Heating under reflux is a reaction procedure that involves heating the reactants of a chemical reaction for a specific amount of time while vapors continuously condense back to liquid form. This procedure is primarily done to synthesize products that do not form under normal (liquid) conditions.
The reactants for the reflux procedure may be solid, liquid, or gases but they react only under a certain (high) temperature. The temperature of a ‘heating under reflux’ mechanism depends on the boiling points of the reactants and the ‘reflux ring’ formed.
Reflux in Chemistry
Reflux is the experimental process of heating an ongoing chemical reaction for a specific amount of time, while vapors continually cool back into the liquid form using a condenser. The purpose of reflux is to maximize the yield of a chemical reaction by stopping the reactants (volatile) from escaping the reaction chamber without the formation of the products.
Organic reactions are generally slow. They need more time to synthesize products and are generally low-yielding reactions. The primary cause of this low yield is the escape of reactants as vapors without the formation of products. These vapors are trapped in a specially designed vertical condensation chamber where the evaporated vapors are brought back into the reaction mixture as a condensate. This increases the product yield of a chemical reaction (mostly organic), and the wastage of reactants is efficiently reduced.
A magnetic stirrer can be implied if the reactant mixture is not too viscous. In that case, a magnetic plate is used instead of a simple burner which provides the magnetic field for the spinning of the magnetic stirrer.
Experimental setup for a Reflux
In the experimental setup of a reflux mechanism, the most commonly used apparatus consists of;
- Round bottom flask
- A condenser having a water inlet and an outlet
- Stopper
- Bunsen burner
- Stand
The flask used here has a rounded bottom in order to supply equal heat to the complete reaction mixture. The heat is provided by a burner, a hot magnetic plate, or a heating mantle. When the vapors go to the condenser, water flowing on the sides of these vapors condenses them down into the reaction mixture again and again. A stopper is implied on the upper end of this vapor tube to stop the vapors from escaping.
A heating mantle is generally preferred in organic reactions because direct heating of volatile organic reactants can be fatal.
Reflux ring
The reflux ring is a region separating the condensed vapors (liquid) and the evaporated vapors (gas) phases. It is a clear ring (mostly white) that forms in the condenser which may change with the changing reactants and products. Its level in the condensation tube is increased with an increase in temperature and decreases with cooling.
Additional topics
- Steady-state and equilibrium
- How to Find Heat of Reaction
- Difference Between Heat and Thermal Energy
Applications of Reflux mechanism
The most important application of heating under reflux is the increase in yield of a chemical reaction. The reactants are contained (enclosed) and therefore are not wasted by evaporation because most of the reactions implied in reflux have volatile reactants.
Reaction studies are accompanied by reflux for the kinetic and thermodynamic studies.
This mechanism is mostly used in organic synthesis.
Organic synthesis
The production of organic compounds in a laboratory or on an industrial scale is termed organic synthesis. The most common organic synthesis reactions using reflux are as follows:
- Esters formation
- Aldehydes formation
- Alkyl halides formation
- Acyl halides formation
- Denaturation of alcohols to prepare alkenes
- All reactions using the Grignard reagents
- Complexes formation from proteins (pharmacology)
- Azo dyes formation
- Retro synthesis
- To prepare ethanoic acid from ethanol
- Reactions where the solvent (medium) amount is meant to be kept constant especially if it is volatile.
Concepts Berg
What is the purpose of heating under reflux?
The main and the foremost purpose of heating under reflux is to increase the yield of an organic chemical reaction.
Why do you need to heat under reflux?
Heating under reflux is needed to keep the volatile reactants from escaping and to make more and more products.
What does heat do in an organic reaction?
In organic reactions, heat generally increases the rate of reactions. When the temperature is increased, the kinetic energy of molecules is increased which eventually leads to more collisions and thus, more products.
Why must the water be on in the condenser while refluxing this reaction?
Water must continually keep flowing in a reflux mechanism because the heat energy from vapors needs to be absorbed by this water which then makes them condense. Otherwise, no reflux reaction is seen.
When do we heat under reflux?
We heat under reflux to increase the products and react more and more reactants with each other.
Why is refluxing necessary?
Refluxing is necessary to insure product formation especially when reactions occur at a temperature near the boiling points of reactants.
What does boiling under reflux mean?
Boiling under reflux primarily means the regular vaporization-condensation occurring to make products near or after the boiling points of reactants.
How do you boil under reflux?
Boiling is done in reflux by the heat supplied by a burner or a magnetic hot plate (heating mantle).
Do you need a heating mantle for acid reflux?
The heating mantle is primarily used for acid reflux because heating some acids directly causes vigorous burning. For example, sulfuric acid.
What is the purpose of reflux in distillation?
The purpose of reflux in distillation is Non-reflux in which the product formed in vapor form is distilled into another chamber.
How do you heat a solution under reflux?
A solution under reflux is heated via;
- A hot plate
- Heating mantle
- Sometimes, a burner
At what temperature does toluene reflux?
Toluene refluxes at almost 60-80 °C.
How to obtain cumene (isopropyl benzene) from benzene?
When benzene reacts with propylene in the presence of anhydrous aluminum chloride in reflux, it forms cumene.
How to convert the ethanol to ethanoic acid?
Ethanol is treated under reflux with a strong oxidizing agent such as K2Cr2O7. An intermediate i.e. acetaldehydes are formed, which then oxidize to ethanoic acid.
How to change CH3COOH to ClCH2COOH?
Acetic acid is converted into chloroacetic acid by reaction with PCl3 at high temperature, and under reflux.
How to convert butanoic acid to propanoic acid?
Butanoic acid is converted to propanoic acid by reacting it with NaOH, and then CaO under reflux.
What is the product of a reaction between ethylene and hypochlorous acid?
The product of ethylene and hypochlorous acid is 2-chloroethanal and it is done under reflux.
How to convert propanoic acid to propane?
The reaction of propanoic acid with a reducing agent (LiAlH4) under reflux changes it to propane.
How to convert ethene to propanoic acid?
Ethene can be reacted with ozone to form formaldehyde which when reacted with Grignard reagent, forms 1-propanol. This 1-propanol upon oxidation forms propanoic acid. All these reactions are carried out under reflux.
References
- Vogel’s textbook of Quantitative Chemical Analysis – Fifth Edition By J.Mendham and J. D. Barnes
- The interactive lab primer – heating under reflux (Royal Society of Chemistry)
- What is Reflux? (University of Toronto, Scarborough, Canada)
