Midterm Quiz
An INSOLUBLE impurity (like sand or glass) has what effect on the melting point?
NO EFFECT!
What happens if the solvent chosen allows impurities and the desired compound to be insoluble at low (cold) temperatures?
NO purification will occur, the impurities will crystallize with the pure material
How are anhydrous inorganic salts (drying agents) removed?
by decanting or gravity filtration
> actively seek better chemical processes > use more benign reagents > generate less toxic waste > be efficient as possible & use renewable resources Are all ways to make ______?
chemistry greener
Re-crystallization uses a solvent and a high temperature to...?
dissolve impurities but NOT the compound
What is the difference between drying and washing in organic chemistry?
drying: removal of water from organic solvent washing: removal of impurities from organic solvent
Define Green Chemistry.
focuses on finding ways to improve chemical products/processes and help reduce/eliminate potential hazards to human health and the environment
Define extraction.
removal of a particular compound (component) from a mixture
How could you differentiate among 2 compounds based on SIMPLE experiment?
*Take a mixed melting point temp* Take melting point of each compound separately then mix compounds together and take mixed melting point temp. > if same melting point, compounds are the same > if lower, different melting point, compounds are different
Strong bases deprotonate (extract) _____.
weak acids Ex. phenol
What is back-extraction?
when a precipitate doesn't occur the compound must be extracted from the water using a fresh portion of organic solvent
What is the melting point range?
when the 1st drop of liquid is observed -to- when the last bit of solid has liquified
How are organic solvents dried?
with anhydrous inorganic salt (anhydrous magnesium sulfate), it absorbs/removes the water that is dissolved in the organic solvent after extraction
What does crystal formation depend on?
1. concentration of solute 2. temperature of solvent 3. solvent used
What are the steps of Re-crystallization?
1.) An impure sample is dissolved at boiling point in a minimum amount of solvent 2.) Insoluble impurities are removed by hot gravity filtration through fluted filter paper 3.) The hot, saturated solution cools at room temp. forming pure seed crystals 4.) Seed crystals serve as nucleation sites where more compound is deposited and crystal growth continues 5.) Solution is put in ice bath to maximize crystal formation 6.) Crystals are then collected by vacuum filtration
What are some properties of a good solvent to be used for re-crystallization?
1.) The solvent should dissolve impurities at high/low/room temperatures. 2.) The solvent should NOT dissolve the desired organic compound at room/low temperatures (insoluble). 3.) The solvent can dissolve the desired compound and organic impurities at high temperatures.
Why do you need to tap the capillary tube before taking the melting point?
1.) make sure sample is packed at the bottom so it can heat evenly 2.) makes sure sample is tightly packed
What are the 2 types of spectroscopy?
> Infrared Spectroscopy > Nuclear Magnetic Resonance Spectroscopy
What are some factors that affect melting point?
> impurities *decrease melting point temp. > pressure > human error > solvent present *wet sample
Having a wet sample can affect ...?
> packing into capillary tube not efficient > wetness may dissolve compound making you think its melting, but not really (the compound may actually have higher mp temp)
How do you ensure accurate determination of melting point?
> sample must be finely powdered > proper amount must be packed into capillary tube > sample must be dry (presence of solvent can cause sweating - appearance of melting)
What are some criteria that need to be followed to ensure that a liquid-liquid extraction is successful?
> the 2 chose solvents must be immiscible (NOT be soluble in each other) Ex. typically 1 polar & 1 non-polar solvent > the compounds to be extracted must have different affinities for the solvents. (Ex.polar compounds are more soluble in polar solvents)
Why NOT use too much drying agent?
???
A pure crystalline solid compound has a fixed, narrow (sharp) melting point temperature range because...
ALL of the bonds break at the same time at the same temp.
What can happen if too much decolorizing Carbon is added?
Because then our starting material will be removed as well from the solution (the carbon will also bind to the amine functional group)
Why do we dissolve the compound p-phenetidine in dilute HCL in the 1st step of re-crystallization procedure?
Dilute HCL is used to dissolve compound into solution by protonating the amine group of p-phenetidine to dissolve it into the compound
What type of spectroscopy gives us information about what functional groups are present in the molecule?
Infrared Spectroscopy (IR)
Re-crystallization is a process that is used to remove _____.
Insoluble Impurities and purify organic solids
Ligroin is a nonpolar solvent that can be used to extract ______?
