CHEM 2445 - Quizzes, Practice MidTerm, Midterm, Practice Final (Final)

Write the 7 steps of recrystallization.

1. Choose a solvent or solvent pair 2. Dissolve solute in solvent or solvent pair 3. Decolorize the solution 4. Filter to remove any impurities and/or suspended solids 5. Recrystallize the solute 6. Collect crystals & wash with cold solvent 7. Dry crystals & calculate % yield

In the nitration of methyl benzoate, ice is added to the reaction mixture after the reaction is complete. What is the purpose of adding ice?

1. Ice is added to dilute the acid. Ice helps to control the heat evolved when the water mixes with the acid. 2. To precipitate the product by diluting the reaction mixture

A student sublimed an unknown and found that the melting point of the unknown fell between the melting points given in the list of possible structures. Give *three methods* for confirming the structure of the unknown compound. Clearly indicate what you would do and what you would observe. You may assume that samples of the compounds given in the list of possible structures are available.

1. Mixture m.pt. - mixture m.pt. will remain sharp and the same, if it is the same compound. 2. TLC - TLC side by side and as a mixture on the same plate should give a single spot in several different solvent systems. 3. IR - take IR of the unknown and a suspected known compound (NMR is also acceptable) 4. Make a derivative of the unknown and compare m.pt. and other properties with literature reports.

Describe two different techniques that are helpful when running a chemical reaction under anhydrous conditions.

1. Oven-drying of glassware prior to use 2. Using drying tube to trap moisture (before it can get into the reaction flask) 3. Using a "cold flame" to dry glassware immediately before use 4. Using dropping funnel to add reagents.

Rank 1˚, 2˚, and 3˚ for SN2

1˚ > 2˚ >> 3˚ (SN2 is concerted, no carbocation) (type of molecule refers to where the halide is)

Rank 1˚, 2˚, and 3˚ for SN1

3˚ > 2˚ >> 1˚ (SN1 forms carbocation) (type of molecule refers to where the halide is)

Which compound would you expect to have the largest vapor pressure at 25˚C? A. C7H16 (BP = 98.4˚C) B. C8H18 (BP = 125.7˚C) C. C9H20 (BP = 150.8˚C) D. C10H22 (BP = 174.3˚C) E. It is impossible to tell which compound would have the largest vapor pressure because there is no relationship between vapor pressure and the boiling point of a compound.

A. C7H16 (BP = 98.4˚C)

Impure solids generally melt at A. lower B. higher C. the same temperature as a pure solid.

A. lower

What is the effect of an insoluble impurity, such as sodium sulfate, on the observed melting point of a compound?

An insoluble impurity, such as sodium sulfate, will not affect the m.pt. of a compound.

What term is used for a mixture of two or more liquids whose proportions cannot be altered by simple distillation?

Azeotrope (ie H2O & EtOH)

What type of footwear is required in the lab? A. either shoes or sandals B. hard-soled, covered shoes C. sandals D. something in a low heel

B. hard-soled, covered shoes

Water is not used to put out most fires in the organic laboratory, because A. it takes longer to clean up B. it can react with some organic chemicals and most organic solvents float on water and will continue to burn C. water will ruin some of the instruments in the lab D. all of the above E. none of the above

B. it can react with some organic chemicals and most organic solvents float on water and will continue to burn

Why is water not used to extinguish most fires in an organic laboratory?

Because H2O can react with some organic chemicals. Most organic solvents float on water and will continue to burn.

You have been given 2.0g of a mixture containing acetylsalicylic acid (aspirin) and acetaminophen (tylenol), both of which are high melting solids and insoluble in water. From the methods you have studied in CHEM 2445, choose one or more methods to separate these two compounds and prepare pure samples of them. Explain in detail the steps you would follow.

Best method is extraction. Dissolve sample in ether. Then extraction with aqueous NaHCO3. Ether Layer 1. dry (MgSO4) 2. evaporate ether 3. gives acetaminophen 4. recrystallize if necessary Water Layer 1. acidify with dilute HCl 2. collect solid by filtration 3. gives aspirin 3. recrystallize if necessary

How can you destroy the excess sodium hypochlorite after oxidizing cyclohexanol to cyclohexanone?

