This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.
Operation method
Ultraviolet absorption spectra of aromatic compounds and solvent effect experiments
Principle
As one of the four major spectra for structural analysis of organic compounds, UV absorption spectroscopy is characterized by simple method, high penetration rate of instrument, easy operation, high detection sensitivity with high absorption intensity of UV absorption spectra, and qualitative and quantitative analysis. Although the number of spectral bands of UV spectra is small, no fine structure, poor characteristics, can only reflect the molecular chromophores and chromophores and their nearby structural characteristics, can not reflect the entire molecular properties, relying solely on UV spectral data to infer the structure of the unknown material is very difficult, but UV spectroscopy for the judgment of the organic chromophores and chromophores in the type, location, number, as well as the distinction between saturated and unsaturated compounds, determination of the degree of conjugation and thus determine the structure of unknown material. However, UV spectroscopy is unique in determining the type and number of chromophores in organic substances, distinguishing saturated and unsaturated compounds, and determining the degree of conjugation in molecules to determine the structural skeleton of unknown substances. Therefore, ultraviolet absorption spectroscopy is an important means of qualitative identification and structural analysis of organic compounds in conjunction with infrared, mass spectrometry and nuclear magnetic. Qualitative use of organic spectroscopy is based on the absorption spectral characteristics of compounds, the main step is to draw a pure sample absorption spectrum curve, by the spectral characteristics of the general law to make a judgment; comparison method to compare the absorption spectra of the unknown and the known pure compounds, or will be the absorption spectrum of the unknown and the standard spectrum comparison, when the concentration and solvent is the same, if the two spectra are the same (the shape of the curve, the number of peaks, λmax and εmax, etc.), indicating that the unknown compound structure and structural analysis. εmax, etc.), it means that they are the same compound. For further confirmation, the solvent can be changed for comparative determination. A commonly used spectral atlas is the Sadtler Atlas, a collection of UV absorption spectra of more than 46,000 compounds with five indexes, which is easy to use. Finally, other chemical, physical or physicochemical methods are used for cross-checking in order to draw the correct conclusions. UV absorption spectra of organic compounds: 1. If the compound has no absorption band in 200-400nm, it can be inferred that the unknown may be a saturated straight chain hydrocarbon, alicyclic hydrocarbon, or olefin containing only one double bond. 2. If the compound has a weak absorption band only in 270-350nm (ε = 10-100L .mol-1.cm-1), which is a characteristic of the absorption of the R-band, it can be inferred that the unknown may be a simple, non-co-absorbed compound. 3. It may be a simple, non-conjugated compound containing heteroatoms with double bonds, such as: carbonyl, nitro, etc. This band is the absorption band generated by the n → Π? jump. 3. If the compound has a strong absorption band (ε ≥ 104L .mol-1.cm-1) within 210-250nm, which is a characteristic of the absorption of the K-band, it can be deduced that the unknown substance may be a compound containing a conjugated double bond. If there is a strong absorption band within 260-300 nm, it indicates that the compound contains three or more conjugated double bonds. If the absorption band enters the visible region, the compound may contain a long conjugated chromophore or a thick ring compound. 4. If the compound has a moderately strong absorption band (ε = 103-104L .mol-1.cm-1) within 250-300 nm, which is characteristic of the benzene ring B band, it can be deduced that the unknown compound tends to contain a benzene ring. 5. Aromatic compounds have a ring-like conjugated system, and their UV absorption spectra are characterized by three characteristic absorption bands generated by the Π → Π? jump, for example: benzene at 184 nm (ε = 47000L .mol-1 .cm-1), in the vacuum ultraviolet region. There is a moderately strong absorption band at 204 nm (ε = 7900L .mol-1 .cm-1) in the terminal absorption range. and a weak absorption band at 254 nm (ε = 204L .mol-1 .cm-1). When there is a substituent group on the benzene ring, it can affect the original three absorption bands of benzene, simplify the B band, and move to the long wave while the absorption intensity increases.
Materials and Instruments
Benzene Cyclohexane Hexane Ethanol Butanone Chloroform Move 1. Absorption spectra of benzene and its derivatives Common Problems WFZ-26A UV-visible spectrophotometer operating procedures For more product details, please visit Aladdin Scientific website.
