Ch. 7: The Reactions of Alkynes, An Introduction to Multistep Synthesis Flashcards
What is an alkyne?
a hydrocarbon that contains a C–C triple bond
What is the general molecular formula for an acyclic alkyne?
CnH2n-2
What is the general molecular formula for a cyclic alkyne?
CnH2n-4
What is a terminal alkyne?
When the triple bond of an alkyne is located at the end of the chain
What is an internal alkyne?
When the triple bond of an alkyne is located elsewhere beside the end of the chain
What is the common name of an alkyne?
acetylene
Which has a higher boiling point and why: alkenes or alkynes
Alkynes have a higher boiling point because they’re more linear and a triple bond is more polarizable than a double bond. These two features cause an alkyne to have stronger LDFs and, therefore a higher BP than an alkene with the same number of carbons.
Are alkynes nucleophiles or electrophiles?
Nucleophiles, and undergo electrophilic addition reactions because of their relatively weak pi bonds.
What is the rate limiting step for electrophilic addition of HCl to an alkene?
the addition of the proton to the alkene to form an alkyl cation
What is the rate-limiting step for electrophilic addition of HCl to an alkyne?
the addition of a proton
What is a vinylic cation?
Addition of a proton to an alkyne forms a vinylic cation. A vinylic cation has a positive charge on a vinylic carbon.
What is an allylic carbon?
the carbon atom that is next to the double bonded carbon atom
What is a vinylic carbon?
the carbon atom that is double bonded to another carbon
What’s more electronegative: an sp carbon or an sp2 carbon?
an sp carbon is more electronegative and is, therefore, less able to bear a positive charge
Explain the relative stabilities of carbocations.
Explain the pi-complex that forms when a proton is added to an alkyne.
What’s more reactive, an alkene or alkyne?
An alkyne. While a pi-complex is more stable than a vinylic cation, it’s still less stable than an alkyl cation.
Under what circumstances can you assume that the less stable of two compounds is the more reactive compound?
For the less stable compound to be the more reactive compound, the less stable compound must have the more stable transition state, or the difference in the stabilities of the reactants must be greater than the difference in the stabilities of the transition states.
What is the product of the electrophilic addition reaction of an alkyne with a hydrogen halide? And if excess hydrogen halide?
1) alkene
2) a geminal dihalide, the H+ adds to the less substituted sp2 carbon (the C–H)
What is a geminal dihalide?
A molecule with two halogens on the same carbon.
Is the halo-substituted alkene more or less reactive than an alkyne and why?
The halo-substituted alkene is less reactive than an alkyne because the halo-substituent withdraws electrons inductively (through the sigma bond), thereby decreasing the nucleophilic character of the double bond.
Is the first electrophilic reaction of a hydrogen halide to an alkyne regioselective or no?
It is regioselective, the H+ adds to the less substituted sp carbon.
What is the intermediate of the addition of a hydrogen halide to an alkyne?
A pi-complex
Explain the mechanism for electrophilic addition of a hydrogen halide to an alkyne?
Is the electrophilic addition of a hydrogen halide to an alkyne syn or anti? Mark or anti-mark?
Mark, anti
Is the addition of a halogen to an alkyne syn or anti? What are the reagents?
Anti, X2 and CH2Cl2
What is an enol?
Has a C–C double bond with an OH group bonded to one of the sp2 carbons. alkene + alcohol
What is a tautomer?
constitutional isomers that are in rapid equilibrium. A ketone and enol differ only in the location of a double bond and a hydrogen.
What are keto-enol tautomers?
A ketone and its corresponding enol
What are the reagents for the addition of water to an alkyne?
H2SO4 (and I think H2O too?)
What are the products of the addition of water to an alkyne?
Initially you got an enol, then you got keto-enol tautomers
Is the keto or enol tautomer more stable?
the keto
What is the mechanism for acid-catalyzed keto-enol interconversion?
What’s the product of the addition of water to a symmetrical internal alkyne?
A single ketone.
What’s the product of the addition of water to a unsymmetrical internal alkyne?
Two ketones.
Explain mercuric-ion-catalyzed hydration of an alkyne.
Terminal alkynes are less reactive than internal alkynes toward the addition of water. Therefore, the addition of water to a terminal alkyne is generally carried out in the presence of mercuric ion (Hg2+). The mercuric ion is a catalyst - it increases the rate of the addition reaction.
Explain the mechanism for the mercuric-ion-catalyzed hydration of an alkyne.
What are the reagents for a hydroboration-oxidation reaction of an alkyne?
- THF/R2BH
- HO-, H2O2, H2O
What is the intermediate and product of a hydroboration-oxidation reaction of an alkyne? Is it Mark or Anti-Mark? Syn or Anti?
Intermediate: boron-substited alkene
Product: an aldehyde (the carbonyl group is on the terminal carbon)
Anti-Mark, Anti
When is an alkene the product of the addition of hydrogen to an alkyne instead of an alkane?
The initial product of hydrogenation is an alkene, but it is difficult to stop the reaction at this stage because alkenes are more reactive than alkynes. The final product of the hydrogenation reaction, therefore, is an alkane. But the reaction can be stopped at the alkene stage if a “poisoned” (partially deactivated) palladium catalyst is used. The most common partially deactivated palladium catalyst is known as Lindlar catalyst.
Is the product of the hydrogenation of an alkyne syn or anti?
Syn because the alkyne sits on the surface of the metal catalyst and the hydrogens are delivered to the triple bond from the surface of the catalyst, both Hs are delivered to the same side of the double bond.
How can internal alkanes be converted to trans alkenes in the hydrogenation of an alkyne?
Using liquid ammonia (NH3). The reaction stops at the alkene stage because Na or Li reacts more rapidly with triple bonds than with double bonds. This reaction is called a dissolving metal reduction.
Explain the mechanism for the conversion of an alkyne to a trans alkene (hydrogenation).
What is a radical anion?
a species with a negative charge and an unpaired electron
What is a vinylic radical?
the unpaired electron that’s on a vinylic carbon
Explain the difference in acidity between ethyne, ethene, and ethane.
think s-character and how that effects electronegativity
Explain the mechanism for an alkylation reaction?
What reagants can you use to turn an alkyne into a acetylide ion?
NaNH2
What reactions results in a ketone?
acid-catalyzed addition of water
hydroboration-oxidation
How can 3-hexyne be produced in a synthesis with 1-butyne as the starting material?
remove a proton from its sp carbon, followed by alkylation.
Starting with ethyne, how could you make 2-bromopentane?
2-bromopentane can be prepared from 1-pentene –> 1-pentyne –> ethyne and an alkyl halide with 3 carbons
What are the reagents for alkylation?
- NaNH2
- [alkyl]Br
How can you prepare an anti-Mark alcohol?
hydroboration-oxidation
How can you prepare a Mark alcohol?
acid-catalyzed addition of water
How do you prepare a trans alkene?
from the reaction of an internal alkyne with Na/NH3(liq)
How do you prepare a cis epoxide?
from a cis alkene and a peryoxyacid
How do you prepare a geminal dibromide?
from an alkyne
What reactions do we know that form a ketone?
1) acid-catalyzed addition of water to an alkene
2) hydroboration-oxidation
