Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up. Many organic reactions are unreasonably slow and can take an extended period of time to achieve any noticeable effect so heating is often used to increase the rate of reaction.
However, many organic compounds have low boiling points and will vaporise upon exposure to such high heat, preventing the reaction from proceeding in full. To address this, heating under reflux is used. This refers to heating a solution with an attached condenser to prevent reagents from escaping. As seen above, any vapor will condense on the cool surface of the attached condenser and flow back into the flask. The hot water bath pictured is an optional component of heating under reflux and is usually only used for particularly sensitive reactions.
Also, using it limits the reaction temperature to degrees Celsius.Lumbini park images
Temperature control is important for reactions, especially in organic chemistry. Some reactions are strongly exothermic or have notable side-reactions that can be suppressed at a low temperature.
Thus, increasing the temperature is often favourable. Almost all organic reaction are conducted in a solvent. The choice of solvent dictates the temperature range you can reach; e.
Often, a published reaction will have a set of conditions most likely to work; they typically come with a preferred solvent and a preferred temperature. When heated to the boiling point, the solvent will partially evaporate and recondense on colder surfaces. But since also the concentration of reactants is important, one typically wants to recollect the evaporating solvent. This is where heating under reflux comes into play.
It is important to connect the cooling water circuit correctly. For some reason, most images found on the internet, including the one in the other answer, suggest a suboptimal cooling. The most optimal cooling efficiency is given in a countercurrent setup.Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up. I had a quick query regarding the setup of a reaction.
The procedure that I am working from states that I need to put my reaction mixture under reflux for 3 hours. How do you determine the temperature needed? Do I go via the boiling point of diethyl ether or THF? If more details are needed regarding the reaction I am carrying out then please let me know. But my query leans more towards the determination of mixed solvent systems in general and how one would determine the reflux temperature needed. The absolutely correct answer would be to look at a phase diagram, determine whether you have a positive or negative azeotope or non at allfind out where on the phase diagram you are and then derive the temperature.
That would tell you approximately at what temperature your mixture will boil — after you factored in boiling point increase due to dissolved compounds. In practice, you need to set your oil bath to some 5 or 10 degrees higher than the boiling point of the lower-boiling solvent anyway.
So you can just ballpark it, selecting a temperature that would keep the lower-boiling solvent refluxing. Come back after 15 minutes, if it is not refluxing, increase by 5 or 10 degrees. Sign up to join this community.
The best answers are voted up and rise to the top. What is the temperature needed for a reflux using a mix of solvents as the reaction medium? Asked 4 months ago. Active 4 months ago. Viewed times. Improve this question. Mithoron 4, 12 12 gold badges 34 34 silver badges 52 52 bronze badges. Jamie Jamie 39 1 1 bronze badge.
This gives a simple self-calibrating way to get the right temperature where you don't even need to measure the temperature.A simple reflux apparatus is shown in Figure 1 on the right. Place a heating mantle on the stirrer.
Insert a round bottom flask distilling flask into the heating mantle and clamp the neck of the flask to the ring stand.
Be sure to position the flask over the center of the stirrer plate. Measure out the reactants and solvent that will be used in the reaction and add them to the reaction flask, using a funnel. Add a magnetic stir bar to the flask and continue to set up the reflux apparatus as shown in Figure 1.
Insert a condenser tube into the flask and clamp at the top joint of the condenser, if necessary, to stabilize the apparatus. Connect the heating mantle to the Variac, and set the Variac at approximately 50V. Check your apparatus against the diagram in Figure 1. Carefully turn the water on, turn on the magnetic stirrer and the Variac heat. As the apparatus is heated, the liquid in the reaction flask will begin to bubble. The actual refluxing will begin when the solvent volatilizes and moves up into the condenser tube where it experiences a lower temperature.
As the vapor rises through the condenser which is cold from the water running through it the solvent will condense back to the liquid phase and drip into the reaction flask.A reflux setup Figure 1. A reflux setup is analogous to a distillation, with the main difference being the vertical placement of the condenser. The liquid remains at the boiling point of the solvent or solution during active reflux. A reflux apparatus allows for facile heating of a solution, but without the loss of solvent that would result from heating in an open vessel.
In a reflux setup, solvent vapors are trapped by the condenser, and the concentration of reactants remains constant throughout the process. The main purpose of refluxing a solution is to heat a solution in a controlled manner at a constant temperature.
But often the boiling point of the solvent is chosen as the reaction temperature because of its practicality. Figure 1. Do not turn off the water flowing through the condenser until the solution is only warm to the touch.
After a few minutes of air cooling, the round bottomed flask can be immersed in a tap water bath to accelerate the cooling process Figure 1. Attach the condenser, and connect the hoses so that water travels against gravity cooling water comes into the bottom and drains out the top.
