Why Melting Point Analysis Matters in Organic Chemistry
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.
FAQs on Determination of Melting Point of Organic Compounds
1. Why is determining the melting point an important technique in organic chemistry?
Determining the melting point is a fundamental and crucial technique in organic chemistry for two main reasons. Firstly, it serves as a primary criterion for ascertaining the purity of a crystalline organic compound. A pure substance melts at a sharp, specific temperature. Secondly, it is used for the identification of unknown compounds. By comparing the experimentally determined melting point with known literature values, a chemist can identify or confirm the identity of a substance.
2. What is the difference between a 'melting point' and a 'melting range'?
A melting point is the single, precise temperature at which a pure crystalline solid transitions into a liquid. In contrast, a melting range is the span of temperature from when the solid first begins to melt until the last crystal disappears. For a highly pure compound, this range is very narrow (typically less than 1°C). The presence of impurities broadens this range significantly.
3. What factors influence the melting point of an organic compound?
Several factors influence the melting point of an organic compound. The key determinants are:
- Intermolecular Forces: Stronger forces like hydrogen bonding and dipole-dipole interactions require more energy to break, leading to a higher melting point.
- Molecular Weight: Generally, as molecular weight increases within a homologous series, the melting point also increases due to stronger van der Waals forces.
- Molecular Symmetry and Packing: Symmetrical molecules can pack more efficiently and tightly into a crystal lattice, resulting in a higher melting point compared to their asymmetrical isomers.
- Presence of Impurities: Impurities disrupt the crystal lattice, weakening the intermolecular forces and thus lowering and broadening the melting point.
4. How do impurities affect the melting point of an organic solid?
The presence of impurities has two primary effects on the melting point of an organic solid:
- Melting Point Depression: Impurities disrupt the uniform structure of the crystal lattice, which makes it easier to break the intermolecular forces. This results in a lower overall melting point compared to the pure substance.
- Broadening of the Melting Range: An impure substance does not melt at a single temperature but over a range of temperatures. The more impurity present, the wider this range becomes.
5. What is the Thiele tube method for determining a melting point?
The Thiele tube method is a common laboratory technique for determining the melting point of a solid. A small amount of the powdered solid is packed into a capillary tube, which is then attached to a thermometer. Both are suspended in a high-boiling point liquid (like paraffin oil or silicone oil) within the Thiele tube. The side arm of the tube is gently heated, creating a convection current that ensures uniform heating of the liquid. The temperature at which the solid begins to melt and the temperature at which it completely liquefies are recorded to determine the melting range.
6. Why do symmetrical molecules often have higher melting points than their asymmetrical isomers?
Symmetrical molecules often have higher melting points because their regular shape allows them to pack more efficiently and tightly into a crystal lattice. This close packing maximises the intermolecular forces of attraction between the molecules. More thermal energy is required to overcome these strong, well-organised forces and break down the lattice structure, resulting in a higher melting point compared to less symmetrical isomers which cannot pack as effectively.
7. What does observing a wide melting range, for example 120-125°C, indicate about the sample?
Observing a wide melting range, such as 120-125°C, strongly indicates that the sample is impure. A pure crystalline solid should melt over a very narrow range (typically 0.5-1°C). The wide range suggests the presence of foreign substances that disrupt the crystal lattice, causing the melting process to begin at a lower temperature and extend over several degrees until all of the substance has melted.
8. What are some essential precautions to take during a melting point determination experiment?
To ensure an accurate melting point determination, several precautions are necessary:
- The organic compound must be finely powdered and packed tightly in the capillary tube.
- The heating of the liquid bath should be slow and uniform, especially near the expected melting point (about 1-2°C per minute).
- The thermometer and capillary tube should be placed so that the sample is level with the middle of the thermometer bulb.
- The thermometer used must be accurately calibrated.
- Never heat the Thiele tube directly at the bottom; always heat the side arm to facilitate proper convection.
