Dehydration of Dehydration of

22--methylcyclohexanolmethylcyclohexanol

Dehydration ReactionsDehydration Reactions

Alcohol dehydration is an acid catalyzed reaction, which can be

performed by strong, concentrated mineral acids such as sulfuric

acid (H2SO4) and phosphoric acid (H3PO4).

BackgroundAlcohol dehydration proceeds by an elimination mechanism called E1.

OH

H-O-PO3H2

O-H

CH3CH3

H+

+

H

CH3

H

secondary carbocation

CH3

H

+

CH3

CH3

tertiary carbocation

carbocation rearrangement

O-PO3H2

O-PO3H2

O-PO3H2

rate determiningstep

minor productmajor product

1st step - Protonation changes the very poor leaving group (-OH) into a good leaving group (H2O).

2nd step - Water departs, leaving behind a carbocation. This step is the rate determining step (rds).

3rd step - The base H2PO4- removes a proton

from the carbon adjacent to the positively charged carbon, forming an alkene and regenerating the acid catalyst H3PO4.

oxonium ion

Saytzev’s rule: The more substituted alkene is the more stable product (major product).

Dehydration of Dehydration of 22--methylcyclohexanolmethylcyclohexanol

carbocation rearrangement

major productbp 110° C

minor products

bp 165° C

Experimental ProcedureExperimental Procedure

Changes from the Manual procedures:Changes from the Manual procedures:

Use Use heating mantelheating mantel instead of sand bath. Don't forget instead of sand bath. Don't forget boiling chipsboiling chips..

Use Use 6 6 mLmL of 2of 2--methylcyclohexanol and 5 methylcyclohexanol and 5 mLmL of phosphoric acidof phosphoric acid. .

No sulfuric acid is usedNo sulfuric acid is used..

Only the Only the qualitative test with qualitative test with BrBr22 is performed. Test is performed. Test acetoneacetone as a as a blankblank, , and the and the starting materialstarting material, and the , and the productproduct..

No refractive indexNo refractive index will be measured.will be measured.

GCGC is performed on the product mixture.is performed on the product mixture.

Distillation Set UpDistillation Set Up

Heating mantel sitting on lab jack

Do not allow temperature to rise above 100 ºC.Do not distill to dryness.

Set thermometer Set thermometer bulb even with bulb even with the joint of the the joint of the distilling headdistilling head

(NOTE: this (NOTE: this drawing shows it drawing shows it to be too low!)to be too low!)

Transfer distillate to a clean centrifuge tube, then allow layers to separate.

Add 0.5-1.0 g CaCl2 to the centrifuge tube, shake, allow to stand for 10 min.(Tube must be stoppered). (NOTE: Even if you cannot see any water separated from the distillate, you need to add CaCl2, shake it up, and let it sit for 10 min.)While the product stands, dissolve the pot residue in water and transfer to the same waste bottle as the aqueous layer.Transfer the dried product to a preweighed sample vial. Do not transfer any CaCl2. Weigh the product, calculate % yield and then perform the unsaturation test.

Water layer (Use pipet to discard into waste bottle)

Organic layer containing all products(KEEP!!!!)

Bromine test:Bromine test: To distinguish between saturated and unsaturated compounds, employ the observation of color changes.

Label three small test tubes, one for the blank, one for the starting material (methylcyclohexanol), and one for products.

Place 2 drops starting material and the product in each test tube.

Add 20 drops of acetone into each test tube and shake.

Add 2 drops of Br2 test reagent to each tube and shake.

Observe any color change within five minutes.

GC analysis:GC analysis: Dissolve 2-3 drops of product in 1 mL of acetone, then inject 1 µL into GC.

Identify peaks by comparing retention times with those of the pure alkenes (These will be posted).

Report the ratio of the products (equal to the ratio of peak areas).

Do not use the calculated areas on your chromatogram. Measure the peak height (h) and width (w) (at half the peak height).

Area = h X w

Cautions: Phosphoric acid is highly corrosive.

Methylcyclohexenes are highly fammable.

Br2 is toxic and an oxidizer.

Grading is 50% for the written report and 50% for results and observations.