Exam 3 Quizlet Study Guide PDF

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CH 235 Exam 3 Study online at quizlet.com/_2pes6x 1.

What are the two types of elimination reactions?

E1 and E2

2.

Describe the basic idea of elimination reactions:

The molecule gets smaller, the halogen is removed but the c arbon chain is kept

What is nucleophilic substitution?

In any reaction in which a nucleophile replac es another electron-ric h group c alled a leav ing group (Lv)

4.

What is a nucleophile (Nu:-)?

An electron-rich molecule or ion that donates a pair of electrons to another atom or ion to form a new covalent bond

5.

What are the two types of substitution reactions?

SN1 and SN2

6.

How do you determine how many steps you have in a reaction?

How many peaks are in the energy diagram (one peak: one s tep; two peaks: two s teps)

Describe what SN2 means: (3)

1. S: subs titution 2. N (s ubscript): nuc leophilic 3. 2: both nucleophile and substrate are involv ed in the trans ition s tate (bimolec ular)

3.

7.

8.

Where do SN2 reactions occur?

With invers ion at the reac ting c enter (chiral c enter)

9.

Why are SN2 reactions bimolecular?

Because both nuc leophile and haloalkane c oncentrations influence reaction rate

10.

What happens if you double the concentration of either haloalkane or nucleophile in an SN2 reaction?

It will double the rate of the reaction because an SN2 reaction is bimolec ular

In an SN2 reaction, how does the nucleophile approach the carbonleaving group?

The nucleophile must approach the leaving group bond form the backs ide in order to populate the carbon leav ing group anti-bonding orbital and allow reaction

11.

16.

The structure of a _______ always affects an SN2 reaction

Substrate

17.

What type of halides are most reactive for an SN2 reaction?

Methyl halides

18.

After methyl halides, what type of carbons are most reactive for SN2 reactions?

Primary

19.

For SN2 reactions, is there a reaction at vinyl halides (C=C)?

No

20.

The more stable the ____ produced upon SN2 reactions, the better the _______ group ability

Anion Leaving

21.

The more ____ groups connected to the reacting carbon in an SN2 reaction, the _____ the reaction

Alkyl Slower

22.

What type of hybridized carbon will never be an SN2 reaction?

sp2 hy bridized c arbon

23.

What type of nucleophile is best for SN2 reactions? (2)

1. More bas ic nucleophiles react faster (for s imilar struc tures) 2. Anions are better than neutral nuc leophiles

24.

Why is acetate not a good nucleophile for SN2 reactions even though it's negatively charged?

It has a negative charge but it also has res onance struc tures that mak e it stable

25.

What are excellent leaving groups in SN2 reactions?

Stable anions that are weak bases , they c an delocalize charge

26.

In an SN2 reaction, if a leaving group is very basic or very small what does it do?

It prevents the reaction

27.

What types of compounds never undergo SN2 reactions? (4)

1. 2. 3. 4.

28.

What is the best substrate for an SN2 reaction?

A primary or secondary carbon

29.

What is the best nucleophile for an SN2 reaction?

The nucleophile is preferred to be v ery strong, normally with a negative charge

30.

What are the most common nucleophiles for SN2 reactions? (3)

1. I2. CN3. HS-

R-F R-OH R-OR' R-NH2

What happens because an SN2 reaction is attacked from the backside?

The reaction at a chiral c enter inverses the configuration (from S to R or R to S)

13.

What is an SN2 substrate sensitive to?

Sensitiv e to steric effects

14.

What does steric hindrance do to SN2 reactions?

It mak es it harder for molecules (nucleophiles) to interact with the molec ule (chiral center)

31.

What is the best leaving group for SN2 reactions?

The leaving group should be very reactive, large, and acidic

15.

For SN2 reactions, what type of carbons are preferred?

Primary carbons , a secondary carbon might work , but a tertiary carbon will nev er work

32.

What are the most common leaving groups for SN2 reactions? (3)

1. Br2. I3. TosO-

12.

33.

What are the best solvents for SN2 reactions?

Polar aprotic solv ent (c ontains no OH or -NH groups)

34.

What do solvents that donate hydrogen bonds do to SN2 reactions?

They slow down the reactions by interacting with the nucleophile

35.

What are good solvents for SN2 reactions? (3)

1. DMSO 2. DMF 3. CH3CN

36.

Describe the SN1 reaction:

1. Tertiary alky l halides react rapidly in protic solv ents by a mec hanism that involve a leaving group and a nucleophile 2. Oc curs in two s teps 3. Nucleophile is pres ent in large concentration

48.

Describe elimination of alkyl halides: (4)

1. Elimination is an alternativ e pathway to subs titution 2. Oppos ite of addition 3. Generates an alkene 4. Can c omplete with s ubstitution and decrease yield, especially for SN1 reactions

49.

What does Zaitev's Rule for elimination reactions say?

1. The more highly subs tituted alkene product will predominate 2. The double bond in the middle of the c hain has a higher c hance at forming than a double bond on a terminal carbon because it is more substituted

50.

What is a base in an E2 mechanism?

