Dehydration notes-1

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Transcript Dehydration notes-1

Dehydration of
-- 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
Major product
Dehydration of alcohols is …
Acid catalyzed – creates a good leaving group (i.e. water)
Carbocation intermediate formation.
First two steps of the mechanism at the same as for SN1.
Carbocation will rearrange for increased stability, if
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)
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
1. 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
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 test for the presence
of alkenes by bromine test for unsaturation.
9. Using a glass pipette transfer 5 drops of the
original 2-methylcyclohexanol into test tube
labelled 1, then transfer 5 drops of the unsaturated
alkenes product mixture into test tube labelled 2.
10. Add 2-3 drops of liquid bromine in DCM
(Br2/DCM) into each of the test tubes above.
11. Record your observations and conclude.
Show the test results to your instructor before leaving.
12. Clean up. No product is turned in today.
Chemical Reactions
1. Dehydration of alcohol:
H2O + H3PO4
2. What about the washes?
• Neutralization of H3PO4 :
H3(PO4) + 3 Na(HCO3)
Na3(PO4) + 3 H2O + 3 CO2
Bromine test for unsaturation
Bromine adds itself across the double bond converting
it to a single bond. See video link below.
If the double bond is absent, the solution remains brown.
If the double bond is present, the solution turns colorless
Calculations Section of NB
% 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.)