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A-level Chemistry y1 > 17:Alkenes > Flashcards

17:Alkenes Flashcards

1
Q

What is a sigma bond (σ-bond)

A

2 s orbitals, each with 1 electron, overlap to form a sigma bond
or
2 p orbitals, each with 1 electron overlap end-over-end

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2
Q

What is a pi bond (π-bond)

A

2 p orbitals with 1 electron overlap sideways

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3
Q

Why is rotation not possible around a double bond

A

Rotation would destroy the sideways overlap of the p-orbitals and hence break the π-bond

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4
Q

What is the general formula for alkenes

A

Cn H₂n

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5
Q

Are alkenes saturated or unsaturated

A

unsaturated

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6
Q

Definition of stereoisomers

A

Compounds with the same structural formula but with a different arrangement in space

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7
Q

How do you assign priority in stereoisomers

A

Whichever atom bonded to the carbon has the higher atomic number.
If they are the same then go to the next atom until priority is assigned

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8
Q

Where are the higher priority groups in a Z-isomer

A

On the same side

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9
Q

Where are the higher priority groups in a E-isomer

A

On opposite sides

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10
Q

What does Cis mean when naming stereoisomers

A

two identical groups are on the same side

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11
Q

What does trans mean when naming stereoisomers

A

two identical groups are on opposite sides

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12
Q

What other name is dehydration called

A

elimination

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13
Q

What conditions are needed for dehydration of alcohols

A

Heating in the presence of an acid catalyst (usually concentrated sulfuric acid)

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14
Q

What happens when you add hydrogen to an alkene in the presence of a nickel catalyst

A

It turns into the corresponding alkane

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15
Q

What conditions are needed for addition of hydrogen to alkene reaction

A

Nickel catalyst and at (typically) 60℃

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16
Q

What conditions are needed for addition of halogens to alkene

A

Alkenes will react readily with chlorine and bromine at room temperature and in the dark

17
Q

What observations will there be for addition of bromine to an alkene

A

The orange bromine solution will immediately decolourise

18
Q

How do you test for unsaturation (alkenes)

A

Add bromine water.

Bromine water is orange. When it is added to an alkene, the solution will immediately become colourless

19
Q

What happens when hydrogen halides are added to alkenes

A

Turn into a haloalkane

20
Q

What happens when steam is added to an alkene

A

Turns into an alcohol

21
Q

What conditions are needed for addition of steam to an alkene

A

Acid catalyst

most commonly phosphoric acid

22
Q

What is the name of the mechanism for addition reactions for alkenes

A

Electrophilic addition

23
Q

What are the 3 steps of the mechanism of adding a halogen to an alkene

A

1) Polarisation of the halogen
2) Formation of the intermediate
3) The carbocation intermediate rapidly reacts with the bromide ion, forming the product

24
Q

What is an electrophile

A

An electron pair acceptor

25
Q

What is heterolytic fission

A

Both electrons stay with one atom and the other atom retains neither

26
Q

What is a carbocation

A

The organic intermediate formed in electrophilic addition, it is a positive ion with the charge on carbon

27
Q

What is the order of stabilities of carbocations

A

Tertiary > secondary > primary

28
Q

What is Markownikoff’s rule

A

The major product of the addition reaction will be the one formed via the more stable carbocation intermediate

29
Q

What is a polymer

A

A long molecule formed by chemically linking together many smaller molecules, called monomers

30
Q

What is the bracketed fragment of a polymer chain called

A

The repeat unit

31
Q

What are the advantages of combustion of polymers

A

They are good fuels, like other hydrocarbons, and burning them releases energy (which can be used for heat or electricity generation) and reduces demand for landfill sites.

32
Q

What are disadvantages of combustion of polymers

A

Toxic combustion products may be formed. This is particularly an issues for chlorinated plastics such as PVC, because toxic, acidic hydrogen chloride is formed when this burns

33
Q

What are 3 sustainable uses for polymers

A

Combustion

Feedstock for production of new plastics and other organic chemicals

Biodegradable and photodegradable polymers

A-level Chemistry y1 (26 decks)

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