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

Chapter 13 - Alkenes Flashcards

The properties of alkenes, Stereoisomerism, Reactions of alkenes, Electrophilic addition in alkenes, Polymerisation in alkenes.

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1
Q

Alkenes are which kind of hydrocarbons?

A

Unsaturated

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2
Q

What is the general formula for alkenes?

A

CnH2n

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3
Q

How is a double bond formed?

A

Three sigma bonds formed. One electron left in a p-orbital. A pi bond forms from the sideways overlap of two p-orbitals.

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4
Q

What prohibits rotation in alkenes?

A

The pi bond as it locks the two carbon atoms in position.

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5
Q

What is the shape around the carbon atoms in the double bond and why?

A

Trigonal planar Three regions of electron density Repel each other equally In the same plane 120 degree bond angle

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6
Q

What is stereoisomerism?

A

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

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7
Q

What are the two types of stereoisomerism?

A

E/Z isomerism - only occurs in compounds with a C==C double bond. Optical isomerism - can occur in a much more wider range of compounds, including alkanes with no functional groups.

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8
Q

When is cis-trans isomerism applicable?

A

Molecules have a C=C double bond Each carbon in the double bond is attached to two groups that are the same

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9
Q

What is the cis isomer?

A

Hydrogen atoms on each carbon on the same side.

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10
Q

What is the trans isomer?

A

Hydrogen atoms diagonally opposite each other.

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11
Q

What are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?

A

Higher atomic number is higher priority. Groups of higher priority on the same side of the double bond, then Z isomer.

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12
Q

Describe the hydrogenation of alkenes

A

An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.

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13
Q

Describe the halogenation of alkenes

A

Rapid addition reaction between an alkene and chlorine or bromine at room temperature to form a halogenoalkane.

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14
Q

How are alkenes tested for?

A

When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.

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15
Q

Describe the hydration of alkenes

A

An alkene is reacted with steam in the presence of a phosphoric acid catalyst forming an alcohol (two products can be formed).

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16
Q

What is the mechanism for addition reactions?

A

Electrophilic addition

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17
Q

Outline an electrophilic addition reaction between an alkene and hydrogen bromide

A

H-Br is a polar molecule Electron pair in pi bond attracted to H Double bond breaks Bond forms H and C atoms H-Br bond breaks by heterolytic fission Bromide ion and carbocation formed Br- reacts with carbocation

18
Q

How does electrophilic addition take place with molecules that are non polar?

A

When the molecule approaches the pi bond, the electrons in the bond are repelled and this induces polarity.

19
Q

What is Markownikoff’s rule?

A

In electrophilic addition reactions, when the carbocation is formed it can be primary, secondary or tertiary. The tertiary is the most stable so is the most likely product.

20
Q

Why is the tertiary carbocation the most stable?

A

The connecting alkyl groups donate and push electrons towards the positive charge of the carbocation. Therefore more alkyl groups means a more stable ion.

21
Q

What is the name of the reaction for a polymers formation from alkenes?

A

Addition polymerisation

22
Q

How is the monomer unit displayed?

A

Displayed formula with an n before it to signify many units.

23
Q

How is the repeat unit of the polymer displayed?

A

Displayed formula In square brackets An n placed after to show a large number of repeats

24
Q

What are the conditions for a polymerisation reaction?

A

High temperature and pressure

25
Q

What are the uses of polypropene?

A

Childrens toys, packing crates, guttering, uPVC windows, rope fibre.

26
Q

What are the uses of polyphenylethene aka polysterene?

A

Packaging material, food trays and cups.

27
Q

What are the uses of polytetrafluoroethene aka PTFE or Teflon?

A

Coating for non stick pans, permeable membrane for clothing and shoes, cable insulation.

28
Q

What is the downside of current disposal of waste polymers?

A

Very unreactive so are non-biodegradable, with serious environmental effects.

29
Q

What are the benefits of recycling polymers?

A

Reduces environmental impact by conserving fossil fuels and decreasing waste.

30
Q

What are the weaknesses of recycling polymers?

A

Polymers have to be sorted by type. Process is undermined if polymers are mixed as renders product unusable.

31
Q

How are polymers recycled?

A

Once sorted, they are chopped into flakes, washed, dried, melted and then cut into pellets for use.

32
Q

What are the dangers of PVC disposal?

A

High chlorine content and range of additives present make it hazardous. When burnt it releases the corrosive hydrogen chloride gas and toxic dioxins.

33
Q

What is the method of recycling PVC?

A

Solvents used to dissolve the polymer and the high grade PVC is recovered by precipitation and the solvent can be used again. Previous recycling, PVC was ground down and then reused.

34
Q

How can polymers be used as fuel?

A

Waste polymers can be incinerated to produce heat for power plants.

35
Q

What is feedstock recycling?

A

Chemical and thermal processes that can reclaim monomers, gases and oils from waste polymers.

36
Q

What is a major advantage of feedstock recycling?

A

Able to handle unsorted and unwashed polymers

37
Q

What are biodegradable polymers?

A

Produced from renewable sources and can be broken down by microrganisms. Made from starch or cellulose.

38
Q

What are photodegradable polymers?

A

Contain bonds that are weakened by absorbing light to start degradation or light-absorbing additives are used.

39
Q

State the addition reactions of the alkenes

A

1) hydrogen in the presence of a nickel catalyst at 423K 2) halogens 3) hydrogen halides 4) steam in the presence of an acid catalyst

40
Q

what are the conditions required for E/Z isomerism to take place?

A

a C==C double bond

different groups attached to each carbon atom of the double bond.

A-Level Chemistry - OCR (35 decks)

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