Lab F: Fischer Esterification Flashcards
what is the general reaction formula for a fischer esterification reaction?
give two examples of the fischer esterification reaction
what is the general reaction formula for ester synthesis?
give 6 examples of esters
- isoamyl acetate / isopentyl acetate: banana
- methyl butyrate: apple
- n-propyl acetate: pear
- isobutyl propionate: rum
- ethyl butyrate: pineapple
- octyl acetate: oranges
give two fun facts about esters
- have pleasant FRUITY odors
- used as flavoring agents and in the perfume industry
what methods are involved in the preparation/synthesis of esters?
- reflux
- extractions
- simple distillation
what type of reaction is the synthesis of esters?
acid catalyzed reaction (aka the acid is regenerated and is never consumed)
what is impossible in the preparation of esters?
impossible to obtain a 100% yielf
what is the reaction scheme of this lab? (synthesis of isopentyl acetate: banana oil)
why is glacial acetic acid called glacial?
EXPAND
- excess reactant (economical)
- BP 118C
what are the health hazards for glacial acetic acid?
flammable: 2
health: 3
corrosive
what are the health hazards for sulfuric acid?
corrosive
health: 3
why do we use boiling stones in this lab/reaction?
the pores in the boiling stones…
1. help prevent solution from bumping too vigorously during reflux
2. helps maintain even boiling
draw the reflux schematic for fischer esterification
what is the purpose of the reflux?
to heat a reaction mixture at its boiling temp (the boiling point of the solvent, or in the case, excess reactant used in the reaction) to form the product without losing any material inside the reaction flask
what are the 2 advantages of a reflux?
- maintains a constant temp in the reaction flask
- reaction is able to reach equilibrium with minimal evaporation of material inside the reaction flask
draw the flowchart for fischer esterification reaction
what are 3 purification techniques?
- recrystallization
- thin layer chromatography
- extraction
what is the purification technique for liquids?
simple distillation
what is simple distillation used for?
used to separate liquids where the boiling points differ by more than 20-30C at 1 atm
what type of liquids should be used for simple distillation?
the two liquids must be MISCIBLE in each other
distillation definition
a technique in which a liquid is vaporized by heating to its boiling point, then re-condensed back to a liquid (called distillate), which is collected in a receiving flask
draw the schematic for simple distillation
if a volatile compound is being distilled what should you do?
if a volatile compound is being distilled, the receiving container can be chilled in an ice-water bath to minimize loss of material through vaporization
what are notes for simple distillation?
- if a volatile compound is being distilled, the receiving container can be chilled in an ice-water bath to minimize loss of material through vaporization
- do not distill to dryness! - leaving a small amount of residue will prevent overheating and breaking the flask and also prevent formation of pyrolytic tars that are difficult to wash out
- cover your distilling RB flask with foil at the start of distillation
- make sure the ring clamp holds the heating mantle flask and firmly against the round bottom flask
why do you not distill to dryness?
leaving a small amount of residue will prevent overheating and breaking the flask and also prevent formation of pyrolytic tars that are difficult to wash out
when do you cover your distilling RB flask with foil? why do you use foil?
- at the start of distillation
- foil insulates the RB flask to get it to boil quicker
what types of compounds is vacuum distillation used for?
used for compounds that either boil at too high a temp or decompose near their boiling points
what does under vacuum mean?
compounds can be distilled at temps lower than their atmospheric boiling points
- the only feasible solution for distilling compounds with BPs > 200C
what is a fractional distillation used for?
a fractional distillation is used to separate liquid mixtures where the different in boiling points is less than 25C at 1 atm
describe the setup for fractional distillation?
a fractionating column (aka vigreux column) is inserted between the 2-way distillation adaptor and the RB flask (distillation flask) to provide a larger SA over which a number of separate liquid-vapor equilibria can occur
how is the efficiency of a fractionating column expressed?
expressed by the number of theoretical plates
what is one theoretical plate equivalent to?
equivalent to one simple distillation cycle
what are the components of a fractional distillation setup?
- distillation
- water condenser: has CHWS (water in at bottom) and CHWR (water out at top)
- vigreux column (aka fractionating column): has indentations
- round bottom flask
what are 2 factors affecting boiling points?