By adding saturated sodium bisulfite (NaHSO3) solution.

When two miscible liquids such as cyclohexane (b.pt. 81˚C) and toluene (b.pt. 110˚C), are mixed, the boiling point of the mixture will be A. higher than 110˚C B. lower than 81˚C C. Somewhere between 81˚C and 110˚C D. None of the above

C. Somewhere between 81˚C and 110˚C

What should be done if a chemical gets in the eye? A. notify the TA; then use the eye-wash fountain B. Use the eye-wash fountain, then return to the experiment C. Use the eye-wash fountain; then notify the TA D. Nothing, unless the chemical causes discomfort

C. Use the eye-wash fountain; then notify the TA

Some of the chemicals and solvents that form peroxides in the presence of oxygen and light are A. diethyl ether B. THF C. cyclohexene D. all of the above E. only B & C

D. all of the above

What are some toxic compounds?

DCM (CH2Cl2) CHCl3 (Chloroform) o Teratogen = cause no effect on mother, but cause abnormalities in fetus o Benzene o Toluene o Xylene o Aniline o Nitrobenzene o Phenol o Formaldehyde o Dimethylformamide (DMF) o Dimethyl sulfoxide (DMSO) o Polychlorinated biphenyls (PCBs) o Estradiol o Hydrogen sulfde o Carbon disulfide o Carbon monoxide o Nitrites o Nitrous Oxide o Organolead and mercury compounds o Thalidomide o Ethanol

What is the purpose of making derivatives? What are the 3 main types of derivatives?

Derivatives allow you to make crystalline solids with high, specific mp's to allow you to identify aldehydes & ketones. Oximes, Semi-Carbazones, 2,4-DNP

Name three drying agents commonly used after extraction (to remove water from an organic solvent).

Drierite (CaSO4) Magnesium Sulfate (MgSO4) Calcium Chloride (CaCl2) Sodium Sulfate (Na2SO4) Molecular Sieves (Zeolite)

For the recrystallization experiment, why is gravity filtration rather than suction filtration used to suspended impurities and charcoal from a hot solution?

During suction filtration of a hot solution, evaporation fo the solvent takes place with rapid cooling. This will cause crystals to form, which will result in clogging the buchner or hirsch funnel.

General rules in the organic laboratory include A. eye protection with safety goggles B. wearing sneakers or shoes that cover your feet completely (ie not wearing sandals or open-toed shoes) C. not eating, drinking, smoking, or applying cosmetic in the lab D. not using cell phones, iPod, or any other music players E. all of the above

E. all of the above

The functional groups that render compounds explosive are A. azides B. peroxides C. nitro compounds D. nitroso compounds E. all of the above

E. all of the above

Which of the following compounds can cause explosions? A. azides B. peroxides C. nitro compounds D. ozonides E. all of the above

E. all of the above

__ groups activate aromatic compounds for electrophilic substitution and they are __-directing Where did we see this?

Electron-donating they are *ortho- and para-directing* Friedel-Craft Alkylation of dimethoxybenzene to 1,4-di-t-butyl-2,5-dimethoxybenzene

__ groups, on benzene and other aromatic compounds make the compound meta-directing. Where did we see this?

Electron-withdrawing groups Nitration of methyl benzoate

A sample of 4-chloroaniline is contaminated with a large amount of 1,4-dichlorobenzene which cannot be removed by recrystallization. Describe in detail the easiest experimental procedure for getting a pure sample of 4-chloroaniline.

Extraction 1. Dissolve sample in diethyl ether or MTBE 2. Extract ether with aqueous dilute HCl solution 3. Separate the organic layer, which now has 1,4-dichlorobenzene 4. Treat water layer with NaOH solution to neutralize and regenerate 4-chloroaniline. 5. Collect precipitate by filtration and wash with water 6. Allow to dry and take m.pt. (Recrystallize if m.pt. is not sharp)

A sample of benzoic acid is contaminated with a large amount of neutral compound which cannot be removed by recrystallization. Describe in detail the easiest experimental procedure for getting pure sample of benzoic acid. (When pure, benzoic acid has a mp of 121-122˚C).