Be sure there is a secure connection between the round bottomed flask and condenser, as vapors escaping this joint have the potential to catch on fire. Circulate water through the condenser, then begin heating the flask by using a heating mantle, sand, water, or oil bath. Use an adjustable platform so the heat can be lowered and removed at the end of the reflux, or if something unexpected occurs.
After air cooling somewhat, the flask can be quickly cooled by immersing in a container of tap water.Goodman fielder contact
Table 1. Complete text is available online. Overview of Reflux A reflux setup Figure 1. Step-by-Step Procedures Figure 1. Pour the solution to be refluxed into a round bottomed flask, and clamp it to the ring stand or latticework with an extension clamp Figure 1.
The flask should be no more than half full. Add a stir bar or few boiling stones for bump prevention. Boiling stones should not be used when refluxing concentrated solutions of sulfuric or phosphoric acid, as they will colorize the solution.Argumentacion persuasiva concepto
For example, when a stir bar is used for bump prevention with concentrated sulfuric acid, the solution remains colorless Figure 1. When the same reaction is conducted using a boiling stone, the solution darkens during heating Figure 1. Place rubber hoses on a condenser wet the ends first to allow them to slide onthen attach the condenser vertically to the round bottomed flask. If using a tall condenser, clamp the condenser to the ring stand or latticework Figure 1.
Be sure the condenser fits snugly into the flask. Safety note: if the pieces are not properly connected and flammable vapors escape, they may be ignited by the heat source. Do not connect the round bottomed flask and condenser with a plastic clip, as shown in Figure 1.
Plastic clips can sometimes fail especially when they are heatedand this setup does not allow for the flask to be reliably removed from the heat source at the end of the reflux. Connect the hose on the lower arm of the condenser to the water faucet and allow the hose on the upper arm to drain to the sink Figure 1. It is important that water goes in the bottom of the condenser and out the top so water flows against gravity or else the condenser will be ineffective as it will not fill completely.
To accomplish this, the upper arm of "Setup A" which normally drains to the sink is instead connected to the lower arm of "Setup B. Connecting apparatuses in series minimizes the use of water, as water exiting one condenser enters the next. Several reflux setups can be connected in series, and the water flow should be monitored to ensure that all setups are adequately cooled. Begin circulating a steady stream of water through the hoses not so strong that the hose flops around from the high water pressure.
Check again that the pieces of glassware securely fit together, then position the heat source under the flask.Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial  and laboratory  distillations.
It is also used in chemistry to supply energy to reactions over a long period of time. The term reflux    is very widely used in industries that utilize large-scale distillation columns and fractionators such as petroleum refineriespetrochemical and chemical plantsand natural gas processing plants.
In that context, reflux refers to the portion of the overhead liquid product from a distillation column or fractionator that is returned to the upper part of the column as shown in the schematic diagram of a typical industrial distillation column. Inside the column, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation column.
The more reflux provided for a given number of theoretical platesthe better is the column's separation of lower boiling materials from higher boiling materials. Conversely, for a given desired separation, the more reflux is provided, the fewer theoretical plates are required. A mixture of reactants and solvent is placed in a suitable vessel, such as a round bottom flask. This vessel is connected to a water-cooled condenserwhich is typically open to the atmosphere at the top.
The reaction vessel is heated in order to boil the reaction mixture; vapours produced from the mixture are condensed by the condenser, and return to the vessel through gravity. The purpose is to thermally accelerate the reaction by conducting it at an elevated, controlled temperature i.
The diagram shows a typical reflux apparatus. It includes a water bath to indirectly heat the mixture. As many solvents used are flammabledirect heating with a Bunsen burner is not generally suitable, and alternatives such as a water bath, oil bathsand bathelectric hot plate or heating mantle are employed.
What Is the Purpose of Reflux in Chemistry?
The apparatus shown in the diagram represents a batch distillation as opposed to a continuous distillation. The liquid feed mixture to be distilled is placed into the round-bottomed flask along with a few anti-bumping granulesand the fractionating column is fitted into the top.
As the mixture is heated and boils, vapor rises up the column.
The vapor condenses on the glass platforms known as plates or trays inside the column and runs back down into the liquid below, thereby refluxing the upflowing distillate vapor. The hottest tray is at the bottom of the column and the coolest tray is at the top. At steady state conditions, the vapor and liquid on each tray is at equilibrium.
Only the most volatile of the vapors stays in gaseous form all the way to the top.3513 percy st
The vapor at the top of the column then passes into the condenserwhere it cools until it condenses into a liquid. The separation can be enhanced with the addition of more trays to a practical limitation of heat, flow, etc. The process continues until all the most volatile components in the liquid feed boil out of the mixture. This point can be recognized by the rise in temperature shown on the thermometer.We received all of the materials we needed and our travel consultant marked our hotels and train stations on maps to assist us in new cities.
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