Chem midterm

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

What is distillation used for?

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

Types of distillation

Simple, fractional, steam, vacuum

Boiling/normal boiling point

Occurs when the vapor pressure of the liquid being heated equal to the external pressure

What would happen if the external pressure was lowered?

Less energy is needed to boil

How do we find the boiling point at a different pressure?

Use a temperature-pressure nomograph

How to use a nomograph

Pressure then boiling point at usual atmospheric pressure, connect lines and draw to observed boiling point

Simple distillation

Liquids in the mixture have about a 50 deg C difference in their boiling point

When a liquid mixture is distilled, the temperature...

Increases

Is the composition of the vapor phase different or the same as that of the liquid phase?

Different

Is the vapor composition of simple distillate all A, B, or both?

Both - it is often impractical to completely separate a mixture by means of simple distillation

When could simple distillation be ideal for separation?

1\. If the boiling point difference is 100 deg C or greater \n 2. When the mixture contains 90% of one compound and 10% of the other \n 3. If the mixture contains pure liquid and a non-volatile solid

Fractional distillation

Multiple simple distillations

What is the difference between the simple and fractional distillation apparatus?

Fractionating column

What does the fractionating column provide?

Temperature gradient, surface area for repeated vaporizations and condensation to take place before collecting the final product - at the top it is nearly pure compounds

Why is it important to maintain a temperature gradient?

Temperature decreases toward the top allowing the compound to condense and drip back down the column

What needs to happen to the temperature to distill liquid B?

Increased

How can we measure how "good" a column is for separation?

Theoretical plates, the more theoretical plates the better the separation

How many theoretical plates does simple distillation have?

One

Types of fractionating columns

Packing condenser with glass beads or metal mesh, metal mesh provides more theoretical plates, glass beads can have differing diameters, needs to be insulated

Hold-up volume

Amount of liquid that remains inside the column after the distillation is complete

Azeotropic mixtures

Many liquids when combined exhibit different behavior, namely a constant boiling point

In this distillation experiment we start the experiment with 10 mL of liquid mixture of cyclohexane and toluene. However, it is not possible to obtain all 10 mL back under the current procedure particularly for fractional distillation. Explain why that is the case for distillation and in particular for fractional distillation.

Some material is lost to evaporation and some is left behind in the apparatus as "hold-up", a small amount of material is purposefully left in the flask

Plot a theoretical distillation curve of temperature (y-axis) vs. volume in mL (x-axis) for a 15 mL of a mixture containing 60% 1-propanol and 40% 2-propanol. Are these two compounds easier to separate by distillation than cyclohexane and toluene? Explain your answer.

take 60% and 40% of 15mL to determine the volume of each compound, create graph T vs V, the larger boiling point difference is preferred

In this experiment you were asked to collect two fractions and early fraction and a late fraction. Which of the two fractions would be richer in cyclohexane? Would there be a difference between the percent of cyclohexane in the fractions collected from fractional vs. simple distillation.

Early fraction is richer in cyclohexane, percent of cyclohexane is higher in fractional distillation

A researcher at a chemical company was asked to prepare several binary mixtures of liquids for a distillation demo but he forgot to label the composition. He then took 10 mL of the mixture he prepared and ran a fractional distillation on the mixture. Given below is the data he obtained from the distillation. Plot the data as volume (X) vs. Temperature (Y) and determine the composition and identity of the two liquids in the mixture. Explain why he did not need to collect more than 5 mL of distillate. The following liquids are the possible liquids used for making the mixture: water, aetone, dichloromethane, diethyl ether, etheyl acetate, methanol and ethanol.

Spike indicates change in compound being distilled, can tell which compounds by boiling points that correspond to the data

Hot gravity filtration

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Simple distillation apparatus

Fractional distillation apparatus

Gas chromatography (GC)

Analytical technique of separation and analysis

Does GC have a mobile and stationary phase?

Yes

What is the basis of separation for GC?

Boiling point/volatility

Is GC destructive or constructive?

Destructive

What can GC be used for?

Find the amount of a compound or how many separate compounds a sample contains and the purity of a sample, determine the structure of an unknown sample, identification tool by comparison of a compound with known compounds

How much sample does GC require?

A minuscule amount

What are the steps of a GC?

1. The sample is put into the injection port

2. Volatized

3. Enters the column

4. Pushes through columns with different retention times

5. Enters FID and is burned

6. Current is measured

Stationary phase

Coated on the column walls, very thin coating of 95% silicone oil and 5% phenyl methyl silicone oil, very high bp, relatively non-polar, can change polarity

Mobile phase

Inert gas (helium gas in our lab)

How does the sample travel in the column?

Interacts with the stationary phase while being pushed by the mobile phase, the more volatile a compound the more time it is in the gas phase and the smaller the retention time

How can we control the time and resolution in GC?

By changing the flow rate of the mobile phase gas and the temperature in the column

Resolution

The degree of separation of the compounds in the mixture being analyzed

How to decrease resolution

Increasing the temperature and increasing the flow rate, vary the temperature

What is a variable that influences retention time?