Special group of nuc leophile that can only attack hydrogen and sometimes carbon

51.

What is a nucleophile?

A broader definition of a bas e, it can attac k any target that is positiv ely charged

52.

What happens in an E2 mechanism?

The leaving group will leave, a hydrogen will be taken from a c arbon, and a double bond will form between the two carbons

53.

How many steps does an E2 mechanism have?

One

54.

What is the geometry of an E2 mechanism?

The hydrogen and leaving group mus t be on the same plane opposite to each other (anti-periplanar)

37.

What step is the rate determining step in an SN1 reaction?

The first step

38.

What is the slowest step in an SN1 reaction?

The ionization (the first step)

39.

What is the stereochemistry for SN1 reactions?

Rac emic (retention and inv ersion product)

40.

What is more prominent in SN1 products, retention or inversion?

Invers ion

41.

What does the Hammond postulate say about SN1 reactions?

Any factor that stabiliz es a highenergy intermediate should also stabiliz e the transition state leading to that intermediate

55.

Why are E2 reactions anti-periplanar?

Overlap of developing pi orbital in the transition state requires preiplanar geometry, anti-arrangement, in order to reac t

42.

The more stable the _______ intermediate, the _____ the SN1 reaction

Carboc ation Fas ter

56.

Can an E2 reaction take place at an unstable conformation?

Yes, the leav ing group must be in the axial pos ition and the hy drogen must be anti-periplanar to it

43.

What is the best nucleophile for an SN1 reaction?

The reaction rate normally isn't affec ted by the nucleophile s o it doesn't matter

57.

A ring flip will occ ur to put the leaving group in the axial position

44.

What are the best leaving groups for an SN1 reaction?

A large reac tivity, larger halide ions are better leaving groups

What happens to an E2 reaction where the leaving group is in the equatorial position?

58.

Where the tri-substituted molec ule is

45.

What is the best solvent for SN1 reactions?

Polar protic solv ents

In an E2 mechanism where is the double bond more likely to form?

59. 46.

How is solvent polarity measured?

Dielec tric polarization (P)

What is the E1 mechanism closely related to and why?

47.

What are good solvents for SN1 reactions? (2)

1. Alc ohols 2. Water

SN1 reaction because they both break a connection between the substrate and the leav ing group which leaves behind a pos itive charge

60.

Is E1 stereospecific?

No

61.

What kind of products do E1 mechanisms have

A Zaitsev orientation becaus e the step that controls products is a los s of proton after formation of carbocation

62.

How can a carbocation get rid of the positive charge?

It can us e SN1 or E1 reactions

83.

What produces an absorption spectrum?

Changing wav elengths to determine whic h are abs orbed and which are transmitted

63.

Is a strong base needed for E1 or E2?

E2 84.

What is the highest peak in a UV spectroscopy graph?

The degree of conjugation

64.

Is E2 stereospecific?

Yes

65.

What type of orientation do the products of E1 have?

Zaitsev

66.

What type of reactions do alkyl halides undergo?

Different reactions depending on the reacting molec ule and the conditions

85.

The more pi bonds a molecule has the more ______ it has and the less ____ it has

Energy Wav elength

67.

What happens when there is a reaction of RX with Mg in ether or THF?

The product is RMgX which is an organometallic compound (alky l-metal bond)

86.

What does the Beer Lamber Law determine?

It can us e a UV s pectrum to monitor reactions and tell the concentration of molec ules in the reac tion

68.

What happens when RLi reacts with copper iodide?

It gives lithium diaky l copper (Gilman reagent)

69.

Lithium diakyl _____ reagents react with alkyl halides to give ______

Copper Alkanes

87.

What is the formula for the Beer Lamber Law?

A = clε

70.

What happens when two organometallic molecules come together?

They produce larger molec ules of more defined struc ture

88.

What is the formula A = log10 (Io/I) for absorbance (A)?

89.

What is the formula for energy?

E = hv = hc / λ

71.

What happens when you couple lithium diakylcopper molecules?

It mak es a trialky lcopper intermediate

90.

What does IR energy correspond to?

What physical methods can determine the structure of molecules? (4)

1. Mass spec trosc opy 2. Ultra-v iolet s pectros copy 3. Infrared s pectros copy 4. Nuclear magnetic resonance spectroscopy

It corresponds to specific modes, corresponding to c ombinations of atomic movements suc h as bending and stretching of bonds between groups of atoms called normal modes

72.

91.

Energy

What physical method is most widely used in organic chemistry?

Nuc lear magnetic spectroscopy (NMR)

In infrared energy modes what characteriz es the atoms in the group and their bonding?

92.

74.

What are the two types of nuclear magnetic spectroscopy?

1. Proton 2. Carbon

What can infrared spectroscopy be used to do?

To identify elements of a molec ule but not to identify the molecule itself

93.

What happens when organic compounds are exposed to electromagnetic radiation?

They can absorb the energy but only at certain wav elengths (unit of energy)

What bonds are at 4000-2500 cm-1?

N—H, C—H, O—H (stretching)

75.

94.

