Ochem 2 Lab Questions

In a hypothetical molecule, protons A and B are coupled to each other. Proton A has a chemical shift of 6.02 ppm. What chemical shift for proton B will give the largest leaning effect for the signals of both protons? 4.02 ppm 5.79 ppm 6.12 ppm 8.99 ppm

6.12 ppm (the one closest to 6.02 ppm)

Shown below are some of the steps necessary to prepare your column. Order them as steps 1, 2, 3, or 4 of the process (for example, A1, B2, C3, D4) A: Add dry slurry of silica plus sample, using weigh paper to transfer B: Add pre-elution solvent C: Add first elution solvent D: Attach bulb and push solvent level down to just above silica. Repeat until all silica is wet, topping up solvent as needed

B1, D2, A3, C4

A student has a mixture of two compounds and has determined that the best solvent system to elute the first compound is solvent system 2, hexanes/ethyl acetate 80:20. Instead, they began the separation by eluting the column with hexanes/ethyl acetate 50:50. Both compounds came off in the first 5 fractions. Explain the results.

Because the student used a solvent that was far more polar than the correct one (that would have separated the two compounds), both of the compounds to move faster through the column (and not separate sufficiently from eachother). This cause both compounds to be eluted in the first 5 fractions.

What reagents would you use to synthesize the following ester? CH3CH2COOCH2CH2CH3

CH3CH2CH2COOH (the carboxylic acid) + H3CH2COH (ethanol) + H+ (from H2SO4) to create the ester above.

Of the following compounds, which will be the first, second, and last to come off an alumina chromatography column? Explain. Aromatic ring carboxylic acid alcohol

Carboxylic acid compound will elute first. Alcohols second, aromatic compound third due to polarity.

Under irreversible (kinetic) conditions, will the exo or endo product be preferred for this reaction? a. Impossible to tell b. Neither c. Endo d. Exo

Endo

Explain why concentrated H2SO4 (98% solution in water) is a far superior esterification catalyst compared with concentrated HCl (37% solution in water).

H2SO4 is 96% by weight in aqueous solution, and HCl is 37% by weight in aqueous solution. Therefore, sulfuric acid has less water; water would drive the equilibrium towards the reactants. Another reason that sulfuric acid is a better catalyst is that sulfuric acid is dehydrating, or has a great affinity to water. Since it is a powerful dehydrator which binds water, it drives the equilibrium to the right.

From what solvent will you recrystallize the benzhydrol? Methanol Acetone Hexanes Water

Hexanes

It is very important that the column never runs dry during column chromatography. Why is this?

If the column runds dry, air gets forced into the column and may cause the absorbent in the column to not be uniform (it cracks or bubbles). This causes the separation to not be nearly as efficient as it could have been.

What is the name of the functional group you will form during today's experiment? Enamine Amine Imide Imine

Imine

Imagine you are setting up a reaction that uses an acid catalyst to create your desired product. In order to maximize atom economy, should you choose an acid with a high or low molecular weight?

It doesn't matter - atom economy does not include MWs of catalysts

What are the advantages of using water as a solvent for this experiment? (select all that apply) It is more environmentally friendly. a. It dissolves the starting materials more effectively than an organic solvent would. b. It allows the molecules to be activated into an excited state where the LUMO is more accessible. c. It causes the reaction rate to increase by squeezing together the reacting molecules. d. It is more environmentally friendly.

It is more environmentally friendly. It causes the reaction rate to increase by squeezing together the reacting molecules.

How will you purify the crude product of today's experiment? TLC Distillation Recrystallization Column chromatography

Recrystallization

The solubility of a compound with a melting point of 65°C is as follows: 8 g/100 mL hexanes (68˚C), 1 g/100 mL hexanes (0˚C). If you tried to recrystallize 1 g of this compound from 10mL of hexanes, you would notice an oil forming in the hot mixture. What is this oil? What would you do to correct the problem so that the compound will recrystallize properly?

The oil is the compound oiling out of the solution or forming a liquid because the solution is saturated and above the melting point of the compound (so it forms an oil). To correct this problem, I would add more hexanes until the oil disappears (there is enough to dissolve all of the compound).

A column chromatography procedure for the separation of a polar and a non-polar compound calls for sequential elution with hexanes, followed by ethyl acetate. Would the two compounds separate if the ethyl acetate were used as the first eluting solvent? Why or why not?

The two compounds probably wouldn't separate if ethyl acetate were used as the first eluting solvent. Ethyl acetate is more polar than hexanes, therefore when you elute with ethyl acetate first, it will cause both the polar and non-polar compound to move rapidly through the column.

