Chemistry 281 quizlet practice final exam PDF

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Chem 281 Final Exam Study online at quizlet.com/_zu6dv 1.

Beyond what value are peaks on an IR spectra uninformative?

Less than 1500 - these are fingerprint peaks and do not give structural information

2.

Can PBr3 and SOCl2 be used to convert tertiary alcohols to alkyl halides?

No - it's an SN2 reaction, not reactive

3.

Distinguish between a high and low chemical shift value in terms of frequency, field, and shielding.

High chemical shift = high frequency, downfield, deshielded Low chemical shift = low frequency, upfield, shielded

Distinguish between enantiotopic and diastereotopic protons.

4.

5.

6.

Distinguish between heterotopic and homotopic protons.

Distinguish between the preparation of liquid and solid samples for IR.

10.

Explain how to tell if two protons are diastereotopic or enantiotopic.

Shortcut: if there is a chiral centre in the molecule, any H's will be diastereotopic for sure Otherwise, do the deuterium though experiment - sequentially replace each H with a D If you generate enantiomers, they're enantiotopic, if you generate diastereomers, they're diastereotopic

11.

Enantiotopic = have identical chemical shifts so cannot be distinguished by NMR Diastereotopic = have different chemical shifts so can be distinguished by NMR

Explain intermolecular dehydration ether synthesis. Draw mechanism.

1. OH of one alcohol is protonated by acid catalyst 2. Un-protonated other alcohol attacks (SN2)

12.

Homotopic = reside in identical chemical environments and have identical chemical shift values Heterotopic = reside in chemically distinct environments and do not have same chemical shift values

Explain oxymercurationdemercuration ether synthesis. Draw mechanism.

1. Identical beginning to alcohol formation, bridged Hg2+ structure formed 2. Alcohol attacks more substituted carbon 3. NaBH4 must be added to replace Hg with H

13.

Explain the mechanism of acid-catalyzed hydration. Draw it.

1. Acid forms H3O+ from water molecules 2. Pi bond of alkene attacks H of H3O+, adds to less substituted carbon 3. Carbocation formed rearrangement possible here 4. Water attacks carbocation, forming alcohol

14.

Explain the mechanism of alcohol production via oxymercurationdemercuration. Draw it.

1. Pi bond attacks Hg on Hg(OAc)2, causing departure of one OAc 2. Hg+ added to less substituted carbon, but forms a bridging structure between both carbons of the DB, so NO full carbocation 3. H2O attacks more substituted carbon, breaks apart bridging structure 4. NaBH4 added to cleave C-Hg bond and replace it with H

Liquid - just place a drop on the plates Solid - dissolve in organic solvent, apply to plates and wait for solvent to evaporate; if it will not dissolve, grind it into mineral oil

7.

Distinguish between the products formed by molecular halogen addition to a cis alkene vs a trans alkene.

Molecular halogen addition is anti Cis will make enantiomers Trans will make a meso

8.

Distinguish between the products formed by OsO4 addition to a cis alkene vs a trans alkene.

OsO4 addition is syn Cis will make a meso Trans will make enantiomers

9.

Explain capturing a carbocation with an alcohol ether synthesis. Draw mechanism.

carry out reaction at lowtemp 1. Generate tertiary carbocation either from alkene + acid catalyst or alcohol + acid catalyst 2. Add primary alcohol 3. O of primary alcohol attacks carbocation

15.

Explain the mechanism of converting an alcohol to an alkyl halide using another acid and a salt of X-, mentioning any stereochemical/regioselective requirements. Draw it.

1. O of OH attacks H or other Lewis acid, becoming protonated or positive 2. Now, mechanism depends on substitution - if alcohol is primary, X- will attack in SN2 (stereospecific), if alcohol is secondary or tertiary, Xwill attack in SN1 (racemate)

19.

Explain the mechanism of just hydroboration. Draw it.

1. At the same time, pi bond electrons attack the empty p orbital on BH3 and H on BH3 attacks the more substituted carbon 2. Tight, 4-membered ring intermediate formed with BH2 attaching to less substituted carbon 3. Process can repeat two more times 3. Trialkyl borane, where B is attached to 3 alkyl groups at the less substituted position, is formed

16.

