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Chemistry OCR A A level > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

What do alkenes contain?

A

One or more carbon-to-carbon double bond

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2
Q

What is the general formula of aliphatic hydrocarbons?

A

C(n)H(2n)

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3
Q

Where is a pi bond formed from?

A

The sideways overlap of two p-orbitals one from each carbon atom of the double bond

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4
Q

Where is pi bond electron density concentrated?

A

Above and below the plane of nuclei of bonded atoms

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5
Q

What do pi bonds prevent? How?

A

Rotation around the double bond, due to the lock of the two carbon atoms leading to concentrated electron density above and below the plane of atom.

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6
Q

What is the shape around the carbon atoms in double bond of an alkene?

A

Trigonal Planar- three areas of electron density around each of the carbon atoms

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7
Q

Define stereoisomers

A

Same structural formula but a different arrangement of the atoms in space

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8
Q

Why does stereoisomerism occur?

A

Due to the restricted rotation around the double bond, and therefore the groups attached to each carbon are fixed relative to each other.

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9
Q

What two conditions have to be fulfilled for E/Z isomerism?

A

A C=C double bond

Different groups attached to each carbon atom of the double bond

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10
Q

What condition has to be different for cis-trans isomerism than E/Z isomerism?

A

In cis-trans isomerism one of the attached groups on the each carbon atom on the double bond must be the same.

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11
Q

What does a cis-isomer have?

A

The hydrogen atoms and methyl groups on each carbon in the double bond are on the same side.

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12
Q

What doe a trans-isomer have?

A

The hydrogen atoms and methyl groups on each carbon in the double bond are opposite each other.

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13
Q

What occurs in a Z isomer?

A

The groups with higher priority are on the same side

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14
Q

What occurs in a E isomer?

A

The groups with higher priority are diagonally placed across the double bond

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15
Q

How is priority assigned?

A

Based on atomic number. The higher the atomic number, the higher the priority.

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16
Q

What happens if the atom attached to carbon atom is the same when assigning priority?

A

Keep going along the molecule until reaches a difference

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17
Q

Why are alkenes more reactive than alkanes?

A

Pi bonds are the overlap of p orbitals above and below the plane of the sigma bond. Being above and below means more exposed electrons. Reacts more easily.

18
Q

What conditions are needed in the hydrogenation of an alkene?

A

423K

Nickel Catalyst

19
Q

What is hydrogenation?

A

The addition of H2 across a double bond

20
Q

How is unsaturation tested for?

A

Addition of bromine water. Colour change from orange to colourless if a double bond is present.

21
Q

What conditions are required for hydration of alkenes?

A

Steam

Concentration phosphoric acid catalyst

22
Q

Define ‘electrophilic addition’

A

An addition reaction in which the first step is attack by an electrophile on a region of high electron density

23
Q

Where is high electron density found?

A

Double bond of the alkene

24
Q

Define ‘electrophile’

A

An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair

25
Q

What is a carbocation?

A

Containing a positively charged carbon atom

26
Q

What is the most stable carbocation?

A

Tertiary carbocation

27
Q

What makes a carbocation more stable?

A

Each alkyl group pushes electrons towards the positive charge of carbocation
Positive charge is spread over alkyl groups
More alkyl group= more spread of charge= more stable

28
Q

What does Markownikoff’s rule state?

A

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

29
Q

What are polymers?

A

Long chains of repeating units of monomers

30
Q

What type of polymerisation do alkenes undergo?

A

Addition polymerisation to produce long saturated chains containing no double bonds.

31
Q

How does recycling alkene based polymers help?

A

Reduces environmental impact
Conserves finite fossil fuels
Reduces waste in landfill

32
Q

How are polymers recycled?

A 
Discarded polymers are sorted by type
Polymers chopped into flakes
Washed
Dried 
Melted
Recycled polymers cut into pellets
33
Q

Why is disposal of PVC hazardous?

A

High chlorine content

34
Q

What does PVC produce when burnt?

A

Corrosive hydrogen chloride gas

35
Q

How is PVC disposed of?

A

Solvents dissolve polymer

High grade polymers are recovered by precipitation from the solvent

36
Q

What is feedstock recycling?

A

Chemical and thermal process of reclaiming monomers from waste polymers

37
Q

What is the biggest advantage of feedstock recycling?

A

Can handle unsorted and unwashed polymers

38
Q

What are bioplastics?

A

Plastics produced from plant starch, cellulose, plant oils and proteins which are renewable and sustainable.

39
Q

How are biodegradable polymers broken down?

A

By microorganisms into carbon dioxide, water and biological molecules

40
Q

What are photodegradable polymers?

A

Polymers containing bonds that are weakened by absorbing light to start degradation

Chemistry OCR A A level (20 decks)

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