Practical Quiz - Chem 232 UMD

What lab is lab one?

Recrystallization of an impure solid

What is recrystallization of an impure solid?

a purification technique in which a crude solid is dissolved in minimum amount (volume) of hot solvent, and upon cooling, a purer solid precipitated from the solution

What is the most important consideration for good recrystallization?

Choosing a good solvent based on

• Polarity

• Differential solubility of solid in solvent

• Solvent Bp

• Chemical interaction between solid and solvent

• Solubility of impurity in solvent

insoluble, soluble

The solid should be infinitely _____ at low temperatures and infinitely _____ at high temperatures

What can you do if there are purities that result that do not dissolve?

Use hot gravity filtration to filter them out, or a decolorizing agent

What is the experimental set-up for experiment 1?

four test tubes in a warm/hot water bath, then cooled to room temp, then finally in an ice bath, best solvent should be observed (which one is solid at cold and room temp but no precipitate at hot)

• THERE MUST BE 4 TUBES, ONE FOR EACH SOLVENT

Then, for recrystallization, .40g of crude solid was placed in a 25 ml Erlenmeyer flask, best solvent was used, small magnetic stir bar was added, heated on hot plate until solid was completely dissolved

• Cool to room temp, any insoluble are filtered out via gravity filtration

• Recrystallized sample was collected via vacuum filtration using a Buchner funnel and filter flask

What is lab 3?

Extraction

What is extraction?

A technique defined as transferring solute from one solvent to another

• We match polarity as much as possible (like dissolves like)

• We want something to be completely solvated at room temp, not partially precipitated at room temp like for experiment 1

• Make sure the solvent has a high boiling point so we can remove it later

What is selectivity?

The solvent extracts only our compound of interest and not other components

• typically achieved by experience and trial and error

• In order to extract the max amount of solute into the solvent, we help the process along b raising the temperature, grinding the solid, and mixing well

What kind of extraction did we use?

Liquid-liquid extraction - use two immiscible solvents to separate a mixture of solids (must have different densities, refractive indices)

• MOST ORGANIC COMPOUNDS ARE NOT WATER SOLUABLE

• In order to separate the mixed solutes, we increase the solubility of one solid in water and “pull” it to the aqueous phase, thus separating it from the other organic solids in the mixture

• The process is repeated (take advantage of the acidity/basicity of the solution

What bases are used for what compounds experiment 3?

Strong acids, like carboxylic acids, use weak bases (NaHCO3) to deprotonate it

Weak acids, like phenol, require strong bases like NaOH

Amines are protonated with strong acids like HCl

What reagents are used for experiment 1?

Benzoic acid, p-toluic acid, acetanilide, and benzyl

What solvents are used in experiment 1?

Hexanes, acetone, ethanol, and water

What reagents are used in experiment 3?

3-nitrobenzoic acid, 2-napthol, and 1,4-dimethyoxybenzene

What are the solvents used in experiment 3?

NaHCO3, NaOH, and HCl

What is experiment 4?

Thin layer chromatography of spinach

What is chromatography?

separation and purification technique

• thin layer chromatography is based on partitioning the sample between two phases, the mobile and stationary

• Planar version

• Stationary phase is the silica paper, and the mobile phase is capillary action

• Separates based on polarity (silica plate is polar, so the more polar a substance, the less distance it will travel.

What are the pigments that we plan to extract in experiment 4?

Chlorophyll-a, lutein (xanthophyll) and B-carotene.

What is experiment 5?

Column chromatography of spinach

What is column chromatography?

Another common technique for separation of liquids - we use a stationary phase and mobile phase

• Is quantitative

• Column is the Pasteur pipet

• Stationary phase (silica) is packed and the mobile phase (organic solvents) is applied at the top, traveling down the column

• GRAVITY IS A HUGE COMPONENT HERE

How does polarity relate to column chromatography?

Compounds that are more polar tend to stick to the stationary phase and lag behind, while less polar compounds interact with the mobile phase more and travel faster down the column with the mobile phase

What is retention time?

Time it takes for a compound to travel down the column and be collected

What must you push into the pipette before adding the sand and MgSO4?

A small piece of cotton

After your pigments are extracted using column chromatography, what do you do?

A TLC plate with one “control” and blot the pigments next to the control to see how well the extraction was

• You’re extractions should match up with the pigment results on the sample.

What is experiment 6?

Distillation

What is distillation?

A process in which a liquid is heated to its boiling point and the resulting vapor phase is conducted to and condensed in a different container.

• Separation and purification technique for mixture of liquids that have different boiling points

What are the two kinds of distillation?

Simple distillation - a useful technique when the liquids in the mixture have ~50 degrees Celsius difference in their boiling point

• Temperature often rises as distillation occurs because the flask is having more and more of the high temperature solvent

Fractional Distillation - simple distillation but using a fractionating column to repeat the processes of distillation so that it is a more pure distillate that results

• More surface area for distillation = more accurate results (repeated cycles of evaporation and condensation as the liquid moves up the tube)

• Make sure that you have a temperature gradient so that there is no reflux back into the RB flask

What is important to note about the speed of distillation?