Extraction: 1. Dissolve contaminated benzoic acid in diethyl ether (or MTBE) 2. Add to a separatory funnel along with sufficient H2O. 3. Add NaHCO3 to funnel to form sodium benzoate, which will enter the lower aqueous layer. 4. Separate the aqueous layer from the funnel and acidify (ie with HCl) to regenerate benzoic acid. 5. Filter to obtain precipitate of benzoic acid. 6. Determine melting point of product. If mpt is not sharp, recrystallize.

T/F: A single extraction with 100 mL of solvent can extract more compound than two separate extractions with 50 mL portions of the same solvent.

False

T/F: A single extraction with 60mL of solvent can extract more compound than three separate extractions with 20 mL portions of the same solvent.

False

T/F: A student had an unknown that gave a silver mirror with tollen's reagent and positive test with sodium hydroxide/iodide (iodoform reaction). The sample is probably a ketone.

False

T/F: An azeotrope mixture can be easily separated by fractional distillation.

False

T/F: If the TLC of a mixture of biphenyl and benzophenone is eluted with acetone, the benzophenone will move further along the plate (ie have a higher Rf value) than the biphenyl because benzophenone is more soluble in acetone than is biphenyl.

False

T/F: In order to dilute a concentrated acid, always add water gradually to acid.

False

T/F: Since the melting points of urea and cinnamic acid are the same (132.5˚C-133˚C), the urea-cinnamic acid mixture will always melt at 132.5˚C-133˚C.

False

T/F: A mixture of ethanol and water can be completely separated by fractional distillation.

False EtOH and H2O form an azeotrope, which canNOT be separated by fractional distillation.

T/F: Naphthalene has a lower Rf value than B-naphthol on a silica gel TLC plate developed with dichloromethane (look up structures of compounds, their structures were given on the exam)

False Naphthalene will have a higher Rf (because it is less polar) Beta-Naphthol will have a lower Rf because its polar -OH will interact with the polar silica gel

Hydrochloric acid is a strong mineral acid like sulfuric acid and phosphoric acid. Why would HCl not be a satisfactory catalyst in the formation of aspirin?

Hydrochloric acid is 20% HCl and 80% H2O. The water will react with acetic anhydride and hydrolyze it, impeding the synthesis of aspirin.

For the synthesis of acetylsalicylic acid, either sulfuric acid or phosphoric acid is used. Hydrochloric acid is nearly as strong as a mineral acid as sulfuric acid. Why would HCl not be a satisfactory catalyst in this reaction?

Hydrochloric acid is 20% HCl and 80% H2O. Water will hydrolyze the acetic anhydride.

What is the effect of dissolved NaCl on the boiling point of H2O?

It will *raise* the boiling point of water.

What is the effect on the insoluble substance such as sand on the boiling point of water?

It will have no effect.

Why should you never use a beaker for recrystallization?

Large opening of the beaker allows solvent to evaporate easily and reduces the volume of the solvent. A crust of impure material appears at the edge on cooling.

What does Iodoform test identify?

Methyl Ketones (colorless -> yellow)

How do you account for the smell of vinegar when an old bottle of aspirin is opened?

Moisture in the air hydrolyzes the aspirin over time to salicylic acid and acetic acid. The acetic acid smells like vinegar.

Electron-withdrawing groups, such as __, on benzene and other aromatic compounds make the compound __-directing. Where did we see this?

NO2 Carbonyls (R-C=O-R') CN CF3 SO3R NR3+ *meta*-directing Nitration of methyl benzoate

Which one would give the highest Rf value for the following compounds? naphthalene, benzoic acid, and phenyl acetate

Naphthalene, the least polar

The pKa of p-nitrophenol is 7.15, whereas the the pKa of 2,6-dinitrophenol is 5.15. Which one, if any, will be dissolved in bicarbonate solution? [The pKa of carbonic acid is 6.35]

Only 2,6-dinitrophenol will be dissolved.

