Final Exam Flashcards
What is the GC stationary phase?
a high-boiling liquid film supported on an inert solid and packed in either a fused silica capillary column or in a copper or stainless steel metal column
What is boiling point?
Temperature at which the vapor pressure of a liquid equals the external pressure applied to the surface of the liquid (atmospheric pressure)
Which is more volatile cyclohexane or toluene?
cyclohexane is more volatile. Lower boiling point means a compound is more volatile
What is the eutetic composition?
Composition at which solid X(impurity) and solid Y (desired compound) are in equilibrium with the liquid mixture (at eutetic temperature)
How can you visualize compounds on TLC?
- directly 2. with UV light 3. with iodide
What is gas chromatography?
a common technique used to separate and identify volatile organic compounds.
Why do you dissolve recrystallizing compound in MINIMUM volume of boiling solvent?
too much means it may be very difficult to get product to recrystallize at the end
Adsorbant vs. eluent
adsorbants are typically polar, hence polar compounds are attracted to adsorbant and move less far. However, the more polar the eluent, the more rapidly a compound moves.
Compounds with less attraction for the adsorbent move ____ with the eluent Compounds with more attraction to the adsorbent move ______ with the fluent
Compounds with less attraction for the (polar) adsorbent move RAPIDLY with the eluent (ie. nonpolar compounds move farther) Compounds with more attraction to the adsorbent (polar) move SLOWLY with the eluent (ie. polar compounds move less far)
What happens if you add too much compound? (TLC)
tailing
What is evaporation?
Occurs at temperature below boiling point. Transition from liquid to vapor of molecules at liquid surface.
What is the difference in simple and fractional distillation?
Incorporation of fractional column with glass beads that increase surface area and cause vapor to condense and reevaporate with hot vapor. Fractional is a series of simple distillations
What is steam distillation?
a type of special distillation for temperature sensitive materials like natural aromatic compounds
What is SN1?
-Rate determining step entails one molecule (R-X) -3>2>1 -polar protic solvents, like ethanol -weak Nu OK -racemic mixture
What is the RDS of SN1?
formation of carbocation (LG leaves)
Wat is the eutetic temperature?
the lowest possible melting point for a mixture of 2 compounds
What is require of the two solvents in an extraction?
they must be immiscible
HCO3- reacts only with what in the extraction lab?
reacts only with p-toluic acid (strongest acid)
In the preparation of eugenol benzoate lab, which layer contains the eugenol benzoate? (ether or aqueous)
ether layer
order of polarity? (toluene,acetone, cyclohexane, methanol, hexane, ethyl acetate, petroleum ether, acetone, dichloromethane
C-cyclohexane P-petroluem ether H-hexane T-toluene D-dichloromethane E-ethyl acetate E-ethanol A-acetone M-methanol
What will be the effect on retention time of a change in detector attenuation
no effect
In GC chromatography, what will happen if the temperature is set too low for the column?
Compounds with lower boiling points will spend more time in the mobile phase than compounds with higher boiling points
What is melting point range?
Span of temperatures from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid
What is codistillation?
distillation performed on mixtures in which the two compounds are not miscible
What is recrystallization?
purification process used to remove impurities from organic compounds which are solid at room temperature
What is washing?
the removal of impurities from a solvent containing your compound by a second immiscible solvent.
What will be the effect on retention time of a change in the size of injected sample?
no effect
What is extraction?
a process that selectively dissolves one or more compounds into an appropriate solvent. refers to the transfer of these compounds from one liquid solvent to another one
In GC chromatography, what will happen if the temperature is set too low for the injector?
the compound will not instantly vaporize and the compound will slowly move onto the column in a brand bond, making separation and quantification difficult
What will be the effect on retention time of a change in column temperature?
all compounds will move through the column faster so retention time will be shorter
What is the GC mobile phase?
an inert gas (N2, He, Ar, CO2)
In GC chromatography, what will happen if the temperature is set too low for the detector?
a low boiling compound could condense and clog the detector
What is dalton’s law?
Ptotal= Po 1 +Po 2
When should you use fractional distillation rather than simple distillation?
fractional: boiling points less than 40 degrees apart
What is distillation?