Explain the mechanism of converting an alcohol to an alkyl halide using HX directly, mentioning any stereochemical/regioselective requirements. Draw it.

1. O of OH attacks H of HX, becoming protonated 2. Now, mechanism depends on substitution - if alcohol is primary, X- will attack in SN2 (stereospecific), if alcohol is secondary or tertiary, Xwill attack in SN1 (racemate)

20.

Explain the mechanism of molecular halogen addition in a nonnucleophilic solvent. Draw it.

1. Pi bond electrons attack the delta-positive X, which separates from the other X 2. Cyclic halonium intermediate formed 3. X- attacks more substituted carbon of the intermediate, it breaks open 4. Vicinal di-halide formed

17.

Explain the mechanism of converting an alcohol to an alkyl halide using PBr3, mentioning any stereochemical/regioselective requirements. Draw it.

1. O of OH attacks P of PBr3, causing departure of one Br 2. Br- attacks C attached to O in an SN2 fashion (stereochemical inversion) 3. HOPBr2 departs and can be used 2 more times, ultimately forming 3 alkyl halide equivalencies and P(OH)3

21.

Explain the mechanism of OsO4 addition to an alkene. Draw it.

1. At same time, pi bond electrons attack Os and O attacks carbon of alkene (concerted cycloaddition) 2. Ring intermediate formed 3. H2O hydrolyzes, producing a diol

22.

Explain the mechanism of oxidation of a trialkyl borane. Draw it.

Explain the mechanism of converting an alcohol to an alkyl halide using SOCl2, mentioning any stereochemical/regioselective requirements. Draw it.

1. O of OH attacks S of SOCl2, causing departure of one Cl 2. Weak base (pyridine) removes H from O 3. Cl- attacks C attached to O in an SN2 fashion (stereochemical inversion) 4. Cl- departs the OSO group during this, forming O=S=O (sulfur dioxide gas)

1. NaOH and H2O2 (peroxide) are added to the mixture 2. OH- abstracts a proton from H2O2, forming HOO-, a very reactive nucleophile 3. HOO- attacks B, forming a Bintermediate 4. The B-O bond is weak, and so an alkyl shift occurs where one alkyl group migrates onto the O 5. This repeats 3 times, forming a trialkyl borate ester 6. OH- attacks the B, causing departure of one of the alkyl ester groups, making an alkoxide anion 7. Alkoxide anion removes a proton from the OH, producing the alcohol

23.

Explain the principle of how an NMR spectra is generated.

A certain amount of electromagnetic radiation (radio frequency) applied to spinactive nuclei in a magnetic field will cause their magnetic moment to just tip out of the xy plane, and this generates a signal in a receiver coil located in the NMR spectrometer. As the nuclei is allowed to relax back, the signal in the coil decays, and this generates an oscillation signal called the free induction decay.

18.

24.

Explain what the Markovnikov product is and why it is favoured in most addition reactions.

When the first atom/group added is put onto the less substituted carbon, this occurs because of carbocation stability, it is better for the positive charge to be on the MORE substituted carbon

33.

How can the oxymercurationdemercuration process be modified to form an ether?

Instead of using THF/H20, use THF/alcohol, then the product formed will be an ether

25.

Explain why the addition of BH3 occurs the way it does.

H- from BH3 attacks the more substituted carbon because it has more positive charge built up. Also, it is easier for BH2 to bond with the less substituted carbon because of sterics.

34.

How can the value of J tell us about cis or trans?

Cis (dihedral angle of 0) typically has a smaller chemical shift/J value than trans (dihedral angle of 180)

35.

How can we check for ionizable hydrogens using NMR?

React the molecule with D2O or CDO3D (deuterated protic solvents) and it will exchange with the H if there is an ionizable one present H can be detected by NMR, D can't, so your signal will disappear

36.

How do alkynes react with HX?

Exact same way as alkenes, but can selectively use 1 equivalency to stop at the alkene

37.

How do alkynes react with molecular halogens?

Exact same way as alkenes, but can selectively use 1 equivalency to stop at the alkene If you do this, trans product will be formed due to anti addition

38.

How does IR spectroscopy work?

Certain functional groups absorb IR radiation at certain frequencies, causing them to stretch/vibrate at a greater amplitude, so based on the frequencies that get absorbed by a molecule, we know what functional groups are present on it A graph is presented plotting percent transmittance against frequency (wavenumber), and when we see spikes, it indicates percent transmittance has shot down because of being absorbed

39.

