Ochem Lab Exam 1

why sublimation under vacuum instead of atmospheric pressure?

vaccum is needed to lower the pressure so that phase change from a solid to a gas can occur. without a lowered pressure we go straight to liquid instead

What is the most recognizable signal in the IR spectrum of the starting molecule that will not be present in the product spectrum?

Approx: 3400 Stretch: OH

What does Rf stand for?

Retardation factor. Rf = spot distance/total distance

Upon drying the organic layer, what are two indications that the organic solution is dry?

1. Remaining solution will appear translucent. 2. The drying agent will float and not stick to the glassware.

List one or two criteria needed in order for two liquids to form an ideal solution that follows Raoult's law.

1. The intermolecular forces between the molecules must be the same, without this in common they will be more likely to escape the liquid. 2. It takes the same amount of energy to break and form bonds between the two.

What are the two most probable contaminants in the benzoic acid sample?

1. Water 2. NaCl or HCl

How much 6M HCl would you expect to use to neutralize the combined aqueous extracts (don't forget to consider the benzoic acid)?

15.64mL.

Starting with 2.5g of benzoic acid, what is the theoretical amount of 5% sodium hydroxide solution needed to convert all of it to sodium benzoate? Given this amount of sodium hydroxide needed, why is more used (ie. 3 X 25 mL extractions)?

16.36 mL NaOH is needed. More is used because benzoic acid is a weak acid, meaning it needs more washes in order to prevent it from returning back into itself instead of becoming sodium benzoate.

Define extraction.

A technique used to separate an existing compound from a mixture.

During the extraction of the pigments, when the acetone pigment mixture was mixed with hexane and allowed to stand, what happened?

An emulsion formed because the two did not combine, changes in polarity impact this.

How is infrared spectroscopy useful to a chemist?

It can be used to determine specific functional groups that are present in a substance based on the peaks that are recorded.

what is sublimation and why are we using it

It is the process of isolating one from other compounds through the use of a phase change. the phases change from solid to gas and back to solid. this prevents us from needing to separately change the liquid into a solid.

What is the biggest potential hazard in the 4-methylcyclohexanol experiment?

Phosphoric acid and sulfuric acid are corrosive and may cause burning of the skin.

At what volume of distillate collected, during fractional distillation, would a rapid increase in temperature be expected for 100mL of a 1:1 mixture of an ideal solution?

Since it is a 1:1 ratio you would expect 50mL.

Explain how intermolecular forces are important in gas chromatography and how they affect retention times.

Strong/many intermolecular forces leads to a higher temp needed for a compound to boil. With a higher temp needed to boil, the time it takes (retention time) to pass through the gas chromatography increases. low bp = fast high bp = slow

After crystallization why are the crystals washed with COLD solvent?

The cold solvent prevents the crystals from melting while being able to absorb more impurities than it would if the solvent was hot.

If pure acetone and hexane are mixture (hint: they are mixed in a separatory funnel), what should happen and why didn't this occur in this experiment?

The mixture would be almost miscible, but the separatory funnel allows us to separate hexane on top and acetone on bottom.

What is the function of the potassium permanganate test? What functional group does it detect? Draw the reaction of this functional group and the resulting product.

The test allows us to detect double and triple bonds which are helpful for identifying molecules. It detects alkenes. Note card 5

What is an azeotrope?

They are mixtures in which their vapor and liquid states have the same concentrations. i.e. their boiling point is constant and can't be used for fractional distillation.

For a water cooled condenser, the condenser is arranged so that water enters the condenser at the lowest point and leaves at the highest point. Why?

This allows water to be present at all times. Condensation takes places as the vapor passes through the water molecules towards the bottom, creating a liquid.

Why is a background scan performed on the IR before scanning a sample?

We want to remove background noise that could be falsely attributed to our samples.

What is the reaction used to form benzoate from benzoic acid? What reaction is used to form benzoic acid from benzoate? What does biphenyl look like?

reverse the reaction as needed

Why is the flask containing the hydroxide extracts cooled before adding HCl?

Because the reaction is exothermic and we want to make sure glassware does not break when he heat is given off.

why was ethanol used for sn1 reactions

sn1 reactions use polar portic solvents

why is acetone used for sn2 reactions

sn2 reactions prefer a polar aprotic solvent

for sn1 reactions what reacts the fastest

tertiary then secondary then primary

what impurities from tea leaves may cause the isolated caffeine to have a brown or green color instead of white

the brown = flavonoids green = chlorophyll Also Gallic acid

When a solid compound is contaminated what happens to its melting point?

The melting point is lowered and the melting point range is increased (less specific/narrow).

List the two reasons why biphenyl stays in the organic layer and doesn't move to the basic aqueous layer.

1. Biphenyl is not soluble with water. 2. Biphenyl does not react with NaOH to make a salt like benzoic acid does.

What are the two most probable contaminants in the biphenyl sample you isolated?

1. Ether 2. Sodium sulfate.

What are two problems that could arise in the preparation of a KBR pellet for IR analysis?

1. Impurities will impact the IR and create an inaccurate reading. 2. If the sample is not dried fully the IR will not be accurate.

Without knowing the densities of the liquid layers, what is an empirical method that could be used to determine which layer is the organic layer?

Generally, organic layers are going to be less dense compared to water. So, water droplets could be used to identify the layers. If the water droplet passes through top layer and remains in the second layer, then layer 1 is organic and 2 is aqueous. This worked for all but one lab.

How can the separation of compounds be improved [in simple/fractional] using one of the packing materials in the experiment?

If a packing material has a polarity, the compound that is attracted to that polarity will move through the column slower than the compound that is not attracted to the polarity.

What is a main characteristic of a good solvent used in crystallization?

The solvent is able to remove impurities from the sample without removing the actual sample itself.

why were tea leaves boiked in water containing calcium carbonate instead of just plain water

prevents the caffeine and tannins from being dissolved directly into the water

for sn2 reactions, what reacts the fastest?

primary then secondary then tertiary

The versatility of column chromatography results from many factors that can be adjusted. Name three of these factors.

1. Polarity of the solvent chosen. 2. Absorbent chosen. 3. Rate of the flow/elution.

Thin layer chromatography has several important uses in organic chemistry. Name four of them.

1. To determine the number of compounds in a mixture. 2. To establish two compounds as identical. 3. To monitor the progression of a reaction. 4. To determine the appropriate solvent needed for the compounds to separate (by polarity).

Circle the correct type of mechanism for the dehydration of 4-methylcyclohexanol.

E1

What is the pKa of benzoic acid? Rather than use a visual indicator for complete precipitation, one could use pH as an alternative. Given the pKa of benzoic acid what pH should you adjust the aqueous solution to when adding 6M HCl to the sodium benzoate solution to make sure all of the benzoic acid is precipitated?

The pKa is 4, therefore the pH should be less than 4 to achieve complete precipitation.