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OCR A-level Chemistry > Chapter 13 - Alkenes > Flashcards

Chapter 13 - Alkenes Flashcards

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1
Q

What is the general formula of alkenes?

A

CnH2n

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2
Q

Are alkenes unsaturated or saturated hydrocarbons?

A

Unsaturated

Contain at least one double carbon to carbon bond

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3
Q

Describe bonding in an alkene

A

In one C=C bond:
- 1 sigma bond formed direct overlap of orbitals. Head on overlap - giving highest possible electron density
- 1 Pi bond formed by sideways overlap of two p-orbitals.
Form two parts - one below and one above molecule’s axis.
Pi bonds cannot rotate

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4
Q

What kind of bond is a C-H bond?

A

Sigma

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5
Q

What kind of bond is a C-C bond?

A

Pi

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6
Q

Which one has lower enthalpy? Sigma or Pi bond?

A

Pi bonds have a lower enthalpy as electron density is spread out above and below the nuclei. Meaning electrostatic attraction between the nuclei and shared pair of electrons is weaker.

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7
Q

What is the shape around each carbon atom in a double bond (C=C)?

A

-3 regions of electron density around each of the carbon atoms.
repel each other as far apart as possible, so the bond bond angle = 120 degrees
giving trigonal planar shape

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8
Q

What are stereoisomers?

A

Stereoisomers have the same structural formula but a different arrangement of atoms in space.

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9
Q

What is an E isomer?

A

When the 2 highest priority groups are on OPPOSITE sides of a C=C bond

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10
Q

What is a Z isomer?

A

When the 2 highest priority groups are on the SAME side of the C=C bond

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11
Q

What are the requirements for a molecule to have E/Z isomerism?

A
  • A C=C bond - as this cannot rotate

- 2 different groups attached to each carbon atom of the double bond

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12
Q

What is cis/trans isomerism?

A

Special case of E/Z isomerism where one of the attached groups on each carbon atom of the double bond must be the same.

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13
Q

Which one of cis and trans isomers are E and Z isomers?

A

Cis - Z isomer

trans - E isomer

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14
Q

What are the CIP rules used to find?

A

Priority rules used to find E and Z stereoisomers

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15
Q

How are the CIP rules used to assign priority?

A
  1. ) look at the atoms directly attached to carbon atoms in the double bond and decide which 2 atoms have the highest priority - the higher atomic number = higher priority.
  2. ) If 2 atoms assessed are the same then find the first point of difference - and the higher the atomic number = higher priority.
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16
Q

What is the name - CIP rules short for?

A

Cahn - Ingold - Prelog rules

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17
Q

Why are alkenes more reactive than alkanes?

A

Pi bond electrons are more exposed than the electrons in the sigma bond.

18
Q

Give examples of what alkenes undergo addition reactions with.

A
  • Hydrogen
  • Halogens
  • Hydrogen halides
  • Steam in the presence of an acid catalyst
19
Q

What is hydrogenation of an alkene?

A

Alkene mixed with hydrogen and passed over nickel catalyst at 423K. Addition reaction where hydrogen is added across a double bond

20
Q

Draw hydrogenation of buta-1,3-diene

A

Look at paper flashcards

21
Q

What is halogenation of alkenes?

A

Rapid addition reaction with halogen. The halogen adds across the double bond.

22
Q

What test is used to detect C=C bond (unsaturation)?

A

Bromine test

bromine adds across C=C bond and orange colour disappears.

23
Q

What addition reaction occurs with alkenes that forms a haloalkane?

A

(gaseous) hydrogen halide addition

24
Q

What does hydration of alkenes with steam in presence of a phosphoric acid catalyst form?

A

Alcohol

25
Q

Draw hydration of propene

A

Look at paper flashcards

26
Q

What is electrophilic addition?

A

Double bond in alkenes open up and atoms are added to the carbon atoms

27
Q

What is an electrophile?

A

Atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair
Usually a positive ion or has slight positive charge

28
Q

Describe electrophilic addition mechanism of but-2-ene and hydrogen bromide.

A
  1. )Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains the dipole H + - Br -
  2. ) The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break.
  3. ) A bond forms between the hydrogen atom of H-Br molecule and a carbon atom that was part of the double bond.
  4. ) The H-Br bond breaks by hetrolytic fission, with the electron pair going to the bromine atom
  5. )Bromide ion (Br-) and a carbocation are formed. A carbocation contains positively charged carbon atoms.
  6. ) The br- ion reacts with the carbocation to form the addition product.
29
Q

Draw mechanism of electrophillic addition of hydrogen bromide to but-2-ene

A

Look at paper flashcards

30
Q

Whats the difference when a hydrogen halide is added to a symmetrical alkene to when it’s added to an unsymmetrical alkene?

A

Two possible products when added to a unsymmetrical one.

31
Q

What does the amount of product formed in a electrophilic addition reaction depend on?

A

How stable the carbocation formed in the middle of the reaction is

32
Q

Are carbocations more stable with more or less alkyl groups? And why?

A

More alkyl groups

Alkyl groups feed electrons towards the positive charge

33
Q

State Markownikoff’s rule.

A

The major product from addition of a hydrogen halide (HX) to an unsymmetrical alkene is one where hydrogen adds to the carbon with the most hydrogens already attached.

34
Q

What is addition polymerisation?

A

Joining of monomers to form chain - called polymers

35
Q

How do you write polyethene?

A

Monomer in brackets. With ‘n’ outside representing the repeating unit.

36
Q

How can we reduce environmental impacts of waste polymers?

A
  • Recycling
  • Using waste polymers as fuel
  • Feedstock recycling
37
Q

How does recycling decrease environmental impact of waste polymers?

A

Conserves finite fossil fuels and decreases amount of waste going into landfill

38
Q

What are the downsides to recycling waste polymers?

A

Pointless if polymers are not sorted by type and are mixed.

39
Q

How does feedstock recycling reduce environmental impacts?

A
  • Reassemble products produced from crude oil. Can be used as raw materials for production of new polymers
  • Able to handle unsorted or unwashed polymers.
40
Q

How can waste polymers be used as a fuel?

A

Incinerated to produce heat, generating steam to drive a turbine producing electricity.

41
Q

What are 3 developments that can be made to benefit the environment in polymers?

A
  • Bioplastics
  • Biodegradable polymers
  • Photodegradable polymers

OCR A-level Chemistry (30 decks)

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