Lab A: Synthesis of Salicylic Acid Flashcards
what are 4 uses of salicylic acid?
- as a keratolytic agent
- as an antibacterial agent
- as an anti-dandruff agent
- as a precursor molecule
draw the reaction pathway for the synthesis of salicylic acid
draw the transformation of functional groups
draw the reaction for base promoted hydrolysis of methyl salicylate
hydrolysis is base ___, NOT base ___. and explain why
Hydrolysis is base promoted, not base catalyzed
because the base (OH–) is the nucleophile that adds to the ester and forms part of the product. It PARTICIPATES in the reaction and is not regenerated later like a catalyst would be
what is the most common type of lipid?
triacylglycerol
draw the reaction and products of the hydrolysis of triacylglycerols
what are 4 facts about the hydrolysis of triacylglycerols?
- Each triacylglycerol is a triester containing three long hydrocarbon side chains (Typically, the R groups have 12-20 C’s) .
- Metabolism of a triacylglycerol: Cleavage of three ester bonds forms Glycerol and three long-chain fatty acids.
- Hydrolysis of a Lipid is the same as the Hydrolysis of an Ester Hydrolysis.
- This lipid hydrolysis is catalyzed by the enzyme lipases.
what is saponification?
base-promoted hydrolysis of triacylglycerols
theoretical yield
the yield of a reaction assuming 100% conversion of the limiting reactant to the product
limiting reagent
The reactant that is present in the smallest amount and would limit the amount of product possible
the moles of product (i.e. theoretical yield) is determined by the moles of “limiting reactant” on a molar equivalent basis
percent yield
% yield = actual yield / theoretical yield * 100
stoichiometry review: multistep synthesis (2 tips)
the overall percent yield is determined by multiplying the yield of each step
expressed as a decimal.
multiply the answer by 100 to convert final value back to % yield.
what is recrystallization? (Definition)
Convenient way to purify a solid organic compound by using a suitable solvent
an impure solid compound is COMPLETELY dissolved in a boiling solvent and then ALLOWED TO COOL SLOWLY to form beautiful pure crystals
- impurities not soluble at hot temp may be removed by hot gravity filtration
- impurities soluble at cold temp will remain in the bulk of the solution/filtrate
what is the technique/2 steps of recrystallization?
Step 1. Impure solid is dissolved in an appropriate hot solvent – the impure crystal lattice breaks down
Step 2. Slow cooling of the solution (first on bench top, undisturbed, and later, once substantial crystal formation is observed- cool in an ice-water bath)– allows for the formation of purer, larger crystals. This is because impurity molecules have sufficient time to return to the solution
why do you SLOWLY cool the solution for recrystallization and not RAPIDLY cool it?
slow cooling allows for the formation of purer and larger crystals since the impurity molecules have sufficient time to return to the solution
rapid cooling results in PRECIPITATION and the impurities are trapped due to the rapid formation of the lattice
- aka the impurities stay trapped by the solute molecules that are crystallizing as the solid lattice reforms
how does the “like dissolves like” concept relate to recrystallization?
the polarity of the solvent and the polarity of the compound being recrystallized should be similar
Q. What is the rule for selecting a good solvent for recrystallization? (Is it answer choice A or B?)
A. The solid you are trying to purify should be soluble in the hot solvent and insoluble in a cold or room temperature solvent.
B. The solid you are trying to purify should be insoluble in the hot solvent and soluble in a cold or room temperature solvent.
A. The solid you are trying to purify should be soluble in the hot solvent and insoluble in a cold or room temperature solvent
what is vacuum filtration? (definition)
a technique used to quickly and completely separate a solid from the liquid with which it is mixed
what are 4 technique points to keep in mind when doing a vacuum filtration?
Direction of Ring stand (front facing)
Vacuum filter flask is firmly clamped to prevent from tipping over
Uses an Adapter to ensure a tight seal between Buchner funnel and vacuum filter flask
Employs Heavy-walled rubber tubing to prevent the tubing from collapsing from atmospheric pressure on the outside when the vacuum is applied
what are the 2 sources of vacuum for vacuum filtration?
- water aspirator
- compressor-driven vacuum system
vacuum filtration: when washing the filtered crystals with solvent….
always use SMALL portions of solvent to avoid washing away small amounts of your product
vacuum filtration: to ensure efficient drying of the product, after the 1st round of filtration…
- empty the mother liquor from the vacuum filter flask and then connect the filter flask back to the vacuum line
- pull air only over the crystals inside the filter funnel
what are the steps for drying your product after vacuum filtration?
NOTE: your product is still wet even after vacuum filtration
1. weigh an empty watch glass
2. transfer the product to the water glass
3. weight the product and record this weight as WET mass in lab notebook
4. break down big clumps of product and leave to airdry in drawer until next lab
5. the following week: weight the DRY product and compare the WET and DRY mass of the product
what are 5 types of sample characterization?
Melting Point Range/ Boiling Point Range
Thin Layer Chromatography (TLC)
Column Chromatography
Nuclear Magnetic Resonance (NMR)
Spectroscopy
Infra-Red (IR) Spectroscopy
the presence of impurities will INCREASE or DECREASE the observed melting point range of a compound?
DECREASE: the impurities disrupt the lattice and makes it easier to melt
determination of a melting point range can be describes in 2 ways?
- solid phase changes to liquid phase
- the loss of order in crystalline lattice
always record the observed melting point as a ___
RANGE
what is the first reading of the melting point range?
the temp at which the solid block first begins to liquefy
what is the second reading of the melting point range?
temp at which the solid is completely melted
what are the advantages of a Digital MeltTemp?
