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