Orgo Lab - Exam #1

Analysis of TLC plate

If the lanes share a dot in the same adjacent location, then one is a component of the other. If the dots don't line up, then the components have nothing in common.

What is the purpose of boiling stones?

It has uneven surfaces that will help form bubbles and prevent a heated liquid from erupting violently as it starts to boil. It prevents superheating of the liquid.

What happens if a boiling stone is added to a hot liquid?

It may cause the liquid to erupt violently because of rapid creation of bubbles. If you forget to add boiling stone before heating, remove the heat source and allow the liquid to cool below the boiling point before adding the stone.

When is simple distillation used? When is fractional distillation used?

Simple distillation is used to remove low boiling point solvent from an organic compound with a high boiling point or as the last step in the purification of a liquid compound to obtain a pure product and determine its boiling point. Fractional distillation is used when separating mixture with components whose boiling points differ by less than 70 degrees.

Diethyl ether

intermediate polar solvent immiscible with water

Dichloromethane (DCM, CH2Cl2)

intermediate polar solvents immiscible with water

Ethyl acetate (EtOAc)

intermediate polar solvents immiscible with water

How are compounds separated based on GC?

Compounds with lower boiling points will generally elute faster in GC.

How do you remove acetic acid from ether?

Convert acetic acid to its conjugate base., the acetate ion, by extracting the ether layer with NaHCO3. Sodium acetate is water soluble but not soluble in ether, whereas acetic acid is soluble in both.

Distillation

Distillation is a technique used to separate a mixture of liquids. It uses vaporization/condensation cycles (theoretical plates) to separate liquid components based on different boiling points. Distillation involves vaporizing components of a mixtures, allowing the vapor to travel through a distilling column and finally re-condensing and collecting the distillate. As the vapor travels through the distillation column, it becomes more enriched in the material with the lower boiling point (more volatile). Columns with greater theoretical plates (fractional distillation) tend to separate liquids more efficiently than columns with fewer plates (simple distillation), although fractional takes longer and shouldn't be done is there is a large different between the boiling points of the components in the mixture.

Why are Hirsch funnels better than Buchner funnels?

Hirsch funnels come in smaller sizes so there is inherently less loss when using it for small scale filtrations.

Strength of IMF

Hydrogen bonding (between H and electronegative atom) > dipole dipole (between polar molecules) > induced dipole - dipole (polar with non polar) > induced dipole - induced dipole (two non polar compounds)

Problems with distillation

1. Boiling stones - provide even and controlled boiling a. Must be added before turning on the heating 2. Thermometer reading is lower than expected boiling point - vapor hasn't reached thermometer bulb. a. important to keep thermometer below the side arm, not above 3. rubber seals are attached tightly so the vapors don't escape - outside temperature can be let in and recordings will be skewed 4. distillation flask should NOT reach dryness because the peroxide residue in the flask could ignite or explode and it can cause overheating and breaking; leaving liquid in flask will prevent charring and make it easier to clean.

Problems with recrystallization

1. Choose a recrystallization solvent that dissolves sample sparingly at room temperature, but well at elevated temperature a. If the solubility is low at all temperatures, a very large volume of solvent is needed to dissolve the compound. b. If the solubility is high at all temperatures, then it is difficult to recover the compound 2. do not use too much solvent during recrystallization! the crystals may not come back out of solution 3. allow solution to cool slowly to room temperature before completing recrystallization on ice

Retention time

1. Defined as the amount of time from the injection to the time at which the peak maximum occurs 2. Depends on compounds structure, kind and amount of stationary liquid phase used in the column, the length of column, carrier has flow rate, temperature, sample size. 3. Greater flow rate and higher column temperature produce a shorter retention time. 4. If there is a low boiling point, there is a low retention time. If there is a high boiling point, there is a high retention time.