If a polar compound gives an Rf value of 0.5 using elution with 1:1 hexane:ethyl acetate, what will happen to its Rf value if the amount of ethyl acetate is increased?

Rf will be larger than 0.5 (spot will travel further)

__ reactions form carbocations

SN1

Ethanolic Silver nitrate for nucleophilic substitution favors __ reactions

SN1 (because ethanol is a polar protic solvent)

__ reactions work best in polar protic solvents.

SN1 (ie alcohols)

__ reactions are concerted (do not form carbocations)

SN2

__ reactions work best in polar aprotic solvents.

SN2

NaI in acetone for nucleophilic substitution favors __ reactions

SN2 (because acetone is a polar aprotic solvent)

Show the mechanism of the conversion of 1-butanol with sodium bromide and sulfuric acid to synthesize 1-bromobutane? Calculate the % yield of 1-bromobutane from the amounts of starting materials indicated. SHOW ALL WORK. Indicate clearly which component is the *limiting reactant*. The following were used in the experiment: 24.8g NaBr (MW = 102.9); 14.8g 1-butanol (MW = 74.12); 20mL H2SO4 (density 1.84 g/mL) (MW = 98.08). The yield of 1-bromobutane (MW = 137) was 22g. Consider the separation of p=dimethoxybenzene, 4-tert-butylphenol, and benzoic acid you performed. What is the structure of the compound in the water layer after you extracted with sodium bicarbonate? What are the names of the following laboratory equipment or glassware?

See exam #1

What device is used in the chemical laboratory to separate two immiscible liquids of differing density (a device you used)?

Separatory Funnel

What are two common stationary chromatography phases for TLC?

Silica [(SiO2)x] Alumina [(Al2O3)x]

For the nucleophilic substitution experiment, what would be the effect of carrying out a reaction of sodium iodide in acetone with 1-bromobutane, when the iodide solution is half concentrated.

Since the rate of SN2 reaction is proportional to the concentration of both reactants, the rate will be *decreased by half*.

Consider the separation of p-dimethoxybenzene, 4-tert-butylphenol, and benzoic acid you performed. What is the structure of the compound int he water layer after you extracted with sodium bicarbonate?

Sodium Benzoate

What are some solvents that can form peroxides?

THF Allylic Compounds Benzylic Compounds Aldehydes Ketones

Three test tubes labeled X, Y, and Z contain substances with approximately the same melting points. Although the TA says that the compounds are pure, there is a possibility that the tubes may contain the same compound. How could you prove the test tubes contain three different chemical compounds?

Take the m.pt. of the three mixtures: xy, yz, and zx. If any two of them are identical, there will be no depression of the m.pt. If each mixture shows depress of m.pt., they are all different compounds.

What is the main purpose of recrystallization?

The main purpose is to purify solid organic compounds

What is a key feature of a good recrystallization solvent?

The solute should be soluble when the solvent is hot and not when the solvent is cold.

What problem will ensue if the level of the developing liquid is higher than the applied spot in a TLC analysis?

The spot will dissolve in the developing solvent and wash away. Since the spot travels by capillary action, it will not travel up the TLC plate.

What will be the result of applying too much compound to a TLC plate?

The spots of different compounds will overlap, so that they cannot be distinguished. In other words, there will be trailing instead of distinct spots.

Why is the boiling point of two immiscible liquids below the boiling point of either pure liquid? Explain briefly.

The two liquids exert their vapor pressure independently. Boiling begins when the sum of the vapor pressures of the two immiscible liquids equals the atmospheric pressure.

What is the 'salting out' procedure?

To decrease the solubility of a compound (such as cyclohexanone) in water, inorganic salts such as K2CO3 (potassium carbonate) or NaCl is added. Then most of the inorganic compound can be extracted out with ether or another non-miscible organic solvent.

In what order (from top to bottom) would you expect to find naphthalene, benzoic acid, and phenyl acetate on a silica gel TLC plate developed with dichloromethane?

