Ochem Lab Final

What does melting point tell us

How pure a substance is

What statement fits Melting point depression the best

When an impurity lowers the melting point of a substance

Which statement describes refractive index

Measures how much a liquid chemical bends light to test for impurity

For recrystallization to work well, the compound to be purified should be ___________ in hot solvent and ________ in cold solvent

soluble; insoluble

Benzoic acid was almost completely______ in ice-cold water

insoluble

Benzoic acid is an example of a _______ acid

carboxylic

If a student started with 1.0 g of impure benzoic acid and recovered 0.50 g of benzoic acid after the recrystallization. What was the percent recovery? (0.5/1.0)x100

50%

An impure compound should have a melting temperature that is ____ and ______ in range than a pure compound

lower; broader

Pure benzoic acid is a white crystalline ____

solid

Why did we place the benzoic acid solution into an ice-water bath

to maximize the recovery of benzoic acid

Why was the collected benzoic acid placed on a watch glass and then heated on a plate

to remove water by evaporation

A mixture of four pure liquid compounds labeled A, B, C, and D is being considered for distallation. Given the boiling points below, which of the four compounds in this mixture could be purified by distallation?

A…bp…94 C

B….bp 78 C

C..bp..85 C

D…bp…78 C

Compound A and C

Why should boiling chips be added to the distallation flask before beginning the distallation?

To reduce “bumping” and insure a smooth boiling process

Refractive index is a measurement of the ability of the ability of a transparent substance to do what?

Change the path of light rays

The refractive index at (20 C) of pure methanol is 1.3314. The refractive index at 1-butanol (at 20 C) is 13.993. What would be the approximate refractive index of a 50/50 mixture (half and half) of methanol and 1-butanol be at 20 C?

1.3654

The solution of ether and the solid mixture was extracted with an aqueous solution of 3 M NaOH. Which compound in the solid mixture reacted with the sodium hydroxide to form an ionic compound that was soluble in the aqueous phase?

Benzoic Acid

If the solution of diethylether and the solid mixture was FIRST extracted with an aqueous solution of 3 M NaOH, then which compounds from the solid mixture would have remained in the ether layer (organic phase) after the extraction was completed?

dibenzil and meta-nitro-aniline

What was this ionic compound that dissolved in the aqueous 3 M NaOH solution?

Sodium benzoate

The 8 mL of aqueous 3 M NaOH solution (used to extract the ether solution) was acidified by addition of 6 M HCL (aq) until the solution turned acidic. Then a white precipitate was observed and collected by vacuum filtration. What was this white crystalline compound that formed a precipitate after neutralization of the aquous solution?

Benzoic Acid

After the ether solution (containing the solid mixture) was extracted with aqueous 3 M NaOH solution and with 3 M HCl solution, the ether solution was washed with water and then dried with sodium sulfate. After removing the sodium sulfate by gravity filtration, the collected ether solution was evaporated in the hood using a stream of compressed air. What was the compound that was left as a solid after the ether was completely evaporated?

dibenzil

What drying agent was used to remove traces of water from the ether layer before the solvent evaporation step?

sodium sulfate

Diethylether and aqueous solutions (mostly water) did not mix in the separatory funnel and remained as separate phases. The term that describes the fact that diethylether and water did not mix is

immiscible

The literature value for the melting point of benzoic acid is given as 122.4 C. Supposed the benzoic acid recovered from the extraction experiment had an experimental melting range of 117.3-121.2 C. The purity of the sample of benzoic acid should best be described as

fairly pure

Fluorene is a compound with a molecular formula of C13H10 and therefore it only contains carbon and hydrogen atoms. The lack of very electronegative atoms such as oxygen, nitrogen, sulfur, chlorine or fluorine means that fluorene is a ___________ compound.

non-polar

Fluorenone has a molecular formula of C13H8O1 and it contains a carbonyl group (C=O). The carbonyl group has a dipole moment and it can act as a hydrogen bond acceptor but cannot act as a hydrogen bond donor. Fluorenone is best described as a _________________ compound.

moderately polar

Fluorenol has a molecular formula of C13H10O1 and it contains a hydroxyl (-OH) group. The hydroxyl group has a dipole moment and it can act as both a hydrogen bond donor and a hydrogen bond acceptor. Fluorenol is best described as a _______________ compound.

polar

Dichloromethane has the molecular formula CH2Cl2 and it has a small dipole moment. Dichloromethane is best described as a _______________ solvent.

very polar

Silica is a polymer with general formula (-SiO2-)n. Silica is best described as a _____ absorbent

polar

When fluorene, fluorenone and fluorenol were tested by thin-layer chromatography (TLC), which compound has the highest Rf value?

Fluorene

When fluorene, fluorenone and fluorenol were tested by thin-layer chromatography (TLC), which compound had the lowest Rf value ?

Fluorenol

For a thin-layer chromatogram (TLC strip) the distance moved by the solvent (from origin to final solvent line) was 5.0 cm. If an unknown compound travelled a distance (from origin to final spot position) of 2.5 cm, what would be the Rf value ?

0.50

The technique of thin-layer chromatography separates compounds based on their differences in ___________________.

polarity

Could thin-layer chromatography be used to separate compounds that were gases at room temperature and atmospheric pressure?

