3. Alkenes

Question 1

why does cis-trans isomerism occur in alkenes?

Answer 1

presence of π bonds -> rotation of C=C restricted. at room temp, insufficient energy to break π bonds, so cannot rotate

Question 2

when drawing structures for cis-trans isomers, you must remember to draw ___?

Answer 2

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

Question 3

cis isomers are more/less stable than trans isomers? why?

Answer 3

less stable. due to steric strain between 2 alkyl groups

Question 4

cis isomer has higher/lower boiling point than trans isomer?

Answer 4

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

Question 5

cis isomer has higher/lower melting point than trans isomer?

Answer 5

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]

Question 6

how can alkenes be formed? [2]

Answer 6

2 elimination reactions
- elimination of HX from halogenoalkane
- elimination of H2O from alcohol

Question 7

reagent and conditions for elimination of HX from halogenoalkanes

Answer 7

ethanolic KOH/NaOH, heat

Question 8

reagent and conditions for elimination of H2O from alcohol

Answer 8

excess conc. H2SO4, 170°C

Question 9

chemical reactions of alkenes

Answer 9

1. combustion
2. reduction
3. oxidation (mild/strong)
4. electrophilic addition (X2/HX/H2O)

Question 10

are alkenes reactive? why?

Answer 10

yes. have high electron density at C=C bond that can attract electrophiles

Question 11

alkenes undergo electrophilic __

Answer 11

addition

Question 12

reagent and conditions for electrophilic addition of halogens
[alkene to dihalogenoalkane]

Answer 12

X2 in CCl4, in the dark

Question 13

reagent and conditions for electrophilic addition of HX
[alkene to halogenoalkane]

Answer 13

HX(g)

Question 14

what is Markonikov’s rule? [2]

Answer 14

when an asymmetrical alkene undergoes electrophilic addition, the more stable carbocation intermediate is formed

Question 15

reagent and conditions for formation of halohydrins

Answer 15

X2(aq), room temp, in the dark

Question 16

reagent and conditions for electrophilic addition of water
[alkene to alcohol]

Answer 16

cold, conc H2SO4, followed by H2O, boiling

Question 17

reagents and conditions for mild oxidation
[alkene to diol]

Answer 17

KMnO4 (aq), H2SO4/NaOH(aq), cold

Question 18

reagents and conditions for strong oxidation

Answer 18

KMnO4(aq), H2SO4/NaOH(aq), heat

Question 19

reagents and conditions for reduction
[alkene to alkane]

Answer 19

H2(g), Ni, heat OR H2(g), Pt

Question 20

for reduction of alkene to alkane, Ni/Pt acts as a _____ catalyst

Answer 20

heterogenous catalyst. the reactants are adsorbed onto the catalyst surface to weaken the bonds and lower the activation energy of the reaction.