Dehydration of Cyclohexanol PDF

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Summary

Report on dehydration cyclohexanol...

Description

I. II.

Dehydration of an Alcohol (Cyclohexanol)

III.

Introduction The purpose of this experiment is to dehydrate cyclohexanol using sulfuric acid via an E1 mechanism reaction to form cyclohexene. E1 is a unimolecular elimination reaction, in which the removal of an HX substituent results in the formation of a double bond or an alkene as the product. Dehydration reactions begin with the protonation of the hydroxyl group in the alcohol, which will then act as a good leaving group. The water molecule is then lost generating a carbocation. (The generation of a carbocation is the rate determining step) The carbocation will act as a strong acid, a weak base such as water or a bisulfate ion can abstract an alpha proton yielding an alkene. C6H12O+H2SO4  C6H10 Procedure A distillation set up was set up. Next, 10 mL of water was added to a 50 mL round bottom flask, along with a slow addition of 8 mL of sulfuric acid. Once the sulfuric acid cooled 6.6 mL of cyclohexanol was added. Two boiling chips were added to the mixture and placed onto the distillation set up for gentle heating. The mixture was heated until there was a significant change in temperature >10°C. The contents in the collection flask were transferred into a large test tube where two layers formed. A pasture pipette was used to remove the bottom layer. The remaining top layer was washed with 2.6 mL of sodium carbonate. The bottom layer was then removed and washed with 3 mL of water removing the bottom layer again. 1 g of anhydrous calcium chloride was used to dry the cyclohexene. The weight, yield, and HNMR was then taken of the dried cyclohexene.

IV.

Table of Chemicals

V.

Chemical

Quantity

MW (or % composition if a solution)

bp °C

mp °C

density

Limiting or excess

Sulfuric acid Sodium carbonate Calcium chloride Cyclohexano l Cyclohexene

8 mL 2.6 mL

98.08 g/mol 10%

337 --

10 850

12.26 4.08

Excess Limiting

1g

147.01 g/mol

--

--

147.01

Limiting

6.6 mL

100.16 g/mol

160.8

25.4

15.18

Limiting

1.673 g

82.14 g/mol

83

--

0.02

Limting

Mechanism

VI.

Observations

VII.

Percent Yield

VIII.

Results The actual product yielded was 1.673 grams (g) of cyclohexene. The theoretical yield calculated was 5.33 g. The percent yield calculated was 31.38%. Cyclohexene evaporates when exposed to air, thus an IR spectrum was not obtained in this experiment. However, IR spectrums of cyclohexene and cyclohexanol were obtained via web and were used to further examine. The NMR for cyclohexene shown below depicts three peaks and orange scribbles created by hand by a toddler that are not part of the data collected.

c

c

a b

b

c

b

a a

IX.

Conclusion/Discussion A yield of 0.7-1.0 g of cyclohexene was expected, I obtained 1.673 g. The amount of product is higher than expected, but the percent yield of 31.38 % is low. While distilling the mixture I turned off the heat when their was a significant change in temperature not until there was no more distillate that could have caused a loss in product and low percent yield. However, a successful NMR was obtained. The NMR shows 3 peaks which were expected for cyclohexene is a symmetrical molecule, proton a integrates for 2 protons, proton b and c integrates for 4 protons. Cyclohexene has a total of 10 hydrogen atoms (2+8.25=10.25) which can all be accounted for looking at the NMR. The IR spectrum of Cyclohexanol shows the OH peak at ~3300 cm-1. In the product this peak is absent since the OH was protonated forming a leaving group with water. The IR spectrum of Cyclohexene shows a strong C-H stretch at ~3010 cm-1 and a weak alkene C=C stretch at ~1600 cm-1 confirming our product is cyclohexene. The spectrum of cyclohexene also shows a small peak at approximately 3400 cm-1 which could be alcohol or water from the original mixture. A

small amount of either could have remained in the product from the high temperature while distilled or from the water wash. X.

Questions 1. Calculate the percent yield of your reaction. 31.38% 2. What are the stretching frequencies you should observe in your IR spectrum for Cyclohexene and cyclohexanol? Label the appropriate frequencies on your IR spectrum. Is there any starting material present? The stretching frequencies observed are shown in the following table in cm-1: Cyclohexanol Cyclohexene ~ 3300 O-H stretch ~3010 =C-H stretch ~ 2900 C-H stretch ~2900 C-H stretch ~ 1450 C-H bend ~1600 C=C alkene stretch In the IR spectrum for cyclohexene the small peak in the green box at ~3400 could indicate a small amount of the remaining starting alcohol or water.

O-H stretch

C-H stretch

C-H bend

C=C alkene stretch

=C-H stretch C-H stretch

3. What are the H NMR resonances you should observe in your HNMR spectrum for cyclohexene and cyclohexanol? Label the appropriate resonances on your NMR Spectrum.

c

c

a

OH

a

d b

c

b

a d

4.

I s there any starting material present? No, the NMR for cyclohexene does not show any starting material from the cyclohexanol and sulfuric acid mixture.

5. What would be the major product obtained from the E1 dehydration of 2methylcyclohexanol? 1-Methylcyclohexene

6. Draw the mechanism for the dehydration of 1-methylcyclohexanol. Would you predict this reaction to be faster or slower than the reaction you performed?

This reaction would be slower than the dehydration of cyclohexanol because of the tertiary carbocation in 2-methylcyclohexanol....