Tatiana Pachova BSc‐2, chemistry Assistant : Chandan Dey Sciences II – lab. A 31/12/11 Dehydration of alcohol into olefin (n°14) 1. INTRODUCTION 1.1)
1.2)
Purpose The objective of this experiment is to synthesize the methylcyclohex‐1‐ene and to compare the abundance of the 2 products. Scheme OH
H3PO4
1.3)
2-Methylcyclohexanol
Mechanism +
1-Methylcyclohex-1-ene (a) 3-Methylcyclohex-1-ene (b) H
OH
O
+
H
H+
H
+
H2O
+
H2O
H
+H3O+
2. PROCEDURE 2.1)
Reaction Reagents molar mass [g/mol] wt/vol taken n [mmol] 2‐methyl‐cyclohexanol 114.19 (d=0.930) acide phosphorique 98 2.2)
10.8mL 2.5mL 87.6 ‐ In a 25mL bottom flask, 10.8mL of 2‐methyl‐cyclohexanol and 2.5mL of phosphorous acid were mixed. A fractionating column with a thermometer and a jack on an overhead was fixed to the flask. The solution was then heated under agitation at 80‐90°C and maintained at this temperature for 20min. Then the temperature was increased until the mixture’s distillation 92‐95°C. Isolation The distilled solution was dried on MgSO4 and then filtered through a pipette containing a little piece of cotton. 3. DISCUSSION AND RESULTS 3.1)
3.2)
3.3)
Observations It was impossible to separate the two products a and b. Therefore the final product is a mixture of the two isomers. The obtained product was a transparent liquid with a very strong smell. Yield After distillation and filtration, 5.2776g of product were collected. molar mass [g/mol] n th [mmol] n exp [mmol] yield [%] 96.17 87.6 54.9 63% The protocol said that the yield should be “around 75%”, therefore we can conclude that the experience went well. Vapor phase chromatography Using VPC we can determine the composition of the product (i.e. the percentage of isomers a and b). Two big peaks were obtained: the first one corresponding to compound b, showed 17.2% and the second one corresponding to the compound a, gave 79.5%. We can notice that the isomer a is therefore largely in the majority. This shows that the first is more stable, its alkene being more substituted. 4. SPECTROMETRY DATA 4.1)
4.2)
NMR 1H (CDCl3, 400MHz) It is difficult to analyse the spectrum, since the two products were mixed. The compound a, has 6 hydrogens, whereas the compound b has 7 hydrogens, so we cannot really conclude on anything. What can be observed is that there is a peak between 1.52 and 1.71ppm and corresponding to literature we should get peaks at 1.60‐1.70 and 1.49‐1.74 for compound b and at 1.74 for 2 hydrogens of the compound a. IR (neat, cm‐1) 2924‐2858; 1440; 1376; 1141; 1087; 917; 796; 668 The peak at 1440 cm‐1 could correspond to the C=C stretch in the aromatic ring (theoretically at around 1600 & 1475). The peak at 2924 cm‐1 could correspond to the C‐H stretch for the methyl group attached to the benzene ring. 5. REFERENCES [1] Travaux Pratiques de Chimie Organique 3ème Semestre, 21 Novembre 2011 – 16 Mars 2012, 27 [2] Silverstein, Bassler, Morrill, Spectrometric identification of organic compounds [3] Vollhardt, Shore, Traité de Chimie Organique, 2004, 4ed, 325 6. ANNEXES IR spectrum, NMR spectrum SDBS NMR spectrum for 1‐methyl‐cyclohex‐1‐ene Assign.
A
B
C
D
E
F
Shift(ppm)
5.379
1.96
1.89
1.627
1.61
1.54
SDBS IR spectrum for b :