Organic Chemistry Lab Final

Electronegative substituents (halogens, oxygen), aromatic ring

_______________ move chemical shift downfield (higher ppm); _______ ______ also moves chemical shifts downfield

vapor

_____is condensed and collected to give a liquid with higher composition of the lower boiling compound

emulsion

a dispersion of very fine droplets of one immiscible liquid within another. These usually occur with alkaline solutions and with chloroform, benzene, and carbon tetrachloride.

superheating

a metastable condition when a small localized section of the liquid rises to a temperature somewhat above the boiling point, turns to gas, causing a rapidly expanding bubble to occur in the liquid. This bubble then "explodes" out of the solution causing liquid to fly up into the condenser. Such bumping can lead to contamination of the distillate.

Pressure-temperature alignment nomograph

a way to determine the boiling point

1. The following table shows the density of a number of solvents that are immiscible with water. All are used as extraction solvents with water. Why might it not be a good idea to extract a toluene solution with methylene chloride even though both solvents are used separately with water and they have significantly different densities?

a. Methylene chloride and toluene are miscible, meaning that when mixed they will not form two separate layers. In order to do an extraction, the two liquids must be immiscible in order to form two layers on contact. 32

2. What is the purpose of washing an ether solution with brine (saturated aqueous NaCl) during an extraction with an aqueous solution?

a. The purpose of washing an ether solution with a saturated aqueous NaCl solution was to remove the water from the organic layer. The NaCl solution pulls water from the organic layer into the NaCl solution because the salt concentration is higher in the solution than in the organic layer. Water travels into the NaCl solution from the organic solution because water travels from areas of low salt concentration to high salt concentration in order to dilute the solution.

Salting out (adding ionic compound to the aq. Layer)

also used to promote solutes moving into extracting solvent

melting point and boiling point

are intensive properties easy to measure that can be used to help identify a compound

splitting of peaks

caused by 1 H on adjacent carbons except when 1 H are chemically equivalent

fractionating column

causes the vaporization-condensation cycle to repeat by providing multiple surfaces for the cycle to take place

NMR

good for telling you the number of types of 1 H in a molecule and information about 1 H on adjacent carbons

simple distillation

has a single cycle of boiling of the liquid and condensation of the vapor

fractional distillation

has multiple boiling, condensation cycles and results in a better separation of the liquids

original solution to added solvent

in extraction solute moves from...

impurities

increase the observed boiling point

Coupling constant, J

is a measure of how much the peak is split

Extraction

is a way to remove a solute from a solution, agitating the solution with an immiscible solvent that attracts the solute you want to isolate

KD

is measured between an organic solvent and an aqueous solution

normal boiling point

is the boiling point when atmospheric pressure is 1 atm

Thin layer chromatography

is the first technique used for rapid qualitatative analysis of compounds.

1H

is the most common nuclei studied by NMR and the only one we've used this semester

melting point

is the temperature a solid changes to a liquid

Boiling point

is the temperature the vapor pressure of a liquid equals atmospheric pressure

impurities or mixtures

lower melting point and broaden melting point range

Infrared spectroscopy

measures excitation of vibrational motion in molecules; mostly used to identify functional groups in molecules

position

of NMR peak along the x axis is due to shielding of 1 H by electrons in the environment, chemical shift is name of x-axis

size

of NMR peak is proportional to the number of 1 H of that type

NMR (nuclear magnetic resonance)

only works for samples in magnetic field and nuclei with non-zero spin

temperature-composition plots

show composition of liquid mixture and the vapor above it at the same temperature

rapid separation of small amounts of material, qualitative analysis, monitoring reaction progress, checking purity of sample

steps of TLC

1. magnetic stir bar or spin vane 2. boiling stones

ways to keep the liquid temperature uniform

C≡C stretch

2100-2250, w, sharp

C≡N stretch

2200-2300, m, very sharp

O-H stretch, carboxylic acid

2500-3500, w/m, very very broad, rough shaped

sp3 C-H stretch

2800-3000, m/s, sharp

H on carbon next to oxygen

3.3

sp2 C-H stretch

3000-3150, w/s, sharp

O-H stretch, alcohol

3200-3500, strong, broad and symmetric

N-H stretch, amine 2 absorption for primary 1 for secondary

3300-3500, w/m, symmetric medium not nearly as wide or strong as alcohol O-H

H on aromatic ring

6.5-8

a. If the compound is impure, it will give a false reading on the melting point range. Some parts of the sample would melt faster or slower than others since the impurities will have different melting point ranges.

B. If a compound was impure, what effect would this have on the melting point measurement?

theoretical plate

Each cycle of boiling, condensation is called a

pH

Extraction may depend on _____ of aqueous solutions;_____changes protonation of solute which changes its solubility in water (same for both)

distillation / Gas chromatography

Method for separating two miscible liquids

upfield (left), downfield (right)

More shielded 1 H are _____ (lower ppm), less shielded 1 H are _____ (higher ppm)

intermolecular forces, the solvent with stronger IMF to the solute attracts the solute better

Partition between the original solvent and the extracting solvent depends on

boiling points on a graph

Pure substances will exhibit a constant boiling point. Mixtures will exhibit changes in the boiling point. An idealized curve (at the right) would show a vertical rise with an infinite slope. Actual distillations (shown below) exhibit lower slopes.

extraction

Rarely yields a pure product. Distillation, recrystallization, or chromatography of the crude product is usually necessary.

