IR Spectra

University of New Haven

Department of Chemistry and Chemical Engineering

RE: Infrared Spectroscopy

Abstract:

In this experiment, an infrared spectrum was obtained for unknown compounds 14-32, 14-38 and 14-34 by using three different methods; KBr pellet, mineral oil mull and Chloroform. The point of running different spectra was to be able to compare the different methods to determine the best method of sample preparation. The primary objective was to obtain acceptable spectra for the three organic unknowns so that the unknowns could be identified. In order to help identify the unknowns an NMR and a mass spectrum were also obtained. It was not easy to determine the unknown compounds, instead important peaks were determined.

Procedure:

To prepare the mineral oil (Nunjol) mull, 5-20 g of sample were grinded to a fine powder with a smooth mortal pestle. One to two drops of mineral oil were added and the mixture was grinded again until the sample was the consistency of Vaseline. A sample size equal to the head of the spatula was placed between two salt plates, which were then pressed together to form a thin film. The plates were then placed in the sample holder and the spectrum was obtained.

A KBr pellet was prepared by obtaining an infrared KBr powder, and agate mortar and pestle, and a metal spatula were obtained. KBr was the standard salt of choice because it transmits radiation down to 400 cm-1. Agate was used in infrared work because it is highly nonporous, and thus helped to avoid contaminating samples with previously prepared samples or with moisture. The piece of glassine weighing paper was placed on an analytical balance. Using the pointed end of the spatula, 2.5 mg of the sample was approximately weighed on the weighing paper. The sample was transferred to the clean and dry mortar and the sample was grinded with the pestle until it was extremely fine. The weighing paper was returned to thebalance again, and using a rounded end of the spatula, 350 mg of KBr powder was weighed onto the weighing paper. The KBr was then transferred to the mortar, but not on top of the sample. The KBr was gradually mixed into the sample; it was started with an amount about equal to the amount of sample and mixed with the spatula. Then more KBr equal to the amount of the mixture was added and mixed again. The process of doubling the amount of mixture was continued until all KBr was mixed with the sample. The weighing paper was once again returned to the balance, the KBr mixture was grinded with the pestle using a circular motion. Approximately 100 mg of the mixture was transferred to the weighing paper using the rounded end of the spatula for preparation of a 13 mm diameter pellet. The sample spectrum transmittance was obtained.

Two unknown organic compounds were provided; 14-32 and 14-38. For one of the liquids two spectra were obtained: one neat, and one as a 5% solution in CHCl3. The

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