Addition to alkenes

Question 1

What general product is formed when bromine reacts with alkene?

Answer 1

1,2-dibromide

Question 2

How does the reaction between an alkene and bromine take place even though both molecules are neutral?

Answer 2

The alkene has a good source of electrons, the pi bond.

Question 3

Where does bromine accept the electrons into?

Answer 3

The sigma star orbital of the Br-Br bond

Question 4

What is the initial product of the bromination reaction?

Answer 4

Cyclic bromonium ion

Question 5

What does the bromination reaction occur by?

Answer 5

overlap of the filled orbital of the nucleophile (alkene) and the empty sigma star orbital of electrophile (Br-Br)

Question 6

What type of reaction is bromination?

Answer 6

Concerted reaction

Question 7

What is a concerted reaction?

Answer 7

Both bond making/breaking occurs in one single step

Question 8

Why is the addition of Br- to the cyclic bromonium ion a fast reaction?

Answer 8

It is breaking the ring strain of the cyclic bromonium ion

Question 9

What increases the attraction between the alkene and bromine?

Answer 9

there is an induced dipole

The delta positive Br attracted to delta negative alkene

Question 10

What type of substitution is bromination?

Answer 10

SN2

Question 11

What are the two steps in forming the bromination

Answer 11

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

Question 12

What type of addition is bromination?

Answer 12

antiaddition

Question 13

What is the anti-addition due to?

Answer 13

Both steps of the mechanism being highly stereocontrolled

Question 14

What is anti-addition?

Answer 14

The two bromines are on different faces of the bond

Question 15

What is syn-addition?

Answer 15

The two bromines are on the same face of the bond

Question 16

What is “relative” stereochemistry?

Answer 16

Bromine being attached to both faces

Question 17

What is “absolute” stereochemistry?

Answer 17

Bromine being attached to only one face

Question 18

What type of stereocontrol gives an anti-addition product?

Answer 18

Relative stereocontrol

Question 19

What does the SN2 reaction cause?

Answer 19

Inversion of stereochemistry

Question 20

When can the initial reaction to form the cyclic bromonium ion only occur?

Answer 20

If both C-Br bonds are formed on the same face of the alkene

Question 21

What type of addition is the first step of bromination always?

Answer 21

Syn-addition

Question 22

What is the only exception to the first step being syn-addition?

Answer 22

If the reaction starts by protonation of the alkene as these go via a carbenium ion

Question 23

Why does protonation of the alkene go by a carbenium ion?

Answer 23

The H+ is too small to form a three membered ring

Question 24

If the reactant is anti-addition what will the product be?

Answer 24

Anti-addition

Question 25

If the reactant is syn-addition, what will the product be?

Answer 25

Syn-addition

Question 26

What type of product will be formed from an E-alkene?

Answer 26

Anti-addition

Question 27

What type of product will be formed from a Z-alkene?

Answer 27

syn-addition

Question 28

Where do electrophiles add to alkenes?

Answer 28

To form the most stable carbenium ion intermediate

Question 29

What is the reaction like if I2 is used as the halogen?

Answer 29

Reaction occurs via an iodonium ion | Equilibrium is unfavourable and only reactive alkenes will react with iodine

Question 30

What is the reaction like if Cl2 is used as the halogen?

Answer 30

Addition occurs via a chloronium ion

Question 31

What is the reaction like if the HI and HCl are used as the halogens?

Answer 31

Same as HBr | Anti-addition reaction

Question 32

What is the reaction like if BrOH or ClOH are used as the halogens?

Answer 32

The halogen-OH bond is polarised with the halogen being delta positive so reactions occurs via a bromonium or chloronium ion

Question 33

What is the reaction like if Br-Cl , Br-F or I-Cl are used as the halogens?

Answer 33

The bond between two different halogens is polarised with the heavier halogen being delta positive so reactions occur via a bromonium or iodonium ion

Question 34

What are the most stable carbenium ion?

Answer 34

Tertiary and allylic

Question 35

What do you have to match the nucleophile to?

Answer 35

The electrophile

Question 36

What makes an alkene hard?

Answer 36

Most hindered | Higher charge density

Question 37

What makes an alkene soft?

Answer 37

Low charge density | Least hindered

Question 38

Where do Nu- and E+ tend to be in relation to each other?

Answer 38

Opposite ends of the conjugated system

Question 39

Where does initial protonation occur and why?

Answer 39

Most substituted double bond to give the most stable carbenium ion

Question 40

How can you make an alkene react with H3O+ to give an alcohol?

Answer 40

If the carbenium ion intermediate is tertiary and in strong acid

Question 41

What type of nucleophile is an alkene generally?

Answer 41

Soft

Question 42

What type of electrophiles are mercury reagents? Why?

Answer 42

Soft | They have a 2+ charge over a huge molecule

Question 43

When mercury reagents are reacted with alkenes, what is the initial product?

Answer 43

Cyclic 3 membered mercurinium ion ring

Question 44

Where does water attack during oxymercuration? Why is this odd?

Answer 44

Most hindered end | This is very unique and hard to do in other circumstances

Question 45

How do sterics influence the oxymercuration reaction?

Answer 45

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

Question 46

Which carbon does water react at in the oxymercuration reaction?

Answer 46

Carbon atom which has the greater positive charge | This will be the more substituted carbon atom as alkyl groups stabilise positive charges

Question 47

What are the conditions and reagents for the hydroboration of alkenes?

Answer 47

1. BH3, THF | 2. H2O2, NaOH

Question 48

What does the hydroboration result in?

Answer 48

Overall addition of water across the alkene

Question 49

Is Borane an electrophile or a nucleophile? Why?

Answer 49

Electrophile | 6 outer shell electrons and has a vacant p-orbital

Question 50

Where does Borane react?

Answer 50

At the less substituted end of the alkene so that delta positive builds up at the more substituted carbon atom