Alkenes Flashcards
Free radical substitution
UV light/high temp/heat, X2
Alkene reduction
H2 (g), heat and Ni Catalyst
OR H2 (g), Pt/Pd Catalyst
Why alkene cannot be reduced by LiAlH4 or NaBH4?
C=C is electron rich, there are no electron deficient areas for Al/B to attack + electrons on C=C also repel the approach of the negatively charged Al-/B-, so no reduction by either of them.
Alkene halogenation (g)
X2 in organic solvent
ie X2 (alc)
Alkene halogenation (aq)
X2 (aq)
Alkene addition of HX
HX (g)
Alkene Hydration
Conc H3PO4, high temp and pressure, steam
OR conc. H2SO4 (aq), heat with water
Alkene mild oxidation
cold KMnO4 (aq), NaOH (aq)
Products of alkene oxidative cleavage?
+ observations?
CO2 (terminal), Carboxylic acid (1 R group), Ketone (2 R groups)
Decolourisation of purple KMnO4
Alkene mild oxidation reagents
+ observations?
hot KMnO4 (aq), H2SO4 (aq)
Decolourisation of purple KMnO4 + formation of brown ppt (MnO2)
Why excess conc H2so4 in the elimination reaction with alkenes?
H2SO4 is a dehydrating agent. When rection happens, water is evolved which is absorbed by the H2SO4. Hence excess and conc to ensure that H2SO4 doesn’t become dilute and allow water to react back with alkene to form alchohol.
Alkene preparation (from?)
Alchohol or Halogenoalkane
Alkene preparation (from OH)
Elimination (dehydration)
Al2O3, heat
OR excess conc H2SO4, heat
Dehydrohalogenation of HX
KOH/NaOH (alc)
Why dehydrogenation has that specific medium? And which medium specifically? (alc? aq?)
alc, because OH in water acts as a nucleophile (lewis base) but OH in alc acts as a bronsted base which reacts with HX to give KX + H2O
Zaitsev’s rule
Form the more substituted alkene because it’s more stable (more substituted = more alkyl groups bonded to the C=C = experiences more e donating I effect = strengthen the electron density of the alkene)
Alkene oxidation (combustion)
In excess O2, CO2 + H2O
In limited, CO + H2O + Soot (C), flame more smoky than alkanes
Markovnikov’s rule
Form the more stable carbocation (in most cases by bonding to the C with more H)
Create more alkyl groups surrounding the carbocation which can exert more e donating I effect to disperse the positive charge on the carbocation
mp and bp of cis vs trans configuration
cis higher bp (higher net dipole moment)
cis lower mp (larger steric hinderance due to arrangement of larger groups = less closely packed together)
Density of alkene
Less dense than water
Density of arene
Less dense than water
Density of halogenoalkanes
generally more dense than water
Straight vs branched chain Alkene (bp)
Branched lower bp because mroe spherical, less surface area = decrease in id-id (less extensive)
As no of C increase, alkene bp?
increase. no of electrons increase = electron cloud size increase = polarisability increase = id-id increase (stronger)
Alkene mainly what type of reaction + why + reactivity compared to alkanes?
Electrophilic addition
C=C bond is a region of high electron density + pi electrons are much more exposed than those in sigma bond so highly susceptable to attack
availability of pi electrons also makes it much more reactive than alkanes
Solubility of alkenes
Soluble in non-polar solvent, insoluble in polar solvent
mechanism for alkenes
slow and fast steps,
electron pair,
electron movement
carbocation formation