Problems with Thin Layer Chromatography

1. Don't see any spots of developed plate - a. Not enough sample was on the plate b. Make sure solvent level is high enough to submerge the origin line containing the spots 2. Spots are too close to solvent front - developing solvent was too polar 3. Spots look like big blobs - used too much sample 4. Rf values are very similar - another solvent can be tested in order to distinguish between them or co spotting can be used

Problems with drying agents

1. Drying agent particles clumped together or organic liquid is cloudy - not enough drying agent used so decant organic liquid and add more drying agent

Problems with filtration

1. Filtrate looks cloudy - tiny solid particles can go through the filter paper when the wrong type of filter paper is used. Additionally, if you used water to wet the filter paper it will contaminate the filtrate. A. During vaccum filtration, the filtered solution can evaporate and become cold. This will cause the solute to crystallize from the filtrate. 2. Funnel or filter paper is clogged - during vacuum filtration, the solvent evaporate so quickly that dissolved allure precipitates in the paper and clogs the funnel

Problems for melting point test

1. Inconsistent melting ranges - a. Heating faster than 1-2 degrees per minute may lead to an observed melting range that's higher than the correct one. b. If the rate of heating is higher than 10 degrees per minute, the temperature of the liquid will fail to increase as rapidly as the temperature of the metal heating block, causing the melting point range to be lower than it actually is. c. A greater amount of sample in the melting point tube can result in a broad melting range so just fill up the tube to 2-3 mm in height. 2. Sample disappeared - sublimation 3. Sample turned brown - compound decomposed instead of melting

What does the amount of time a compound spends in the free or adsorbed state depend on?

1. It depends on the strength of attraction to the stationary and mobile phase. 2. The strength of attraction depends on polarity and intermolecular forces of compounds to be separated, stationary phase, and mobile phase. 3. the more tightly a compound binds to the stationary phase, the more slowly it moves on the plate 4. in our TLC experiment, the stationary phase is polar and able to form hydrogen bonds so... a. the more polar the compound, the MORE strongly it binds to the silica gel, and the SLOWER it moves b. the less polar the compound, the LESS strongly it binds the silica gel, and the FASTER it moves

Problems with extraction

1. Knowing which layer is organic and which later is aqueous - it's critical to obtain the density of the organic solvent you are using. a. If density is higher than water, the organic phase is on the bottom. b. If the density is lower than water, the organic phase is on the top. 2. Three layers are visible - middle is an emulsion of organic and aqueous phases 3. Can't see separation of phases in funnel or vial - a. Adding solvent to solvent - when organic layer is on top and aqueous is on the bottom. If the aqueous solution is not returned to the extraction vessel and the organic layer is not removed from the funnel, no interface appears because the second portion of solvent is the same as the first solvent. b. Upper layer is too small to see - drain some of the lower layer or add additional solvent that will form part of the upper layer 3. Can't tell which is organic and which is aqueous - add droplets of water

Problems with Gas Chromatography

1. No peaks appeared - a. Injected sample into wrong injection port b. Clogged syringe 2. Peaks run together - a. Overloaded the column b. Injected sample too slowly c. Temperature too high so sample runs through column too quickly with insufficient interaction with stationary phase 3. More peaks than expected - a. Made a second injection before the first was complete b. Sample not injected immediately after needle has been inserted into injection port

Retention factor

1. Rf = distance traveled by compounds / distance traveled by solvent 2. similar Rf values show similar compounds 3. good Rf value is between 0.3 and 0.6 (shows good solvent system)

Dyes used in TLC and polarity

1. Tartazarine - 2. Indigo carmine - 3. Fast green - 4. Allura red - 5. Sunset yellow 6. Amaranth - 7. Erythorsin B - 8. Eriogluacine -

Why do losses occur for recrystallization?

1. Too much solvent added 2. Premature crystallization occurs during gravity filtration 3. Crystals are filtered before recrystallization is compete

Liquid-liquid extraction

1. Transfer of solute form one liquid phase to another 2. Exploiting difference in solubilities (think IMF) of components in two immiscible solvents 3. a solution is mixed thoroughly with an immiscible solvents and then the compound (solute) is transferred into the solvent in which it's more soluble 4. typical extraction solvents are water and an organic solvent (dichloromethane, diethyl ether, ethyl acetate,) 5. two types of extractions a. neutral b. pH dependent

How do I know when organic layer is dry?

