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Organic Chemistry > Addition Reactions of Alkynes > Flashcards

Addition Reactions of Alkynes Flashcards

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1
Q

How do I form an alkyne from an alkane?

A

An alkyl dihalide must be treated with excess NaNH2/NH3, followed by H2O. This prompts a series of 2 elimination reactions. First, the first halogen serves as a leaving group while a base attacks a H and the proton leaves; electrons from the H create an alkene. Then, the second halogen leaves and a H is attacked by a stronger base, creating an alkyne.

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2
Q

What are the reagents for the formation of an alkyne from an alkyl dihalide?

A

Excess NH2/NH3 followed by H2O

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3
Q

What is the process that reduces alkynes to alkanes?

A

catalytic hydrogenation

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4
Q

What are the reagents in a catalytic hydrogentation that produce an alkane?

A

H2 followed by a metal catalyst (Pt, Ni, or Pd)

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5
Q

What are the reagents in a catalytic hydrogenation of an alkyne into a cis alkene?

A

H2 followed by a poisoned catalyst (Lindlar’s catalyst OR Ni2B)

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6
Q

What are the reagents in a catalytic hydrogenation of a geminal alkyne into a trans alkene?

A

Na followed by NH3 (l)

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7
Q

What does of hydrohalogenation produce?

A

Markovnikov reaction of halogens

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8
Q

What is the difference between treating alkynes with HX and treating them with excess HX?

A

Treating with just some HX prompts a Markovnikov addition and creates an alkene.
Treating with excess HX prompts a Markovnikov addition, a geminal dihalide is produced

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9
Q

What is the mechanism for the hydrohalogenation of alkynes?

A

Termolecular one-step process during which a carbocation intermediate is NOT created, but the transition state has some partial carbocationic behavior, leading to Markovnikov regioselectivity.

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10
Q

What are the reagents for an anti-Markovnikov hydrohalogenation of an alkyne?

A

HX followed by peroxides (ROOR)

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11
Q

How to create a geminal dihalide?

A

Treat an alkyne with excess HX.

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12
Q

How to turn a dihalide into a terminal alkyne?

A

Treat a dihalide with excess NaNH2/NH3 followed by H2O.

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13
Q

What are the reagents for an acid-catalyzed hydration reaction?

A

Either H3O+ or H2SO4,H2O, followed by HgSO4.

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14
Q

What is the effect of an acid-catalyzed hydration reaction of an alkyne?

A

A ketone is added to the more substituted side.

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15
Q

Describe the acid-catalyzed hydration reaction of an alkyne.

A

The alkyne attacks an Hg+2 ion and a resonance-stabilized mercurinium ion is formed. The cation on the intermediate is attacked by H2O acting as a nucleophile, then another H2O molecule participates as a base, deprotonating the overall molecule. Then the remaining alkene attacks a proton from H3O+. 2 resonance structures form, with the oxygen atom attacking the newly formed cation. The Hg+ acts as a leaving group, leaving behind an enol that rapidly converts into a ketone via acid-catalyzed tautomerization.

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16
Q

What is an enol?

A

a compound with a double bond and an alcohol that rapidly converts into a ketone via acid-catalyzed tautomerization

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17
Q

How does an enol become a ketone via acid-catalyzed tautomerization?

A

The alkene on the enol attacks H3O+ during a proton transfer, generating a resonance-stabilized intermediate with a transferring positive charge. The intermediate is deprotonated by H2O to give a ketone. This entire process migrates a proton and an alkene from one location to another.

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18
Q

What are tautomers?

A

constitutional isomers that rapidly interconvert via the migration of a proton

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19
Q

What is the product of the acid-catalyzed hydration of unsymmetrical internal alkynes?

A

a mixture of ketones, placed on opposite ends of the former triple bond

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20
Q

What is the product of the acid-catalyzed hydration of a terminal alkyne?

A

a methyl ketone

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21
Q

What are the reagents of a hydroboration-oxidation of alkynes?

A
  1. Dialkyl borane (either Disiamylborane or 9-BBN)
  2. H2O2, NaOH
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22
Q

What is the result of a hydroboration-oxidation of alkynes?

A

the addition of an aldehyde at the less substituted side of the alkyne; converts a terminal alkyne into an aldehyde

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23
Q

Is tautomerization during acid-catalyzed hydration acid or base-catalyzed?

A

acid

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24
Q

Is tautomerization during hydroboration oxidation acid or base-catalyzed?

A

base

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25
Q

How does an enol become an aldehyde during a base-catalyzed tautomerization?

A

A hydroxide attacks the H on the alcohol in a proton transfer, deprotonating it and generating a resonance-stabilized enolate ion with a transferring a negative charge. The enolate ion is then protonated by H2O to give an aldehyde.

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26
Q

What is the difference in mechanism between acid-catalyzed hydration and hydroboration oxidation?

A

Acid-catalyzed hydration is a Markovnikov hydration and hydroboration oxidation is an anti-Markovnikov addition.

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27
Q

What happens during hydroboration oxidation of an alkyne?

A

A molecule of R2BH is added across one of the pi bonds in the alkyne. OH is added onto the less-substituted side due to H2O2, producing an enol that is base-catalyzed tautomerized into an aldehyde.

28
Q

To convert a terminal alkyne into an aldehyde, which process would I use?

A

hydroboration oxidation

29
Q

To convert a terminal alkyne into a ketone, which process would I use?

A

acid-catalyzed hydration

30
Q

What are the reagents for the halogenation of alkynes, resulting in an tetrahalide?

A

excess X2 and CCl4
X is either Cl or Br

31
Q

What are the reagents to produce a dihalide from an alkyne?

