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Chemistry > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What are alkenes?

A

Unsaturated hydrocarbons because of the presence of at least one carbon-carbon double bond

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2
Q

What is the general formula of alkenes?

A

CnH2n

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3
Q

How does the number of alkyl groups attached to C=C bond affect the stability of alkenes?

A

The more alkyl groups attached to C=C bond, the more substituted the alkene, the more stable the alkene

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4
Q

Is cis-isomer or the trans-isomer more stable and why?

A

Trans-isomer is more stable than the cis-isomer
- For cis-isomer: bulky groups are on the same side of the double bond, causing steric hindrance, thus less stable
- For trans-isomer: less steric hindrance as bulky groups separated on opposite sides of double bond, thus more stable

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5
Q

What happens to the boiling and melting point of alkenes as the number of C atom increases?

A

As the number of C atom increases, the number of electrons increases, more energy is required to overcome the stronger id-id between molecules, thus boiling and melting point of alkene increases

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6
Q

Do straight-chain alkene or branched isomer have higher boiling point and why?

A

Straight chain alkene has a larger surface area compared to the branched, more spherical alkene, thus more energy is required to overcome the more extensive id-id interactions between alkene molecules

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7
Q

Do cis-isomer or trans-isomer have a higher boiling point and why?

A

Cis-isomer typically have a higher boiling point because trans-isomer’s dipole moment will cancel out, forming weaker id-id interactions while cis-isomer will have net dipole moment forming stronger pd-pd

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8
Q

Do cis-isomer or trans-isomer have a lower melting point?

A

Cis-isomer has a lower symmetry, so the cis-isomer does not pack as well into a crystalline structure compared to the trans-isomer, poor packing of cis-isomer meant that intermolecular forces are not as extensive so less energy is required to melt the solid

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9
Q

What is the industrial preparation of alkene?

A

Cracking of crude oil

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10
Q

What are the laboratory preparation methods for alkenes?

A
  1. Elimination of H2O
  2. Elimination of HX
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11
Q

What is the reagents and conditions for cracking of crude oil?

A

Thermal cracking: 700-900 degree celsius
Catalytic cracking: Al2O3 + 500 degree celsius

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12
Q

What are the reagents and conditions for the elimination of H2O to form alkenes and what does the reagent act as?

A
  • Pass alcohol vapour over heated Al2O3 (Al2O3 acts as a heterogeneous catalyst)
  • Heat alcohol with excess conc. H2SO4 (conc. H2SO4 acts as a dehydrating agent)
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13
Q

What are the reagents and conditions for the elimination of HX to form alkenes?

A

ethanolic KOH + heat under reflux

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14
Q

What is the major product from the elimination reaction to form alkenes?

A

By Saytzeff rule, major product is more substituted alkene

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15
Q

Are alkenes more or less reactive than alkanes and why?

A

Alkenes are more reactive than alkanes because C=C bond is electron rich is vulnerable to attack by electrophiles

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16
Q

What reactions do alkenes undergo?

A
  1. combustion
  2. reduction
  3. electrophilic addition
  4. oxidation with KMnO4 (aq)
  5. addition polymerisation
17
Q

What are the reagents and conditions for reduction reaction?

A

H2 (g), Ni/Pd/Pt catalyst, heat

18
Q

What are the types of electrophilic addition alkenes can undergo?

A
  1. electrophilic addition of HX
  2. electrophilic addition of halogen
  3. electrophilic addition of steam
  4. electrophilic addition of conc. H2SO4, followed by heating with water
19
Q

What is reagents and conditions for electrophilic addition of HX?

A

HX (g), room temperature

20
Q

What is the order of reactivity of HX (from least to most reactive) and why?

A

HCl < HBr < HI
Bond strength:
E(H-Cl) > E(H-Br) > E(H-I)

21
Q

What is the major product of electrophilic addition to unsymmetrical alkene and why?

A

X adds to the more substituted carbon
Alkyl groups are slightly electron-donating and help to reduce the positive charge on the carbocation, thereby stabilising the carbocation. Thus, the greater the number of alkyl groups attached to the electron-deficient carbon, the more stable the carbocation

22
Q

What are the reagents and conditions for electrophilic addition of halogen?

A
  • Cl2(g) or Br2(l), room temperature
  • X2 dissolved in CCl4, room temperature
23
Q

What is the observation of the electrophilic addition of halogen?

A

__(colour)__ (in what medium) will be decloursied [refer to data booklet for the colour]

24
Q

What is the major product of the electrophilic addition of halogen in aqueous medium and why?

A

halohydrin because since water is the solvent, there will be much more H2O molecules than halide ions

25
Q

What are the reagents and conditions for electrophilic addition of steam?

A

steam, H3PO4 catalyst, heat at high pressure

26
Q

What is electrophilic addition of steam a method of?

A

Industrial method of obtaining ethanol from ethene

27
Q

What is electrophilic addition of alkene and conc. H2SO4 a method of?

A

Laboratory method of obtaining alcohols from alkenes

28
Q

What are the reagents and conditions of oxidation reactions with KMnO4 (aq) and the respective observations?

A
  • heat, acidified KMnO4 [Purple KMnO4 turns colourless (Mn2+)]
  • cold, alkaline KMnO4 [Purple KMnO4 turns colourless and brown ppt (MnO2) formed]
29
Q

What are the identification tests to distinguish alkenes?

A
  1. Using Br2 (aq) [Orange Br2 (aq) turns colourless]
  2. Using cold KMnO4 in Na2CO3 [Purple KMnO4 turns colourless and brown ppt. of MnO2 formed]
30
Q

What are the identification tests to distinguish terminal alkenes from non-terminal alkenes?

A

Using KMnO4, dilute H2SO4and heat, pass gaseous products into limewater
- Terminal alkenes: Purple KMnO4 turns colourless and white ppt formed with limewater
- non-terminal alkenes: Purple KMnO4 turns colourless, no white ppt formed with limewater

31
Q

What is the product of the oxidation of alkenes that has two alkyl groups (2R)?

A

Ketone

32
Q

What is the product of oxidation of alkenes that has one alkyl group (1H 1R)?

A

aldehyde then carboxylic acid

33
Q

What is the product of oxidation of alkenes that has no alkyl group (2H)?

A

CO2 + H2O

Chemistry (21 decks)

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