Separation of compounds based on BP differences
When is water on the bottom of immiscible mixture and when is it on top?
Water is more dense than ethers and hydrocarbons (therefore on bottom). Dichloromethane is more dense than water (therefore on bottom)
Where should the thermometer be in the distillation experiment?
at the curve of the equipment, not too low or too high
What is melting point?
Temperature at which solid is in equilibrium with its liquid
How do impurities affect MP range?
decrease and broaden it
What is SN2?
-Rate determining step entails two species (R-X and Nu) -methyl>1>2>3 -acetone and other polar aprotic solvents -strong nucleophile necessary (I) (only partially positive) -inversion of product
What is RDS of SN2?
Nu attacks R-X (Nu–R–X)
Chromatography?
several related techniques for analyzing, identifying, or separating mixtures of compounds
What is clove oil?
consists of several compounds such as eugenol (85-90%) and eugenol acetate (9-10)
What will be the effect on retention time of a change in length of the column?
longer column means slower retention rate
What will be the effect on retention time of a change in detector temperature?
no effect
How can you take care of an emulsion in extraction experiment?
- gently swirl container 2. add drops of saturated aqueous NaCl 3. add drops of ethanol 4. addition of more solvent to dilute solutions
What is vapor pressure?
Pressure exerted by a vapor in equilibrium with its liquid
column chromatography?
technique uses an adsorbent packed in a glass columb, and a solvent that moves slowly through the packed column compounds form a band which moves down the column. Separation is visible
What is the mobile phase?
The sample mixture is placed into a liquid or a gas. (fluent)
What is the advantage of steam distillation?
Allows distillation of organic compounds with high boiling points to distill below 100 degrees celsius
What is Na2So4 used for?
anhydrous sodium sulfate is a drying agent
What is Raoult’s law?
Pt= Xo1*P1+Xo2*P2 where Xo: molar fraction of compound P: partial pressure of compound
TLC?
a simple and relatively inexpensive analytical technique that can quickly and efficiently separate quantities of lass than 10 micrograms of material
What are the two changes that affect GC chromatography retention time?
- change in column temperature 2. length of the column
Should impurities be soluble or insoluble in solvent? and at what temperatures?
Either soluble at all or insoluble at all temperatures
What are the 3 criteria for selecting recrystallizing solvent?
- Abundant quantity of crystals must be recoverable 2. The compound being purified must be insoluble at room temperature but soluble in boiling solvent. (aka, it needs to be able to recrystallize but melt with heat) 3. Solvents boiling point must be lower than the compound’s melting point. (else oiling out)
What is percent composition in GC lab?
corrected integral of compound/ total of all corrected integrals corrected integral: y/x (comp/retention time)
How do you stimulate crystal formation? (3 ways)
- ice bath 2. scratch the glass 3. add a seed crystal of pure substance
Alcohol dehydration proceeds via which mechanism?
E1 elimination
When is percent recovery used?
recrystallization
What demonstrates the different compounds in a chromatography method?
compounds in sample mixture move through stationary phase at different rates due to different attractions for the mobile and stationary phase.
What are eluents?
organic solvents of various structures and polarities
How can extraction be used?
to separate unwanted impurities or to separate mixtures of compounds
Why do you add activated carbon?
absorbs dissolved organic substances. Don’t add too much because it can adsorb your compound
What is the stationary phase?
Mobile phase carrie the sample through a solid support
What are essential oils?
volatile oils usually having the characteristic odor of flavor of the plant. Frequently is a water-immiscible liquid separated by ditillation from the plant material
What is miscible?
When two liquids create a homogeneous solution
With trans and cis-stilbene, which was solid?
cis: liquid trans: solid
What are the 3 principles in the recrystallization lab?
- solubility of a compound in a solvent increases with temperature 2. as the solution cools, crystals form and grow 3. molecules in a crystal have a greater affinity for molecules of the same kind than for impurities
What is steam distillation?
distillation when one of the compounds is water
What will be the effect on retention time of a change in injector temperature?
no effect
What are applications of TLC?
- analysis of reagant and product purity 2. determine number of compounds in mixture 3. follow reation progress 4. select a solvent before larger scale column chromatography
