Topic 8: Alkenes: Synthesis, Properties and Reactions Part 1 Flashcards
Order of carbocation stability
tertiary > secondary > primary > methyl
alkyl groups exert a +I inductive effect, stabilising the positive charge
Stabilised ylids tend to be … while unstabilised ylids tend to be …
E-selective
Z-selective
The Wittig reaction involves the reaction between a … and a …
phosphorus ylid
carbonyl compound (aldehyde / ketone)
Phosphorus ylids are synthesised in a … reaction
When reacted with an alkyl halide, a … is produced
The protons next to … are … so can be removed by a …
nucleophilic substitution
phosphonium salt
phosphorus
reasonably acidic
strong base
Wittig reaction
Aldehyde/ketone + phosphorus ylid –>alkene + triphenylphosphine oxide
Structure of phosphorus ylid
R1 - C (=PPh3) - R2
Structure of triphenylphosphine oxide
O=PPh3
The driving force of the Wittig reaction is …
the formation of a strong P=O bond
Wittig reaction conditions
mild conditions
room temp or gentle heating
Order of bond strength in alkyne
(Weakest to strongest)
Pi bond (second)
Pi bond (first)
Sigma bond
Pi bonds more similar than sigma
Catalyst that stops reaction (alkyne to alkene) at alkene
Lindlar’s catalyst
Pb to reduce activity
Dissolving metal reduction with Na and NH3
Sodium dissolves in liquid ammonia to form blue solution of solvated electrons.
Electrons reduce alkynes to E-alkenes
(Selective for E-alkenes)
Ammonia acts as proton source
Synthesis of alkyne from alkene (vinyl bromide) requires … and … of …
strong base such as LDA
elimination
HBr
Synthesis of simple alkyne (ethyne) to more complex. … of ethyne is … . … with … to form … . … is good … that reacts with … such as … to form …
Terminal proton
relatively acidic
Deprotonate
NaNH2
acetylide anion
Acetylide
nucleophile
electrophile
alkyl halides
alkyne
Hyperconjugation contributes to …
By…
Cation stability
The overlap of the filled C-H sigma orbital with the empty p orbital