- size (aka molecular weight)
- intermolecular interactions
what are the boiling points for pentane, hexane. and heptane? what is the trend?
pentane: 35-36C
hexane: 69C
heptane: 98C
–> higher molecular weight = higher boiling point
compare the boiling points of pentane (alkane), butyraldehyde (C=O group) , and butanol (OH group) and what makes them different (what is the trend?)
highest boiling point
1. butanol (OH) - hydrogen bonding
2. butyraldehyde (C=O) - dipole-dipole interactions
3. pentane (alkane) - London dispersion forces
bond strength increase = boiling point increase
hydrogen bonding > dipole-dipole interactions > london dispersion forces
what does it mean for a solution to boil?
vapor pressure = atmospheric pressure
write the Clausius-Clapeyron equation
define normal boiling point
the temp at which a liquid’s vapor pressure equals 1 atm
draw the vapor pressure vs temp of H2O graph
what is the vapor pressure of water at 25C, 50C, and 100C?
22mmHg at 25C
92mmHg at 50C
760mmHg (aka 1 atm) at 100C
1atm is equal to __ mmHg?
1 atm = 760 mmHg
why do you need to make sure your joints (aka blue keck clip) is tight for simple distillation?
loose joints will cause your product to escape!
when should you read the thermometer for simple distillation?
read the thermometer when the material starts COLLECTING, and NOT when the solution starts boiling
what should the position of the thermometer bulb be?
thermometer bud should be pointed DOWN (at the bottom)
- the bulb should be completely below the side arm of the 3-way distillation adaptor
what is the variac?
variac - a variable voltage transformer that provides a voltage-adjusted source of alternating current
a heating mantle is connected to what?
a heating mantle is connected to a VARIAC
where do you plug in the heating mantles?
in the variac outlets located below the fume hood
what is the starting setting for the variac?
start at 65
why do we not transfer the 2 boiling stones into the separatory funnel?
to prevent the sep funnel from clogging
can boiling stones be reused?
NO
what happens after you add cold water into the sep funnel with the reflux solution?
phase separation occurs between the bottom aq layer (containing H2O) and the top organic layer (containing glacial acetic acid)
glacial acetic acid density: 0.87g/mL
water density: 1.0g/mL
describe the color changes during extraction
when it is freshly from reflux: solution is dark purple
1. after adding cold water H2O –> top (yellow) bottom (purple)
2. after adding sodium bicarb NaHCO3 –> top (lighter, oilier yellow), bottom (clear)
3. after adding 25mL H2O + NaCl –> top (oily yellow), bottom (clear)
4. drain the final top layer with final product into 125mL erlen flask (oily yellow)
why is there fizzing after adding NaHCO3 to the sep funnel?
due to the formation of CO2 gas from the reaction from NaHCO3 + glacial acetic acid
what is the purpose of adding NaHCO3?
to neutralize any left over glacial acetic acid (to remove acetic acid)
why do you check the pH of the aqueous waste from beaker #2 (or after 2nd extraction)?
to make sure the solution has a neutral pH (pH around 5 to 6)
what is the purpose of adding 25mL of H2O for the 3rd extraction?
to remove any left over acid
what is the purpose of adding saturated NaCl (aka brine)?
to begin the drying process since brine is able to draw out large amounts of water that is mixed in with the organic layer
what is brine?
saturate NaCl
what are the components of a simple distillation setup?
3-way distillation adaptor
thermometer adaptor
vacuum adaptor
thermometer
blue keck clips (2)
labeled vial (to collect final product - banana oil)
boiling stones (2)
why do we need to carry out a simple distillation for this lab/reaction
because we are assuming that there is NONE or VERY LITTLE traces of the starting material isopentyl alcohol (it is small enough to make a simple distillation more beneficial compared to losing a large amount of isopentyl acetate in a fractional distillation)
what is the boiling points of isopentyl alcohol (starting material) and isopentyl acetate (product)?
- isopentyl acetate bp: 142C
- isopentyl alcohol bp: 132C
why do we use a simple distillation instead of fractional distillation to collect isopentyl acetate?
- isopentyl acetate bp: 142C
- isopentyl alcohol bp: 132C
the boiling point differences between isopentyl alcohol and isopentyl acetate is only 10C –> which is by definition, small enough to use a fractional distillation.
HOWEVER, because the bp difference is so small, if we were to use fractional distillation which involves a Vigreux column (inserting this column that has many indentations or theoretical plates) and using this column where each theoretical plate responds to a simple distillation cycle –> we would end use doing multiple simple distillations to the product ==> and ultimately LOSE A LOT OF PRODUCT (isopentyl acetate) to due column hold up and the vapors that are left in the system!
why do we start collecting isopentyl acetate when the thermometer reads 125-138C despite isopentyl acetate’s literature bp being at 142C?
because the thermometers we use in lab are not well calibrated at temps over 100C so it it collected at a lower temp range of 125-138C during simple distillation
what is a disadvantage of alcohol thermometers (the ones we use in lab)
they are not well calibrated at temps over 100C
how are the CHWS and CHWR positioned for simple distillation?
