heat up sample in mel-temp too fast: what happens?
lag on thermocouple
inaccurate reading
mel temp capillary tube for inorganic compounds?
not usually, bc inorganic compounds often surpass mel temp MP range
name 2 immiscible pairs
water-hexane
pentane-diethyl ether
name 2 misciple pairs
water-ethanol
toluene-hexane
why is DMSO not routinely crystallized?
bc its boiling point is not 10 degrees C below its melting point
so it may "oil out" in solution
2 advantages of craig tube
able to be centrifuged
able to fit in a beaker water bath
disadvantages of craig tube
some mass lost in transfer
very small, not good if wanting to make a lot of crystals
purpose of activated charcoal
to remove colored impurities from compounds
if not added, black clumpy specks
seed crystals for recrystallization
provides a surface for re-crystallization to begin, esp. if not working
boiling stones
prevent rapid boiling and superheating
good for recrystallization and distillation
celite for recrystalization
keeps particles from getting thru filter paper during hot filtration
if used too much recrystallization solvent:
if tapping the flask, scratching with glass rod, and seeding didn't help
to fix, evaporate excess solvent, starting with about half of what needs to come off
stemless funnel purpose for recrystallization
less distance for product to fall down: prevents premature crystallization
boiling points are ___ at higher elevation
lower
three reasons to perform a vacuum distillation
compounds w/ boiling points > 200 decrees C. bc vacuum distillation allows it to be carried out at a lower/more convenient temp
when a compound decomposes at or near its boiling point
compounds unstable when exposed to atmosphere
when to use a simple distillation (2)
when separating a volatile substance from a non-volatile impurity, or when separating two volatile substances that have boiling points at least 30 degrees C apart
when to use fractional distillation
when two or more volatile substances need to be separated
RF formula
distance traveled by spot X/distance traveled by the solvent first
chromatography is based on the interaction of the components in a mixture with a ___ and ___ phase
stationary and a mobile phase
order of polarity for ascorbic acid, acetylsalicylic acid, acetaminophen
least to most polar: acetylsalicylic acid, acetaminophen, ascorbic acid
ascorbic acid polarity:
most polar bc polyhydroxylated nature
acetaminophen polarity due to:
second most polar due to OH and amide groups
if two compounds have identical boiling points, are they able to be separated by GC?
yes... factors such as polarity allow for dual separation
how to separate two compounds with identical retention times?
1. heat it to one's boiling points, and the one with the lower boiling point will travel first
2. make sure the proper column was chosen... this may take trial and error
longer carbon chain polarity
longer carbon chain=less polar
Higher retention time=
(gas chromatography)
the one with the most carbons, so one with the least carbons elutes first
After visualization, it is found that the spot did not move from the origin. What might you do to get the spot to move up the plate?
increase the SOLVENT polarity
a compound that has a high affinity for the stationary phase will come off the column BEFORE or AFTER a compound that has a low affinity for the stationary phase?
after
likes to be stationary so it waits
if you mix miscible solvents together, what happens?
a homogenous solution results
if two immiscible solvents are mixed:
less dense solvent is on top
this extraction takes advantage of:
This type of extraction takes advantage of the fact that most organic acids and bases are soluble in organic solvents while their conjugate acid or conjugate base ions are soluble in water.
so can get the acid and base separate at the end!
partition ratio
compound X in where its most soluble/compound X in what its less soluble
x partitions itself in btwn the two
- Sodium bicarbonate (NaHCO3) can be used as a base to extract carboxylic acids into the aqueous layer, but it cannot be used to extract most alcohols. Explain. Hint: the pKa of H2CO3 is approximately 6.
- Because the pka of NaHCO3 is larger than H2CO3. But it is not larger than most alcohols!
- Diff btwn extraction and washing:
- Extraction: selective transfer of a substance from one solvent to another solvent in which that substrate is more soluble
- Washing is removing unwanted impurities from solvent that contains a compound of interest
methanol miscibility with water
they are miscible
- In this procedure your organic acid was isolated by precipitation and subsequent filtration. If the resulting organic acid had been soluble in the aqueous solution, how could it have been isolated?
- Solvent such as ether could be added to extract the organic acid from the water layer
- You are given a two-phase system containing an aqueous layer and an organic layer, but you don’t know which is which, nor do you know the identity of the organic solvent. How could you determine which layer is the water layer?
- Take a small drop of one of the layers. Add a drop of water in that layer and see if it dissolves. If so, that is the aqueous layer
dichloromethane and diethyl ether solubility
they are miscible, will form homogenous mixture
which is more dense, diethyl ether or water?
water
is NH2 or NH3+ more soluble in water?
NH3+, bc is conjugate acid of the base
remember, extraction takes advantage of the fact that most conjugate acids/bases are soluble in water, while acids/bases are soluble in ether
see prelab question 1 and post lab question 1 from the two step synthesis lab
...
what solvent needs to be used for the two step synthesis lab and why?
TEG, bc it has a very high boiling point (285 degrees C). ethanol, for example, is too low. need solution with a boiling point higher than 190 (experiment run at 190)
how does cyclohexane destroy excess bromine in the solution?
reacts to form 1,2-dibromocyclohexane
overall yield
multiply the percent yields of two steps
SN1 reactivity
allylic and benzyllic > tertiary > secondary >primary > aryl
n-butylbromide
the one with a primary bromide group on end
William ester synthesis is a type of
SN2
Why is it important to mix 2-naphthol and NaOh prior to adding 1-bromobutane
so that u can give OH a chance to deprotonate 2-naphthol to generate the desired nucleophile (the two rings with a (-) on it). otherwise, the OH may act as a nucleophile
acidity of 2-naphthol vs ethanol
2-naphthol is more negative bc its conjugate base has a negative charge that is stabilized by resonance
do you have to worry about NaOH deprotonating ethanol to any appreciable extent?
No bc the Keq for mixing those two is about 1, so not a major concern, as even if ethoxide does form, its bascicity is roughly the same as hydroxide and it can act as the base just like hydroxide to deprotonate 2-naphthol
fischer projections: review these!
...
symmetrical
IR inactive bc during the bond stretch there is no dipole moment
unsymmetrical
IR active bc there is a dipole moment during bond stretch
what is different between enantiomers?
optical activity
1-octENE vs 1-octanol ifr
1 octENE has a stretch for double bond that disappears and there is now a BROAD oh stretch
staggered, anti
biggest groups farthest away from each other
cyclopentane bond angles
cyclobutane
pentane: 109
butane: 90
if mirror images but two Hs
are IDENTICAL, not enantiomers, bc there was no chiral center switching bc of two identical groups
how to determine number of stereoisomers
2n where n=number of chiral centers
enantiomers means
ALL chiral centers switched!
diastereomers is just one switched
if asked to draw two diff models of 2 butENE, think...
cis and trans!
cis and trans are...
diastereomers!
boiling points and pressure relationship
directly proportional
what makes it a good choice for crystylization
3
boiling point ten degrees below melting point
miscible solvent pairs
crystallizes in the hot stage, not the cold stage
two techniques if not crystallizing
(3)
add a few pieces of pure compound crystals
scrape the side with a glass rod
celite