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A-level Chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

What is an alkene?

A

An unsaturated hydrocarbon with the general formula CnHn-when there’s only 1 double bond

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2
Q

What does the double bond contain?

A

A sigma bond and a pi bond

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3
Q

What is a sigma bond?

A

When 2 orbitals overlap and align horizontally to give a single covalent bond

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4
Q

Why have sigma bonds got a high bond enthalpy?

A

Because of the overlapping of electrons which give a strong electrostatic attraction between the shared pair of electrons and nuclei

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5
Q

What are Pi bonds?

A

The parallel overlap of 2 orbitals, which are dumbbell shaped and when they merge they form 2 oblong shape at the top and bottom

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6
Q

Why are pi bonds weaker than sigma bonds?

A

Because the electron density is spread over a large area so the electron attraction is weaker

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7
Q

Why are alkenes reactive?

A

Because they have a high electron density due to the double bonds having a sigma and pi bond

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8
Q

What is a sterioisomer?

A

It’s when they have the same structural formula but different arrangement of atoms in space

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9
Q

When do you get E/Z sterisomerism?

A

When you have 2 different atoms or groups of atoms on the same carbon

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10
Q

What are E isomers?

A

Same group opposite the double bond

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11
Q

What are Z isomers?

A

Same group in the same side of the double bond

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12
Q

When do you use the Cahn-Ingold-Prelog rules when deciding if an isomer is cis ot trans?

A

When we have 4 different groups around the double bond

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13
Q

What are the steps for deciding if a sterisomer is E or Z?

A
  1. Label the carbons with double bonds

2.calcukate the atomic number of the first element bonded to the carbon on the double bond, atom with highest atomic number= higher priority

3.if both of the high priority groups on carbon 1&2 are on opposite sides= E isomer, if they are on the same side= z isomer

(may have to go further down the chain if 2 atoms have the same atomic number)

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14
Q

What is an electrophile?

A

It’s an electron pair acceptor

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15
Q

Why are alkenes attacked by electrophiles?

A

Because of there double bond which has a high density of electrons

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16
Q

What are examples of electrophiles?

A

NO2+ & H+

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17
Q

What are the steps of an electrophillic addition reaction?

A

1.Curly arrow starts at the double bond and break it

2.then a curly arrow goes from one of the carbons to the ellecrophile and the ellecrophile is added to the molecule

3.Forming an alkane

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18
Q

What are the reagents and conditions of electrophillic addition?

A

Reagents= hydrogen (H2)

conditions= 150 degrees & nickel catalyst

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19
Q

How do you test for Alkenes?

A

By adding bromine water

20
Q

What is a positive result for a alkene being present?

A

The bromine water will go from brown/ orange to colourless

21
Q

What is the addition mechanism of an alkene and bromine?

A
  1. a curly arrow goes from the double bond to the bromine atoms
  2. curly arrow breaks the bond between br2, so a br attaches to the molecule
  3. a carbocation is formed and Br- is attracted to C+
22
Q

What are the products of electrophilic addition of alkenes?

A

Alkanes

23
Q

How are alcohols produced from alkenes?

A

By hydration

24
Q

What happenea during hydration?

A
  1. The double bond is broken in the alkene
  2. then the H2O is added to the molecule= an alcohol
25
Q

What are the catalyst and conditions used for hydration of alkenes?

A

Catalyst= acid catalyst (H+)

condition= steam (H2O)

26
Q

Is hydration reversible?

A

Yes

27
Q

What is formed when hydrogen halides react with Alkenes?

A

Halogenoalkanes

28
Q

What are the steps for reaction of addition of hydrogen halides to alkenes?

A
  1. Curly arrow goes from double bond to the hydrogen of the hydrogen halide
  2. then curly arrow breaks the bond between the hydrogen halide
  3. A carbocation is formed, then the halide is attracted to the C+ forming a halogenoalkane
29
Q

By reacting hydrogen halides with unsymmetrical alkanes how many products does it produce?

A

2 different producers

30
Q

How is the amount of the 2 products formed by reacting hydrogen halides with unsymmetrical alkane, determined?

A

By the stability of the carbocation intermediate

31
Q

What is the order of stability of the carbocations?

A

Tertiary > secondary > primary

32
Q

Why are teritay carbocations the most stable?

A

More alkyl groups, which push electrons towards the positive carbocation, stabilising it

33
Q

Alkenes are monomers, what do they join to make?

A

Polymers

34
Q

What are the two types of polymers?

A

Natural or synthetic

35
Q

Explain how alkenes are turned into polymers?

A

The double bond opens up to form a polymer

36
Q

Are polyalkenes reactive?

A

No because they are saturated and polar

37
Q

Why do polymers need to be disposed of carefully?

A

Because most are not biodegradable

38
Q

What type of plastics are disposed of using landfills?

A

-ones that are to difficult to recycle

-that are to difficult to separate from other matierals

-when there isn’t enough plastic to extract to make it economically viable

39
Q

What are the negatives of using landfills to dispose of plastic?

A

-Landfills aren’t sustainable as large amount of land is needed

-it’s becoming extremely expensive to use land for waste disposal

40
Q

Why do we need to reduce the dependency on crude oil when making plastics?

A

Because crude oil is a non- renewable source

41
Q

How can plastics be recycled?

A

-can be remoulded

-polymer chains can be broken up (cracked) and the monomers can then be used as organic feed stock for plastics or other feeds stock

42
Q

What is a positive of plastics being disposed of by incineration?

A

-incineration can be used if plastics can’t be recycled, the energy from the burning can be used to generate electricity.

43
Q

What is a negative of disposing plastics by incineration?

A

Burning plastics can release toxic fumes so these need to be monitored

44
Q

What are biodegradable polymers?

A

They decompose naturally

45
Q

Do biodegradable polymers always decompose naturally?

A

No they only decompose under certain conditions by organisms

46
Q

What do biodegradable polymers need to decompose naturally?

A

They need to have a good supply of oxygen and moisture

47
Q

How do photodegradable polymers decompose?

A

When exposed to sunlight

A-level Chemistry (19 decks)

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