1. Wait at least ten minutes because it doesn't work instantaneously 2. When an organic liquid is dry, it will be clear and at a portion of the drying agent will still have the particle size and appearance of anhydrous form.

Fractional Distillation

1. allows you to separate liquid components of a mixture by exploiting the vapor pressure difference of the components 2. should be used when components have small differences in boiling points (less than 60-75 degrees) 3. e.g. pentane and hexane - the initial condensate will appear when the temperature is very close to 36 degrees, which is the boiling point of pentane. The observed boiling point will remain essentially constant at 36 degrees while all the pentane distills into the receiving vessel. Then the boiling point will rise rapidly to 69 degrees, the boiling point of hexane. The abrupt temperature increase in boiling point demonstrates an efficient fractional distillation.

What happens when we dissolve solute X into solvent Y?

1. breaking up of intermolecular forces of X (delta H is greater than zero, endothermic) 2. breaking up of intermolecular forces of Y (delta H is greater than zero, endothermic) 3. forming intermolecular forces between X and Y (delta H is less than zero, exothermic) 4. in order for a favorable reaction to occur, the third process must release enough energy to overcome the energy uptake of process 1 and 2 5. delta H of solution = delta H1 + delta H2 + delta H3 < 0

How do you recover a compound from an organic phase? How do you recover compounds from aqueous phase?

1. carboxylic acid and phenol - add HCl dropwise to neutralize NaHCO3 (gas evolution) and protonate carboxylate --> neutral water insoluble 2. amines - add NaOH --> neutral water insoluble 3. neutral compounds - dry organic layer to get rid of any aqueous residual solvent (wash with brine, dry over MgSO4 or saturated NaCl), evaporate organic solvent

Recrystallization

1. commonly used method of purification for solids - when a solute dissolves in a hot solvent and then returned to a solid in a cooled solvent to crystallizing 2. technique of separation based on increasing solubility of the crystals in the hot solvent and their decreased solubility when the solution cools, thereby causing the compound to recrystallize. As the mixture cools, the impurities tend to remain in solution while the highly concentrated product crystallizes. a. impurities soluble in recrystallization solvent b. sample sparingly soluble in recrystallization solvent 3. percent recovery measures the success of the recrystallization process % recovery = mass of recovered crystallized sample / mass of crude sample X 100% 4. Crystals that form slowly are larger and purer than ones that form quickly.

Solubility

1. compound solubility in polar or nonpolar solvents gives NO information about compound polarity (like dissolves like) 2. nonpolar compounds generally dissolve better in nonpolar solvents, polar compounds generally dissolve better in polar solvents 3. solubility governed by IMF (H bonding, dipole-dipole, van der Waals)

Basic procedure for TLC

1. dissolve compound in an appropriate solvent and make a spot on TLC plate 2. TLC plate coated with adsorbant (stationary phase) 3. developing solvent (mobile phase)

Safety hazards for handling silica

1. dry silica poses an inhalation hazard - prepare silica slurry in the fume hood 2. silica slurry is a mixture of silica gel and elution solvent 3. you clean TLC chips and reisdual silica form the benchtop by using damp paper towel to wipe benchtop clean form residual silica gel 4. dispose of TLC plates and silica in solid waste

TLC versus LC

1. e. main difference is the application - TLC is qualitative whereas LC is quantitative 2. the mobile phase in TLC travel up whereas the compounds in LC travel down

pH dependent extraction

1. functional groups such as carboxylic acid, phenol, and amine are pH sensitive 2. carboxylic acid - a. generally insoluble in H2O (depends on R group) - small is soluble, large is insoluble a. adding a base such as NaHCO3 will make it water soluble and leave all the other organic compounds behind in the organic solvent 3. Phenols - special type of alcohol, less acidic than carboxylic acid so needs a stronger base (such as NaOH) a. adding a base such as NaOH will make it water soluble and leave all the other organic compounds behind in the organic solvent 4. Amines - basic functional group, solubility can be altered with acid a. amines are usually water insoluble but adding HCl creates positively charged ammonium ions soluble in water and not soluble in organic solvents 3. Steps - a. dissolve both compounds in Ch2Cl2 (introduce organic layer) b. add 5% aqueous NaHCO3 (introduce basic aqueous layer) c. shake to mix layers (creates acid base reaction) d. separate layers - make sure you label collection vessels

Enthalpy and entropy of solution

1. in order for delta G to be negative (spontaneous), we want.. a. large, negative delta H b. large, position delta S

What factors influence boiling point?

