Addition to alkenes Flashcards
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
If the reactant is syn-addition, what will the product be?
Syn-addition
What type of product will be formed from an E-alkene?
Anti-addition
What type of product will be formed from a Z-alkene?
syn-addition
Where do electrophiles add to alkenes?
To form the most stable carbenium ion intermediate
What is the reaction like if I2 is used as the halogen?
Reaction occurs via an iodonium ion
Equilibrium is unfavourable and only reactive alkenes will react with iodine
What is the reaction like if Cl2 is used as the halogen?
Addition occurs via a chloronium ion
What is the reaction like if the HI and HCl are used as the halogens?
Same as HBr
Anti-addition reaction
What is the reaction like if BrOH or ClOH are used as the halogens?
The halogen-OH bond is polarised with the halogen being delta positive so reactions occurs via a bromonium or chloronium ion
What is the reaction like if Br-Cl , Br-F or I-Cl are used as the halogens?
The bond between two different halogens is polarised with the heavier halogen being delta positive so reactions occur via a bromonium or iodonium ion
What are the most stable carbenium ion?
Tertiary and allylic
What do you have to match the nucleophile to?
The electrophile
What makes an alkene hard?
Most hindered
Higher charge density
What makes an alkene soft?
Low charge density
Least hindered
Where do Nu- and E+ tend to be in relation to each other?
Opposite ends of the conjugated system
Where does initial protonation occur and why?
Most substituted double bond to give the most stable carbenium ion
How can you make an alkene react with H3O+ to give an alcohol?
If the carbenium ion intermediate is tertiary and in strong acid
What type of nucleophile is an alkene generally?
Soft
What type of electrophiles are mercury reagents? Why?
Soft
They have a 2+ charge over a huge molecule
When mercury reagents are reacted with alkenes, what is the initial product?
Cyclic 3 membered mercurinium ion ring
Where does water attack during oxymercuration? Why is this odd?
Most hindered end
This is very unique and hard to do in other circumstances
How do sterics influence the oxymercuration reaction?
There is a steric clash between the Hg and the methyl group, pushing the Hg the other way, causing the bond to become long, weak and essentially already broken
Which carbon does water react at in the oxymercuration reaction?
Carbon atom which has the greater positive charge
This will be the more substituted carbon atom as alkyl groups stabilise positive charges
What are the conditions and reagents for the hydroboration of alkenes?
- BH3, THF
2. H2O2, NaOH
What does the hydroboration result in?
Overall addition of water across the alkene
Is Borane an electrophile or a nucleophile? Why?
Electrophile
6 outer shell electrons and has a vacant p-orbital
Where does Borane react?
At the less substituted end of the alkene so that delta positive builds up at the more substituted carbon atom
