3. Alkenes Flashcards
why does cis-trans isomerism occur in alkenes?
presence of π bonds -> rotation of C=C restricted. at room temp, insufficient energy to break π bonds, so cannot rotate
when drawing structures for cis-trans isomers, you must remember to draw ___?
the trigonal planar geometry around each C in C=C bond [if not cannot show the top-bottom fo functional groups]
cannot draw perpendicular (90 deg) bonds
cis isomers are more/less stable than trans isomers? why?
less stable. due to steric strain between 2 alkyl groups
cis isomer has higher/lower boiling point than trans isomer?
higher. both cis and trans for ID-ID interactions, but since cis isomer slightly more polar than trans isomer, can form additional pd-pd interaction–> more energy required to overcome
cis isomer has higher/lower melting point than trans isomer?
lower. in cis isomer, alkyl groups on same side -> repel each other -> less spherical, harder to pack into regular crystal lattice –> weaker id-id –> melt faster [lower melting point]
how can alkenes be formed? [2]
2 elimination reactions
- elimination of HX from halogenoalkane
- elimination of H2O from alcohol
reagent and conditions for elimination of HX from halogenoalkanes
ethanolic KOH/NaOH, heat
reagent and conditions for elimination of H2O from alcohol
excess conc. H2SO4, 170°C
chemical reactions of alkenes
- combustion
- reduction
- oxidation (mild/strong)
- electrophilic addition (X2/HX/H2O)
are alkenes reactive? why?
yes. have high electron density at C=C bond that can attract electrophiles
alkenes undergo electrophilic __
addition
reagent and conditions for electrophilic addition of halogens
[alkene to dihalogenoalkane]
X2 in CCl4, in the dark
reagent and conditions for electrophilic addition of HX
[alkene to halogenoalkane]
HX(g)
what is Markonikov’s rule? [2]
when an asymmetrical alkene undergoes electrophilic addition, the more stable carbocation intermediate is formed
reagent and conditions for formation of halohydrins
X2(aq), room temp, in the dark
reagent and conditions for electrophilic addition of water
[alkene to alcohol]
cold, conc H2SO4, followed by H2O, boiling
reagents and conditions for mild oxidation
[alkene to diol]
KMnO4 (aq), H2SO4/NaOH(aq), cold
reagents and conditions for strong oxidation
KMnO4(aq), H2SO4/NaOH(aq), heat
reagents and conditions for reduction
[alkene to alkane]
H2(g), Ni, heat OR H2(g), Pt
for reduction of alkene to alkane, Ni/Pt acts as a _____ catalyst
heterogenous catalyst. the reactants are adsorbed onto the catalyst surface to weaken the bonds and lower the activation energy of the reaction.
