Organic Chemistry-Chapter 11
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Hydrohalogenation of an alkyne.
- In the mechanism it is hydrohalogenated first to an alkene and then to an alkane.
- It is Markovnikov addition for the Br both times.
- Mechanism- One of the pi bonds from the nucleophile (the alkyne) attacks the H in HBr (or HCl) resulting in a carbocation (the most substituted carbon of the alkyne) and a Bromide with the H going to the least substituted C.
- The bromide then attacks the carbocation (the most substituted carbon).
- The above process is then repeated except as an alkene instead of an alkyne with the end product being an alkane with the most substituted c now having added two Br's and the least substituted having added two H's.
Halogenation of an alkyne
- There is no markovnikov because the same thing is being added to both C's.
- There is no anti-addition of the end product, but in the intermediate there is.
- Br or Cl can be used.
- Mechanism- The pi bonds of the nucleophile attack the more positive Br (Cl can be used too) and the Br attacks back forming a ring between that Br and the two C's (this is called a bromonium ion intermediate). A bromide is formed as well.
- The bromide attacks the more substituted C and the bromine that was in the ring becomes attached only to the other C (the least substituted one). The two Br's that are now attached to the two different C's in anti-addition (it is a transalkene).
- The same process is then done to the alkene and the result is an alkane with two extra Br's (or Cl's) on each C.
Hydration of Alkynes (resulting in enol-keto tautomerization)
- Only one equivalent of water is needed.
- Getting to an enol- The base (the alkyne) attacks the H off of a hydronium ion (the acid) resulting in an alkene with the most substituted C as a carbocation.
- A nucleophile (a water) attacks the carbocation resulting in a water being attached to the most substituted C of the alkene (The O is positive).
- A base (water) then attacks the acidic part of the alkene (one of the H attached to the positive O) resulting in an Enol (A hydroxyl group is added to the most substituted C).
- Getting a Keto from an Enol-
- Using an acid catalystThere is a resonance structure where electrons from O make a double bond between O and C and the alkyne makes the least subst. C negative. The negative C (a nucleophile) grabs an H from a hydronium ion resulting in the least subst. C having 3 H's.
- A water grabs the H off of the O making the positive O neutral resulting in a keto.
- Using a base catalystA base (for example a hydroxide) attacks the H connected to the O making the O negative. The resulting structure has an O connected to the most subst. C in the alkene and two H's off of the least subst. C in the alkene.
- The resulting structure has a resonance structure where the extra pair of electrons from the O becomes a double bond between it and the most subst. C and the double bond between the C's goes to the least subst. C making it negative.
- The extra pair of electrons on the C attack an H on a water resulting in a Keto which is an alkane with a double bond to an O on the most subst. C and three H's on the least subst. C.
Alternative Hydration of Alkynes (Markovnikov addition)
The mechanism isn't important but the reagents are and they are H2O, H2SO4, and HgSO4.
Alternative Hydration of Alkynes (Anti-Markovnikov addition)
- The double bonded O (carbonyl) is on the least subst. C rather than the most subst. C and the two extra H's are on the most subst. C.
- The mechanism isn't important but the reagents are and they are 1) disiamyborane and 2) H2H2, OH-.
In the reduction of alkynes/alkenes what do the reagents H2 and Pd/C do?
- They reduce the double and triple bonds completely to alkanes making a fully saturated alkane.
- ALL pi bonds are gone.
In the reduction of alkynes/alkenes what do the reagents H2 and Lindlar's catalyst do?
- Because Lindlar's catalyst is a "poisoned" catalyst it is weaker.
- Alkynes are only reduced to alkenes and the H's are added in syn addition (making a cis product).
- Alkenes aren't reduced with these.
In the reduction of alkynes/alkenes what do the reagents Na (metal) and NH3 (liquid) do?
- Alkynes are only reduced to alkenes and the H's are added in anti-addition (making a trans product).
- Alkenes aren't reduced with these.
What is Epoxidation? (using the reagent of mCPBA)
- She didn't explain the mechanism on this one.
- Alkene is reduced to alkane.
- O is added as a three-membered ring to where the alkene used to be.
- Makes a pair of enantiomers.
- The substituents that were already there are added in syn addition just meaning they were added in the same way they were with respect to each other.
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