During today's experiment, which base will you use? Potassium hydroxide Tribasic potassium phosphate Butyllithium

Tribasic potassium phosphate

When one molecule of benzil reacts with one molecule of 1,3-diphenylacetone, how many aldol condensation reactions will occur in the process of forming the final product? Zero One Two Three

Two

What method will you use to isolate your dopant? Extraction Column chromatography Polarimetry Vacuum filtration

Vacuum filtration

What is the term that describes any compound that has a negatively-charged atom bonded directly to a positively-charged atom? Bases Phosphines Acids Ylides

Ylides

Which of the following can help initiate a sluggish Grignard reaction? (select all that apply) a. A few drops of preformed Grignard reagent b. A sonicator c. Water d. A crystal of iodine

a. A few drops of preformed Grignard reagent b. A sonicator d. A crystal of iodine

A normal Diels-Alder reaction is facilitated by: (select all that apply) a. An electron donating group (EDG) on the diene b. An electron withdrawing group (EWG) on the diene c. An electron donating group (EDG) on the dienophile d. An electron withdrawing group (EWG) on the dienophile

a. An electron donating group (EDG) on the diene d. An electron withdrawing group (EWG) on the dienophile

During this experiment, how will you make sure that the liquid crystals are all lined up in the same direction within the cell? a. By shearing the slide b. By applying an electric field to the slide c. By tipping the slide vertically, so the molecules line up against gravity d. By coating the slide with an alignment layer, then rubbing it in a single direction.

a. By shearing the slide

Grignard reagents can add to: (select all that apply) a. Ketones b. Alcohols c. Aldehydes d. Esters

a. Ketones c. Aldehydes d. Esters

Why is it considered "green chemistry" to perform this reaction by grinding, rather than in solution? (select all that apply) a. No energy is needed to heat the reaction to reflux. b. The product has a green tinge to it. c. No solvent is needed. d. A weaker base can be used.

a. No energy is needed to heat the reaction to reflux. c. No solvent is needed. d. A weaker base can be used.

Which of the following are considered disadvantages of using LiAlH4 for reductions, instead of NaBH4? (select all that apply) a. Reactions using LiAlH4 must be carried out under inert, anhydrous conditions b. LiAlH4 reacts violently with water c. LiAlH4 is selective, reducing aldehydes and ketones but not carboxylic acids or acid chlorides d. LiAlH4 is a weaker reducing agent

a. Reactions using LiAlH4 must be carried out under inert, anhydrous conditions b. LiAlH4 reacts violently with water

Which of the following techniques separate compounds? (select all that apply) a. TLC b. NMR spectroscopy c. IR spectroscopy d. Extraction e. Distillation

a. TLC d. Extraction e. Distillation

Which alkene should show a larger coupling constant between its vinylic protons? a. The E alkene b. Impossible to tell c. They should both be the same d. The Z alkene

a. The E alkene

An aldol condensation between two different aldehydes or ketones, or a "crossed-aldol condensation", generally results in: a. Pure aldol b. A mixture of products c. Exactly two products d. One product being preferentially formed

b. A mixture of products

During today's experiment, when will the reaction mixture change to the color of your dye? a. Immediately after the addition of diethyl carbonate or methyl benzoate b. After the addition of hydrochloric acid c. When the Grignard reagent first forms d. After the fabric has been dipped in the reaction mixture

b. After the addition of hydrochloric acid

Consider the compounds in the following table: Compound MP BP 2-Nitroaniline 71 284 Aniline 61 84 Benzhydrol 69 301 Benzoic acid 122 249 Benzophenone 48 306 Ethylbenzene 95 136 Which separation technique would be the best method to separate a 1:1 mixture of aniline and ethylbenzene? a. Gas chromatography b. Chemically active extraction c. Distillation d. Column chromatography

b. Chemically active extraction

What compounds will you mix with your chiral dopant to form a liquid crystal phase? (select all that apply) a. Cholesterol b. EBBA c. Biphenyl-4-carboxaldehyde d. MBBA

b. EBBA d. MBBA

As the reaction proceeds, what change will occur? (select all that apply) a. The reaction will solidify, since the product is a solid b. The reaction will become darker, since the product is dark purple c. The reaction will become clear, since the starting materials are clear d. The reaction will vaporize entirely, since it is generating heat

b. The reaction will become darker, since the product is dark purple

What aspects of today's reaction make it green? (Select all that apply) a. The final product is green in color b. The solvent is produced from renewable feedstocks c. The reaction has high atom economy d. The base is relatively mild

b. The solvent is produced from renewable feedstocks c. The reaction has high atom economy d. The base is relatively mild