A

one equivalent of X2 and CCl4

32
Q

What are the products of the halogenation of an alkyne to produce an alkene? Which is major and which is minor?

A

The products are dihalides. The E isomer is the major and the Z isomer is the minor product.

33
Q

What are the reagents for the ozonolysis of alkynes?

A
  1. O3
  2. H2O
34
Q

What are the products when a terminal alkyne undergoes oxidative cleavage?

A

A carboxylic acid and carbon dioxide

35
Q

What are the products when an internal alkyne undergoes oxidative cleavage?

A

Carboxylic acids with the R groups attached to both

36
Q

How can a carbon chain be extended?

A

Via alkylation of an alkyne. NH2 - deprotonates a terminal alkyne and a methyl or primary alkyl halide is attached via a nucleophilic attack.

37
Q

What kind of pathway is alkylation?

A

SN2

38
Q

What would happen if one tried to use a secondary or tertiary alkyl halide in alkylation?

A

The alkynide ion would function as a base and elimination products would be obtained.

39
Q

How to convert an alkene into an alkyne?

A

Bromination followed by elimination. The first set of reagents are Br2 and CCl4. That produces a vicinal dihalide. After applying xs NaNH2 and H2O, an alkyne is produced.

40
Q

What are the reagents for hydrogenation of an alkene?

A

H2 and a metal catalyst (Pd, Pt, Ni)

41
Q

What kind of mechanism is hydrogenation of an alkene?

A

syn addition, regiochemistry is irrelevant

42
Q

What happens when an alkene is hydrogenated?

A

2 H atoms are added across the double bond and the alkene becomes an alkane.

43
Q

Describe the mechanism of hydrohalogenation of alkenes.

A

The alkene attacks the H, prompting a proton transfer and creating a carbocation. The carbocation may rearrange, depending on stability. When the carbocation is as stable as possible (at the most substituted C), the -X attacks it.

44
Q

Describe the regiochemistry and stereochemistry of the hydrohalogenation of alkenes.

A

Markovnikov
syn and anti additions

45
Q

Describe the regiochemistry and the stereochemistry of the hydrohalogenation of alkenes using peroxides.

A

Anti-Markovnikov
syn and anti

46
Q

What are the reagents for the anti-Markovnikov addition of H and X across an alkene?

A

HX and ROOR

47
Q

What are the reagents for the hydration of an alkene?

A

H3O+; also displayed as [H2SO4] and H2O

48
Q

What are the products of the acid-catalyzed hydration of an alkene?

A

Markovnikov addition of an alcohol

49
Q

Describe the mechanism of acid-catalyzed hydration of an alkene.

A

The alkene attacks one of the H in the H3O+ molecule, creating a carbocation intermediate at the most stable position (Markovnikov addition). The remaining H2O molecule attacks the carbocation and is subsequently deprotonated by H2O.

50
Q

In a system at equilibrium, should the acid be concentrated or diluted to hydrate the alkene?

A

diluted

51
Q

In a system at equilibrium, should the acid be concentrated or diluted to remove an alcohol and form an alkene?

A

concentrated

52
Q

Describe the regiochemistry and stereochemistry of an acid-catalyzed hydration reaction of an alkene.

A

Markovnikov
both syn and anti

53
Q

What are the reagents for an anti-Markovnikov addition of H and OH across an alkene?

A
  1. BH3 and THF
  2. H2O2 and NaOH
54
Q

Describe the regiochemistry and the stereochemistry of the hydration of an alkene using BH3, THF and H2O2 and NaOH.

A

Anti-Markovnikov
Syn addition

55
Q

Describe the bromination of an alkene.

A

As Br2 approaches the alkene, it induces a temporary dipole that allows the alkene to attack one of the Br atoms. At the same time, a lone pair on the Br attacks the other end of the former-alkene, leading to the formation of a positively charged bromonium ion. The other Br serves as a leaving group. Then, through an SN2 mechanism (back-side attack), the bromide replaces one of the bonds to the bromonium, leaving two Br groups with different configurations.

56
Q

Describe the stereochemistry of the bromination of an alkene.

A

anti

57
Q

What are the reagents in the bromination of an alkene?

A

Br2

58
Q

What are the results of halohydrin formation?

A

Br is added to the less substituted side and OH is added to the more substituted side of a former-alkene. Anti-addition.

59
Q

Why is the more substituted side hydrated in halohydrin formation?

A

Because when the bromonium ion is formed, a tri-substituted carbon is more able to adopt the slight positive charge formed, allowing the H2O to attack as a nucleophile.

60
Q

What are the reagents of halohydrin formation of an alkene?

A

Br2 and H2O

61
Q

Describe the regiochemistry and the stereochemistry of a halohydrin formation from an alkene.

A

The OH ends on the most substituted side and the Br on the least substituted side
Anti addition

62
Q

What are the reagents for an anti-dihydroxylation of an alkene?

A
  1. RCO3H or MCPBA
  2. H3O+
63
Q

Describe the process of anti-dihydroxylation of an alkene.

A

An alkene is treated with a peroxy acid, forming an epoxide. The O in the epoxide is protonated by H3O+. One sigma bond in the epoxide is replaced by the H2O molecule in a back-side attack, leaving an OH group and an OH2+ group with opposite configurations. Then the O with the extra proton group is deprotonated with H2O, leaving 2 OH groups with opposite configurations.

64
Q

What are the reagents for syn dihydroxylation of an alkene?

A
  1. OsO4
  2. H2O2

Or
1. Cold KMnO4
2. NaOH

65
Q

What are the reagents for the oxidative cleavage of an alkene?

A
  1. O3
  2. DMS
66
Q

Describe the stereochemistry of an anti-dihydroxylation reaction.

A

Anti; forms two enantiomers

Organic Chemistry (6 decks)

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