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Chemistry > 3. Alkenes > Flashcards

3. Alkenes Flashcards

1
Q

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

A

CnH2n

Isomeric with cycloalkanes

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2
Q

What is the industrial method of preparing alkenes?

A

Cracking of crude oil

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3
Q

What are the laboratory methods of preparing alkenes?

State the reagents and conditions.

A
  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
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4
Q

What is the Saytzeff rule?

A

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

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5
Q

What reactions can alkenes undergo?

A
  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
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6
Q

State the conditions for hydrogenation using H2 (g)

A

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

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7
Q

State the conditions for electrophilic addition of halogen and observations.

A

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.

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8
Q

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

State the product obtained

A

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

Alkyl halide

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9
Q

What is Markovnikov’s Rule?

A

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.

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10
Q

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

State the products obtained.

A

Industrial:

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

Laboratory:

  • Cold, concentrated H2SO4
  • Heat with water

Obtain alcohol from alkene

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11
Q

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

A

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

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

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12
Q

What are the tests for alkenes?

A

Decolourisation of Br2 / KMnO4

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

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

A

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
Q

What does alkenes attract?

A

Electrophiles.

C=C bond is electron rich.

18
Q

Are alkenes less dense than water?

A

Yes.

19
Q

What are alkenes soluble in?

A

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

20
Q

What determines the stability of alkenes?

A

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
Q

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

A

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

22
Q

H2 (g), Ni, Heat

Name the reaction.

A

Reduction (Hydrogenation using H2 (g))

23
Q

Al2O3, Heat

Name the reaction.

A

Elimination of H2O

24
Q

Excess conc H2SO4

Name the reaction.

A

Elimination of H2O

25
Q

Ethanolic KOH, Heat under reflux, alkyl halide

Name the reaction.

A

Elimination of HX

26
Q

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

Name the reaction.

A

Electrophilic addition of halogen

27
Q

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

Name the reaction.

A

Electrophilic addition of hydrogen halide

28
Q

Steam, H3PO4, Heat at high pressure

A

Electrophilic addition of steam (Hydration)

29
Q

Cold conc H2SO4, heat with water

A

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

30
Q

KMnO4 with Na2CO3

A

Oxidation

31
Q

KMnO4 with H2SO4

A

Oxidation

Chemistry (5 decks)

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