Column length

FID

Flame ionization detector, sample is burned in hydrogen/air flame, produce ions that conduct electricity

How to we determine the amount of sample?

A potential of a few hundred volts is applied across a collector electrode the ions creased in the combustion produce a current which is proportional to the number of ions which is in turn proportional to the amount of sample - translated into peaks on chromatogram

Does FID burning vary?

Yes, depending on its functional group and composition

When do we use a response factor?

When comparing compounds that do not have the same molecular formula

Response factor

Equalizer, enables us to refer to the area under the peak as moles regardless of the compounds structure

Analyzing chromatograms

Pay attention to solvent used, usually the largest peak

How do we determine compounds on a chromatogram?

Know boiling points and retention times corresponding to them

What types of runs do we do when determining retention time of pure compounds?

Pure compound run

Response run

Equalize the chromatograms that contain a mixture of two compounds, equals peak areas divided by one another (one equals 1 while the other does not, the non-1 is the response factor)

Gas chromatography

Did the result from your GC analysis match the theory of distillation? Which fraction had more cyclohexane then toluene? Was the simple distillation method better than the fractional method? Compare the theory and the actual results based on the GC results.

First fractions contained cyclohexane, fractional had more moles of desired compound

Ethylbenzene and o-xylene can be separated and analyzed using GC without injecting an equimolar standard to a high accuracy. Explain why that is the case for these two compounds while we had to inject an equimolar solution for our experiment.

They are constitutional isomers, will ionize in the FID similarly, similar molecule weights

The separation of two liquids on the GC is very similar to the distillation process. Compare the two methods describing the process while pointing out to the similarities and differences between the two methods.

Distillation: compounds are evaporated then condensed, doesn't have a mobile or stationary phase, set-ups are different, theoretical plates, not destructive, requires more sample

Both: uses boiling points to separate components, separation techniques, involve purification of samples

GC: has a mobile and stationary phase, destructive, analytical technique, do not need a large sample, considers retention time

Could we separate the pigments of spinach using GC?

No, they are not volatile liquids and the pigments will degrade

Objective of this lab

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

Different conditions

Room temp, elevated temp, irradiation

What steps compose this mechanism?

1. Initiation

2. Propagation

3. Termination

Free radical halogenation

Hydrocarbons used:

toluene, ethylbenzene, isopropylbenzene, t-butylbenzene, cyclohexane, methylcyclohexane

Consider the molecule given below. How many hydrogens on this molecule could theoretically be substituted by Br in a free radical bromination - circle them. Put an asterisk to mark the one most likely to be substituted first.

Benzylic

When toluene is used in free radical bromination, a very small amount of product is formed that contains only carbons and hydrogens and no bromine. Show the structure of that product and the arrow curved mechanism of how it is formed starting from the alkyl radical intermediate of the reaction.

Similar to picture shown above

Free radical Brormination is a slower reaction than chlorination but it is a synthetic route often used to make alky bromides. Alkyl fluorides are not made by Free Radical Halongenation. Explain why free radical Fluorination is not a viable route of synthesis for these compounds (show your work to support your claim with all math calculations - values are available in the Brown textbook used in lecture). What is the major concern with performing free radical fluorination?

Bonds broken-bonds formed, fluorine is highly exothermic and unstable

The purpose of free radical bromination is to substitute a \___ for a \____

Br, H

How are the initial free radicals formed?

Light/heat exposure

Aryl

On ring

Benzylic

C attached to a ring

Allylic

C attached to a carbon atom that is double bonded

Reactivity of hydrocarbons

t-butylbenzene

What indicates a reaction has occurred?

Red/orange to clear color change

What do substitution reactions compete with?

Elimination reactions

What types of mechanisms exist for substitution?

SN1 and SN2

Conductivity detector

Measures the strength of the electrolyte, red is a weak electrolyte, red-green is a strong electrolyte, can be utilized to indicate that a reaction is taking place

Electrolyte

A solution that contains ions and conducts electricity

Reflux

Allows us to run the reaction at the solvent's boiling point without boiling off, recycling the solvent by attaching a water-cooled condenser to the reaction flask

Why do we not cap the apparatus?

Capping could lead to a fire/explosion

Reflux setup

Objective of this lab

Explore several aspects of the substitution reaction and compare factors that affect the mechanism by which the reaction proceeds as well as the relative rate at which the reaction proceeds

Group A

Finkelstein reaction, SN2, precipitate is formed, CL vs Br

Why does bromine react faster?

Bromine is a better leaving group and has a lower activation energy

Group B

Temperature, room temp - substitution, high temp - substitution and elimination

Group C

Temperature, tertiary vs secondary, Cl vs Br

Carbocation intermediate

SN1

Concerted mechanism

SN2

Stereo-inversion

Both

What are good leaving groups?

Weak bases, H2O, Br, I, HSO4-

What are good nucleophiles?

KOH, NaNH2, NaCN, NaI

What factors favor substitution over elimination?

Primary, aqueous, low heat

What function does the probe serve?

The probe measures the formation of Br/Cl anions that are byproducts of the reaction