3. Alkenes

Question 1

What is the general formula of alkenes and what is it isomeric with?

Answer 1

CnH2n

Isomeric with cycloalkanes

Question 2

What is the industrial method of preparing alkenes?

Answer 2

Cracking of crude oil

Question 3

What are the laboratory methods of preparing alkenes?

State the reagents and conditions.

Answer 3

1. Elimination of H2O
   - Pass alcohol vapour over heated Al2O3 or over hot porcelain/broken brick pieces.
   - Heat alcohol with excess concentrated H2SO4
2. Elimination of HX
   - Heat the alkyl halide under reflux with an alkali dissolved in alcohol e.g. ethanolic KOH

Question 4

What is the Saytzeff rule?

Answer 4

If more than 1 alkene can be obtained from an elimination reaction, the major product is the more substituted alkene.

Question 5

What reactions can alkenes undergo?

Answer 5

1. Combustion
2. Reduction: Hydrogenation using H2 (g)
3. Electrophilic addition of halogen
4. Electrophilic addition of hydrogen halide
5. Electrophilic addition of H2O
6. Oxidation

Question 6

State the conditions for hydrogenation using H2 (g)

Answer 6

H2 (g), Ni (or Pt/Pd catalyst), Heat

Question 7

State the conditions for electrophilic addition of halogen and observations.

Answer 7

X2 (g/l) or X2 dissolved in CCl4 (where X=Cl, Br), room temperature
Reddish brown Br2 (l) / Orange brown Br2 (in hexane) / Orange Br2 (aq) / Orange red Br2 in CCl4 decolourised.

Question 8

State the conditions for electrophilic addition of hydrogen halide and the order of reactivity.

State the product obtained

Answer 8

HX (g) where X= Cl, Br, I, room temperature
Order of reactivity: HCl < HBr < HI

Alkyl halide

Question 9

What is Markovnikov’s Rule?

Answer 9

During the addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon of the C=C bond that has the greater number of hydrogen atoms.

Question 10

State the conditions for both the industrial and laboratory method of electrophilic addition of H2O.

State the products obtained.

Answer 10

Industrial:

• Steam (H2O (g))
• H3PO4 catalyst
• Heat at high pressure

Laboratory:

• Cold, concentrated H2SO4
• Heat with water

Obtain alcohol from alkene

Question 11

State the observations for KMnO4 in both the alkaline and acidic mediums.

Answer 11

Alkaline:
-Purple KMnO4 turns colourless and brown ppt (MnO2) formed.

Acidic:
Purple KMnO4 turns colourless. CO2 may be evolved.

Question 12

What are the tests for alkenes?

Answer 12

Decolourisation of Br2 / KMnO4

Question 13

State and explain the trend in melting and boiling points for alkenes.

Answer 13

MP and BP increases as the no. of carbon atoms increases due to increasingly stronger id-id forces of attraction between the molecules.

Question 14

Straight chain alkenes have higher boiling points than their branched isomers. Explain.

Answer 14

This is due to the more extensive id-id interactions between the straight chain alkenes compared to their branched isomers.

Question 15

Typically, cis-isomer has a higher BP than the trans isomer. Explain.

Answer 15

Cis-isomer is held together by stronger pd-pd interactions compared to trans-isomer held together by weaker id-id interactions.

Question 16

Cis-isomer has a lower MP than trans-isomer. Explain.

Answer 16

In order for intermolecular forces to operate effectively, molecules must be able to pack together efficiently in the solid –> Cis-isomer has lower symmetry and does not pack as well into a crystalline structure compared to the trans-isomer –> Not as extensive intermolecular forces –> Less energy required to melt the solid

Question 17

What does alkenes attract?

Answer 17

Electrophiles.

C=C bond is electron rich.

Question 18

Are alkenes less dense than water?

Answer 18

Yes.

Question 19

What are alkenes soluble in?

Answer 19

Alkenes are insoluble in water but soluble in non-polar solvents e.g. benzene

Question 20

What determines the stability of alkenes?

Answer 20

The number of alkyl groups attached to the carbon atoms of the C=C bond. The more substituted the alkene, the more stable the alkene.

Question 21

Is the trans-isomer more stable than the cis-isomer?

Answer 21

Yes, bulky groups on the same side of the double bond causes crowding (steric hindrance) –> Less stable.

Question 22

H2 (g), Ni, Heat

Name the reaction.

Answer 22

Reduction (Hydrogenation using H2 (g))

Question 23

Al2O3, Heat

Name the reaction.

Answer 23

Elimination of H2O

Question 24

Excess conc H2SO4

Name the reaction.

Answer 24

Elimination of H2O

Question 25

Ethanolic KOH, Heat under reflux, alkyl halide

Name the reaction.

Answer 25

Elimination of HX

Question 26

X2 (g/l) or X2 dissolved in CCl4 (where X=Cl, Br)

Name the reaction.

Answer 26

Electrophilic addition of halogen

Question 27

HX (g) where X= Cl, Br, I, room temperature

Name the reaction.

Answer 27

Electrophilic addition of hydrogen halide

Question 28

Steam, H3PO4, Heat at high pressure

Answer 28

Electrophilic addition of steam (Hydration)

Question 29

Cold conc H2SO4, heat with water

Answer 29

electrophilic addition of conc H2SO4, followed by heating with water (Hydration)

Question 30

KMnO4 with Na2CO3

Answer 30

Oxidation

Question 31

KMnO4 with H2SO4

Answer 31

Oxidation