Dehydration of 2-methylcyclohexanol -- an E 1 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

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 S N 1. 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 (H 3 PO 4 ) lead to elimination (this week’s lab) while reaction with HCl leads to substitution (last week’s lab)? - Hint: Compare HCl and H 3 PO 4 (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

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 o C and maintain temperature. 4.Do NOT allow the temperature to rise above 110 o C. 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!!!!) - 1 st wash 5 mL aq. saturated NaCl (then remove aqueous layer) - 2 nd wash 5 mL saturated aq. NaHCO 3 (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)

Chemical Reactions 1.Dehydration of alcohol: 2.What about the washes? Neutralization of H 3 PO 4 : + H 3 (PO 4 ) + H 2 O + H 3 PO 4 H 3 (PO 4 ) + 3 Na(HCO 3 ) Na 3 (PO 4 ) + 3 H 2 O + 3 CO 2 3. Test for unsaturation (dibromination) in previous slide.

Analysis of Products H 2 O & alcohol 110 O C 104 O C 102 O C Retention Time (mins.)

Calculations Section of NB % composition from Gas Chromatograph –Peak 27.87% of distillate –Peak 5.061% of distillate –Peak 52.89% of distillate –Total % of distillate which is alkenes = 85.82%

Mass of distillate is X g but only 85.82% is alkene products so… * 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.) Calculations Section of NB (cont’d)

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?