Organic Chemistry

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Identification of Unknown Organic Compounds by Melting Point, Boiling Point, and Infrared Spectroscopy

Methods and Background

The goal of this laboratory experiment was to determine the structure and identity of an unknown solid and an unknown liquid by using elemental analysis, index of hydrogen deficiency, infrared spectroscopy, melting point (for solid), and boiling point (for liquid). In this laboratory, we were given unknown solid “C” and unknown liquid “D”.

The empirical formula of the unknown was determined through the process of elemental analysis by using molecular weight and given elemental percentage composition. Next, the index of hydrogen deficiency (IHD) was used to generate a numerical value that determined various combinations of double bonds, triple bonds, and rings within a given molecule. The general formula for IHD is as follows: IHD = 2n+2-X2

n = number of Carbon atoms
X = number of Hydrogen atoms
(Oxygen atoms are ignored)

Once the formulas were obtained, the experiment was started by finding the boiling point of the unknown liquid “D” and the melting point of the unknown solid “C”. Results were used to identify the identities of the unknowns. After this, a Fourier Transform Infrared (FTIR) was used to obtain a spectrum of the unknowns. This spectrum was then used to find the structure of each unknown. The melting point of a substance served as a reference point in determining the unknown solid. It is defined as the temperature at which the first tiny drop of liquid appears and extends to and includes the temperature at which the solid has completely melted. Likewise, the boiling point of the unknown liquid was found by examining the temperature at which the liquid bubbled rapidly. Since each solid/liquid has a unique melting and boiling point, it was useful in determining the structures of the unknowns. Infrared spectroscopy (IR) is a tool used in order to determine the structure of an unknown. IR spectroscopy was very useful because it showed which functional groups are present. It also was used to evaluate the purity of a known compound by the absorption of the radiation; these absorptions correspond to the transitions among different vibrational-rotational levels within the same electronic state of the molecule. The stretching, twisting, and bending that atoms undergo is related to the strength of the covalent bond and is described by Hooke’s Law. When infrared light is absorbed by a bond, the atoms become more excited and cause a change in the dipole moment, which is the cause of the emission of strong infrared absorption bands. Functional groups appear at unique frequencies, so when an IR scan shows certain peaks, the functional groups present in the unknown substance are very clear and can be identified.

Experimental Procedure

One solid unknown and one liquid unknown were assigned by the TA. The mass spectrometry, elemental analysis data, and molar mass were provided in table format. By using the given data, elemental analysis was calculated to determine the molecular formula. After the molecular formula was calculated, IHD was calculated using the formula mentioned above.

Next, the melting point of the unknown solid was obtained by using the melting point apparatus by placing a small sample of the unknown solid inside a capillary tube. The tube was then inverted and tapped so that the substance was evenly spread in the capillary tube before it was placed on the melting point apparatus. The apparatus was then turned on and the solid was observed as the temperature rose. Once the solid began to melt, the temperature range from when the solid began to melt until it melted completely was recorded. This procedure was performed once more for a total of two trials for better accuracy.

In order to determine the boiling point of the unknown liquid, approximately 1mL of this liquid was added to a medium/large test tube and was fastened to a ring...
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