Determination of Melting Point of an Organic Compound

Organic compounds are chemical compounds that contain carbon. Organic Chemistry is the study of the manufacturing and synthesis of chemical compounds. 

Melting points are used to determine the purity or impurity of a substance or a compound. The chemical compound changes its state from solid to liquid and this change is called the melting point of the chemical compound. For instance, when the heat is applied to a solid substance, it will reach the melting point with gradual heating. When excess heat is provided, the solid will convert into liquid without any temperature change.  

What is Melting Point Determination?

Melting point determination of different elements is tested to understand the purity of the object. An extremely pure substance will have a melting range of one or two degrees. Impure substances have a much higher range and tend to depress and broaden the melting range, which is higher and smaller than the original, impure one. The melting occurs at a certain temperature change, but it depends on the structure of the molecule. For that very reason, different compounds have different melting properties. 

In the melting point determination of organic compounds, these compounds are defined as a large set of chemical compounds where more than one atom is covalently linked with other atoms such as hydrogen, oxygen, and nitrogen. The melting point determination of organic compounds can be done by doing a small experiment. 

Two Common Methods for Determining Melting Points

• Mel Temp Apparatus 

• Thiele Tube Set-Up 

Different compounds have different melting points as the temperature solid melts at changes its state to solid is determined by the structure of the molecules. The compounds in solid form have to be overcome by heating at a temperature for them to lose their intermolecular forces that hold solid together. 

Crystalline organic compounds (Pure and Nonionic) have a very sharp and characteristic melting point of 0.5 to 1.0⁰C range. Even if there is a small amount of impurity in the mixture it would result in depression of the melting point and an increase in the melting point range. Hence, the melting point of a compound is a criterion for purity and identification. 

Determining the melting point of an organic compound solid, you can take a tiny amount into a capillary tube and attach it to the stem of a thermometer centered in a heating bath slowly. Observe the temperature at the melting begins and also observe the temperature at the melting is complete. Pure samples have sharp characteristic melting points anywhere between 149.5 to 150⁰C or 189 to 190⁰C. Whereas impure samples of compounds melt at lower temperatures and with a wide range of 145 to 148⁰C or 186 to 189⁰C. 

Melting Point Determination Lab Report 

A small experiment can be conducted to determine the melting point determination of organic compounds. One such is given below. 

Thiele tube method – for this experiment, a tube known as Thiele tube is used, which is designed to contain oil and experiment with heat. A capillary, a thermometer, and a bunsen burner are required to complete the experiment well. The substance whose melting point will be observed is placed in the capillary tube, and a rubber band is tied around it. Ensure that the rubber band is not getting dipped into the oil as with the rise of temperature; the rubber will tend to melt. 

The Thiele tube is designed so that the heat that is being emitted from the bunsen burner is evenly distributed. During the experiment, the rate of the temperature is carefully controlled. In and around the melting point of the substance, the heating will be slow so that the rate of temperature increase is not faster than the ability of the heat to be transferred to the sample being observed. The thermometer bulb and the capillary tube sample must be at thermal equilibrium at the melting point.

The MelTemp Apparatus

A MelTemp Apparatus consists of a Thermometer, Eyepiece, melting point tube, tube channels and a heating control knob. 

Steps to Set Up the MelTemp Apparatus: 

• Insert the thermometer into the thermometer well 

• Insert capillary into the channels located at the front of the thermometer tube

• Switch the Green LED on for power 

• Control the heat using the black heating knob 

Adjust the eyepiece and the sample can be observed through the front of the MelTemp Apparatus. The eye has to be about 15cm away from the lens. 

For accurate measurements remember to use slow heat to observe the melting point of the sample. There are two temperatures we need to make a note of: 

• When the sample starts melting (Initial Phase) 

• When all of the samples have melted

By observing the above two temperatures you can determine the melting point range of your sample. Once you have determined the range and the sample has melted completely ensure to turn the power off and remove the capillary and dispose of it. 

Visual Observations

During melting point determination several noticeable changes take place in the three capillaries.  The first sign of change may be due to dehydration, change of color, change in the crystallization state, individual isolated crystals starting to melt without the liquid showing up as a cohesive phase also known as sintering point and condensation in the coolest points of the tube. 

Tips on Melting Points 

• Never heat the sample too fast. The recommended range is 1 to 2⁰C per minute. 

• You should always use a fresh sample and capillary tube. 

• Never melt a sample that you had melted earlier.

• Use only a clean tube before running the experiment of melting point. The results might show a low melting point or a wide melting range. 

• If doing routine measurements use the same batch of capillary tubes. All capillary tubes are not equal and may not give consistent results if a different batch is used.

Factors Affecting Melting Point of Organic Compounds

The factors that affect the melting point of organic compounds are as follows:

• The Size of the Molecule- Melting point determination of organic compounds helps to identify the organic compound properties, both physical and chemical. The structure of the molecule has a big role to play. The tighter the molecules are packed, the higher the melting point. If the structure of the molecules is loose, then the melting point would be lower. 

For instance, in Isopentane, the molecules are loosely packed, so the melting point is lower. Whereas, in the case of symmetrical neopentane molecules, the melting point is higher. Another point is if all the other factors are equal, smaller molecules' melting point will be lower. For instance, ethanol melts at -114.1 degrees, while the bigger ethyl cellulose molecule's melting point is 151 degrees Celsius.

• The force of Attraction- Another factor that influences the melting point is the force of attraction. If the force of attraction is strong among the molecules, then the organic compound's melting point will be higher. For this reason, the ionic compounds have a higher melting point due to their electrostatic force holding the ions together in a strong bond. The presence of polarity or hydrogen compounds may also lead to a great attraction force, which results in higher melting points. A weaker bond persists between butane molecules, so the melting point is lower. 

• Impurities- The melting point range of pure solids is quite narrow and usually ranges from one to two degrees, known as the sharp melting point. In the case of impure substances or elements, they can melt over lower temperatures and at a wider temperature. For instance, a pure organic crystal has uniform molecules, which are jam-packed together. However, the crystals are impure when they occur in a mixture of two different organic molecules because they don't fit together well. Thus, It takes more heat to melt the pure structure.

Conclusion 

The melting point determination of organic compounds helps people understand the physical and chemical properties of the substance. Many different factors affect the melting point of any substance, such as the force of attraction, impurities present in the substance, and the molecules' size and structure.