What should you observe in the TLC plates you develop as the reaction proceeds? a. A white precipitate should form on the TLC plate b. The starting material spots should disappear and the product spot should grow stronger c. The product spot should disappear and the starting materials spots should grow stronger d. Water should form on the TLC plate

b. The starting material spots should disappear and the product spot should grow stronger

In a Fischer esterification ,what can be done to shift the equilibrium to the right? (select all that apply) a. Keep the reaction in the dark b. Use a large excess of the alcohol c. Remove water from the reaction by adding a drying agent d. Pretreat the carboxylic acid with a strong base to activate it

b. Use a large excess of the alcohol c. Remove water from the reaction by adding a drying agent

Grignard reactions may be slow to initiate because the magnesium turnings may: a. React with ether instead b. Be coiled too tightly c. Be coated with magnesium oxide d. Be insoluble in ether

c. Be coated with magnesium oxide

Sort these phases from most ordered to least ordered: I, Cr, N. a. I, Cr, N b. I, N, Cr c. Cr, N, I d. N, Cr, I

c. Cr, N, I (Crystal, Nematic, Isotropic)

During silica gel chromatography, it is normal for: a. All compounds to elute at identical rates, regardless of polarity b. Less polar compounds to elute more slowly than more polar compounds c. More polar compounds to elute more slowly than less polar compounds

c. More polar compounds to elute more slowly than less polar compounds

In the work up for today's reaction you will be basifying the reaction mixture by pouring it into an excess of aqueous sodium bicarbonate. Why is it important to make the reaction mixture basic prior to extracting the desired product with dichloromethane? a. The basic wash deprotonates the ethanol, making it more soluble in water b. This is a trick question - sodium bicarbonate is a mild acid c. The basic wash deprotonates any unreacted carboxylic acid, making it more soluble in water d. H2SO4 can react violently with dichloromethane

c. The basic wash deprotonates any unreacted carboxylic acid, making it more soluble in water

If you run an IR of your product from this experiment and see a peak at 1700 cm-1, you can conclude that: a. The product is contaminated with NaBO2 b. There is an aromatic ring in the product c. There is starting material in the product d. The product contains an alcohol group, as expected

c. There is starting material in the product

Which of the following contaminants in your glassware react with a Grignard, effectively quenching the reagent? a. Silicone compounds in the glass itself b. Dust particles in the air c. Water d. Vacuum grease

c. Water

How much hexanes should you use to recrystallize 1 gram of benzhydrol? (Solubility of benzhydrol in hexanes is 1g/50 mL at 70˚C; 1g/500 mL at 0˚C) a. 500 mL b. 25 mL c. 10 mL d. 50 mL

d. 50 mL

What is the name for a nematic phase with helical twist? a. MBBA b. Helicoid c. Imine d. Cholesteric

d. Cholesteric

Which statement best describes the carbon-magnesium bond in a Grignard reagent? a. Covalent and nonpolar such that the carbon is positively charged b. Covalent and highly polarized such that the carbon is positively charged c. Covalent and nonpolar such that the carbon is negatively charged d. Covalent and highly polarized such that the carbon is negatively charged

d. Covalent and highly polarized such that the carbon is negatively charged

Consider the compounds ferrocene, monoacetylferrocene, and diacetylferrocene. Which is the least polar? a. Monoacetylferrocene b. Diacetylferrocene c. They are all equally polar d. Ferrocene

d. Ferrocene

The equilibrium constant, K, determines the position of equilibrium in an esterification reaction. Which of these K values describes the most efficient reaction? a. K = 0.0056 b. K = 1 c. K = 0.079 d. K = 5.4

d. K = 5.4

How many fabric strips will you dye in this lab? a. Six: acetate rayon, bleached cotton, nylon, silk, viscose rayon, and wool b. Four: bleached cotton, nylon, silk and wool c. One: cotton d. One: a multifiber strip containing 6 different types of fabric

d. One: a multifiber strip containing 6 different types of fabric

During silica gel chromatography, the following is true of polar solvents: a. They move only polar compounds more slowly than non-polar solvents b. They move all compounds more slowly than non-polar solvent c. They move only non-polar compounds more rapidly than non-polar solvents d. They move all compounds more rapidly than non-polar solvents

d. They move all compounds more rapidly than non-polar solvents