Addition to alkenes Flashcards
What are the products of the addition of HX to an alkene?
A racemic halogenoalkane, as the halide ion can attack from either face of the planar carbocation (Sn1 mechanism).
What type of halogenoalkanes are formed when HX is added to an alkene?
Only secondary and tertiary as the intermediate carbocation is too unstable for a primary halogenoalkane to be formed. The activation energy for the formation is much higher.
What are the two ways in which a carbocation can rearrange to form a more stable intermediate?
A 1,2-hydride shift and a 1,2-alkyl shift.
What happens in a 1,2-hydride shift?
A hydride (H-) movs from one carbon to an adjacent carbon. The hydride will be from a carbon centre with many methyl groups attached, so the carbocation can be formed on this centre instead of the original, less substituted carbocation.
What happens in an 1,2-alkyl shift?
A methyl group migrates to generate a more stable carbocation. (Me-). This can happen is a cation carbon is adjacent to a methyl group.
What is the electrophile in the addition of H2o to alkenes?
H3o+.
What is the nucleophile in the addition of H2O to alkenes?
H2O.
How can the anti-Markovnikov hydration product be generated?
Hydroboration.
What are the general steps in hydroboration?
BH3 is added to the alkene before being oxidised to an alcohol. The BH3 adds, a trialkylborane is formed, the hydroperoxide ion attacks to donate an electron pair and this happens twice more and trialkylborate is formed. It is then attacked by hydroxide and forms the Anti product.
Why is BH3 a good electrophile?
It has an empty P orbital.
How is the hydroperoxide ion formed?
OH- acts as a base and attacks a H on hydrogen peroxide to form water and the hydrperoxide ion.
What type of products are formed from the addition of h2o?
Racemic mixture.
What type of products are formed from the radical addition of br-?
Racemic.
What products are seen when Br2 adds to alkenes and why?
Only the anti addition products, as the mechanism forms a bromonium ion which forces the other bromide ion to attack from the other side.
What is the evidence for the mechanism of the addition of Br2?
A cyclopentene ring used - no c-c bond rotation possible.
