For the following lab please answer those pre- lab questions: Pre-lab Questions
ID: 495647 • Letter: F
Question
For the following lab please answer those pre- lab questions:
Pre-lab Questions
1. Look up general IR spectra for aldehydes and ketones and list wavelengths that correspond to their carbonyl stretches. What additional peak will be observed for aldehydes?
2. Refer to the Spectroscopic Analysis section of the lab and determine the pertinent 1H NMR information for aldehydes and methyl ketones.
Lab : Determination of an Unknown Carbonyl Compound
In this lab you will receive a sample of a pure aldehyde or ketone chosen from the list of possible unknowns (see table for full list). In order to identify the carbonyl unknown you will:
· Determine the atmospheric boiling point of the unknown;
· Obtain (or receive a copy of) the infrared spectrum of the unknown;
· Obtain or receive a copy of the 1H NMR spectrum of the unknown;
· Identify the unknown by interpreting the information collected.
Safety and Waste Disposal
The carbonyl unknowns are not toxic unless consumed in large amounts. Use standard precautions (goggles and gloves) when handling these compounds. Dispose in appropriately labeled containers.
The guidelines below outline the pre-lab steps.
Read the Determination of a Carbonyl Unknown lab and review the following pages from The Organic Chem Lab Survival Manual:
Chapter 32: Infrared Spectroscopy: pp.279-284.
Chapter 33: Nuclear Magnetic Resonance: pp. 314-320.
Determination of an Unknown Carbonyl Compound
In this lab you will determine the identity of an unknown aldehyde or ketone through the interpretation of boiling point measurements and spectroscopic analysis.
Boiling Point Determination
Place about 0.5 mL of the unknown in a dry 10- 75-mm test tube and add several boiling chips. Clamp the tube upright with the bottom resting in a small heating mantle. Suspend a thermometer (200° C scale) inside the tube so that the bulb is about 1-1.5 cm above the surface of the liquid and not touching the walls of the tube. Heat the sample to reflux (= boiling without evaporation). The boiling point is the temperature at which the hot vapors condense on the thermometer bulb and walls of the tube. If the heat input is raised, the temperature will not rise. Be sure to record an estimate of the uncertainty in the boiling point. Since the bp is the temperature at which the vapor pressure of the liquid equals the atmospheric pressure, the experimental boiling point will vary with altitude and weather. As a result, the observed boiling point is most likely to fall below the “true” 1 atmosphere boiling point. Alternatively you may do a short path distillation.
Possible Unknowns and Boiling Points*
Compound
Boiling Point
2-butanone
80
2-pentanone
102
3-pentanone
102
Hexanal
128
Cyclopentanone
131
4-Heptanone
145
3-Heptanone
148
2-Heptanone
151
Heptanal
153
Cyclohexanone
155
Octanal
171
2-Octanone
173
Benzaldehyde
179
Salicylaldehyde
196
* data taken mostly from CRC Handbook of Tables for Identification of Organic Compounds, 3rd ed., Rappoport, Z., editor, CRC Press, Cleveland OH, 1967.
Spectroscopic Analysis
It is difficult to tell an aldehyde from a ketone by IR spectroscopy, but the presence of an aldehyde is very obvious by 1H NMR. Consult a table of chemical shifts to find the ppm value for the RCHO signal.
IR spectroscopy indicates if the compound is aromatic (contains benzene rings) or aliphatic (contains sp3 and sp2 hybridized atoms only). The IR spectrum also shows if the aromatic ring is monosubstituted, ortho-, meta-, or para-disubstituted. Check up on the frequencies for these peaks in the tables in the chapter on infrared spectroscopy. A “methyl ketone” or CH3CO-R has distinctive peaks in both the H NMR and IR spectrum. Look these up.
Compound
Boiling Point
2-butanone
80
2-pentanone
102
3-pentanone
102
Hexanal
128
Cyclopentanone
131
4-Heptanone
145
3-Heptanone
148
2-Heptanone
151
Heptanal
153
Cyclohexanone
155
Octanal
171
2-Octanone
173
Benzaldehyde
179
Salicylaldehyde
196
Explanation / Answer
Solution:
1. The additional peak of cyclohexanone and 2-octanone peak will be observed along with aldehyde hence H-C=O stretch of 1440-1320(s) have to refined for finding precise aldehyde molecule
2. For 1H NMR the aldehyde and methyl ketone is of 3.9 ppm