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Z A Level Chemistry 2 > Organic Synthesis 2 > Flashcards

Organic Synthesis 2 Flashcards

1
Q

How do you prepare an organic solid?

A
  • There are a number of experiments that you may carry out to prepare an organic solid
  • In both the preparation of aspirin and benzoic acid by alkaline hydrolysis a solid forms which has been separated from the reaction mixture before it can be purified
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2
Q

Describe the preparation of aspirin

A

You reflux salicylic acid, ethanoic anhydride and glacial ethanoic acid using Quickfit apparatus

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3
Q

Describe the preparation of benzoic acid by alkaline hydrolysis

A

You use qucikfit apparatus to reflux methyl benzoate with aqueous sodium hydroxide, followed by a acidification

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4
Q

Why is filtration under reduced pressure used?

A

Filtration under reduced pressure is a technique for separating a solid product from a solvent liquid reaction mixture

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5
Q

What do you need to carry out filtration under reduced pressure?

A
  1. Buchner flask
  2. Buchner funnel
  3. Pressure tubing
  4. Filter paper
  5. Access to filter or vacuum pump
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6
Q

How do you carry out filtration under reduced pressure?

A
  1. Connect one end of the pressure tubing to the vacuum outlet or to the filter pump whilst attaching the other end of the rubber tubing to the Buchner flask
  2. Fit the Buchner funnel to the Buncher flask ensuring that there is a good tight fit. This is usually obtained using a Buchner ring or a rubber bung
  3. Switch on the vacuum pump, or the tap, to which your filter pump is attached
  4. Check for good suction by placing your hand across the top of the funnel
  5. Place a piece of filter paper inside the Buchner funnel and wet this with the same solvent used in preparing your solid. You should see the paper being sucked down against the holes in the funnel
  6. To filter your sample, slowly pour the reaction mixture from a beaker into the centre of the filter paper
  7. Rinse out the beaker with the solvent so that all of the solid crystals collect in the Buchner funnel
  8. Rinse the crystals in the Buchner funnel with more solvent and leave them under suction for a few minutes so that the crystals start to dry
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7
Q

Why is recrystallisation used?

A
  • The solid product obtained after filtration will contain impurities which can be removed by carrying out recrystallisation
  • Purification by recrystallisation depends upon the desired product and the impurities having different solubilities in the chosen solvent
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8
Q

How do you carry out recrystallisation?

A
  1. Pour a quantity of the chosen solvent into a conical flask. If the solvent is flammable warm the solvent over a water bath and if the solvent is water, place the conical flask on a tripod and gauze over a Bunsen and warm the solvent
  2. Tip the impure sample into a second conical flask or beaker
  3. Slowly add the solvent to the impure sample until it dissolves in the solvent. You should ass the minimum volume of solvent needed to dissolve the solid
  4. Once the solid has dissolved, allow the solution to cool. Crystals of the desired product should form in the conical flask or beaker. When no more crystals form, filter the crystals under reduced pressure to obtain the dry crystalline solid
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9
Q

Why is melting point determination used?

A

-Chemists determine the meting point of solids to identify where a solid compound is pure

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10
Q

Describe meeting point determination

A
  • A pure organic substance usually has a very sharp melting range of one or two degrees
  • If the compound contains impurities the solid melts over a wide range of temperatures
  • An impure sample also has a lower melting point than pure sample
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11
Q

What is the melting range?

A

The difference between the temperature at which the sample starts to melt and the temperature at which melting is complete

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12
Q

How do you carry out melting point determination?

A
  1. Before taking the melting point of a solid you should ensure that the sample is completely dry and free flowing
  2. Take a glass capillary tube or melting point tube. Hold one end of the capillary tube in the hot flame of a Bunsen burner. Rotate the tube in the flame until the end of the tube is sealed
  3. The capillary tube is allowed to cool, and is then filled with crystals to about 3mm depth. This is usually carried out by pushing the open end of the capillary into the solid sample to force some of the solid into the tube
  4. Once you have prepared your sample you will need to take its melting point. In schools and colleges, one of the two methods will be available depending on the apparatus available
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13
Q

How do you carry out melting point determination using an electrically heated melting point apparatus?

A
  1. Place the capillary tube containing the sample into a sample hole and a 0-300 degrees C thermometer in the thermometer hole of the melting point apparatus
  2. Using the rapid heating setting, start to heat up the sample whilst observing the sample through the magnifying window
  3. Once the solid is seen to melt, record the melting point. Allow the melting point apparatus to cool
  4. Prepare a seance sample in a new capillary tube and apace in the melting point apparatus and agin heat up the sample
  5. As the melting point is approached, set to low and raise the temperature slowly whilst observing the sample. An accurate determination of the melting point can then be obtained
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14
Q

How do you carry out melting point determination using an oil bath or Thiele tube method?