No

Using a solvent of 50/50 isopropyl alcohol/water, t-butyl bromide reacted much faster than t-butyl chloride. What is the reason this was observed ?

The bromo group is a better leaving group than a chloro group

Using 50/50 IPA/water as the solvent, t-butyl bromide (2-bromo-2-methyl-propane) reacted much faster than isopropyl bromide (2-bromo-propane). What is the reason this was observed ?

The t-butyl bromide formed a teritary carbocation which is more stable intermediate

Why did it take t-butyl chloride over 15 minutes to react?

The chloro group is a poor leaving group

Why did it take isopropyl bromide (2-bromo-propane) many hours to react ?

There is a high energy of activation to form the secondary carbocation which is less stable.

The t-butyl bromide reacted slower in 60% IPA/40% water than it did in 50/50 IPA/water. Why was this observed ?

The higher solvent polarity inhibited solvolysis

The t-butyl bromide reacted faster in 40/60 IPA/water than it did in 50/50 IPA/water. Why was this observed ?

The higher water content resulted in a more polar solvent that promoted solvolysis.

What is the first step in the SN1 reaction mechanism for t-butyl bromide.

Solvolysis

What is the second step in the SN1 reaction mechanism of t-butyl chloride ?

Nucleophilic attack

Suppose compound A reacts in one minute and compound B reacts in 2 minutes. What is the relative rate of reaction for compound A as compared to compound B. In other words, what is the ratio of RateA / RateB ?

2

What is the correct rate expression for an SN1 reaction ?

Rate=k [R-X]¹

The original reactant was 1-butanol; however, when this chemical was placed in a cold solution of concentrated hydrobromic acid and sulfuric acid what was the 1-butanol converted to?

an oxonium ion

What was the purpose of placing 1-butanol in a solution of concentrated hydrobromic acid and sulfuric acid?

To convert a poor leaving group (-OH) to a good leaving group (H2O)

After adding the reactants the reaction mixture was heated to boiling and refluxed for about 1 hour. Why was this done?

Heat favored elimination and caused the SN2 reaction to go to completion.

The conversion of 1-butanol to 1-bromo-butane involved an SN2 mechanism. Which of the following statements is true concerning this mechanism.

An SN2 mechanism occurs as a single concerted step

After the reflux period was done, the reaction mixture was transferred to a test tube and two layers were eventually observed. Why did two layers form?

The product, 1-bromo-butane was not soluble in the aqueous phase which contained strong acids

After removing the aqueous acid layer, the 1-bromo-butane layer was washed with aqueous sodium bicarbonate. Why was this done?

To remove residual acids from the product layer containing 1-bromo-butane.

After several washes with water and aqueous sodium bicarbonate, the product layer (containing 1-bromo-butane) was then transferred to another container. What drying agent was added to remove traces of residual water?

anhydrous CaCl₂

The IR spectrum of the product (mostly 1-bromo-butane) showed there was very little of the reactant (1-butanol) present. What key feature allowed this to be concluded?

There was no broad signal at 3300 cm-1 present, which would have been due to O-H stretch.

What key feature of the IR spectrum of the product allowed confirmation that 1-bromo-butane was obtained?

A sharp signal at about 600 cm-1 due to C-Br stretch.

What two physical constants were necessary to use in the calculation of the theoretical yield of 1-bromo-butane?

The molar mass of 1-butanol and the molar mass of 1-bromo-butane.

The preparation of cyclohexene involved acid-catalyzed dehydration of the starting reactant. What was this reactant?

cyclohexanol

As soon as the reactant was added to the solution of concentrated phosphoric acid and sulfuric acid, a certain reaction intermediate was formed. What was this intermediate?

oxonium ion

Upon heating the reaction mixture containing the reactant and the concentrated acids, a dehydration step occurred to form what reaction intermediate (the second reaction intermediate) in the mechanism?

secondary carbocation

Upon forming the second reaction intermediate, what type of chemical species was needed to abstract a hydrogen ion (proton) to form the final product?

Base

As the product was made in the reaction mixture, it was distilled due to the low boiling point. What was the product that was distilled and collected in the receiving flask?

1-bromo-butane

A small amount of water was collected in the receiving flask along with the major product. What drying agent was used to remove water from the major product collected by distillation?

Calcium chloride (anhydrous)

The starting material could be identified by the IR spectrum. What was the key feature in the IR spectrum that identified the starting reactant used to synthesize the product?

Signal at 3300 cm ^-1 due to O-H stretch

The final product could be identified by the IR spectrum. What were the key features in the IR spectrum that identified the final product?

Alkene =C-H stretch (3050 cm-1) and Alkene C=C stretch (1650 cm-1)

If the final product were examined by IR spectroscopy and seemed to have the proper features for the product; however, a weak broad signal at 3300 cm-1 was observed, what would this indicate?

Water or cyclohexanol was still present as contaminants.

Suppose a student weighed the final product and after doing calculations, they obtained a 110% yield. Since it is not physically possible to obtain more than 100% yield, what was the most likely explanation for this finding?

The product must be contaminated with water or cyclohexanol.