Dehydration of 2-methylcyclohexanol

-- an E1 reaction

Elimination Reactions

1. E1 – unimolecular transition state – formation of carbocation intermediate

– Preferred for tertiary carbons

2. E2 – bimolecular transition state – no intermediate formed

– Preferred for primary carbons

CH3

OH

+ H3PO4CH3

OH2+

CH3

+

- H2O

CH3

+

- proton

rearrangement

CH3

+

CH3

+

CH2

Major product

1. Acid catalyzed – creates a good leaving group (i.e. water)2. Carbocation intermediate formation.3. First two steps of the mechanism at the same as for SN1.4. Carbocation will rearrange for increased stability, if

possible.5. Protons can be removed from any adjacent position

leading to multiple products.6. Major product is the most stable alkene from the most

stable carbocation.

Why does reaction with phosphoric acid (H3PO4) lead to elimination (this week’s lab) while reaction with HCl leads to substitution (last week’s lab)?- Hint: Compare HCl and H3PO4 (look at pg. 23 of the PDF file posted on Blackboard)

Dehydration of alcohols is …

To Prepare Your NB…

• 5 Chemicals to include in Table of Properties: 2-methylcyclohexanol, 85% phosphoric acid, 1-methylcyclohexene, 3-methylcyclohexene, methylene cyclohexene

• Handwrite procedure from this Powerpoint and refer to relevant pages in your NB for simple distillation and seperatory funnel set-ups.

• Two chemical reactions

Procedure1. Put 10 mL 2-methylcyclohexanol and 2 mL of 85%

phosphoric acid in a 25 mL round bottom flask (as the stillpot) with boiling stones.

2. Setup a simple distillation: attach the stillhead, thermometer, condenser, adapter, and use a graduated cylinder as the receiver.

3. Heat slowly to 96 oC and maintain temperature.

4. Do NOT allow the temperature to rise above 110oC.

5. Collect distillate until about 2 mL are left in the round bottom flask (this should be the 2 mL of phosphoric acid).

Procedure

6. Transfer distillate to separatory funnel for washes:- 1st wash 5 mL aq. saturated NaCl (then remove aqueous layer)- 2nd wash 5 mL saturated aq. NaHCO3 (then

remove aqueous layer)(What does each wash do?)

7. Remove organic layer and dry it over anhydrous sodium sulfate for at least 15 mins.

8. Carefully separate the product from the dessicant, bottle it in a properly labeled container, and submit it for GC analysis.

Chemical Reactions

1. Dehydration of alcohol:

2. What about the washes?• Neutralization of H3PO4 :

+ H3(PO4)

+ H2O + H3PO4

H3(PO4) + 3 Na(HCO3) Na3(PO4) + 3 H2O + 3 CO2

Analysis of Products

H2O & alcohol

110OC

104OC

102 OC

Retention Time (mins.)

Calculations Section of NB

• % composition from Gas Chromatograph– Peak 1 @ 27.87% of distillate– Peak 2 @ 5.061% of distillate– Peak 3 @ 52.89% of distillate– Total % of distillate which is alkenes = 85.82%

Mass of distillate is 6.50 g but only 85.82% is alkene products so…

0.8582 * 6.50 g = 5.58 g of mixed alkenes

% yield = actual mass of mixed alkenes theoretical yield

(Since all alkenes have the same molar mass, you can calculate the theoretical yield of mixed alkenes just as you would if there were only 1 alkene.)

Calculations Section of NB(cont’d)

Discussion

• The percentages of each alkene shown in the GC can be compared to your expectations from the mechanism.

• (I will hand back your GCs next week, so you do NOT need to write your discussion for this lab until then.)