Electrophilic Addition to Alkenes and Alkynes Flashcards
(42 cards)
why is the C-H bond of an alkene shorter than in an alkane?
in the alkene, the electrons in the bond are held more closely to the carbon as it has more ‘s’ character.
what does it mean if something has more ‘s’ character?
orbitals with more ‘s’ character tend to be more spherical.
more tightly bound to the nucleus.
better at bonding with other atoms.
bond angle and strength are also impacted.
why is the boiling point of cis alkenes normally higher than trans alkenes?
carbon sp² - sp³ bond is polarised towards sp², causing a dipole moment.
in cis, these reinforce, in trans they cancel out.
larger dipole moments = larger dipole interactions = larger boiling points.
what determines the acidity of alkenes?
stability of resulting carbocation after double bond has been protonated.
more stable carbocation = more acidic alkene.
protonation forms + charge on one carbon in the db, = carbocation intermediate.
stabilised by resonance, positive charge is delocalised over adjacent π bonds.
alkenes w/ electron-withdrawing groups stabilise + charge and increase acidity.
alkenes w/ electron-donating groups destabilise the + charge and decrease the acidity.
why are alkenes more acidic than alkanes?
when H is removed, a carbanion is formed, stabilised by the π bonds.
The bond is able to delocalise the - charge, distributing it over a larger area.
in contrast, the alkane conjugate base is destabilised by inability of sp³ to accomodate additional - charge.
what are the key differences between the IRS of alkenes and alkanes?
C-H stretch in alkene is >3000, but lower in alkanes.
C=C stretch in alkenes is between 1620-1680.
what are the differences between the IRS of alkenes and alkynes?
C-H stretch in alkenes = 3000-3100,
but larger in alkynes.
C=C stretch in alkynes is >1900.
tell me about the bromination of alkenes?
Br₂ + alkene -> Br
\__
\
Br
toluene is the solvent.
alkenes turn colourless.
alkanes stay orange/brown.
alkene = Nu
(HOMO is π filled).
Br = E
(LUMO is σ* filled).
why do more electron rich alkenes lead to faster rates of bromination?
alkyl substituents raise the energy of HOMO C=C bond by hyperconjugation.
electronic effects override the steric effects.
what is Markovnikov’s rule?
the addition of a protic acid (HX) to an alkene, the acid H becomes attached to the C w/ the fewest substituents.
The X attaches to the C w/ more alkyl substituents.
what are some examples of naturally occurring and synthetic alkenes?
natural = retinal and cholesterol.
synthetic = polyethene and terpenes.
what part does regioselectivity play in Markovnikov’s rules regarding addition reactions between alkenes and HBr?
HBr + alkene proceeds w/ carbocation intermediate, + charge is sp² hybridised.
empty p-orbital perpendicular to plane of molecule.
stability of carbocation is reinforced through hyperconjugation and inductive effects.
favours more substituted alkyl bromide as major product.
alkenes react with peracids to give what?
epoxides.
epoxides can be opened with water under acidic conditions to give what?
trans diols.
why do more substituted alkenes epoxidise faster?
higher electron density around db.
π electrons of double bond held more tightly, leading to stronger interaction with peroxide agent.
steric hindrance around db in more substituted alkenes is reduced.
means peroxide reagent can approach the db more easily.
what is anti-markovnikov addition?
when the H binds to the most substituted carbon atom and the incoming group binds to the least substituted group.
why is a tertiary cation more stable than a primary one?
3⁰ has charge spread out over a larger area.
alkyl groups provide steric hindrance around C atom.
also has resonance stabilisation with π bonds.
how does one make the antimarkovnikov product?
hydroboration-oxidation.
alkene + BH₃ = boron intermediate, can react with water = alcohol product.
boron intermediate can be attacked by water with oxygen peroxide presence to form alcohol.
what is special about peroxides?
they can stimulate homolytic fission.
peroxides have a weak O-O bond that can be broken by heat or light, making 2 radicals.
radicals can then initiate and propagate.
tell me about the structure and stability of radicals:
resonance stability
inductive effects
hyperconjugation
why is a tertiary radical more stable than a primary radical?
alkyl groups attached to the carbon provide more electron density and inductive stability.
branching results in steric hindrance.
why are allyls more stable than phenyls and vinyls
π electrons in the allyl system can delocalise over the entire conjugated system, provides stability.
π electrons in the phenyl group are delocalised over the ring system, less stable.
What type of fission does peroxide undergo?
Homolytic
Why is a primary radical not feasible in propagation?
There is a greater electron inducting effect in tertiary.
Tertiary have more alkyl groups, stabilises unpaired electron pairs and reduces reactivity
