expoxidation, bromination, dihydroxylation

Question 1

What is the test for alkenes are what colour changes occur?

Answer 1

Addition of bromine turns reaction from red/brown to colourless

Question 2

What is the stereochemistry of addition of bromine and what is used to prove it

Answer 2

Always trans never cis and validated through the formation of bromonium ion (can be isolated)

Question 3

What is the actual mechanism for bromination?

Answer 3

both carbons attack the 1 bromine from the bromine molecule at the same time -> forms bromonium ion (positive charge on bromine instead of carbon, tricyclic intermediate) -> trans product formed after second bromine reacts

Question 4

what is the difference between regio and stereoselectivity?

Answer 4

In summary, regioselectivity refers to the preference of a reaction for a specific position or site within a molecule, while stereoselectivity refers to the preference of a reaction for a specific stereoisomer or spatial arrangement of atoms.

Question 5

What is the stereospecificity of bromination?

Answer 5

E forms mesoisomers
Z forms enantiomers (chiral mixture)

Question 6

What are epoxides?

Answer 6

3 membered ring containing oxygen

Question 7

Why are epoxides reactive?

Answer 7

Due to the strain of the 3 membered ring + the electronegativity of oxygen creates a region of delta positive around the carbons which are prone to nucleophile attack

Question 8

What are epoxide reactions used for?

Answer 8

After breaking the epoxide ring the negatively charged oxygen reacts with acidic water to form hydroxyl group

Question 9

How are epoxides generated? What are the reagents used? Is it concerted? What is the product?

Answer 9

Oxidation of double bonds
- mCPBA (meta-chloroperbenzoic-acid)
- concerted: 1 pot
products: epoxide and carboxylic acid

Question 10

Why can perbenzoic acid ions form epoxides?

Answer 10

The second oxygen is very electrophilic and therefore reacts with the electron dense alkene bond

Question 11

Draw out the mechanism for formation of epoxide

Answer 11

Check p7 of slides

Question 12

What effects the rate of epoxide formation?

Answer 12

More electron rich alkene = faster epoxidation (more substituents means more electrons to be found)

Question 13

What is the epoxide reaction similar to? what is the only product that can be formed?

Answer 13

Bromine bromonium ion reaction as the nucleophile is attached in the opposite side of the epoxide

only syn product can form

Question 14

What is the reaction that allows for syn addition (cis) of oxygens to an alkene and draw the molecule

Answer 14

Alkene + osmium tetraoxide then hydrolysis with water

Question 15

Is the dihydroxylation of alkenes with osmium tetraoxide 1 step or 2 step and what type of product does it form

Answer 15

1 step and concerted CIS ONLY NEVER TRANS

Question 16

How does the oxidation state of the osmium change in the dihydroxylation reaction?

Answer 16

8 to 6

Question 17

What is the difference between the products of epoxidation and addition of osmium tetraoxide?

Answer 17

epoxidation = trans product
osmium tetraoxide = cis product

Question 18

what does the diels alder reaction consist of and why does it only work with those 2 reactants?

Answer 18

cycloaddition with a diene and alkene (works because the LUMO of the alkene and the HOMO of the diene are similar in energy), the LUMO of a diene is too high in energy to be combined with the HOMO of another diene

Question 19

which confirmation of the diene is required for the diels alder reaction?

Answer 19

cis confirmation but trans can be converted in a diene because it only required the rotation of the central single bond

Question 20

if the nucleophile can attack from the top and the bottom in the carbocation intermediate, what is the relationship between the 2 products formed?

Answer 20

enantiomers

Question 21

how was the existence of the bromonium ion proven?

Answer 21

using a bulky substituent that hinders backside attack so that the bromonium ion can be isolated

Question 22

what are the orbitals involved in the bromination steps?

Answer 22

1. pi bonding orbital is HOMO on alkene and it attacked the LUMO of Br-Br which is the sigma antibonding orbital
2. the Br-C antibonding orbital is then attacked by the HOMO of the Br- (backside attack, whichever side the Br- attacks the bond between the C-Br will be broken)

Question 23

if the alkene is unsymmetric, which site will the nucleophile attack?

Answer 23

the side with the carbocation that can have more substitution (test the positive charge on both ends to decide)

Question 24

when is a reaction concerted?

Answer 24

3+2 no. of atoms involved or 4+2 no. of electrons involved

Question 25

what is a chiral compound?

Answer 25

A chiral molecule is a molecule that is not superimposable on its mirror image. In other words, a chiral molecule lacks an internal plane of symmetry.

Question 26

what is the regioselectivity or borane addition?

Answer 26

borane is added to the least substituted end of the alkene

Question 27

draw the mechanism for borane addition

Answer 27

check notes

Question 28

which bond is formed faster in borane addition?

Answer 28

C-B bond is formed faster than C-H which makes the more substituted carbon more delta positive

Question 29

what is a regioisomer

Answer 29

an isomer that differs in the connectivity or arrangement of atoms within a molecule.