A
  1. Set up the Thiele tube or oil bath
  2. Attache the capillary tube containing the sample to a thermometer using a rubber band
  3. Insert the thermometer through a hole in the cork if using a Thiele tube or clamp the thermometer if using an oil bath. The end of the thermometer and the end of the capillary tube should dip into the oil
  4. Using a micro-burner slowly heat the side arm of the Theile tube or the oil bath whilst observing the solid. When the solid starts to melt, remove the heat and record the temerpature at which all of the solid has melted. It is important to heat the oil slowly when approaching the melting point, and it is advisable to repeat the the melting point determination a second time to ensure that you obtain an accurate value
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15
Q

How do you recrystalise?

A

-If the product of an organic reaction is a solid, then the simplest way of purifying it is recrystallisation
-First you dissolve your solid in a solvent to make a saturated solution, then you let it Coll and as the solution cools the solubility of the product falls and when it reaches the point where it cannot stay I solution it starts to form crystals
1. Very hot solvent added to impure solid until it just dissolved and it’s important not to add too much solvent
2. This should give a saturated solution of the impure product
3. This solution is left to cool down slowly and crystal of the product form as it cools. The impurities stay is solution and they are present in much smaller amounts than the product is they would take much long to crystallise out
4, The crystals are removed by filtration under educed pressure and washed with ice cold solvent and then they are directly leaving you with crystals of your product that a are much purer than the original solid
-In a saturated solution the maximum possible amount of solid is dissolved in the solvent

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16
Q

Why do reactions need to be heated?

A
  1. Organic reactions are slow and the substances are usually flammable and volatile (they have got low boiling points) and if you stick them in a beaker and heat them with a Bunsen Bruner and they will evaporate or catch fire and so reflux to get around this
  2. The mixture is heated in a flask fitted with a vertical Liebig condenser, is when the mixture boils, the vapours are condensed and recycled back into the flask and this stops reagents being lost from the flaks and given them time to react
17
Q

What is a problem with refluxing?

A
  • Can cause desired product to react further and if this isa the case you can carry out the reaction in a distillation apparatus instead
  • The mixture is gently heated and substances evaporate out of the in order of increasing boiling point
  • If you know the bp of you of your pure product you can use the thermometer to tell you when it is evaporating and therefore when it is condensing
  • If the product of a reaction has a lower bp than the starting material then the reaction mixture can be heated so that the product evaporates from the reaction mixture as it forms. The starting material will stay in the reaction mixture as long as the temperature is controlled
18
Q

Why is the choice of solvent for recrystallisation very important?

A
  1. When you recrystallise a product, you must use an appropriate solvent for that particular substance and it will only work if the solid is very soluble in the hot solvent but nearly insoluble when the solvent is cold
  2. If your product is not soluble enough in the hot solvent you won’t be able to dissolve it at all
  3. If your product is too soluble in the cold solvent, most of it will stay in the solution even after cooling and when you filter it, you’ll lose most of your product giving you a. very low yield
19
Q

How can you filter an organic solid?

A
  • Under reduced pressure
    1. If your product is a solid you can separate it from any liquid impurities by filtering it under reduced pressure
    2. The reaction mixture is poured into a Büchner funnel with a piece of filter paper in it. The Büchner funnel is on top of a sealed side arm flask which is connected to a vacuum line causing it to be under reduced pressure
    3. The reduced pressure caused suction through th funnel which causes the liquid to pass quickly into the flask leaving being dry crystals of your product
20
Q

How are melting and boiling points good indicators of purity?

A

-Most pure substances have a specific melting and boiling point and if they are impure, the melting point’s lowered and the boiling pout is raised and if they are impure, melting and boiling will occur across a wide range of temperature

21
Q

How do you accurately measure the melting point?

A
  1. Put a small amount of the solid in a capillary tube and place it in a beaker of oil with a very sensitive thermometer
  2. Slowly heat with constant stirring until the solid just melts and read the temperature on the thermometer
    - Once you have measured the mp of your sample, you can compare it to the known melting point of the substance to determine its purity
    - If the melting points are similar then your sample is quite pure but if your value is much lower than the standard value, your sample contains impurities

Z A Level Chemistry 2 (17 decks)

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