Addition to alkenes
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What general product is formed when bromine reacts with alkene?
1,2-dibromide
How does the reaction between an alkene and bromine take place even though both molecules are neutral?
The alkene has a good source of electrons, the pi bond.
Where does bromine accept the electrons into?
The sigma star orbital of the Br-Br bond
What is the initial product of the bromination reaction?
Cyclic bromonium ion
What does the bromination reaction occur by?
overlap of the filled orbital of the nucleophile (alkene) and the empty sigma star orbital of electrophile (Br-Br)
What type of reaction is bromination?
Concerted reaction
What is a concerted reaction?
Both bond making/breaking occurs in one single step
Why is the addition of Br- to the cyclic bromonium ion a fast reaction?
It is breaking the ring strain of the cyclic bromonium ion
What increases the attraction between the alkene and bromine?
there is an induced dipole The delta positive Br attracted to delta negative alkene
What type of substitution is bromination?
SN2
What are the two steps in forming the bromination
electrons donated from nucleophile to sigma star orbital or Br-Br bond causing it to break. The bromine then uses it’s lone pairs of electrons to donate electrons back to the alkene
What type of addition is bromination?
antiaddition
What is the anti-addition due to?
Both steps of the mechanism being highly stereocontrolled
What is anti-addition?
The two bromines are on different faces of the bond
What is syn-addition?
The two bromines are on the same face of the bond
What is “relative” stereochemistry?
Bromine being attached to both faces
What is “absolute” stereochemistry?
Bromine being attached to only one face
What type of stereocontrol gives an anti-addition product?
Relative stereocontrol
What does the SN2 reaction cause?
Inversion of stereochemistry
When can the initial reaction to form the cyclic bromonium ion only occur?
If both C-Br bonds are formed on the same face of the alkene
What type of addition is the first step of bromination always?
Syn-addition
What is the only exception to the first step being syn-addition?
If the reaction starts by protonation of the alkene as these go via a carbenium ion
Why does protonation of the alkene go by a carbenium ion?
The H+ is too small to form a three membered ring
If the reactant is anti-addition what will the product be?
Anti-addition