How does the alkyl shift that occurs in trialkyl borane oxidation change stereochemistry?

It doesn't, it migrates with retention of stereochemistry

40.

How does the amount of material required for C-NMR differ from H-NMR?

More material needed because C-NMR is so much less abundant

26.

Explain Williamson ether synthesis. Draw mechanism.

1. Add NaH to primary or secondary alcohol, which will form alkoxide ion 2. React with primary or methyl alkyl halide or sulfonate ester (SN2)

27.

For what type of nuclei can NMR be spectra be generated?

Spin-active, which comes from having odd atomic numbers and odd or even mass numbers

28.

How are samples for IR plated and what cautions must be taken?

On KBr salt plates, which are obviously at risk of dissolving, so important to handle them only by edges and avoid all water and polar solvents

29.

How can alcohol be produced from a reaction with CONCENTRATED acid (not acidcatalyzed hydration)?

1. Pi bond attacks H on the acid, forming carbocation 2. A- attacks carbocation, forming a substitution product 3. Heat the mixture, add water 4. A- will depart, re-forming carbocation 5. H2O can attack to form alcohol

30.

How can a trans diol be produced from an alkene?

Form an epoxide with mCPBA or another peracid, then do an acidcatalyzed reaction with water

31.

How can epoxides be formed from alkenes?

Addition of organic peroxide/peracid, such as mCBPA

32.

How can ethers be converted to alkyl halides? Explain the mechanism

Reaction with HX 1. O is protonated by H of HX 2. X- attacks one of the carbons, cleaving apart into an alcohol and an alkyl halide 3. The alcohol is re-protonated an attacked again to form second alkyl halide

41.

How do you distinguish between a primary, secondary, tertiary amine on an IR spectrum?

Primary shows two peaks at the 3300 region, secondary shows one, tertiary shows none

42.

If a proton is beside more than one set of chemically distinct protons, does it have to follow the N+1 rule? Explain.

No, because if J values are not equal, it can have as many as (n1 +1) times (n2 + 1) peaks, so we typically refer to just a "multiplet" because of the complexity involved

43.

If you are asked how two compounds can be distinguished via NMR, what is the best approach?

See how many signals each will give - quite often, they will have different numbers of chemically distinct hydrogens

44.

List all the methods we have for synthesizing alcohols from alkenes (6 ways)

1. Acid-catalyzed hydration 2. Oxymercuration-demercuration 3. Hydroboration-oxidation 4. OsO4 oxidation (cis diol) 5. Epoxide formation with peracid, then acid-catalyzed reaction with water (trans diol) 6. Molecular halogen in water (halo alcohol)

45.

Name 3 key deshielding effects for C-NMR.

1. More substituted C's are more deshielded 2. Alkenes are more deshielded than alkynes 3. Ketones are more deshielded than carboxylic acids or esters because of resonance

46.

Name but do not explain the methods of converting an alcohol to an alkyl halide.

1. HX directly 2. Another acid, and a salt of X3. PBr3 4. SOCl2

47.

Outline the procedure for acidcatalyzed dehydration in the laboratory.

1. Measure out alcohol and place it in a round-bottom flask with antibumping granules 2. Measure out small amount of concentrated acid 3. Add acid drop by drop, swirling as you do so 4. Heat the mixture and collect what distills over 70-80 degrees 5. Pour cooled distillate into a new flask and add saturated NaCl 6. Mix, invert, and allow to separate, then drain the bottom layer 7. Dry the remaining liquid with anhydrous CaCl2 8. Clean and dry the distillation apparatus, then carefully decant the liquid from the previous step into it (don't let any drying agent in) 9. Distill, and collect what boils over 80-85 degrees 10. Measure & record volume of pure alkene

48.

What 3 parameters are used to describe an IR band?

1. Location - wavenumber 2. Intensity - how tall the peak is 3. Shape - broad vs sharp

49.

What 3 steps should to be taken to evaluate a C-NMR spectra?

1. Calculate IHD for chemical formula 2. Look at NUMBER of peaks for number of chemically distinct sets of carbons 3. Look at chemical shift for what type of group they might be

50.