It must occur not too fast and not too slow in order collect pure distillate

• Heat setting on 2-3 stir bar

• Distill no longer than an hour

• Collect at 2mL and 7 mL (don’t collect more than 7mL)

What is experiment 7?

Analysis of distillate - gas chromatography

What is Gas Chromatography?

is an analytical technique of separation and analysis

• separation in the GC is based on volatility or boiling point

• Used as an analytical tool rather than a preparatory one

• Used as a identification tool by comparing an unknown with a known standard using the retention times under the same conditions preferably on the exact same instrument

What are the stationary phase and mobile phases of the GC?

Stationary phase - very thin coating of viscous liquid on a long capillary tube (VERY HIGH BP, generally non-polar)

Mobile phase - inert gas (hydrogen or nitrogen)

What is important to know about GC’s?

The solvent that you use - tells us which peak is the solvent peak so we can ignore it when looking to the chromatogram

• Where the solvent peak will be will be based on its boiling point (if you have a mix of two solutions that have high bps, use a low bp solvent, and if you have a mix with low bps, use a high bp solvent like xylenes)

What is the GC detector?

Flame ionization detector - sample is burned in hydrogen flame, producing ions that conduct electricity and can be used by the GC.

How to identify peaks on the chromatograms?

Lowest retention time = lowest BP

• To confirm, we use pure solutions of the two solutions in our mixture to get a baseline for the retention time of the liquid before we make a final judgement

What is the response factor?

What we get when we divide the area of the lower bp liquid by the higher bp liquid

• We apply this number to every molar ratio by multiplying the higher bp liquid by this number to get the adjusted molar ratio

What is experiment 8?

Free radical halogenation of alkanes

What are we doing during free radical halogenation of alkanes?

• Comparing the relative rate of free radical halogenation of six hydrocarbons under three different conditions

  • Red/orange bromine color allows bromination reaction to be easily followed (color disappearing = fully reacted)

What are the three steps in free radical halogenation?

Initiation - X₂ → 2X

Propagation - R-H +X → HX + R radical

R radical + X = X → RX + X radical

Overall reaction - R-H +X₂ → R-X +HX

What hydrocarbons do we study?

Toluene, ethylbenzene, isopropyl benzene, t-butyl benzene, cyclohexane, methyl-cyclohexane

What is the lab set up for Bromination?

At room temp: Label 6 test tubes with the name of the solution on it, add the 1M bromine solution to each of the test tubes, watch for the color to disappear for 20 minutes

• At elevated temperature, add the test tubes to a warm water bath

• For bromination with irradiation, add a lamp

What is lab 9?

Nucleophilic substitution reactions

What is the conductivity probe (experiment 9)

used to measure the solutions ability to conduct electric current between two electrodes

• Strength of the current is directly proportional to the concentration of ions in the solution, so a higher concentration of ions will give higher conductivity values

What is reflux (experiment 9)?

Allows us to run the reaction at the solvent’s BP without boiling it off

• Drips back into the reaction flask when it is heated

• DO NOT CAP THE SETUP BECAUSE ITLL EXPLODE

What are the solvents and procedure used for experiment 9?

NaI for the Finkelstein reaction (SN2) with 1-bromobutane, 1-chloro-butane, 2-bromobutane, and 1-bromo-2-methylpropane

• Add a stir bar and place beaker on stir 3-4, timing when precipitate appears (stop after 25)

Sub or elim for 3-bromopropyl benzene with sodium methoxide in methanol

• Use reflux apparatus then use the separatory funnel to separate the two layers you form

• Repeat with room temp and see what happens

Substitution - methanol and 2-bromo-2-methylpropane, 2-chloro-2 methylpropane, 2-bromobutane and 2-chloro-butane

• USE CONDUCTIVITY PROBE

What is experiment 10?

Elimination of alkyl halides (e1 and e2)

What are we doing exactly in experiment 10?

exploring the effect of the base structure on the distribution of elimination productions

What is the set-up for experiment 10?

Similar to fractional distillation but grease the joints of the column prior to assembly

What are the solvents./bases/nucleophiles for experiment 10?

KOH, KoTBu,1-propanol, 1-butanol

What tests do you perform to each of the solvents that you used after elimination has supposedly taken place?

Bromine in dichloromethane - place a few drops of .1 M bromine in dichloromethane and then add to your test tube, watching for the brown color to change to colorless

Bayer test - place two drops of product in a test tube and add 0.1M KMnO4 drops, watching for the purple color to turn to a brown precipitate

What is experiment 11?

GC analysis of elimination

What is the objective for experiment 11?

To analyze the product the students obtained from elimination experiments and compare the distributions to see which one of the two was favored over the other

• NO RESPONSE FACTOR

• Same set-up to GC of distillate experiment, except we use xylenes as our solvent (high BP) since our products have low BPs.

• Run 3 standards - 2-bromo-2methylbutane, 2-methyl-2-butene and 2-methyl-1-butene

  • 2-butene will be zaitzev, other will be hoffman (lower bp)

Recrystallization set up

Extraction set up

How to use the separtory funnel

Extraction flow chart

What the TLC plate should look like

How column chromatography is set up

How simple distillation is set up

Phase diagram for simple distillation

Phase diagram for fractional distillation

Setup for fractional distillation

Setup for reflux

Set up for elimination

Conductivity probe

GC machine