Alkenes Flashcards
what do alkenes act as?
Nucleophiles- high electron density
What happens in reactions of unsymmetrical alkenes?
2 isometric products could form
the reaction will proceed through the more stable carbocation - the more stable one is where the + is n the carbocation. There are more sigma bonds (i.e more groups) around it allowing the electron density to be spread out around
How to make - bromoalkanes and alcohols from alkenes
Bromoalkanes - alkenes + HBr
Alcohols- alkenes + H30+ (H20 and H+)
2 ways of forming epoxides from alkenes
reaction with m-CPBA or dioxirane
what stabilises carbocations?
hyper conjugation stabilises carbocations and more highly substituted carbocations are more stable due to donating of sigma bond electrons into the empty P orbital of the carbocation
Tertiary > secondary > primary
what does markovnikovs rule help us do?
helps us predict which product will result from electrophilic additions of H-X to unsymmetrical alkenes
Define markovnikovs rule
The hydrogen of the H-X reagent becomes attached to the carbon of the double bond that had more hydrogens attached to it in the alkene starting material
(For heterolytic reactions - markovnikov addition)
What does markovnikov addition and anti-maokovnikov addition require?
markovnikov - requires heterolytic addition of HX to alkene - ionic process
Anti markovnikov= requires homolytic addition – free radical process
What is homolyitc addition?
movement of 1 electron to generate a neutral species - Reacts with radicals
How can homolytic reactions be achieved?
by performing reaction in the presence of. a radical initiator.
Performed from peroxide in the presence of light
Breaks the weak O-O bond to form alkoxy radicals in the initiation step
Can then abstract a hydrogen radical from HBr to form a Br radical
what is the process of forming isomers of alcohols?
if different isomers of alcohols are required via reactions of unsymmetrical alkenes then an alternative reaction called hydroboration is used
Anti-markovnikov = OH attaches to the less substituted carbon
Via BH3, NaOH, H2O2 or H+/H20
why is m-CPBA a good electrophile?
because the carboxylate anion Is a good LG as its stable due to resonance
In epoxidation - is the alkenes stereochemistry conserved?
Yes - it occurs from 1 face of the molecule.
Z alkenes = Z-epoxides
E alkenes = E-epoxides
Why is it advantageous to use dioxirane instead of m-CPBA when forming epoxides?
Dioxirane more advantageous expoxidising agent since it doesn’t produce an acidic by product in the reaction
why do we converts alkenes to diols?
we can alter functional groups In molecules to obtain more water soluble molecules = diols