- rate at which the temp of the heating block increases can be programmed
- you can input the Start temp, End temp, and Ramp Rate
what is the heating rate for the digital MeltTemp?
the heating rate is to be maintained at 2 degrees C per minute –> the sample has to be melted SLOWLY
why does the sample have to be melted SLOWLY to see the observed melting point?
- thermometer lag
- a broad range is observed when melting too quickly
thermometer lag
occurs when the sample melts before the thermometer has a chance to reach that temp
can samples be re-melted for the observed melting point?
NO!
Samples should never be re-melted
used melting point (MP) capillary tubes should be disposed where?
in the Red Sharps container
melting point capillary tubes (MP) should be ____ on one end
melting point capillary tubes (MP) should be CLOSED on one end
melting point range: how do you efficiently pack the sample into the capillary tube?
what should the sample height in capillary tube be?
tap the capillary tube on the bench inside the hood or let if fall through a long glass tube
- Digi-melt also has a “tube tapper” option to pack the MP capillary tubes more uniformly
sample height in capillary tube: approx. 2-3mm
Describe the steps for neutralization of “waste acidic filtrates”
- “Waste acidic filtrates” must be neutralized using NaHCO3 (aka baking soda) ONLY in the NEUTRALIZING HOOD
Procedure:
1. Transfer your filtrate from the Filter flask to a 250 mL Beaker. Label this beaker as “waste acidic filtrates”.
2. Carry your beaker with glass rod and a strip of pH paper to the Neutralizing Hood.
3. Place your beaker inside the bucket provided INSIDE the Neutralizing Hood.
4. Use the Bicarb beaker scoop provided to scoop Bicarb from the Bicarb bag into your Beaker.
5. Add Bicarb, Stir with the glass rod and Check pH.
6. Bring your beaker back to your hood and pour the neutralized solution into the sink adjacent to your hood.
7. Rinse your Beaker with tap water.
- place your beaker inside the plastic bucket and add a scoopful of NaHCO2, stir the solution and then test the pH by placing a drop of solution from the beaker onto the pH paper strip –> color: LIGHT GREEN (pH=7)
why do you use erlenmeyer flasks for reactions?
the SLOPED sides of the erlenmeyer flask help reduce the amount of solvent that evaporates and also reduces the loss of materials due to splashing during the reaction
when neutralizing a compound, what color should the pH strip show?
light green color –> pH = 7
what are the safety hazards for sodium hydroxide and hydrochloric acid?
both are corrosive
At what approximate temperature will the reaction mixture containing methyl salicylate and 30 mL of an
aqueous solution of 4M sodium hydroxide boil?
at 100 degrees Celcius since the mixture of methyl salicylate and NaOH is AQUEOUS
- the mixture may realistically boil at a few degrees higher though since the solute RAISES the boiling point
Why is it important to clamp your vacuum filter flask, even though it has a flat bottom?
it is important to clamp because the buchner funnel makes the set up TOP-HEAVY and more prone to breakage –> the clamp helps support the filter flask
Salicylic acid has some solubility in water. What would happen if you washed your crystals with too
much water, during vacuum filtration?
If you wash your crystals with too much water, you risk washing away actual product –> decreases yield (since salicylic acid is kind of soluble in water)
Why is it necessary to empty out the the filtrate from the vacuum filter flask, before pulling air over the
crystals being dried inside the Buchner funnel?
extra filtrate (liquid released at the bottom of the vacuum) will exert pressure on system –> to MAXIMIZE VACUUM PRESSURE, the filtrate should be emptied before pulling air over the crystals
Are large crystals more likely to form when solutions are cooled quickly or slowly? and Why?
large crystals are more likely to form when solutions are cooled SLOWLY since this prevents soluble IMPURITIES from being incorporated into the forming solid
what do you need to neutralize in this lab?
the “waste acidic filtrate” collected from the vacuum filtration
where will organic compounds be found in the lab? (aka which hood)
Organics Dispensing Hood
where will acids, bases, and other common organic reagents be found in the lab? (aka which hood)
Reagents Hood
on the hot plate, which color represents heat on and which color represents stir on?
heat on - orange (highest setting is 300)
stir on - green
what does the solution of methyl salicylate and sodium hydroxide look like before and after boiling?
before - cloudy white stage
after - clear solution stage
the solution starts boiling after it turns __ and ___
solution starts boiling after it turns CLEAR and is HOMOGENOUS
should you add HCl to the reaction flask while it is sitting on the hot plate or not? explain why
the addition of HCl should NOT be done with the reaction flask sitting on the hot plate
- although hot plate is unplugged, RESIDUAL heat from hot plate will continue to warm the flask –> adding HCl to flask on hot plate will cause the acidic content to spill over from flask and may spill onto your gloves and cause burns
should you stir your solution after each incremental addition of HCl?
YES!
do you want to pour out your content all at once inside the buchner funnel for vacuum filtration or not? explain why
YES! - you should POUR OUT ALL AT ONCE
if you do not add the solution all at once, you risk losing solid that will remain coated on the walls of the flask
what should IMPURE salicylic acid look like?
white powder
when recrystallizing salicylic acid, why do you not want to put the solution into an ice water bath immediately after taking it off the hot plate?
recrystallization is a SLOW COOLING process
what should PURE salicylic acid look like?
white, needle-shaped crystals
what are soaps chemically and how are they formed?
salts of long chain fatty acids
formed from base-promoted hydrolysis of triacylglycerol (H2O, NaOH)
salicylic acid boiling point?
158-161 degrees Celcius
Use Erlenmeyer flask ___ times larger than amount of solvent you will need
Use Erlenmeyer flask 2-3 times larger than amount of solvent you will need
Use ___ times the amount of solvent as compound being recrystallized
Use 20-40 times the amount of solvent as compound being recrystallized
hazards for NaOH, HCl, and methyl salicylate?
NaOH, HCl: toxic, corrosive
methyl salicylate: toxic