What 4 considerations should one make on the first "pass" of an IR spectra?

1. Only C=O stretches have tight, strong absorbance in 1630-1780 range 2. Only nitrile or alkyne bonds appear between 2000 and 2300 3. Only hydroxyl groups of alcohols or carboxylic acids create large, broad signals around 3300 4. Only amines produce small, broad peaks around 3300

51.

52.

What 4 methods exist for synthesizing ethers? Name them and state what type of molecules they are "good for."

What 5 steps should be taken to interpret an H-NMR spectra?

1. Intermolecular dehydration - only good for symmetrical ethers and primary alcohols 2. Oxymercuration-dermurcuration minimal limits, allows for unsymmetrical ethers 3. Capture of carbocation with alcohol - allows for unsymmetrical ethers, but only good for tertiary carbocation and primary alcohol 4. Williamson synthesis - allows for unsymmetrical ethers, but only good for primary or secondary alcohol 1. Calculate IHD for the chemical formula given to get some idea of constraints on the molecule 2. Look at NUMBER of signals to know number of sets of chemically distinct H's 3. Look at chemical shift values to know what kind of structures you have 4. Look at integration heights to get relative numbers of each type 5. Look at coupling patterns to determine how many H's each type is beside

53.

What are the two overarching methods for converting alcohols to good leaving groups?

Converting to a sulfonate ester or converting to a halide

54.

What do alkenes and aromatic rings do to the chemical shift of H?

Make it greater through deshielding, because their circulating pi electrons create shielding "cones" that H are not a part of

55.

What do alkynes do to the chemical shift of H?

Make it lower than an alkene through shielding, because the H now fall within the shielding cone, however still not as shielded as an alkane because of sp hybridization electronegativity

56.

What do electronegative atoms do to the chemical shift of H?

Make it greater through deshielding, because they pull electron density away and therefore "expose" the nuclei to the magnetic field more

What does coupling on the H-NMR spectra tell us?

The number of peaks within one signal tells us how many H's that H is coupled to (n+1 rule, a triplet is coupled to 2 H's for example)

57.

58.

What does integration on an H-NMR spectra tell us?

The relative number of H's of that chemical type present in the molecule - higher integral height = more H's

59.

What does IR spectroscopy tell you about a molecule?

What type of functional groups are present in it

60.

What does the 1500-1600 range on an IR spectra tell us?

C=C bonds show up here, so we will see alkenes (single peak) and aromatics (multiple, usually 3 peaks)

61.

What does the 1650-1800 range on an IR spectra tell us?

C=O bonds show up here (aldehydes, ketones, esters, etc)

62.

What does the 2200-2300 range on IR spectra tell us?

Triple bonds show up here so we will see alkynes (weaker) and nitriles (stronger)

63.

What does the NMR signal for a benzene ring with one substituent look like?

Complex, overlapping signals forming a complex multiplet

64.

What happens if molecular halogen is added in nucleophilic solvent like water?

Halo alcohol or halohydrin is formed instead of vicinal dihalide Mechanism is essentially identical except instead of X- attacking, H2O does

65.

What happens when a magnetic field is applied to spin-active nuclei? Distinguish between alpha and beta spin states.

Spin-active nuclei normally spin around randomly, generating a small magnetic moment that may point in any direction. Once an external magnetic field is applied, they MUST line up with the field, either with their magnetic moment pointed in the same direction as it (alpha, lower energy) or in the opposite (beta, higher energy)

66.

What is a 2D NMR spectrum and what do dots on it represent?

It has one spectrum on one axis and one on the other, and dots in between represent correlations so you can get more explicit structural information

67.

What is acidcatalyzed dehydration?

Loss of a water molecule from an alcohol when it is heated with acid, an elimination reaction

68.

69.

What is a COSY spectra and how do you read it?

What is a HETCOR spectra and how do you read it?

H-H 2D spectra, gives us which H nuclei are coupled to which others To read it, you start at a peak on one axis, draw a straight line to an off-diagonal dot, then draw a straight line to the other axis, and this tells you that these two nuclei are coupled, so you can gather an idea of which are beside each other H-C 2D spectra, gives us which H nuclei are coupled to which C nuclei It does not have a diagonal, so to read it you just start at an axis peak, draw a straight line to a dot, then draw a straight line to the other axis, ...