDuring extraction with NaOH (in sep. funnel)

 PhCO2H + NaOH(aq) PhCO2

− + Na+ + H2O

 

benzoic acid benzoate

soluble in ether insoluble in ether

insoluble in water soluble in water

  

After extraction and during precipitation with HCl (in beaker) 

PhCO2− Na+ + HCl(aq) PhCO2H ↓ + Na+ + Cl−

 

benzoate benzoic acid

soluble in water insoluble in water

 

 

Experiment Notes• Don’t throw anything away. Almost invariably, something gets mixed up and the

product goes down the drain. The experiment can almost always be saved as long as it is not in the Fairfax County waste water stream.  

• Demonstrate using separatory funnel

• Extract the ether phase (which contains benzoic acid), vigorously to ensure mixing, with NaOH, combining the NaOH extracts. Keep the aqueous layer (which contains benzoate). REMEMBER TO VENT the separatory funnel.

• During final heating on hot plate, do this in the Hood! Cool before adding HCl.

 

Experiment Notes• Filter and then recrystallize the whole crude sample, using about 3 ml

H2O per gram of crude product.

• Keep product in beaker in the storage drawer to dry until next week.

• Use “wet” sample weight to calculate % yield for report due next week.

• All ether layer waste is put in the jar in the Hood.

• Clean-up: King Gyasi

 

The Report• Complete reaction showing presumed intermediate compounds:

• Equation to use to calculate theoretical yield:

– Note: it is not necessary to know anything about the Grignard intermediate to calculate a % yield. It should not appear in the equation.

• Since CO2 and HCl were used in large excess; exclude them from the calculation of theoretical yield.

• Remember to include literature spectra for the benzoic acid product (with citations included).

BrMg0

Mg-Brether CO2 CO2M gBr

HCl(aq)CO2H

Br

Mg0 CO 2 HCl(aq)CO2H

+ + +