1. molecular weight - the higher the molecular weight, the higher the boiling point 2. IMF - the stronger the intermolecular forces, the higher the boiling point a. H bonding > dipole dipole interactions > van der Waals forces b. polar molecules have higher boiling point than nonpolar molecules of similar molecular weight 3. surface area - the larger the surface area, the higher the boiling point

Neutral extraction

1. no change in pH is necessary to achieve separation

Melting point analysis

1. obtain identity of compound 2. indication of purity pure compounds have sharp, narrow melting point ranges whereas impure compounds have broad, depressed melting point ranges 3. characterization data if compound is new

Simple Distillation

1. only two or three vaporizations and condensations occur 2. onyl good if components have very different boiling points 3. any liquid that distills at a temperature less than 5 degrees below the product's reported boiling point should be collected in a separate fraction

Effect of pH on solubility

1. pH sensitive functional groups are... a. Acidic functional groups tend to increase compound solubility in aqueous basic solvents. Carboxylic acid and phenol groups respond to basic pH. e.g. benzoic acid is sparingly soluble in water but is water soluble when combined with base NaHCO3 (sodium bicarbonate) or NaOH (sodium hydroxide) b. Basic functional groups tend to increase compound solubility in aqueous acidic solvents. e.g. amine is insoluble in water but is water soluble when combined with HCl

Procedure for Liquid/Column Chromatography

1. prepare properly packaged column - glass wool, sand, then silica slurry (dry and fine powder) 2. Wait until solvent meniscus is just below the surface of the silica before adding sample 3. add concentrated solution of sample carefully to the silica at the top of the column 4. add elution solvent carefully - do not disturb silica surface 5. elute sample from column with elution solent 6. collect eluted compound in fractions (in test tubes) 7. analyze fractions by TLC 8. recover by evaporating elution solvent 9. gray are more polar, blue are less polar

Uses of Gas Chromatography

1. purity of a sample 2. fractions of components in a sample 3. identify unknown samples

Gas Chromatography (GC)

1. quick method of analyzing organic mixtures (like TLC) but it is more liquids and gases and then separation is based on boiling points - generally used for compounds of high volatility in small amounts a. mixtures separate because their compounds interact in different ways with the liquid stationary phase - the partitioning of a substance between the liquid and gas phases depend on both its relative attraction the liquid phase and its vapor pressure b. the greater the vapor pressure, the greater the tendency to go from the liquid stationary phase into the mobile gas phase c. a more volatile compounds spends more time in teh gas phase than does a less volatile compound - lower boiling point compounds with higher vapor pressures travel through a GC column faster than higher boiling compounds 2. a liquid is injected into the GC and vaporized. The vapor travels through a column containing a stationary phase with a mobile phase of an inert gas (usually Helium or Nitrogen) a. unlike LC and TLC, where the mobile phase actively competes with the stationary phase for the compound being analyzed, in GC the mobile phase does not interact with the compounds - the inert gas carries them down the column when they are in the vapor state b. like TLC, GC uses very small amounts of material and is not preparative 4. in our experiment the stationary phase was nonpolar and uses air as its carrier

Procedure for Distillation

1. sample is heated until boiling (liquid to vapor phase) 2. vapors leave flask and rise up the column 3. some vapor condenses and returns to flask as liquid a. vaporization - condenses cycle b. these cycles repeat and the vapor phase becomes enriched with the lower boiling point component 4. vapor reaches thermometer and records temperature of vapor ready to distill 5. vapor reaches cool condensing arm, condenses back to liquid 6. condensed liquid flows into collected vial

Procedure for Gas Chromatography

1. sample mixture is injected into the port 2. sample is vaporized in heated chamber 3. sample vapor is carried to the column by the mobile phase (inert gas, usually HE or N2) that doesn't interact with compounds 4. separation occurs on the GC column - separations based on attraction to column and compound boiling points a. lower boiling point - more time in vapor phase is carried along faster 5. chromatogram recorded

What do you need in order to have a successful extraction?