Top 1. Naphthalene (least polar) 2. Phenyl acetate 3. Benzoic acid (most polar) Bottom

In what order (from top to bottom) would you expect to find naphthalene, phenol, and phenyl acetate on a silica gel TLC plate developed with 5% and 95% dichloromethane? (look up structures of compounds, were given on exam)

Top (highest Rf) 1. Naphthalene 2. Phenyl Acetate 3. Phenol Bottom (lowest Rf)

T/F: Activated charcoal is used to remove colored impurities during recrystallization, since they can absorb high-molecular-weight impurities.

True

T/F: Benzoic acid has a lower Rf value than Beta-naphthol on a silica gel TLC plate developed with dichloromethane.

True

T/F: In the synthesis of 1-bromobutane, water and 1-bromobutane cannot be separated by distillation because their boiling points are very close.

True

T/F: The reflux method is a convenient way to maintain a relatively constant temperature for a reaction.

True

T/F: Two miscible liquids of similar chemical structure that boil at the same individually will have nearly the same boiling point as a mixture.

True

How would you separate two immiscible liquids from each other.

Two immiscible liquids will remain in two layers and a separatory funnel is the best tool to separate them.

What are two common methods of visualizing the spots of molecules on TLC plate?

UV light & Iodine Chamber

To make dilute sulfuric acid, which one of the following is the wrong answer? A. gradually add water to concentrated sulfuric acid B. gradually ad concentrated sulfuric acid to water C. must wear safety glasses D. addition should be carried out in a hood

WRONG answer is asked for! A. Gradually add water to concentrated sulfuric acid

In an extraction involving dichloromethane/water, which phase will be on top?

Water will be on top (D = 1.0 g/mL) DCM on bottom (D = 1.33 g/mL)

Under which circumstances is it wise to use a mixture of solvents to carry out a recrystallization?

When no single solvent has the desired property of rendering the solid soluble when hot and insoluble when cold.

Why do we use a minimum amount of solvent to dissolve the solid that is to be crystallized?

With excess solvent, recovery of the desired solid will be less, since some solid will remain in the solvent.

An organic chemistry student sets up an experiment outside the hood. He is wearing gloves as he dispenses diethyl ether, a volatile flammable solvent. By accident he spills some ether on his hand but continues on with the experiment. After the experiment is complete, he cleans up, pours the ether down the drain, and washes the glassware. He then takes off his gloves and washes his hands thoroughly. List all the safety issues with this scenario, if any.

a. Not doing the experiment in the hood b. Not taking off the gloves and washing hands thoroughly with water immediately after the spill c. Pouring ether down the drain, instead of in the waste bucket

Write the one-step chemical reaction for the synthesis of aspirin from salicylic acid and acetic anhydride, which you performed in the lab.

acetic salicylic acid = aspirin H2SO4 *or H3PO4* are written in place of H+ Don't write the acetic acid in the products (See quiz 5, question #1)

Why is it necessary to perform the Diels-Alder reaction we conducted in the hood?

because it generates poisonous CO gas

Electron-donating groups, such as __, activate aromatic compounds for __ and they are __-directing. Where did we see this?

-OH -OR -NH2 alkyl (ie CH3) -SH *electrophilic substitution* they are *ortho- and para-directing* Friedel-Craft Alkylation of dimethoxybenzene to 1,4-di-t-butyl-2,5-dimethoxybenzene

How should the reaction conditions of the synthesis of 1-bromobutane be changed in order to produce 1-chlorobutane?

Instead of using NaBr, *use NaCl* to react with 1-butanol after protonation with an acid (ie H2SO4). Still use *excess NaCl and acid* to ensure full conversion of 1-butanol to 1-chlorobutane. Also *increase reaction time & temperature*.

What does Schiff's test identify?

Aldehydes (colorless -> magenta)

What does Tollen's test identify?

Aldehydes (silver mirror forms)

What does bisulfite addition test identify?

Aldehydes & Methyl Ketones (forms white precipitate)

What problem will arise if the level of the developing liquid is higher than the applied spot in a TLC analysis?

If the spot is submerged int he solvent, it will wash off the plate and be lost. Also, the spot will not travel as efficiently because the spots travel by capillary action.