Alkenes

Question 1

Free radical substitution

Answer 1

UV light/high temp/heat, X2

Question 2

Alkene reduction

Answer 2

H2 (g), heat and Ni Catalyst
OR H2 (g), Pt/Pd Catalyst

Question 3

Why alkene cannot be reduced by LiAlH4 or NaBH4?

Answer 3

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.

Question 4

Alkene halogenation (g)

Answer 4

X2 in organic solvent
ie X2 (alc)

Question 5

Alkene halogenation (aq)

Answer 5

X2 (aq)

Question 6

Alkene addition of HX

Answer 6

HX (g)

Question 7

Alkene Hydration

Answer 7

Conc H3PO4, high temp and pressure, steam
OR conc. H2SO4 (aq), heat with water

Question 8

Alkene mild oxidation

Answer 8

cold KMnO4 (aq), NaOH (aq)

Question 9

Products of alkene oxidative cleavage?
+ observations?

Answer 9

CO2 (terminal), Carboxylic acid (1 R group), Ketone (2 R groups)
Decolourisation of purple KMnO4

Question 10

Alkene mild oxidation reagents
+ observations?

Answer 10

hot KMnO4 (aq), H2SO4 (aq)
Decolourisation of purple KMnO4 + formation of brown ppt (MnO2)

Question 11

Why excess conc H2so4 in the elimination reaction with alkenes?

Answer 11

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.

Question 12

Alkene preparation (from?)

Answer 12

Alchohol or Halogenoalkane

Question 13

Alkene preparation (from OH)

Answer 13

Elimination (dehydration)
Al2O3, heat
OR excess conc H2SO4, heat

Question 14

Dehydrohalogenation of HX

Answer 14

KOH/NaOH (alc)

Question 15

Why dehydrogenation has that specific medium? And which medium specifically? (alc? aq?)

Answer 15

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

Question 16

Zaitsev’s rule

Answer 16

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)

Question 17

Alkene oxidation (combustion)

Answer 17

In excess O2, CO2 + H2O
In limited, CO + H2O + Soot (C), flame more smoky than alkanes

Question 18

Markovnikov’s rule

Answer 18

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

Question 19

mp and bp of cis vs trans configuration

Answer 19

cis higher bp (higher net dipole moment)
cis lower mp (larger steric hinderance due to arrangement of larger groups = less closely packed together)

Question 20

Density of alkene

Answer 20

Less dense than water

Question 21

Density of arene

Answer 21

Less dense than water

Question 22

Density of halogenoalkanes

Answer 22

generally more dense than water

Question 23

Straight vs branched chain Alkene (bp)

Answer 23

Branched lower bp because mroe spherical, less surface area = decrease in id-id (less extensive)

Question 24

As no of C increase, alkene bp?

Answer 24

increase. no of electrons increase = electron cloud size increase = polarisability increase = id-id increase (stronger)

Question 25

Alkene mainly what type of reaction + why + reactivity compared to alkanes?

Answer 25

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

Question 26

Solubility of alkenes

Answer 26

Soluble in non-polar solvent, insoluble in polar solvent

Question 27

mechanism for alkenes

Answer 27

slow and fast steps,
electron pair,
electron movement
carbocation formation