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A-Level Chemistry > 13 Alkenes > Flashcards

13 Alkenes Flashcards

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1
Q

What are the 2 types of bonds in alkenes?

A

Sigma and pi bonds

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2
Q

Why are there 2 bonds in alkene double bonds?

A

3 of 4 electrons in the outer shell are used for sigma bonds
The fourth electron occupies a p orbital

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3
Q

What is a pi bond?

A
  • Formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
  • Each C atom contributes 1 electron to the pair
  • Electron density is above and below the line joining the nuclei of the bonding atoms
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4
Q

Why can atoms in the double bond not rotate?

A

Pi bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond

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5
Q

What is the shape around a double bond?

A
  • 3 regions of electron density
  • Repel as far apart as possible
  • Trigonal planar shape (120)
  • All atoms are in the same plane
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6
Q

What is structural isomerism?

A

Molecules with the same molecular formula but a different structural formula

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7
Q

What is stereoisomerism?

A

Molecules with the same structural formula but a different arrangement of atoms in space

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8
Q

Why is there stereoisomerism around double bonds?

A
  • The rotation about the double bond is restricted because of the electron density of the pi bond above and below the plane of the sigma bond
  • The groups attached to each carbon atom are therefore fixed relative to each other
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9
Q

What are the conditions for E/Z isomerism?

A
  1. Contains a C=C double bond
  2. Each carbon in the double bond must be bonded to different groups
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10
Q

What is the test for unsaturation?

A

Addition reaction of alkene and bromine
Reacts with alkenes (orange to colourless)
Alkanes no reaction

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11
Q

What is the difference in reactivity between alkenes and alkanes and why?

A

Alkenes more reactive than alkanes
Have a pi bond where the pi electrons are outside of the bond so more exposed

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12
Q

What are the conditions for the hydrogenation of alkenes?

A

Nickel and 423K (150C)

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13
Q

Write an equation for propane and hydrogen

A

Propene + hydrogen –> Propane

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14
Q

Write an equation for alkenes with bromine

A

Ethene + bromine –> dibromoethane

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15
Q

Write an equation for the reaction of propene with hydrogen chloride

A

Propene + hydrogen chloride (HCl) –> haloalkane (chloropropane)

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16
Q

What are the 2 possible products of alkenes and hydrogen halides?

A

1-chloropropane –> minor product
2-chloropropane –> major product

17
Q

What are the conditions for hydration of alkenes?

A

High temp (reacting with steam)
Phosphoric acid
Used to make ethanol from ethene

18
Q

Write an equation of propene and steam

A

Propene + steam –> propan-1-ol or propan-2-ol

19
Q

What is an electrophile?

A

An atom or molecule with a (full or partial) positive charge that can accept a pair of electrons
- Electron-pair acceptor

20
Q

Draw the mechanism of the addition reaction of but-2-ene and hydrogen bromide and describe

A
  • H in HBr is slightly positive due to permanent dipole
    Arrow from double bond to H and from single bond to Br (heterolytic fission)
  • Br has a lone pair is bromide, C is positive
  • Arrow from lone pair to positive C
  • Br bonds to C
    Alkenes –> Carbocation intermediate and Br- –> Alkane
21
Q

What is a carbocation intermediate?

A

Has 3 bonds and a positive charge on the carbon

22
Q

Is the HBr an electrophile or not and explain why?

A

Yes as accepts a pair of electrons from the pi bond

23
Q

When do minor and major products form?

A

Unsymmetrical alkene

24
Q

What happens to the H and Br on the major product?

A

Br attached to the C with less H
H has attached to the C with more H

25
Q

Why does the major product form?

A

More stable carbocation intermediate forms major product (secondary carbocation more stable than primary)

26
Q

Why are secondary carbocations more stable than primary?

A

sigma bond electrons stabilise the positive charge, more sigma donation happening so more product formed

27
Q

What is a polymer?

A

A large molecule made from many small molecules called monomers

28
Q

What do unsaturated alkene molecules undergo?

A

Addition polymerisation
Produce a long, saturated chain

29
Q

What is a repeat unit?

A

The specific arrangement of atoms in the polymer that repeats over and over again, and is always drawn in square brackets

30
Q

What are the 5 ways of disposing of polymers?

A

Recycling
PVC recycling
Using waste polymers as fuel
Feedstock recycling
Biodegradable/
photodegradable polymers

A-Level Chemistry (15 decks)

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