Dehydration of Dehydration of

2-methylcyclohexanol2-methylcyclohexanol

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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).

OH

H-O-PO3H2

O-H

CH3CH3

H

+

+

H

CH3

H

2o carbocation

CH3

H

+

CH3

CH3

3o carbocation

carbocationrearrangement

O-PO3H2

O-PO3H2

O-PO3H2

rate determiningstep

CH2

BackgroundBackgroundAlcohol dehydration proceeds by an elimination mechanism called E1.

minor productmajor product

bp 110°C

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

minor product

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

bp 165°C

Reaction Set Up – Simple DistillationReaction Set Up – Simple Distillation

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 the bulb even with the joint of the distilling joint of the distilling head. (NOTE: this head. (NOTE: this drawing shows it to drawing shows it to be too low!)be too low!)

2-methylcyclohexanol+

Phosphoric acidBoiling chips

Alkenes + water

Experimental ProcedureExperimental Procedure

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

Perform Perform Part BPart B. Use . Use simple distillationsimple distillation..

After drying your product with NaAfter drying your product with Na22SOSO44, transfer your product to a clean pre-, transfer your product to a clean pre-

weighed vial, weigh and calculate yield% later.weighed vial, weigh and calculate yield% later.

Perform Perform p172 Part Dp172 Part D. Only the . Only the qualitative test with qualitative test with BrBr22 is performed. is performed.

Test 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.

GC AnalysisGC 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 given by your TA).

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) and calculate the Triangle Area.

Cautions: Phosphoric acid is highly corrosive.

Methylcyclohexenes are highly fammable.

Br2 is toxic and an oxidizer.