expoxidation, bromination, dihydroxylation Flashcards
(30 cards)
What is the test for alkenes are what colour changes occur?
Addition of bromine turns reaction from red/brown to colourless
What is the stereochemistry of addition of bromine and what is used to prove it
Always trans never cis and validated through the formation of bromonium ion (can be isolated)
What is the actual mechanism for bromination?
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
what is the difference between regio and stereoselectivity?
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.
What is the stereospecificity of bromination?
E forms mesoisomers
Z forms enantiomers (chiral mixture)
What are epoxides?
3 membered ring containing oxygen
Why are epoxides reactive?
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
What are epoxide reactions used for?
After breaking the epoxide ring the negatively charged oxygen reacts with acidic water to form hydroxyl group
How are epoxides generated? What are the reagents used? Is it concerted? What is the product?
Oxidation of double bonds
- mCPBA (meta-chloroperbenzoic-acid)
- concerted: 1 pot
products: epoxide and carboxylic acid
Why can perbenzoic acid ions form epoxides?
The second oxygen is very electrophilic and therefore reacts with the electron dense alkene bond
Draw out the mechanism for formation of epoxide
Check p7 of slides
What effects the rate of epoxide formation?
More electron rich alkene = faster epoxidation (more substituents means more electrons to be found)
What is the epoxide reaction similar to? what is the only product that can be formed?
Bromine bromonium ion reaction as the nucleophile is attached in the opposite side of the epoxide
only syn product can form
What is the reaction that allows for syn addition (cis) of oxygens to an alkene and draw the molecule
Alkene + osmium tetraoxide then hydrolysis with water
Is the dihydroxylation of alkenes with osmium tetraoxide 1 step or 2 step and what type of product does it form
1 step and concerted CIS ONLY NEVER TRANS
How does the oxidation state of the osmium change in the dihydroxylation reaction?
8 to 6
What is the difference between the products of epoxidation and addition of osmium tetraoxide?
epoxidation = trans product
osmium tetraoxide = cis product
what does the diels alder reaction consist of and why does it only work with those 2 reactants?
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
which confirmation of the diene is required for the diels alder reaction?
cis confirmation but trans can be converted in a diene because it only required the rotation of the central single bond
if the nucleophile can attack from the top and the bottom in the carbocation intermediate, what is the relationship between the 2 products formed?
enantiomers
how was the existence of the bromonium ion proven?
using a bulky substituent that hinders backside attack so that the bromonium ion can be isolated
what are the orbitals involved in the bromination steps?
- pi bonding orbital is HOMO on alkene and it attacked the LUMO of Br-Br which is the sigma antibonding orbital
- 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)
if the alkene is unsymmetric, which site will the nucleophile attack?
the side with the carbocation that can have more substitution (test the positive charge on both ends to decide)
when is a reaction concerted?
3+2 no. of atoms involved or 4+2 no. of electrons involved
