Organic Synthesis And Equilibria Experiments Flashcards
What are some considerations when deciding the synthetic method?
- Cost and availability of the reactants
- Product yield
- Ease of separating product from excess reagents or by-products
- simplicity of experimental setup.
What steps do an organic synthesis experiment typically entail?
- Reaction
- Separation (of products from by products/excess reagents)
- Purification
- Identification
What are the general experimental techniques associated with the synthesis of solid vs liquid org compounds?
SOLID organic product
1. Heat under reflux
2. Filtration
3. Recrystallisation
4. Melting pt determination to identify product
LIQUID:
1+2: (a) heat under reflux — when your BP of product higher than BP of reactant
(B) heat w immediate distillation: when product undergoes further reaction upon heating
(C): heat with distillation — when your BP of product lower than BP of reactant, best to faster distillate over
Why is heating under reflux used for organic synthesis experiments?
Organic synthesis expts usually involve organic compounds and solvents which have low boiling pt, yet reaction requires heating to a temperature that is higher than their boiling point. HUR needed to prevent the loss of reagents/solvents during prolonged heating
How do you draw a heating under reflux diagram? What are the key components to include and what are its purposes?
Note: rmb to draw adaptors
Dropping funnel (to introduce liquids dropwise)
Round bottom flask (to ensure even heat distribution during the reflux process
Liebeg condenser — a straight glass tube surrounded by a water jacket.
—> How it works: Water is continuously circulated into the water jacket from the lower end and carries away the heat of vaporisation released from the condensing vapour. Condensed liquids will return to the reaction mixture.
[++] top of the condenser is open to air so as to prevent pressure buildup in the system
-Boiling chips to ensure even boiling, magnetic stirrer to ensure reaction mixture is homogenous
-Oil bath because many organic reactions require temps higher than 100 degree celcius
-Heating plate/heating mantle/heating block because direct heating via Bunsen burner not possible (reactants/solvents are very flammable)
Why use heating under reflux? What is the temperature maintained at and how do you choose a solvent?
(When boiling point of product > reactants/solvents so you don’t have to worry about it evaporating off or anything)
Heating under reflux is often used for org synthesis because reactants and solvents in org synthesis reactions have very low boiling points and the reaction requires heating to temperatures high than these boiling points. HUR enables reaction to take place in a reaction flask WITHOUT LOSS OF ITS CONTENTS DUE TO EVAPORATION.
HUR reflux system, temp is constant at the BP of the solvent. Solvent is determined by the required temperature of the reaction and solubility of reactants in the solvent.
Why use heating under reflux? What is the temperature maintained at and how do you choose a solvent?
(When boiling point of product > reactants/solvents so you don’t have to worry about it evaporating off or anything)
Heating under reflux is often used for org synthesis because reactants and solvents in org synthesis reactions have very low boiling points and the reaction requires heating to temperatures high than these boiling points. HUR enables reaction to take place in a reaction flask WITHOUT LOSS OF ITS CONTENTS DUE TO EVAPORATION.
HUR reflux system, temp is constant at the BP of the solvent. Solvent is determined by the required temperature of the reaction and solubility of reactants in the solvent.
What is usually the objective of a solubility equilibria experiment and how do you go about doing it? (Briefly)
Objective is usually to find solubility of the salt (ie concentration of the salt in a saturated solution), and or Ksp of the salt
Ways to do so involve making a saturated solution and then filtering it to obtain filtrate and residue
1. Iodometric titration to find the concentration of the oxidising agent (in the redox reaction which occurs during titration)
2. Gravimetric analysis to find the mass of solid sample remaining (using the residue), and thus the mass of solid dissolved in the saturated solution.
How do you prepare a saturated solution (for solubility equilibria?)
1.Add a known mass of solid sample into a conical flask containing a known volume of deionised water.
2. Place reaction mixture in a thermostatic water bath to ensure constant temperature (as Ksp varies w temp).
3. Stir the mixture continuously till no more salt dissolves.
The mixture is then left to stand for sufficient periods of time to establish equilibrium
What are some precautions to take note of when filtering a saturated solution?
Ensure conical flask and filter paper is clean and dry. If there is water on it, the filtrate may be diluted. When a portion of the filtrate undergoes titration, requires lower amount of titrant to react with it, affecting accuracy of the titration experiment in determining the equilibrium concentrations of the dissolved ions of the salt
Emphasis on the “portion of the filtrate”! Because of dilution, there is less moles of dissolved ions in the portion extracted from the reaction mixture
How to use gravimetric analysis to determine solubility?
- Analyse residue via GA.
- Drying the filter paper and the residue in an oven followed by cooling in a dessicator.
- Weigh the mass of the filter paper and residue
Repeat heat cool reweigh process till 3 consistent mass readings are obtained.
Subtract the mass of the solid residue away from initial mass used to determine the mass of salt that actually dissolved
When is recrystallisation method used? What are separation methods used before and after it?
Recrystallisation is used to purify and obtain a solid organic product from impure solid
You may end up using hot gravity filtration to remove impurities that are insoluble in the chosen hot organic solvent BEFORE you start recrystallisation.
Can obtain pure crystals via vacuum filtration (obtaining solid product from solid-liquid mixture)
Describe hot gravity filtration and precautions taken
Hot gravity filtration is used to remove insoluble impurities from solid organic product.
- For organic synthesis experiments which requires dry conditions, the impure solid organic product is washed with an appropriate solvent that is not water.
- Fluted filter paper is needed as it ensures rapid filtration and prevents the loss of liquid product. Hot liquid product may form crystals on the cooler filter paper if filtration proceeds slowly.
Describe how recrystallisation works to obtain pure and dry solid organic product. How is organic solvent chosen?
- The impure organic solid is first dissolved in Minimum quantities of hot organic solvent which is added gradually in small volumes, forming a hot saturated solution.
- [choosing of organic solvent] The solvent is chosen such that impure organic solid is soluble in the solvent at high temperatures and insoluble or sparingly soluble at low temperatures. Impurities should be very soluble in the solvent (if impurities are insoluble, hot gravity filtration can be carried out prior to recrystallisation)
- The hot saturated solution is allowed to cool at room temperature or in an ice water bath, allowing the crystallisation of the pure solid product. Impurities remain dissolved in the solvent.
- Purified crystals can be obtained from the solid-liquid mixture via vacuum filtration. A miminal amount of cold solvent can be used to wash the crystals, which is then left to dry by turning on the vacuum pump for a few minutes.
- A dessicator with a drying agent is then used to dry the crystals. If the crystals are stable to heat, an infrared radiation lamp can also be used.
Describe vacuum filtration in detail and explain the use of the cold trap in the vacuum pump
