Lecture 2 - Melting Ranges, Reaction Workups And Distilation Flashcards
Melting point range
Pure covelant organic compounds have definite melting ranges
If compound is not pure the melting range will increase significantly and the upper end of the melting range will be lowered
Most common error in melting point determination is heating the sample too quickly
Decomposition may occur during the melting point determination - do not re-melt samples as decomposed material is not the same as beginning sample
If more than 2 *C melting range - impurities are present
Stages of melting range
1 - solid only
2- first liquid drops
3 - sample begins to melt
4 - meniscus formed
5 - entirely liquid
Steps 3-5 is the melting range
Use a small sample as this increases accuracy and decrease time taken to melt
Mixed melting point
- Take unknown solid and make 1:1 mixture of smaple that may be your compound
- Take melting range of mixture
- If mixture melts at a lower temp or over a broader range it is not the same - try a different compound
- If it melts at the same temp range as unknown, it is most likely the same. Repeat with different ratio of known : unknown to confirm
(Impurities will cause a suppression on the mp and increase melting range)
Melting range
Melting range and upper limit give an indication of sample purity
Compare with literature reference may help to confirm identity of compound
Mixed melting point can confirm identity with reasonable certainty
Reaction work up and extraction
- reaction workups (series of manipulations required to isolate and purify the product(s) of a chemical reaction), quenching reactions, washing /extraction
Must stop reaction as soon as endpoint has occurred - to reduce production of side products and decomposition of product
Quenching
Stopping a reaction and deactivating any reactive reagents
Stops reaction asap - avoid side reactions or decomposition of product
Makes reaction safe to handle
Follow experimental / literature procedure exactly
Choice of quench depends on the reagents used and product stability
Choosing the quenching agent
Done by Follow literature procedure exactly, Cool heated reactions to room temp, Cool reactions that will be exothermic upon quenching, Quench inert reaction before exposing to air, Ensure product is stable in quench conditions
Type of reaction and quench technique:
Strongly basic non-aqueous (e.g. BuLi, LDA)
Cool to 0°C and add ammonium chloride (NH4CI) solution
Strongly acidic non-aqueous (e.g. TiCI4, BF3.OEt2)
Cool to 0°C and add sodium bicarbonate or sodium carbonate solution (weak base)
Neutral
water
Slightly acidic
Water or sodium bicarbonate solution
Slightly basic
Water or ammonium chloride solution (Weak acid)
Liquid - liquid extraction concept
separation of compounds based on their relative solubility in 2 diff immiscible liquids, usually aqueous and organic solvents
Also known as: washing, extraction, partitioning
Product can be extracted into aq or organic phase
Follow experimental/literature procedure exactly
Liquid - liquid extraction - slide 15
Requirements
- separating funnel and stopper
- funnel
- ring
- safety beaker / conical flask
Tips:
Remove any particulate matter before extracting
Remove any water miscible solvent (eg THF, MeOH, EtOH, acetonitrile, acetone) and redissolve in a suitable solvent
Use low bp solvent if possible eg dichloromethane, diethyl ether, ethyl acetate etc (more dense than water therefore will be lower layer)
Make sure tap is shut and you are able to twist it - put a beaker underneath just in case
Acid - base extraction
Covelant molecules are converted into their salts so can be removed from non-polar solvent into water.
Covelant species will remain dissolved in the non-polar solvent
Eg a mixture of ArCOOH and RH - first is acid, second is neutral, both insoluble in H2O but soluble in CH2CL2
Addition of NaOH:
Mixture of ArCOO-Na+ (now soluble in H2O and insoluble in dicholoromethane (CH2Cl2) so is in top layer with water) and RH (still in bottom layer)
Example question: how would you isolate the pure benzoic acid in this reaction?
Ph-COO-CH3 —hydrolysis—> Ph-COOH + MeOH
1- evaporate the MeOH off ass solvable in water so need to get rid of it before partitioning in funnel
2- add aq base (sodium bicarbonate) and then organic solvent to separating funnel: Ph-COO-CH3 will stay in organic layer and Ph-COO- (ionised) will end up in the aq layer.
