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CHM 1721 Study Guide - Final Guide: Reagent, Borohydride, Meso Compound

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carminecrow316
9 Apr 2015
School
University of Ottawa
Department
Chemistry
Course
CHM 1721
Professor
Guerts

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Document Summary

In order to obtain a new compound during a chemical reaction, two different process may occur, oxidation and reduction. Oxidation is the removal of hydrogen atoms from the compound. Reduction is the addition of hydrogen atoms to the compound. During this experiment, reduction will be used to turn benzyl into hydrobenzoin through a series of reaction mechanisms using sodium borohydride (nabh4) as the reducing agent. This is done because sodium borohydride has the capability to reduce four separate functional groups. In order for a reduction reaction to occur interaction between nucleophiles and electrophilic bonds needs to be present. This will allow for the total number of electrons to increase in a functional group, resulting in a reduction reaction. The reaction will be monitored by the use of thin layer chromatography (tlc). The use of tlc will help to determine the stereochemistry of the produced molecules.

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Related Questions

1. (1 point) What are transferred in an oxidation-reduction reaction? *

• atoms

• electrons

• ions

• protons

2. (1 point) In the reaction of beryllium with fluorine, which atom is oxidized? *

• fluorine

• beryllium

• neither of them

• both of them

3. (1 point) Ag - - > Ag ^+ +1 e^- This equation represents the type of reaction called _____. *

• hydrolysis

• oxidation

• redox

• reduction

4. (1 point) What is the reducing agent in the following reaction? 2K + S - - > K2S *

• K

• S

• K2S

• None of the above

5. (1 point) The oxidation number of oxygen when it is in a compound other than a peroxide is ____. *

• -2

• -1

• 0

• +2

6. (1 point) What is the oxidation number for each atom in NH4F? *

• N=+3, H=-4, F=+1

• N=-3, H=+1, F=-1

• N=+3, H+1, F=-1

• N=-3, H=-1, F=+1

7. (1 point) Which atom has a change in oxidation number of +4 in the following redox reaction?K2Cr2O7 + H2O + S - - > KOH + Cr2O3 + SO2 *

• K

• Cr

• O

• S

Metal activity series for question 8

8. (1 point) Of the following metals, which ions are most easily reduced? Use the above table to answer the question. *

• magnesium

• lead

• copper

• sodium

9. (1 point) In a zinc-copper cell, Zn(s) | Zn^+2(1M) || Cu^+2(1M) | Cu(s), which electrode is positive? *

• Cu^+2

• Cu(s)

• Zn(s)

• Zn^+2

10. (1 point) How must a redox reaction that is to be used as a source of electrical energy be set up? *

• One half reaction must involve two metals.

• Two half reactions must use a metal wire electrodes.

• One half reactions must involve more than one electron.

• Two half reactions must be physically separated.

11. (1 point) In a voltaic cell at which electrode does reduction occur? *

• anode only

• cathode only

• both anode and cathode

• Either anode or cathode , depending on the metal

12. (1 point) What assures that there is no charge build up in a voltaic cell as oxidation and reduction occur? *

• one of the half cells

• the electrolyte solutions

• the moving electrons

• the salt bridge

13. (1 point) In the most common dry cell what material is the electrode that is the center rod made of? *

• copper

• graphite

• lead

• zinc

14. (1 point) To charge a lead storage battery you must ________. *

• apply a direct current to it.

• hold a magnet close to it.

• Use alternating current through it.

• increase the oxygen in it.

15. (1 point) In a hydrogen- oxygen fuel cell ______________. *

• hydrogen diffuses through the cathode.

• hydrogen and oxygen are mixed before entering the anode.

• oxygen diffuses through the anode.

• oxygen diffuses through the cathode.

16. (1 point) When electrical energy is used to cause a chemical reaction we call this process _____. *

• electronation

• electrolysis

• hydration

• reduction

17. (1 point) Which of the following is true about an electrolytic cell? *

• It changes chemical energy into electrical energy.

