Experiment 11 - lab 11 PDF

Preview

Summary

lab 11...

Description

Experiment 11 Identification of unknown ketone #202

Amira Lazizi CHEM 222 Section 02 Lab 04 Student ID: 40096525 Partners: Vicky Barrera

TAs Gianna Di Censo Cynthia Messina

Friday, October 5th 2018

Introduction In this experiment, the goal was to identify unknown methyl ketone #202 by putting it through several tests. To obtain the right derivative we needed to use for these tests, we made it react with DNPH (2,4-dinitrophenylhydrazine) then vacuum filtrated it twice, recrystallizing it in between. We used a TLC silica gel plate and measured the melting point range to compare the Rf value and melting point to those of five identified ketones. We also analyzed an H-1 NMR spectrum to confirm the identity of the unknown.

Results Observations: When we mixed DNPH and ethanol with the unknown ketone, it turned yellow and cloudy. This was likely caused by the DNPH. During recrystallization, some magenta coloured spots appeared in our derivative. The derivative we obtained after the second filtration was too scarce (1.00 g). We merged teams on TA’s instructions, but the unknown from the second team had two different spots on the TLC, meaning their derivative was impure. Because of this, their values have not been included in this report. Data: Melting point range: 54.7 to 67.1 C Melting point: 60.9 C Table I: Distances travelled by compounds in TLC plate and respective calculated Rf values Compound name Unknown #202 2-butanone 2-pentanone 2-hexanone 2-heptanone 2-octanone Solvent

Distance travelled (cm) 4.35 3.35 3.70 3.85 4.35 4.4 6.4

Rf value calculated 0.680 0.523 0.578 0.602 0.680 0.688 1

Calculations: Example of Rf value calculation Rf =

distancetravelled by compound distancetravelled by solvent

Rf unknown #202 =

4.35 cm =0.6796875 6.4 cm

Percent error calculation for 2-heptanone and 2-octanone 2-heptanone :

2-octanone :

|Rf Unknown−Rf 2−heptanone | Rf 2−heptanone

|Rf Unknown−Rf 2−octanone | Rf 2−octanone

× 100=

× 100=

|0.6796875− 0.6796875| 0.6796875

|0.6796875−0.6 875| 0.6875

× 100=0 %

× 100=1.137 %

Melting point average Average =

min temp+ max temp 54.7 + 67.1 = 2 2

= 60.9 C

Table II: H-1 NMR Analysis Chemical shift (ppm)

Integration (cm)

# of H

Splitting

Possible structure

3

Triplet

CH3-CH2

0.869

2.98 2.09

1.205

4.15 = 1.99 x 2 2.09

4

Sextant

CH2-CH2

1.510

2.09 2.09

= 1.00 x 2

2

Quintet

CH2-CH2-CH2

2.131

3.00 2.09

= 1.37 x 2

3

Singlet

CH3-C=

= 1.43 x 2

2.401

2.10 2.09

= 1.0 x 2

2

Triplet

CH2-CH2-C=

Discussion According to the TLC test, the possible matches to unknown ketone #202 would be either 2heptanone or 2-octanone, with a closer match to 2-heptanone. As a matter of fact, the Rf values are exactly concordant for unknown #202 and 2-heptanone, as they travelled the same distances on the silica gel plate and both have an Rf value of 0.680. However, 2-octanone’s Rf value of 0.688 is also too close to be disregarded, with a percent error of only 1.137%. This leads one to think our unknown could be 2-octanone or 2-heptanone. However, the NMR spectrum analysis helped us further confirm that our compound is in fact 2heptanone. The total number of hydrogens found with each peak amount to 14, which could only be 2-heptanone, which has the molecular formula of C7H14O. The peak at 2.131 shows the start of the carbon chain with a CH3 linked to a C that has a double bond. This would be the C=O double bond of 2-heptanone. The 2.401 peak continues the chain, and we can reconstruct the CH3COCH2CH2CH2CH2CH3 structure by assembling the possible structures we got from each peak this way, going from deshielded (closer to O, higher ppm) to shielded (further from O, lower ppm). The final 2-heptanone chain can be obtained by making sure we respect the # of H’s found in the analysis, include the double bond in the chain, and this way, inevitably get the seven-carbon chain. The melting point found, which was 60.9 C, was a poor way of identifying our unknown ketone and did not prove useful....