Transcript Dehydration notes

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
O
O
OP(OH)2
H OP(OH)2
+
H OH
+
+
H3O
H O H
H
OH
O H
H
O H
H
OH2
H
H OH2
H
1,2-Hydrogen atom Shift
Hoffman Product
Minor
Major Product
(Zaitsev Product)
Hoffman Product
Minor
H
OH2
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
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.
1.
2.
3.
4.
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
Procedure
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
acid).
Procedure
6. Transfer distillate to separatory funnel for washes:
(drain off bottom layer after each wash into a common
beaker!!!!)
- 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, and pre-weighed vial container, and
weigh the mass of your distillate by substraction.
Procedure Continuation….
9. BROMINATION TEST FOR ALKENES- transfer 5 to 7
drops of your distillate product into a small test tube and add
1 – 2 drops of Br2 in dichloromethane-DCM (CH2Cl2). The
Red-brown bromine liquid gets decolorized if alkenes are
Present.
Do the same for your starting material alcohol which acts as
a control experiment. Record your observations.
(Mechanism is on the next slide)
ANTI-ADDITION DIBROMINATION
(TEST FOR UNSATURATION)
Br
Br
Br
Br
Br2
+
Br
Br
Br
Br
Br
+
Br
Chemical Reactions
1. Dehydration of alcohol:
+
H3(PO4)
+
H2O + H3PO4
2. What about the washes?
• Neutralization of H3PO4 :
H3(PO4) + 3 Na(HCO3)
Na3(PO4) + 3 H2O + 3 CO2
3. Test for unsaturation (dibromination) in previous slide.
Analysis of Products
110OC
104OC
102 OC
H2O & alcohol
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%
Calculations Section of NB
(cont’d)
Mass of distillate is X g but only 85.82% is
alkene products so…
0.8582 * X g = Y g of mixed alkenes
% yield = actual mass of mixed alkenes ‘Y’
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.)
DISCUSSION
• Use the information given in the previous 3 slides as GC
results to calculate overall % Yield for the reaction.
• All observations must be discussed
• Why E1 and not Sn2, Sn1, E2?
• Major product based on Zaitsevs rule?
• Role of all chemicals added during extraction work up?
• Any errors?