1. solubilities of compounds in organic and aqueous phases (acidic, basic, neutral) 2. densities and miscibilities of organic extractions solvents a. densities are to keep track of which layer is which b. miscibilities tell you if it's practical in extraction - must be abel to separate form aqueous layer 3. pH sensitive functional groups present in compounds

What does the distance travelled by a molecule depend on?

1. the developing solvent (mobile phase) 2. nonpolar compounds spend less time interacting with silica and more time in the mobile phase, whereas polar compounds spend more time interacting with silica and less time in the mobile phase a. LESS polar compounds travel FASTER and FURTHER on silica TLC plate and the HIGHER the Rf value 3. nonpolar solvents move nonpolar compounds and polar solvents move polar compounds 4. polar solvents move farther on TLC than nonpolar solvents

Problems in Liquid Chromarography

1. the elution solvent is too polar - sample mixture will elute too quickly and poor separation will result. but if the solvent is not polar enough, the sample will elute too slowly and the bands of compounds will broaden by diffusion. 2. if too much solvent is used, the excess will behave as an elution solvent and start to carry out the mixture's components down the column --> separation will be incomplete because the entire sample was not in the column before its components started to move through the stationary phase. 3. Non-horizontal bands - due to adsorbent not being flat at the top of the column, column not properly clamped, sample not evenly applied to the column. This causes poor separation because the lower part of one band will diffuse with another. 4. Channeling - due to when the sample is applied to the column too vigorously (it should be applied with a piper one drop at a time). It basically means that the bottom of one band co-elutes with the top of a lower band. 5. Column can become dry - solvent level falls below the top of the adsorbent, it can become dry and pull away from the column wall. 6. column not properly clamped - separation of dyes is less effective because the non-horizontal bands develop so the lower part of one band will diffuse with another 7. mobile phase (elution solvent) flows very slowly so the dyes in the column will not effectively separate and bands will broaden by diffusing together 8. if large elution fractions are collected, too much solvent was used and the excess will move the dyes down the column. as a result, separation will be incomplete because the entire mixture of dyes were not at the top of the column at the same time before moving through the silica gel, or stationary phase. Larger fractions have a higher risk of containing more than one dye.

Stationary phase in TLC

A glass, metal, or platic plate coated with a thin layer of adsorbant. SiO2 is the most common adsorbant and it is a polar compound.

Liquid/Column Chromatography

1. the most common method of purification of compounds in a solution based on polarity - generally used for compounds of low volatility and a wide range of sample quantities 2. most of the theory of TLC can be applied to liquid chromatography a. stationary phase - solid adsorbant with a liquid coating packing into a column, silica gel supported in vertical column b mobile phase - elution solvent c. separation based on polarity and intermolecular forces d. generally a good separation of TLC with a particular mobile phase leads to good column chromatography separation with the same mobile phase a. from TLC to LC - the compound that travels farther up (nonpolar) will elute FIRST 3. as the elution solvent is passed through the column, fractions are collected in disposable test tubes and then the purity can be determine by spotting each fraction on a TLC plate 4. the relative polarities of the two phases determine the order in which compounds in the sample elute from the column a. with a polar adsorbant such as silica, b. the less polar compound elutes first

Analysis of GC

1. the order of peak is directly proportional to boiling point a. i.e. the first peak corresponds to component with the lowest boiling point

Mobile phase in TLC

A pure solvent or mixture of solvents that depends on polarities of compounds being separated. If you have a nonpolar compound, you use a nonpoar solvent. Likewise, if you have a polar compound, you use a polar solvent. It also might be possible to use a solvent with an intermediate polarity.

How do i know which is organic later and which is aqueous later?

Add a few drops of water - if the water dissolves, then aqueous later is top layer; if the water droplets fall, then organic layer is top layer.

How is GC similar to TLC?

Both do not identify compounds unless the known samples are available.

Why does the filter flask become cold during vacuum filtration?

If the solvent used is recrystallization is a volatile compound, it will evaporate rapidly during the filtration process. Evaporation is a cooling process.