• It is the type of cell used in electrocution

• It uses an electric current to make a nonspontaneous reaction go.

• All of the above

18. (1 point) The products formed in the net reaction of the electrolysis of water are ____. *

• aqueous hydrogen ion and hydroxyl ion

• hydrogen gas and oxygen gas

• hydrogen gas and liquid oxygen

• water vapor

19. (1 point) What occurs in electroplating? *

• decomposition of an ionic compound

• decomposition of a salt layer

• deposition of a metal layer on a material

• deposition of a salt layer on a metal

Open response Questions : Answer questions 20 through 22 in complete sentences

20. ( 5 points) Write the equation for the reaction of calcium and oxygen ( 1 point). Write the oxidation numbers for both the reactants and the products ( 2 points) What is the reducing agent and why? ( 2 points)

21. ( 5 points) Tell how a voltaic cell works. Be sure to mention the various parts of the voltaic cell and what they have to do with how it works.

22. (4 points) What are the signs given to the electrodes in both the voltaic and electrolytic cells? What reaction (oxidation or reduction) is occurring at each?

What is the purpose of this lab??

Introduction

Metal hydrides (sources of H-) such as LiAlH4 (LAH) and NaBH4, are widely used in reducing carbonyl groups such as aldehydes and ketones. The somewhat reduced reactivity of NaBH4 allows it to be used in both alcoholic and aqueous media in contrast to LAH. For each of these reagents, four hydrogen atoms are available for reaction, and thus one mole of hydride reagent can reduce four moles of ketone.

The stereochemistry of hydride reductions is very interesting. It appears that the alcohol derived from the reaction below appears to result from hydride attack on the carbonyl group on the least hindered side of the molecule. Such effects are called steric approach control (control of stereoselectivity of a reaction by steric hindrance towards attack of the reagent, which is directed to the less hindered face of the molecule). However, in the reduction of simple ketones, the alcohol obtained appears to be the thermodynamically more stable product. Such product formation is called product development control (this term is used of reaction under kinetic control where the selectivity parallels the relative thermodynamic stabilities of the products). In this experiment, approach of the hydride from the bottom (endo) leading to isoboreol appears to be less hindered than that from the top (exo) which leads to the isomer, borneol. In this experiment, the technique of NMR spectroscopy will be used to ascertain the composition of the reduction product in the reaction mixture. Before coming to lab, review the meaning of the terms "exo" and "endo" in your organic lecture text.

Procedure

In a 25-mL Erlenmeyer flask, dissolve 0.40g of racemic camphor in 2 mL of cold methanol. Cautiously add 0.30 g of sodium borohydride, in portions, to the solution. If necessary, cool the flask in an ice bath to keep the reaction mixture at room temperature. When all the borohydride is added, in the hood, heat the methanolic solution to boiling for 2 minutes.

Pour the hot reaction mixture into about 15 g of chipped ice, using small portions of methanol to aid the transfer (if you add to much methanol, the product will not reprecipitate). When the ice melts, collect the white solid by vacuum filtration (rinse with cold water), transfer the solid to a small flask and in the hood add about 20 mL of ether to dissolve the product. Add a spatula full of anhydrous magnesium sulfate to dry the solution, gravity filter to remove the drying agent, and evaporate the solvent from a Petri dish in a hood.

Purify the dry isoboreol by sublimation. The product in a Petri dish is placed on a hot plate with the cover on, and a beaker of ice and water is placed on the cover. Heat the dish on a low setting until the sublimation is complete. If water condenses inside the lid during the sublimation, remove the Petri dish from the heat and wipe away the water. Weigh the purified material and calculate the theoretical and percent yield.

Determine the melting point (sealed tube); pure racemic isoborneol melts at 212°C.

The product will be analyzed by H-NMR in CDCl3. Analyze the spectrum by comparing to the spectra of the pure alcohols obtained from a spectral database. Use the integration to determine the ratio of isoborneol to borneol. Based upon the results obtained, what conclusions can one make concerning the sterochemistry of the reduction?