Theory & Procedure, Boiling point of Organic compound | Additional Study Material for NEET PDF Download

Our Objective

Our objective is to determine the boiling point of an organic compound.

The Theory

What is the importance of knowing the boiling point of organic compounds?

The boiling point of organic compounds can give important information about their physical properties and structural characteristics. Boiling point helps identify and characterise a compound. A liquid boils when its vapour pressure is equal to the atmospheric pressure. Vapour pressure is determined by the kinetic energy of a molecule.

Kinetic energy depends on the temperature, mass and velocity of a molecule. When the temperature increases, the average kinetic energy of particles also increases. When the temperature reaches the boiling point, the average kinetic energy becomes sufficient to overcome the force of attraction between the liquid particles. As the force of attraction decreases, the molecules in the liquid state escape from the surface and turn into gas.

The boiling point of a liquid varies with the surrounding atmospheric pressure. A liquid at a higher pressure has a higher boiling point than when that liquid is at lower atmospheric pressure.

The normal boiling point of a compound is an indicator of the volatility of that compound. The higher the boiling point, the less volatile is the compound. Conversely, the lower the boiling point, the more highly volatile is the compound. At a given temperature, if a compound’s normal boiling point is lower, then that compound will generally exist as a gas at atmospheric pressure. If the boiling point of the compound is higher, it then exists as a liquid or a solid.

What are the general trends that affect the boiling point?

1. Strength of intermolecular forces

The relative strength of intermolecular forces such as ionic, hydrogen bonding, dipole-dipole interaction and Vander Waals dispersion force affects the boiling point of a compound. The influence of these forces depends on the functional group present. We can explain the effect of these forces on the boiling point of compounds with the help of some examples.

Consider butane and its three derivatives such as diethyl ether, n- butanol and sodium n- butoxide. 

n-butane (C₄H₁₀) contains no polar functional group. The only attraction between the butane molecules is weak Vander Waals dispersion forces. The result is that butane boils at a temperature at which water freezes, and is much lower than diethyl ether. In the case of diethyl ether, the molecules are held together by dipole-dipole interaction which arises due to the polarized C-O bond. Its boiling point is 35^oC.  Compare its boiling point with that of n-butanol. The boiling point of n-butanol is 117^oC. The greatly increased boiling point is due to the fact that butanol contains hydroxyl group, which is capable of hydrogen bonding. But the boiling point of sodium butoxide is higher than that of butanol because the attractive force in sodium butoxide is very strong ionic bond.

The intermolecular forces go in the order Ionic > Hydrogen Bonding > Dipole-Dipole > Van der  Waals dispersion force. 

2. Length of carbon-carbon chain

As the number of carbon atoms increases or the length of carbon-carbon chain increases, the boiling point also increases. This is because the force of attraction between the molecules increases as the molecule gets longer and has more electrons. It takes more energy to overcome the force of attraction, and so the boiling point rises.

3. Branching decreases the boiling point

As the length of carbon chain increases, the surface area of the compound will also increase. Van der Waals dispersion force is proportional to the surface area. So the increase of surface area increases the ability of individual molecules to attract each other. Branching in molecules decreases the surface area thereby decreasing the attractive force between individual molecules. As a result, the boiling point decreases.

Consider the boiling point of n-pentane and neo-pentane (2,2-dimethyl propane). These are isomers having the same molecular formula (C₅H₁₂), but differ in their structures. 

The boiling point of neopentane is much lower than that of n-pentane. 

4. Polarity

Polarity of the molecule determines the force of attraction between the molecules in the liquid state. In polar compounds, the positive end of one molecule is attracted by the negative end of another molecule. That means polar molecules are attracted by opposite charge effect. The polarity of a molecule is determined by its functional group. The greater the polarity, the higher is the boiling point.

Boiling point of some common organic compounds 

CompoundBoiling Point (^oC)CompoundBoiling Point (^oC)

 Learning Outcomes

1. Students understand the term boiling point from this experiment.
2. Students understand the procedure to determine the boiling point in other organic liquids.
3. Students understand that the boiling point of a liquid is a constant.

Materials Required

The Procedure

Real Lab Procedure

• First fill two-thirds of the small test tube with the given liquid whose boiling point needs to be determined.
• Fix this test tube to the thermometer with a rubber band in such a way that the bottom of the tube is at the middle of the thermometer bulb. The rubber band should be fixed near the mouth of the tube so that it remains outside the acid bath.
• Fill half of the beaker with Con. sulphuric acid and place it over a wire gauze placed over a tripod stand.
• Clamp the thermometer carrying the test tube to an iron stand through a cork. Lower the thermometer along with the tube into the acid bath.
• Adjust the thermometer so its bulb is well under the acid and the open end of the tube with the rubber band is sufficiently outside the acid bath.
• Take the capillary tube and seal at it about 1 cm from one end by heating it in flame and giving it a slight twist.
• Place the capillary tube in the test tube containing the  given liquid so that the sealed part of it stands in the liquid.
• Start heating the acid bath slowly and stir the bath gently. Keep an eye on the liquid and the test tube and also on the thread of the mercury in the thermometer.
• At first a bubble or two will be seen escaping at the end of the capillary tube dipped in the liquid, but soon a rapid and continuous stream of air bubbles escapes from it.  At this stage the vapour pressure of the liquid just exceeds the atmospheric pressure.
• Note the temperature (t₁) when continuous stream of bubbles starts coming out.
• Remove from the flame and note the temperature (t₂) when the evolution of bubbles from the end of the capillary tube just stops.
• The mean of these two temperatures gives the boiling point of the liquid.
• Allow the temperature to fall by 10^oC and repeat the heating and again note the boiling point.

Simulator Procedure (as performed through the Online Labs)

• You can select the compound from the ‘Select the unknown compound’ drop down list.
• To start the experiment, click on the ‘Start’ button.
• The temperature begins to rise.
• Note the temperature ‘t1’ at which a continuous stream of air bubbles appear from the end of the capillary tube.
• Now turn off the burner by clicking on the ‘Knob’ of the burner.
• Note the temperature ‘t2’ at which the air bubbles completely disappear.
• Enter the values in the respective text boxes.
• The boiling point of the compound is shown in the text box.
• You can select the actual compound from the ‘Select the actual compound’ drop down list.
• You can verify your result by clicking on the ‘Show chart’ button.
• To redo the experiment, click on the ‘Reset’ button. 

Observations

Record your observations in the table given below.

Inference

The boiling point of the given organic liquid = ... ^oC.

Precautions

• If on placing the sealed capillary tube in the test tube, the liquid is seen rising in the capillary tube, it indicates that the capillary tube is not properly sealed. Reject this capillary tube and use a new one.
• The seal point of the capillary tube should be well within the liquid.
• The acid bath must be heated very slowly and the acid is stirred to ensure uniform heating.