Ligroin = non-polar extraction solvent
What type of spectroscopy gives us information about the structure and bonding in a molecule?
Nuclear Magnetic Resonance Spectroscopy (NMR)
Rf calculation formula
Rf = distance of compound from origin/ distance of origin to solvent front
An Amorphous solid has a powdery consistency and has a broad range for its melting point because...
Some bonds take more time and a higher temperature to break
How does one give positive identification to an organic compound?
Spectroscopy
The melting point can NEVER be higher than the expected temperature of the synthesized compound? T/F
TRUE
What is the ideal ratio for an eluent in TLC extraction?
The ideal ratio for each sample is determined by the polarity of the compounds you want to isolate. The spots should be separate when the ideal ratio is used.
If the capillary tube is not packed tightly at the bottom, what will happen to the melting point of the sample?
The sample will take longer to melt and need more heat so melting point range will be broader
In recrystallization, why do we use decolorizing Carbon on our p-phenetidine sample first?
To remove the colored impurities from the original sample which is reddish-brown due to impurities cause by air oxidation of amino FG
Which type of filtration is faster?
Vacuum filtration
When do we use back extraction?
When neutralization of your compound is not acidic enough and NO precipitate happens
If you read in literature that a compounds melting point was 160 degrees C, what melting point would you not expect to get?
a melting point exceeding 160 degrees C
2 students took the melting point of the same sample compound on different instruments and got 2 different melting point temps. Why different ranges? Student 1 = 145 - 147 degrees C Student 2 = 143 - 149 degrees C a.) sample was impure b.) Student 2 used a lot of sample c.) Student 2 used high ramp rate d.) Student 1 used small ramp rate e.) Student 2 had solvent in sample
b, c, d all correct reasons as to why the students achieved different melting point ranges
The melting point range for a pure compound is a unique property of the compound and helps ______ & ________.
identify the compound & assess its purity
When do we use gravity filtration?
if the material to be filtered is granular
Why is it important to use a stemless funnel when performing hot gravity filtration?
if there is a stem, premature cooling could occur and re-crystallization of our sample could occur inside the stem and clog the funnel
Why should you not under-fill or overfill the capillary tube before taking melting point temp. of your sample?
if you under-fill the tube: not accurate if you overfill the tube: broad temp range
What is the purpose of hot gravity filtration?
it is used to remove INSOLUBLE IMPURITIES from granular samples
Why do we use fluted filter paper in hot gravity filtration?
larger surface area and faster filtration
Solid-Liquid extractions are often performed using ________ as a solvent.
liquid CO2
How does the presence of a soluble impurity in a sample effect the melting point of that compound?
lowering and broadening the melting point range
The mobile phase aka eluent phase in TLC is a mixture of _____ & ______ solvents.
polar & nonpolar
Why are there inaccuracies when measuring melting point temperature?
poor melting point determinations can be made due to poor heat transfer to the sample, too much sample, excess solvent in compound, ramp rate too high
In TLC, the stationary phase is the _________, it is polar so more polar compounds retain more and move slower on it.
silica
Liquid C02 is highly volatile (very reactive/easily evaporated) and is used to perform _____________ with the orange rind.
solid-liquid extraction
When the melting points is broad and has a range of 5 degree or more, it means the sample has a _______.
soluble impurity
Why should you try and dissolve the compound in as little hot solvent as possible?
solution needs to be saturated for efficient recrystallization and too much solvent makes the solution dilute and the crystals won't form which lowers the percent yield
Weak bases deprotonate (extract) _______.
strong acids Ex. Carboxylic acid
Define melting point of a pure substance.
temperature at which the solid phase is in equilibrium with the liquid phase
Why does the melting point lower and become more broad when there is an soluble impurity in the compound.
the impurity disrupts the crystalline lattice of the sample so less energy is required to convert the solid phase into liquid phase (crystals begin to melt at a lower temp, and melting point range will be broader, 5 degrees or more)
How do you know if a compound is pure?
the melting point range will be sharp, the range is narrow (1-2 degrees range at most)
When taking melting point temperature, why is it important to have the ramp rate at 1-2 degrees C per minute?
this temperature setting is more accurate than a higher ramp rate
Why is the solvent hot during hot gravity filtration?
to avoid the crystallization of our compound
Why do we use decolorizing carbon (charcoal)?
to remove colored (oxidized) impurities by binding with polar functional groups
