3. Alkenes Flashcards Preview

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Flashcards in 3. Alkenes Deck (31):
1

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

CnH2n
Isomeric with cycloalkanes

2

What is the industrial method of preparing alkenes?

Cracking of crude oil

3

What are the laboratory methods of preparing alkenes?
State the reagents and conditions.

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

4

What is the Saytzeff rule?

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

5

What reactions can alkenes undergo?

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

6

State the conditions for hydrogenation using H2 (g)

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

7

State the conditions for electrophilic addition of halogen and observations.

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.

8

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

State the product obtained

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

Alkyl halide

9

What is Markovnikov's Rule?

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.

10

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

State the products obtained.

Industrial:
-Steam (H2O (g))
-H3PO4 catalyst
-Heat at high pressure

Laboratory:
-Cold, concentrated H2SO4
-Heat with water

Obtain alcohol from alkene

11

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

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

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

12

What are the tests for alkenes?

Decolourisation of Br2 / KMnO4

13

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

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

14

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

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

15

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

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

16

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

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

17

What does alkenes attract?

Electrophiles.
C=C bond is electron rich.

18

Are alkenes less dense than water?

Yes.

19

What are alkenes soluble in?

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

20

What determines the stability of alkenes?

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.

21

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

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

22

H2 (g), Ni, Heat
Name the reaction.

Reduction (Hydrogenation using H2 (g))

23

Al2O3, Heat
Name the reaction.

Elimination of H2O

24

Excess conc H2SO4
Name the reaction.

Elimination of H2O

25

Ethanolic KOH, Heat under reflux, alkyl halide
Name the reaction.

Elimination of HX

26

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

Electrophilic addition of halogen

27

HX (g) where X= Cl, Br, I, room temperature
Name the reaction.

Electrophilic addition of hydrogen halide

28

Steam, H3PO4, Heat at high pressure

Electrophilic addition of steam (Hydration)

29

Cold conc H2SO4, heat with water

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

30

KMnO4 with Na2CO3

Oxidation

31

KMnO4 with H2SO4

Oxidation