CHWS - at BOTTOM of incline
CHWR - at TOP of incline
describe where each of the arms for a 3-way distillation adaptor goes
side arm goes into condenser
bottom arm goes into RB flask neck/opening
top arm is attached to thermometer adaptor and has a thermometer stuck into it
describe the two ends of a thermometer adaptor and where each end goes
rubber end - for inserting the thermometer into
glass end - for fitting into the 3-way distillation adaptor’s top arm
what is the ring clamp used for for simple distillation setup?
the ring clamp is used to raise the height of the heating mantle (where the heating mantle is placed on)
which clamps are used for simple distillation?
1 normal clamp around RB flask neck
1 normal clamp around condenser
1 ring clamp supporting the heating mantle below
where do you collect the liquid (if any) that comes out of simple distillation BEFORE the 125-138C range for isopentyl acetate collection?
into a separate beaker
why do we use a normal clamp around the RB flask neck and not a blue keck clip?
because the RB flask will get really hot once you start heating and it could melt the blue keck clip which is PLASTIC
how to get the RB flask solution to boil?
insulate with tin foil again until boiling
describe the pathway of the vapors in simple distillation
the vapors rise up from the RB flask when heated, reach the thermometer bulb and then go into the condenser (same concept of inner hollow tube) and the cold water circulating in the outer jacket of the condenser to cool and condense the vapors again back into a liquid –> so when we collect isopentyl acetate, it is in the form of a clear colorless LIQUID
describe the appearance and smell of isopentyl acetate
- clear colorless liquid
- strong banana smell
where do you begin taking apart the simple distillation setup?
start from the RIGHT (away from the RB flask) since it will still be very hot to the touch from residual heat
why do we use glacial acetic acid in excess for this reaction?
- because the synthesis of isopentyl acetate is an EQUILIBRIUM REACTION and we need excess glacial acetic acid to drive reaction for the formation of isopentyl acetate
- the separation of isopentyl alcohol (starting material) and isopentyl acetate (product) is tricky since their bp are only 10-13C apart –> to make this separation easier, we want to significantly LIMIT the amount of isopentyl alcohol that we have to separate from the product –> can achieve this by using excess amount of acetic acid to push reaction to more product formation
what is the purpose of concentrated sulfuric acid H2SO4?
serves as an acid catalyst
what type of reaction is the synthesis of isopentyl acetate?
an acid catalyzed esterification reaction
fischer esterification definition
an acid-catalyzed equilibrium reaction between a carboxylic acid and an alcohol in the presence of a strong acid catalyst to produce the corresponding ester and water (byproduct)
how do we remove all of the glacial acetic acid from product (in extraction)?
since glacial acetic acid is a carboxylic acid (pKa near 5) and can be easily DEPROTONATED, we use sodium bicarbonate NaHCO3 to deprotonate the acetic acid and convert it into its sodium salt so that we can extract it into the AQUEOUS layer and then drain/extract it out of the organic layer in extraction
the deprotonation reaction that occurs between glacial acetic acid + NaHCO3 forms CO2 as a byproduct which is why there is fizzing and thus venting is necessary
what happens if you do not vent frequently enough during extraction?
the glass stopper may pop out when you’re shaking the sep funnel –> large product loss
how much in excess of glacial acetic acid do we use?
more than 2:1 molar equivalents
safety hazards of concentrated sulfuric acid H2CO4?
extremely CORROSIVE
when do we check pH in this lab?
after the second extraction with NaHCO3 to test if all of the acidic glacial acetic acid has been removed
- check the pH of beaker #2 aqueous waste
- pH should be ORANGE or GREEN
waste disposal:
all aqueous waste –>
anhydrous CaCl2 pellets –>
gravity filter paper –>
low boiling distillate from simple distillation –>
MP capillary tubes –>
all aqueous waste –> special container “Aqueous Waste (Fischer Lab)”
anhydrous CaCl2 pellets –> biohazards box
gravity filter paper –> biohazards box
low boiling distillate from simple distillation –> CHO nonhalogenated container
MP capillary tubes –> red SHARPS container
why do we use “glacial” acetic acid for this reaction? what does “glacial” mean?
use “glacial” because more economic (aka cheaper)
glacial = it is anhydrous so its dry and free of oxygen
what are 2 ways to drive reaction to products?
- remove water as it forms (since water is the byproduct, decreases product conc drives reaction to the right)
- use an excess of the cheaper reagent
–> for this lab, we did 2.
what is the alcohol thermometer measuring? vapor temp or liquid temp?
VAPOR TEMP!!