What bonds are at 3300-3600 cm-1?

N—H, O—H

76.

What happens when a compound absorbs energy?

It is dis tributed internally in a distinct and reproducible way

95.

What bonds are at 3000 cm-1?

C—H

96.

Why do molecules want to be at the ground state?

Because it has a lower energy

What bonds are at 2500-2000 cm-1?

C≡C and C≡N (stretching

77.

97.

What happens when molecules go down an energy level?

They release light as a form of energy (photons )

What bonds are at 2000-1500 cm-1?

C=O, C=C

78.

98.

What is the range for UVC?

200 nm — 280 nm

What bonds are at 1680-1750 cm-1?

C=O

79. 80.

What is the range for UVB?

280 nm — 320 nm

99.

What is the range for UVA?

320 nm — 400 nm

What bonds are at 1640-1680 cm-1?

C=C

81. 82.

What is the only safe form of UV light for humans?

UVA

100.

What bonds are below 1500 cm-1?

The fingerprint region

101.

What is the wavenumber?

The inversion of wavelength and proportional to frequenc y

73.

102.

Describe the usages of NMR spectroscopy: (3)

1. Used to determine relative location of atoms within a molecule 2. Most helpful in spectroscopic tec hnique in organic c hemis try 3. Related to MRI in medic ine

115.

Describe the parts of the NMR spectra: (2)

1. Left part is downfield (deshielded) 2. Right part is upfield (shielded)

116.

What are nuclei on the upfield side?

They are s trongly shielded

117.

What is an NMR chart calibrated with?

Tetramethylsilane (TMS)

103.

What does NMR map?

It maps the carbonhydrogen framework of molec ules (not func tional groups)

118.

The nuclei to the right on an NMR chart have more what?

The more electronegative pulling there is on the hydrogen the more downfield (to the left) the peak will be

104.

What does NMR depend on?

Strong magnetic fields

119.

An odd number

It stands for the number of hydrogens on a c arbon

105.

How many protons are needed for NMR?

In the n + 1 rule, what does n mean?

120.

106.

What happens if the number of neutrons and protons are even?

Then there is no NMR

What does the n + 1 rule give for NMR graphs?

It shows how many splits (singlet, doublet, triplet, quartet) a peak will have

107.

How does NMR work?

1H and 13C nuc leus s pins and the internal magnetic field aligns parallel to or against an aligned ex ternal magnetic field

121.

108.

What orientation is lowest in energy in NMR?

Parallel orientation

109.

What causes nuclei to flip into an anti-parallel state in NMR?

Radio energy of exac tly correct frequenc y (resonance)

110.

What two options do the spinning nuclei have in NMR?

1. Follow the direction of external magnetic field (+½ spin), whic h lasts longer becaus e it has low energy 2. Going against the direction of ex ternal magnetic field (-½ spin), whic h doesn't last long because it has high energy

Which of the following d. They react at identical rates statements is not true with chiral reagents regarding pairs of enantiomers a. They have identical melting points b. They have identical boiling points c. They rotate plane polarized light in opposite directions d. They react at identical rates with chiral reagents

122.

Which of the following is the definition of a pair of diastereomers? a. A pair of structures that are superposable mirror images of one another b. A pair of stereoisomers that are no-superposable mirror images of one another c. A pair of stereoisomers that are not mirror images of one another d. A pair of stereoisomers that have equal specific rotations

111.

The ______ the magnetic field of NMR machine the ______ because the energy gap is larger, which makes it easier to observe the nuclei

Larger Better

112.

What do electrons in bonds do to the magnetic field?

Electrons in bonds shield nuclei from the magnetic field

113.

What is the area of a 1H-NMR proportional to?

It is proportional to the number of equiv alent hydrogens giving ris e to that signal

114.

What is a chemical shift?

The relative energy of resonance of a particular nucleus resulting from its local env ironment

c. A pair of stereoisomers that are not mirror images of one another

Which of the following is the definition of a meso compound? a. A molecule with stereocenter centers which is chiral b. A molecule with stereocenter centers which is not chiral c. A diastereogmer with no stereocenter centers d. A chiral compound with more than one stereocenter center

b. A molec ule with stereoc enter c enters whic h is not chiral

Which of the following statements is true? a. All mirror images are enantiomers b. All molecules that have stereocenter centers are chiral c. Isomers that are not superposable on their mirror images are enantiomers d. Superposable structural isomers are enantiomers

c. Isomers that are not superposable on their mirror images are enantiomers

What happens when replacement of each H with "X" gives different constitutional isomer in NMR?

If nonequivalent hydrogen are formed then there are different chemical shifts

What happens when replacement of each H with "X" gives different diastereomers in NMR?

If diastereotopic hy drogens are formed then there are different chemical shifts

127.

What happens when replacement of each H with "X" gives an identical molecule in NMR?

If homotopic equiv alent hydrogens are formed then they are the same chemical s hifts

128.

What happens when replacement of each H with "X" gives a pair of enantiomers in NMR?

If enantiotopic equivalent hydrogens are formed then they are the same chemical s hifts