WFZ-26A Ultraviolet-visible Photometer Quartz Absorbent Cell Plugged Colorimetric Tube Graduated Pipette
Add 0.5 ml of benzene, toluene, phenol, aniline, nitrobenzene, benzaldehyde, benzoic acid, naphthalene and cyclohexane into eight 5 ml stoppered colorimetric tubes, dilute to the scale with cyclohexane and shake well. The ultraviolet absorption spectra of the eight substances were obtained by wavelength scanning in the ultraviolet region using a 1 cm quartz absorption cell with cyclohexane as the reference solution. The absorption spectra of benzene and its derivatives were observed and compared, and the effects of substituents on the original absorption bands of benzene were discussed.
2. Solvent polarity on ultraviolet absorption spectra
(1) solvent polarity on n → Π?
The effect of solvent polarity on the n → Π? transition was investigated by adding 0.02 ml of butanone into three 5 ml stoppered colorimetric tubes, diluting each tube with cyclohexane, ethanol and water, and shaking well. The UV absorption spectra of butanone in three different polar solvents were obtained by wavelength scanning in the UV region using a 1 cm quartz absorption cell with each solvent as the reference solution. Observe and compare the effect of different polar solvents on the n → Π? jump. Discuss the reasons.
(2) The effect of solvent polarity on Π → Π?
leaps
In three 5 ml stoppered colorimetric tubes, 0.2 ml of isopropylidene acetone was added, and diluted with cyclohexane, ethanol and water to the scale, and then shaken well. The UV absorption spectra of isopropylideneacetone in three different polar solvents were obtained by wavelength scanning in the UV region using a 1 cm quartz absorption cell with each solvent as the reference solution. Observe and compare the effects of different polar solvents on the Π→Π? jump. The reasons are discussed.
(3) Effect of solvent polarity on the keto and enol interconversion isomers of β-carbonyl compounds:
Add 0.5 ml of ethyl acetoacetate into three 5 ml stoppered colorimetric tubes, dilute each to the scale with hexane, ethanol, and water, and shake well. The UV absorption spectra of ethyl acetoacetate in three different polar solvents were obtained by wavelength scanning in the UV region using a 1 cm quartz absorption cell with each solvent as the reference solution. Observe and compare the magnitude of ε value of K-band absorption (λmax=243 nm) of ethyl acetoacetate in different polar solvents in the enol form of ethyl acetoacetate. Discuss the reasons.
(4) Effect of solvent on the fine structure of absorption spectra
Take 2 drops of benzene with a dropper and add it to a 1 cm quartz absorption cell, cover it with a lid, place it in the sample optical path after 2-3 min, and then scan the wavelength in the ultraviolet region with the empty quartz absorption cell as a reference to obtain the absorption spectrum of benzene vapor.
Add 0.01 ml of benzene into two 10 ml stoppered colorimetric tubes, dilute each tube with cyclohexane and ethanol, and shake well. The UV absorption spectra of benzene in two different polar solvents were obtained by wavelength scanning in the UV region using a 1 cm quartz absorption cell with each solvent as the reference solution. Observe and compare the above three absorption spectra, discuss the effect of solvents on the fine structure of the absorption spectra and explain the reasons.
(5) Effect of acidity and alkalinity of a solution on the absorption spectrum of phenol.
Add 0.5 ml of aqueous phenol solution into two 5 ml stoppered colorimetric tubes, dilute each solution with 0.1 mol.L-1HCl and NaOH to the scale, and shake well. The absorption spectra of phenol in two different acidic solutions were obtained by wavelength scanning in the ultraviolet region using a 1 cm quartz absorption cell with water as the reference. Observe and compare the two absorption spectra and discuss the reasons.
1. Turn on the computer power, in WINDOW98 desktop, double-click the wfz-26 icon, run the WFZ-26A UV-visible spectrophotometer operating software, according to the requirements of the boot prompt interface, turn on the UV host power.
2. On the computer screen, click "OK" on the boot prompt interface, the UV host for system initialization, the items detected correctly into the main screen of the UV-Visible Spectrophotometer operating software.
3. In the spectral measurement, should be carried out before the "parameter settings", the settings include: measurement mode, power supply, scanning speed, wavelength range, measurement range, spectral bandwidth.
4. Put the blank test solution into the reference and sample chamber for "baseline scanning".
5. Put the blank and sample cuvettes into the reference and sample chambers for spectral scanning.
6. When shutting down the UV instrument, select "Exit System" in the File menu, and click "OK" to turn off the power of the UV main unit when the shutdown prompt appears on the computer screen.