3. You are a biochemical researcher and must isolate a metabolite from an aqueous solution. The distribution constant for the metabolite between ligroin and an aqueous solution is 11.8. You extract 50.0 mL of an aqueous solution that is 0.0350 M in the metabolite with 25.0 mL ligroin. What is the concentration of the metabolite in the aqueous solution and in the ligroin?

See Experiment 2

extraction

Separation technique used to isolate one or more compounds from a mixture.

immiscible

Solvent added must be _____ so that it can be physically removed and thereby remove the solute of interest

3 σ bonds

Splitting is not observed if the 1 H are separated by more than

distillate

The vapor obtained from a boiling liquid, once cooled, will re-condense to a liquid known as the

Micro scale boiling point determination

This method is more accurate than distillation or mini scale (+/- 0.2oC) and requires a small amount of sample (approx. 0.2mL). The procedure consists of determining the temperature at which the external pressure on the boiling liquid is large enough to overcome the vapor pressure in a capillary tube inserted closed side up in the liquid. When this happens, the vapor will condense and liquid will rise into the capillary. Usually a melting point apparatus is used to heat the sample.

Ch3

0.7-1.2

Theory of Fractional Distillation

1. As the hot vapors leave the distilling flask, they condense on the first cold surface, completing one vaporization-condensation cycle. 2. This surface begins to heat from the condensed vapors which are now 55:45 toluene-benzene. This benzene enriched liquid now has a boiling point of ~94 °C (lower than the incoming vapors) and it begins to boil off this higher surface 3. These vapors are even further enriched in benzene (now 35:65, toluene:benzene) and condense on the next cold surface 4. This condensed liquid has an even lower boiling point (86 °C) and as this surface heats it begins to boil off this next higher surface 5. This vapor now condenses on the next cold surface (now 18:82, toluene:benzene) and the cycle continues 6. This cycle will continue until the top of the column is reached. The liquid collected after seven cycles is now 99% benzene!

Ch2

1.3-1.8

C-O stretch

1000-1300, s, m

C=C stretch, aromatic

1400-1600, m, m often 2

C=C stretch, alkene

1600-1700, m/w, medium and often 2

C=O stretch

1650-1800, s, medium/symmetric

H on carbon next to C=O

2.1

H on carbon attached to aromatic ring

2.3

a. The thermometer has mercury in it and the capillary tube is made of glass. The thermometer will heat up faster than the capillary tube because the specific heat capacity of glass is more than mercury. The specific heat capacity of glass is .84 J/gm K and the specific heat capacity of mercury is .140 J/gm K. b. The observed melting point would be less than the actual melting point since the thermometer heats up quicker than the glass.

A. If the Mel-Temp apparatus is heated too quickly errors occur due to a difference in the actual temperature of the sample and the temperature read on the thermometer. a. Which do you think changes temperature more quickly as the Mel-Temp apparatus, the capillary tube with the sample or the thermometer? In other words, which has the larger heat capacity? b. Based on your answer to part a, would the actual melting point be above or below the observed temperature of the thermometer?

salt

Adding _____ makes aqueous layer more polar and often increases KD by moving

a. The two samples A and B can be mixed together in equal amounts, and that sample could be placed in a capillary tube or on a glass cover slip to be analyzed in the Mel-Temp apparatus or the Fischer-Johns apparatus to determine the melting point range. If the melting point range is higher than what is known or lower than what is known, that indicates that there is an impurity in the mixture, either A or B, and therefore A and B are not the same substance. If the melting point range matches what is known, that indicates that A and B are identical, and that they represent the same chemical compound.

C. You have two samples, called A and B, which have the same melting point. How can you determine if they are the same by using melting point measurements? Explain in detail. (Adapted from Pavia et al. Organic Laboratory Techniques, page 637, question 1)

lower boiling

Compound with _____ _____ point is a larger proportion of the vapor over the heated mixture than it's proportion in the liquid

distribution coefficient, KD

Concentration of solute in original solution and extracting solvent measured by

a. At 154°C, the air that was in the bell started expanding and leaves the tube, which is why the bubbles are present. When the heating was stopped, the pressure in the bell was larger than the pressure in the atmosphere, so the liquid rises into the tube. Heating began again, so the pressures went back to equilibrium, so the liquid left the tube. At 165°C, the air that remained trapped in the tube started expanding again and left the tube which generates the bubbles. After cooling until 161°C, the liquid entered the bell again. This happens because upon cooling, the liquid vapor pressure decreased below atmospheric pressure, which compelled the liquid to enter the tube. The temperature at which the liquid enters the tube represents the boiling point of the liquid. The boiling point of the liquid in this problem is 161°C.

D. Question number 5 on page 703 in Pavia et al. Organic Laboratory Techniques, 4th Ed. During the microscale boiling point determination (see page 697-699 of Pavia et al.) of an unknown liquid, heating was discontinued at 154°C and the liquid immediately began to enter the inverted bell. Heating was begun again at once, and the liquid was forced out of the bell. Heating was discontinued at 165°C, and at this temperature a steady stream of bubbles from the bulb is observed. On cooling, the rate of bubbling gradually diminished until the liquid reached a temperature of 161°C when liquid entered and filled the bell. Explain the sequence of events (what is happening at 154 °C, 165 °C and 161 °C?). What was the boiling point of the liquid? Hint, look at the explanation and difficulties associated with the microscale boiling point method on pages 698-99.

position in cm-1 , shape of absorption (broad, sharp..) and strength (depth of absorption along y axis)

Infrared absorptions characterized by their