Addition Reactions of Alkynes Flashcards
How do I form an alkyne from an alkane?
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.
What are the reagents for the formation of an alkyne from an alkyl dihalide?
Excess NH2/NH3 followed by H2O
What is the process that reduces alkynes to alkanes?
catalytic hydrogenation
What are the reagents in a catalytic hydrogentation that produce an alkane?
H2 followed by a metal catalyst (Pt, Ni, or Pd)
What are the reagents in a catalytic hydrogenation of an alkyne into a cis alkene?
H2 followed by a poisoned catalyst (Lindlar’s catalyst OR Ni2B)
What are the reagents in a catalytic hydrogenation of a geminal alkyne into a trans alkene?
Na followed by NH3 (l)
What does of hydrohalogenation produce?
Markovnikov reaction of halogens
What is the difference between treating alkynes with HX and treating them with excess HX?
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
What is the mechanism for the hydrohalogenation of alkynes?
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.
What are the reagents for an anti-Markovnikov hydrohalogenation of an alkyne?
HX followed by peroxides (ROOR)
How to create a geminal dihalide?
Treat an alkyne with excess HX.
How to turn a dihalide into a terminal alkyne?
Treat a dihalide with excess NaNH2/NH3 followed by H2O.
What are the reagents for an acid-catalyzed hydration reaction?
Either H3O+ or H2SO4,H2O, followed by HgSO4.
What is the effect of an acid-catalyzed hydration reaction of an alkyne?
A ketone is added to the more substituted side.
Describe the acid-catalyzed hydration reaction of an alkyne.
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.
What is an enol?
a compound with a double bond and an alcohol that rapidly converts into a ketone via acid-catalyzed tautomerization
How does an enol become a ketone via acid-catalyzed tautomerization?
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.
What are tautomers?
constitutional isomers that rapidly interconvert via the migration of a proton
What is the product of the acid-catalyzed hydration of unsymmetrical internal alkynes?
a mixture of ketones, placed on opposite ends of the former triple bond
What is the product of the acid-catalyzed hydration of a terminal alkyne?
a methyl ketone
What are the reagents of a hydroboration-oxidation of alkynes?
- Dialkyl borane (either Disiamylborane or 9-BBN)
- H2O2, NaOH
What is the result of a hydroboration-oxidation of alkynes?
the addition of an aldehyde at the less substituted side of the alkyne; converts a terminal alkyne into an aldehyde
Is tautomerization during acid-catalyzed hydration acid or base-catalyzed?
acid
Is tautomerization during hydroboration oxidation acid or base-catalyzed?
base
How does an enol become an aldehyde during a base-catalyzed tautomerization?
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.
What is the difference in mechanism between acid-catalyzed hydration and hydroboration oxidation?
Acid-catalyzed hydration is a Markovnikov hydration and hydroboration oxidation is an anti-Markovnikov addition.
What happens during hydroboration oxidation of an alkyne?
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.
To convert a terminal alkyne into an aldehyde, which process would I use?
hydroboration oxidation
To convert a terminal alkyne into a ketone, which process would I use?
acid-catalyzed hydration
What are the reagents for the halogenation of alkynes, resulting in an tetrahalide?
excess X2 and CCl4
X is either Cl or Br
What are the reagents to produce a dihalide from an alkyne?
one equivalent of X2 and CCl4
What are the products of the halogenation of an alkyne to produce an alkene? Which is major and which is minor?
The products are dihalides. The E isomer is the major and the Z isomer is the minor product.
What are the reagents for the ozonolysis of alkynes?
- O3
- H2O
What are the products when a terminal alkyne undergoes oxidative cleavage?
A carboxylic acid and carbon dioxide
What are the products when an internal alkyne undergoes oxidative cleavage?
Carboxylic acids with the R groups attached to both
How can a carbon chain be extended?
Via alkylation of an alkyne. NH2 - deprotonates a terminal alkyne and a methyl or primary alkyl halide is attached via a nucleophilic attack.
What kind of pathway is alkylation?
SN2
What would happen if one tried to use a secondary or tertiary alkyl halide in alkylation?
The alkynide ion would function as a base and elimination products would be obtained.
How to convert an alkene into an alkyne?
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.
What are the reagents for hydrogenation of an alkene?
H2 and a metal catalyst (Pd, Pt, Ni)
What kind of mechanism is hydrogenation of an alkene?
syn addition, regiochemistry is irrelevant
What happens when an alkene is hydrogenated?
2 H atoms are added across the double bond and the alkene becomes an alkane.
Describe the mechanism of hydrohalogenation of alkenes.
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.
Describe the regiochemistry and stereochemistry of the hydrohalogenation of alkenes.
Markovnikov
syn and anti additions
Describe the regiochemistry and the stereochemistry of the hydrohalogenation of alkenes using peroxides.
Anti-Markovnikov
syn and anti
What are the reagents for the anti-Markovnikov addition of H and X across an alkene?
HX and ROOR
What are the reagents for the hydration of an alkene?
H3O+; also displayed as [H2SO4] and H2O
What are the products of the acid-catalyzed hydration of an alkene?
Markovnikov addition of an alcohol
Describe the mechanism of acid-catalyzed hydration of an alkene.
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.
In a system at equilibrium, should the acid be concentrated or diluted to hydrate the alkene?
diluted
In a system at equilibrium, should the acid be concentrated or diluted to remove an alcohol and form an alkene?
concentrated
Describe the regiochemistry and stereochemistry of an acid-catalyzed hydration reaction of an alkene.
Markovnikov
both syn and anti
What are the reagents for an anti-Markovnikov addition of H and OH across an alkene?
- BH3 and THF
- H2O2 and NaOH
Describe the regiochemistry and the stereochemistry of the hydration of an alkene using BH3, THF and H2O2 and NaOH.
Anti-Markovnikov
Syn addition
Describe the bromination of an alkene.
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.
Describe the stereochemistry of the bromination of an alkene.
anti
What are the reagents in the bromination of an alkene?
Br2
What are the results of halohydrin formation?
Br is added to the less substituted side and OH is added to the more substituted side of a former-alkene. Anti-addition.
Why is the more substituted side hydrated in halohydrin formation?
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.
What are the reagents of halohydrin formation of an alkene?
Br2 and H2O
Describe the regiochemistry and the stereochemistry of a halohydrin formation from an alkene.
The OH ends on the most substituted side and the Br on the least substituted side
Anti addition
What are the reagents for an anti-dihydroxylation of an alkene?
- RCO3H or MCPBA
- H3O+
Describe the process of anti-dihydroxylation of an alkene.
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.
What are the reagents for syn dihydroxylation of an alkene?
- OsO4
- H2O2
Or
1. Cold KMnO4
2. NaOH
What are the reagents for the oxidative cleavage of an alkene?
- O3
- DMS
Describe the stereochemistry of an anti-dihydroxylation reaction.
Anti; forms two enantiomers
