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what is the purpose of carrying out a melting point analysis?
- 1. characterize a known compound if choosing between several known
- 2.record physical characteristic for new compound
- 3.establish purity of a known compound
melting point of a pure compound
if a compound melts over a narrow temp range, usually less than one degree.
what is the criteria for impurity?
must be soluble in a given compound to depress the purity
what forces affect melting point?
- van der waals
- h-bonding (more = higher temp)
- size of molecules
point at which two solids of liquid solution are in equilibrium and at what temperature they melt
how to find unknown?
if you have 3 knowns and must choose between the 3, mix unknown with all different, one with the least/no change is the known
what if you heat the sample too quickly?
incorrect temperature reading, sample at a diff temp than thermometer
from first drop of liquid until complete liquefication
lowest temperature at which a mixture of a given composition will start melting
why carry out a recrystallization?
to purify a product
picking solvent for recrystallization
like dissolves like
requirements of solvents for solvent pairs?
- dissolves solvent completely at solvents melting point
- precipitates solvent completely when solvent is cold
- does not react with solute
- does not dissolve impurities
why cool slowly?
if cooled too quickly solid will ppt with impurities crashing out
remedies for unsaturated solution
boil off excess solvent in respirator tube
remedies for supersaturation?
use seed crystals to induce crystallization and scratch bottom of test tube with glass rod
- above boiling point without boiling
- to prevent, use boiling stick
oiling out.. remedy? prevention?
- boiling point of solvent is higher than melting point of solute
- add more solvent
why remove boiling stick during crystallization?
so that you dont lose crystals that form on the boiling stick
two methods to filtering out solid impurities from hot solution are
- gravity filtration
7 steps of recrystallization
- 1.choose solvent pair
- 2.dissolve solute
- 3.decolorize the solution
- 4.filter out suspended solute
- 5.crystallizing the solute
- 6.collecting and washing crystals
why do we carry out distillations?
- purify liquids
- separate based on boiling points
why wrap apparatus with aluminum foil?
insulate and minimize heat loss
why wrap sidearm with wet towel?
to ensure that vapor condenses in side arm
when do you use simple distillation?
when boiling point difference between the two are large and one liquid composition is less than 10%
when do you use fractional distillation?
- when liquid composition is substantial
- boiling point difference between the 2 is small
- mixture with a fixed composition that can't be altered by simple distillation
- has a constant boiling point
when is steam distillation used?
- for immiscible liquids such as water
- used when substances decompose before their melting points
when do you use vacuum distillation?
- for compounds that decompose before their boiling points
- slowly ascending to obtain equilibrium
never distill to ______? because...
dryness, dry residue of liquids can explode
never distill in a _________? because..
closed, apparatus can rupture
sources of error for distillation?
- 1. loose connectors
- 2. temp variation from airflow in hoods
- 3. large apparatus, lose heat
boiling point of a liquid
temperature at which vapor pressure of the liquid equals the force exerted on it
what to avoid in tlc?
- smeared spots will make it hard to obtain Rf
- flaked adsorbent won't allow solvent to rise
why should solvent be below the spotting line?
so that solvent doesnt dissolve samples
make sure the top is on the container for tlc
so that solvent doesnt evaporate and make a messy plate
filter paper must be in tlc chamber to..
make sure chamber is saturated with solvent
solvents with _________ migrate rapidly.
low boiling points and low viscosity
silica is good for
alumina is good for
uses of tlc?
- 1. see if a solid is pure
- 2. see what compounds are in sample
- 3. determine identity of compound
- 4. monitor progress of reaction
- 5. determine effectiveness of purification
- 6. determine appropriate conditions for column chroma
- 7. monitor column chroma
too polar solvent in tlc
spots will move towards solvent frnt
too nonpolar a solvent in tlc
spots will stay at bottom
reasons to carry out extractions?
separate a mixture
order of carrying out extractions? why?
- weak base then strong base
- to avoid strong acid reacting and neutralizing weak base
requirements for extraction solvents
- 1.readily dissolve others to be extracted
- 2. not miscible with water
- 3. not reacting with solute
- 4.low boiling point
- 5. non flammable
- 6. non toxic
- 7. cheap
emulsions.. how do we fix it?
- suspension of one liquid as droplets in another caused by too much shaking
- fix by adding nacl to aq layer
how do you decrease solubility of a subtance in organic layer?
add nacl or cacl2
what indicates that the organic layer is dry?
choosing drying agent?
- 1.must not react with solute
- 2.must remove water quickly
- 3.easy to recover solute from agent
advantages of t-butyl ether
- forms azeotrope with water
- doesnt need trying
- doesnt form peroxides
disadvantages of diethyl ether
- forms peroxides
in extractions, why not 100% recovery?
solute is nearly never completely transferred to organic layer
- polar protic solvent- carbocation stabilized
- rearrangements can occur
- racemic mixture
- aprotic polar solvent
- no rearrangements concerted
- all in one step- fast
- inverted configuration
why add same amount of alkyl halide and nuc
for sn2 amount of nucleophile affects rate and adds another variable
possibility of a false sn1sn2?
too much alkyl halide added can make halide act as a solvent in which nuc isnt soluble
vinyl and aryl halides dont undergo sn1 because
they are unstable carbocations
vinyl and aryl halides dont undergo sn2 because
electrons in double bonds repel nucleophile
why dont the br- and cl- attack alkyl iodides that form in sn2?
- because they are taken up to form nacl and nabr
- none free
esterification lab.. how could le chatelier principle be used in the reaction?
- 1. use of excess cheap reagent: boiling points are close and excess of starting material would be an impurity
- 2. remove product water by drying with molecular sieves of azeotropic distillation
how to remove water?
- 1. molecular sieves
- 2. azeotropic distillation
why was the esterification apparatus set up at an angle?
so that water could be removed as distillation was happening
purpose of IR?
to identify the products
how could we have used tlc in esterification lab?
identify what was in the product
role of resin in esterification?
- acid catalyst
- easily removed by filtration or pipetting out of flask
disadvantage of using traditional acid catalyst?
difficult to remove, would need to dilute with ether then dry then evaporate
reason reduction may not go to completion?
entire reaction is reversible
how is a reduction characterized in orgo?
- increase in the number of bonds to H
- decrease of number of bonds bonded to electronegative atoms
why is nabh4 used rather than lialh4?
- more selective in reducing aldehydes and ketones
- does not react violently with alcohols and water
- lialh4 produces hydrogen gas
why was ethanol used in stead of ether?
its insoluble and reduces ketones and aldehyde
why was nabh4 used in excess?
- to ensure the reaction goes to completion
- account for nabh4 with solvent
- account for decomposition
role of each addition of water
- to hydrolyze ester
- saturate solution to start recrystallization
role of ice bath?
to slow down rxn with sodium hydroboride so that its not explosive
how do you minimize getting a mixture of products in aldol lab?
- use excess of reactant with no hydrogen
- use more reactive aldehyde in presence of unreactive ketone
- deprotonate carbonyl with lda to convert the enolate then add second carbonyl slowly
role of sodium hydroxide in dibenzalacetone synthesis?
reason for washing crystals with water?
to remove base catalyst that can cause oiling out during recrystallization
requirements for starting materials for aldol addition and condensation
- carbonyl must have one alpha hydrogen for it to form enolate
- carbonyl that forms enolate must have at least 2 aplha hydrogens to form condensation product
why does the enolate form
because carbon is deprotonated easily (acidic alpha hydrogen because of resonance when removed)