3- acidify and back extract
Extraction and back extraction: acid eg PH-COOH - diagram on slide 19
pKa of Ph-COOH is 4.2
Add Na2CO£ (aq) to form Ph-COO- which is aq soluble
Add HCl (aq) to reform into Ph-COOH
Want to move 2 pH values away from pKa value to he sure that you have either the protonated or deprotonated material
Extraction and back extraction: acid eg PH-OH - diagram on slide 20
Ph-OH has pKa value 10
Add strong base (NaOH) (aq)
Forms Ph-O- (aq. Soluble)
And HCl (aq)
Forms Ph-OH which is organic soluble
You always want to extract the material into an organic solvent because the solvent is volatile - can use a drying reagent to get rid of any residual water to separate that out and dry it, filter it then evaporating the organic solvent - leaving product pure
Extraction and back extraction: acid eg PH-NH2 - diagram on slide 21
Ph-NH2 has a pKAH value of 4.6
Add 10% HCL (aq)
Forms Ph-NH3+ which is aq soluble
Add NH4OH (aq) - deprotonates to extract material into organic layer
Forms Ph-NH2 which is organic soluble
Extraction tips
Always have safety flask underneath the separating funnel
Remove the stopper when you empty the funnel
Test pH of aq layer - not organic
If the organic compound is slightly soluble in water, try washing with saturated sodium chloride solution
If an emulsion forms (no distinct layers but a fog of particles)
- try adding acid, or base, or ethanol.
- stir with a glass rod or gravity filter through filter paper. Check product stability in acids/bases before you start
Monitor by TLC (To see how things have separated)
Several smaller extractions are better than one big one.
Extracting three times is usually enough
Partition coefficient (K)
Liquid - liquid extraction theory is based on the equilibrium of dissolved component in the two immiscible liquids. Also called the distribution coefficient.
K = ( conc of solute in liquid 1 ) / ( conc of solute in liquid 2 )
Drying agent examples: pros and cons
Anhydrous calcium chloride
Cheap and rapid.
Forms fine powder which can pass through filter paper.
Don’t use to dry alcohols.
Anhydrous sodium/potassium carbonate
Basic.
Avoid acidic products.
Anhydrous sodium sulfate
Neutral. High Capacity. Slow.
Loses water above 32°C
Anhydrous nagnesium sulfate (hexahydrate salt is formed)
Best all-round drying agent.
Large surface area for product to become trapped.
Don’t add too much
Drierite
Commercial.
Blue when dry, pink when wet
Rotary evaporator
Used for gentle and efficient removal of solvents by evaporation. Done under reduces pressure
Equipment:
Water bath temperature control
Pot
Speed control
Reservoir where solvent collects
Tube to vacuum
Stop clock
diagram on slide 27)
Distillation - slide 28
Purification of solvents and separation of mixtures of liquids
Equipment:
Stirrer bar in rb flask
Round bottomed flask attached to still head
Still head attach to thermometer adaptor which is attached to thermometer
Also attached to condenser
Condenser attached to receiver adaptor with a vent
Adapter attached to collecting vessel
Distillation at atmospheric pressure is good for separating products with boiling points at least 50 *C apart OR separating products from an involatile residue
Fractional distillation - slide 30
Used for deprecating compounds with close bp
Uses longer vigreux distillation column
Increases temp gradually and collect distillate slowly
Collects lots of small fractions rather than a few large ones
Collection vesselS in this case is attached to pig adapter (with tubing to vacuum pump) as it allows for 3/4 collecting vessels
Therefore don’t need to take apparaturs apart leading to loss of product etc
Distillation under high vacuum
Used to separate high bp liquids (>150 *C)
Purification of compounds which decompose when heated
Same appartus as fractional distillation
Boiling point / pressure chart (nomograph) - slide 34
Pressure in Torr = 1 mm Hg = 1.33 mbar
Gives you 3 scales: bp at reduced pressure ( *C), bp at 760 Torr ( *C) and pressure in Torr
Need to know 2 out of the 3 to work out the third
