Preparing Alkenes Flashcards
How can alkenes be prepared from alkynes?
By partial reduction of alkyne using a Lindlar catalyst and H2
By partial reduction of alkynes using sodium and liquid ammonia
How can alkenes be prepared from aldehydes and ketones?
From the reaction of a phosphonium ylilde with an aldehyde or ketone In a wittig reaction
What control do we have of the products of a wittig reaction?
Have regiocontrol of alkene as alkene always replaces the carbonyl
How regiocontrol of double bond geometry
What is the stereochemistry of the products a wittig reaction?
If R of the ylide is non stabilising alkyl group you get a z alkene as the major product
If R of the ylide is stabilising ester, ketone, nitrile we get E alkene as the major product
(Need to stabilise negatively charged carbon)
What is saytzev elimination?
The more stable alkene is the most highly substituted one
When X is leaving group
What is Hoffman elimination?
The less substituted alkene is the more stable alkene
Positively charged leaving group
How can an alkene form from a halogenoalkane or alcohol?
This can occur by elimination reactions of the halogenoalkane or alcohol
Which is the favoured stereochemistry of the halogenoalkane elimination reaction?
The E isomer is the major product due to less steric strain
What is meant by stereospecific?
This is a species that can only adopt one conformation
Why is the elimination of halogenoalkanes not useful?
It is not synthetically useful as they give a number of products
