Experiment 6: Dehydration of2-Methylcyclohexanol

Reading: Mohrig, Hammond & Schatz Ch. 19 pgs 256-276 Carey & Guiliano Ch. 5 pgs 184-207

Ch. 6 pgs 250-253

Dehydration of 2-Methylcyclohexanol• This week's reaction:

- dehydration of a 2° alcohol to give a mixture of alkene isomers- H3PO4 is a catalyst (facilitates reaction, but is not consumed)- water is lost as reaction proceeds

• Which product will be major?

• Why? Zaitsef's rule - most highly substituted (most stable) product is preferred

A B

A

• How does this reaction occur (what mechanism)? E1

Reaction Mechanism• E1 elimination:

• Note: reaction is reversible! - drive to completion by removing product from reaction mixture

e.g. distill - products are volatile! (boiling points?)

Types of Chromatography

1. Thin Layer Chromatography (TLC) • stationary phase: spread over glass or plastic sheet

• mobile phase: liquid; drawn up plate by capillary action

2. Column Chromatography • stationary phase: contained in a column

• mobile phase: liquid; passes through column (gravity or pressure)

3. Gas Chromatography (GC) • stationary phase: contained in a column

• mobile phase: gas; passes through column (pressure)

Gas Chromatography• Useful analytical technique

- follow reaction progress (requires a standard)- use for product identification (requires a standard)- determine product purity- determine ratio of product or isomers (quantitative measure!)

• Extremely sensitive - can separate very complex mixtures (hundreds of components)

- don't need much material (typically 10-6 - 10-15 g µL)

• Often used in environmental & forensic labs- identification of trace amounts of material

• Basic principles, same as other forms of chromatography, but: - stationary phase: non-volatile polymer on glass or solid support - mobile phase: inert gas - typically He - column length: 10 - 30 feet - temperature: 25-300°C

Gas Chromatography

• Longer GC column leads to better separation

• Compounds travel through column at different rates depending on: - volatility - strength of interaction with stationary phase

lower boiling compounds tendto come off first

• How does it work? - long column packed w/stationary phase & placed in an oven - inert gas (mobile phase) passed through column at a controlled flow rate - liquid sample injected (µLs), vaporized as passes through column, & detected as it emerges

Gas Chromatography• Detector response plotted vs. time

• Retention time: time it takes a compound to emerge length of time compound is retained in the column

Retention time = dcs

= distance from injection point to sample peak (cm)chart speed (cm/min)

• RT will remain constant for a given set of conditions (column type & temp)

Gas Chromatography• detector response is proportional to amount of compound passing through it can quantify amounts by comparing peak areas

Peak Area X = (1.2 cm) x (0.4 cm) = 0.48 cm2 Peak Area Y = (4.1 cm) x (0.5 cm) = 2.05 cm2

Peak Area = (Peak Height) x (Peak Width at 1/2 Height)

4.1 cm

0.5 cm0.4 cm

1.2 cm

Ratio X : Y = 1 : 4.3

Sample GC Trace - cyclohexane:toluene (1:1)

injectionpoint

note baseline separation of peaks

retention time

peak areas

Next Week (October 18 - 22)

A. Synthesis of a mixture of alkene isomersdehydration of 2-methylcyclohexanolisolate products by distillation

B. Analysisbromine test for unsaturationTLCIRGC Analysis: determine ratio of two double bond isomers

Experiment 6: Dehydration of an Alcohol/Analysis by GC

DUE: Column Chromatography Lab Report (exp 5)

Lab Reports are due at the beginning of your regular lab session

Experimental Details1. Combine reactants in long neck roundbottom flask

carefully combine 2-methylcyclohexanol and 85% phosphoric acidwear gloves! phosphoric acid can cause burns

2. Add a boiling chip & assemble a modified fractional distillation apparatus(a long neck flask with stainless steel sponge in the neck)will drive reaction forward by removing product alkenes as they formwater co-distills with your product

4. Before you begincheck to be sure all joints are sealed & clampedcheck the thermometer position!!

5. Proceed with distillationcool the collection vial in ice (products are volatile!)do not allow temperature of distillate to exceed 96°CCHANGE!! do not tent apparatus with foildo not distill the flask to dryness!;

6. Stop distillation; Keep the collection vial COLD!!

Apparatus

keepcold

long-neckroundbottom flask

packed withstainless sponge

distillationhead

collectionvial

ice bathboiling chip

sand bath

thermometeradapter

connector withsupport rod

DO NOT TENT THE APPARATUS WITH FOIL DURING DISTILLATION

Experimental Details7. Wash distillate with NaHCO3 (you are doing a mini extraction!)

add solid NaCl to product vial to saturate the aqueous layeradd 1 mL saturated NaHCO3mix well by drawing layers in & out of a disposable pipetcheck pH to be sure any acid has been neutralized

8. Draw off aqueous layer using a pipetwhich layer is the aqueous layer??? the bottom layer!keep everything just to be sure!

9. Dry organics over anhydrous Na2SO4cap & keep vial cold while drying

10. Transfer the dried product to a clean, dry, preweighed (with cap) vialinclude the cap when you pre-weigh the vial

11. Weigh product in capped vialmass of product can be used to calculate the percent yield

12. Analyze the productbromine test, TLC, IR, GC can be done in any order

Bromine Test for Unsaturation• Qualitative Test - helps identify what functional groups are present

in this case we are looking for the presence of a C=Ccan be detected by addition of bromine to a solution of an organic

compoundpositive test is decolorization of the solution (colored colorless)

+ Br2

(reddish-brown)

Br

Br

(colorless)

• How it Works: Halogenation of the Double Bond

Thin Layer Chromatography• Will visualize spots using a chemical stain - KMnO4

dip plate, let dry on paper toweldo this in the hood ONLY!! KMnO4 is a strong oxididant!

• This is a destructive method of visualizationcompounds undergo a chemical reaction on the TLC plate

will see light spots on a purple backgroundmay take some time to develop

Writing the Lab Report: Exp #5 Column Chromatography

Purpose - technique experiment: what will you learn? - what conclusions will you reach? - a general discussion of theory/expected results is not a purpose!

Results & Discussion - Identify contents of each flask (flask #1 and flask #2)

which is ferrocene? which is acetylferrocene clearly explain how you reached this conclusion what data can you use?

TLC - correlate Rf to structurecolor (what color should it be?)IR

discuss success of separation are the fractions pure? how efficient was the separation (e.g. discuss your % recovery)

be sure to justify your answers using the data/info available

Writing the Lab Report: Exp #5 Column Chromatography

Conclusion - a brief recap of your findings - include a general statement about column chromatography

(what did you learn?) - do not include a lot of theory

Appendix A: Calculations - Rf

- Percent Recovery

Appendix B: Spectra - Page of IR spectra (provided)