What can you conclude if two substances have the same Rf value?

If the two substances moved with the solvent front, you could say very little about their being identical or not. The use of a less polar developing solvent would lower the Rf values and provide a better opportunity to distilled finish the two compounds from one another.

What happens when you use too little drying agent? Too much drying agent?

If too little drying agent is used, the organic layer will still contain water which may interfere with the use of liquid and act as an impurity. If too much drying agent is used, the product is adsorbed into the surface of the drying agent and results in significant loss and reduced yield.

What happens if a compound is crystallized too quickly?

Impurities can be trapped in the crystal lattice. Slow crystallization allows for maximum selectivity and therefore purity in the process.

Simple Distillation verus Fractional Distillation

In simple distillation, there are fewer vaporization-condensation cycles that occur during si. sot here is less effective separation. In fractional, there are more vaporization condensation cycles that occur because there is greater surface area o there is better separation of components. Fractional distillation is suitable for separating liquids with boiling point differences of less than 70 degrees Celsius.

Which compounds will show up as a dark spot under UV light?

Look for the conjugate that is stabilized because it will have lower energy. Essentially want alternating double and single bonds.

How do you chose the right drying agent?

MgSO4 is generally a good drying agent. Do not use a basic drying agent for an acid or an acidic drying agent for a base. It is better to use KOH for amines than carboxylic acid.

Why does the seal need to be broken in vacuum filtration before the water flow is turned off?

Otherwise water may be sucked back into the filter flask.

Why won't phenols react with NaHCO3 if carboxylic acids do?

Phenols are a weaker acid so they need a stronger base such as NaOH.

What precaution should be observed when an aqueous sodium carbonate solution is used to extract an acid?

Production of CO2 results due to acid-base reaction. Unless the funnel is periodically vented, the gas will build up pressure in the funnel and could blow out the stopper.

What is TLC used for?

Purity, separation, and following the course of a reaction, to determine appropriate conditions for chromatographic separation, monitor column chromatography. It doesn't work for liquid compounds.

What is the most important variable in determining melting point range?

Rate of heating

What has the lower Rf value? What has the higher Rf value?

The MORE polar compound will have the lower Rf value. The LESS polar compound will have the higher Rf value.

Free state versus adsorbed state

The free state is when molecule is dissolved in the mobile phase. The adsorbed state is when molecule is stuck to surface of solid phase. The molecule has to be in the free state in order to move on the plate. The molecule's preference will determine how quickly it moves.

Which component elutes first in LC?

The less polar component elutes BEFORE the more polar components. So the dots on the top part of the TLC plate elute before the dots on the bottom.

Solid-liquid extraction

Transfer of material form solid phase to liquid phas

Chromatography

The process of partitioning components in a mixture between a stationary phase and a mobile phase. Components are introduced at one end of the stationary phase and, as they interact with mobile phase, different compounds migrate at different rates. The mobile phase can be liquid or gas and moves in a definite direction and passes over the stationary phase.

Why would a hot recrystallization solution be filtered by gravity filtration over vacuum filtration?

The solution would cool too rapidly during vacuum filtration and crystallize prematurely. Vacuum filtration cools the solution to a much greater extent and produces excessive crystallization of the desired compound.

What is the purpose of a mixed melting point experiment?

To determine whether the two compounds share the same identity.

What is superheating?

When a liquid is heated above its boiling point but doesn't boil because bubbles are kinetic ally prevented from forming. This is commonly encountered in vessels with smooth surfaces.

What is the easiest way to evaluate a TLC plate?

When the mixture of components have a large separation of Rf values.

Solubility in polar compounds

main IMFs are H bonding and dipole dipole

Solubility in nonpolar compounds

main IMFs are induced dipole interactions, some polar solvent can induce dipoles in nonpolar compounds and aid solubility

Hexanes

nonpolar solvents immiscible with water

Toluene

nonpolar solvents immiscible with water

Acetonitrile

polar solvent miscible with water

Dimethyl sulfoxide (DMSO)

polar solvent miscible with water

Ethanol (EtOH)

polar solvent miscible with water