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AQA A Level Chemistry > Alkenes > Flashcards

Alkenes Flashcards

1
Q

General formula of alkenes

A

CnH2n

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2
Q

What are alkenes

A

Unsaturated hydrocarbons

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3
Q

What does bonding in alkenes involve

A

a double covalent bond and a centre of high electron density

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4
Q

What do alkenes consist of

A

They have a carbon-carbon double bond which consists of a sigma bond and a pi bond

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5
Q

Which is weaker the pi bond or the sigma bond

A

the pi bond

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6
Q

Why is a compound with the general formula CnH2n not necessarily an alkene

A

It could be a cycloalkane as they are isomeric with alkenes

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7
Q

How would you name the alkene if it has 2 double bonds

A

pent-2-ene becomes penta-1,2-diene

-DIENE

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8
Q

How would you name alkenes using IUPAC

A

select the longest carbon chain containing the double bond

Number the carbon atoms giving the lower number to the carbon atom containing the double bond

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9
Q

What is the arrangement of bond around the carbon-carbon double bond

A

Trigonal planar

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10
Q

What is the bond angles in an alkene

A

120 degrees

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11
Q

Why does ethene have a trigonal planar shape

A

Each carbon has 3 orbitals containing 1 unpaired electron. They repel each other as far apart as possible to a position of minimal repulsion and take up a trigonal planar shape

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12
Q

Why is no free rotation possible around the carbon-carbon double bond

A

The double bond restricts the rotation

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13
Q

How is a pi bond formed

A

By the sideways overlap of adjacent p orbitals above and below the bonding carbon atoms and there is high electron density above and below the line between the 2 nuclei in a pi bond

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14
Q

Explain why alkenes are more reactive than alkanes considering the strength of the bonds in these compounds

A

The C-C bonds in alkanes are sigma bonds.

The C-C double bond in alkenes has a pi bond.

A pi bond is weaker than a sigma bond so less energy is needed to break the pi bond and therefore electrophiles attack the pi bonds in alkenes

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15
Q

Why do electrophiles attack alkenes

A

The carbon-carbon double bond has a high electron density, so alkenes are attacked by electrophiles. The electrophile attacks the pi bond in carbon-carbon double bond.

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16
Q

What are the similarities between pi bonds and sigma bonds

A
  1. Both involve the overlap of 2 atomic orbitals each containing a single electron
  2. Both involve an attraction between a bonding pair of electrons and 2 nuclei
  3. In both bonds the 2 electrons are localised(fixed in position) between 2 atoms in the bond
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17
Q

What are the differences between pi and sigma bonds

A
  1. sigma bond is formed by the head on overlap of 2 atomic orbitals whereas a pi bond is formed by adjacent overlap of p orbitals
  2. There is high electron density along line between 2 nuclei in sigma bond but in a pi bond it is above and below the line between 2 nuclei
  3. sigma bond is formed by overlap of a single lobe, but pi bond is formed by overlap of 2 lobes from a single orbital
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18
Q

What is the shape of any alkane molecule

A

Tetrahedral

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19
Q

Why is the shape of all alkanes tetrahedral

A

There are 4 bonding pairs of electrons around the central C atom. These electron pairs repel each other as far apart as possible to a position of minimum repulsion

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20
Q

What is the shape of any alkene molecule

A

Trigonal planar

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21
Q

Why is the shape of any alkene molecule trigonal planar

A

There are 3 regions of electron density around the central C atom. These electron regions repel each other as far apart as possible to a position of minimum repulsion

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22
Q

Steroisomerism

A

Same structural formula but different spatial arrangement of atoms

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23
Q

Why does E-Z isomerism occur

A

Due to restricted rotation about the carbon-carbon double bond due to the pi bond preventing it

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24
Q

Why does geometric isomerism occur

A

Due to the non-rotation (restricted rotation) of the carbon-carbon double bond because the pi bond prevents it.

25
Q

where are the H atoms in a cis isomer

A

Both the hydrogen atoms are on the same side of the carbon-carbon double bond

26
Q

Conditions needed for geometric isomerism

A

There should be 2 different atoms/groups bonded to each unsaturated carbon atom

There should be carbon-carbon double bond which restricts rotation

27
Q

How would you distinguish between 2 steroisomers

A

they have different BP/MP
There is a difference in their dipoles
They have different infrared spectra

28
Q

Why does one isomer not convert to the other at room temperature

A

The carbon-carbon double bond restricts rotation
There isn’t enough energy to break the pi bond at room temperature

29
Q

When can you not use the cis-trans system and have to use the E-Z nomenclature

A

When the 4 groups around the carbon-carbon double bond are different

30
Q

Z-isomer

A

The highest priority groups on each carbon of the C=C double bond are on the same side of the carbon-carbon double bond. Z corresponds to cis

31
Q

E-isomer

A

The highest priority groups on each carbon of the C=C double bond are on opposite sides of the C=C bond. E corresponds to trans

32
Q

Types of structural isomerism

A

Chain, Position and Functional group isomerism

33
Q

Types of steroisomerism

A

Geometric and optical isomerism

34
Q

Electrophile

A

A species which accepts a pair of electrons

35
Q

How do alkenes react

A

By electrophillic addition

36
Q

What happens in electrophilic addition

A

A molecule adds across the C=C bond to give a single saturated product

37
Q

Conditions needed for addition reaction of alkenes with hydrogen

A

Nickel catalyst and heat at 150 degrees

38
Q

Explain why bromine molecules react with the double bonds in alkenes

A

C=C is an area of high electron density
Br-Br becomes polarised
The delta positive bromine molecule is attracted to the C=C double bond

39
Q

How to test for unsaturation / double bond in an organic compound

A

Add bromine water to the compound and shake. It turns from orange to colourless.

40
Q

What happens when a hydrogen halide is added to an unsymmetrical alkene

A

The more electropositive atom with be added to the unsaturated carbon atom with the greatest number of hydrogen atoms

41
Q

Conditions needed for electrophilic reaction fo alkenes with hydrogen halides, forming haloalkanes

A

gas phase, room temperature

42
Q

What does the reaction between alkenes and steam in the presence of a catalyst form

A

Alchohols

43
Q

Conditions needed for reaction between alkenes and steam to form alcohols

A

High pressure and high temperature and H3PO4 catalyst

44
Q

Explain why but-1-ene can form 2 products when it reacts with HBr

A

Hydrogen could add to either carbon in the double bond

45
Q

How to determine which product is the major/minor product

A

teritary>secondary>primary

This means tertiary will always be major and primary will be minor product

46
Q

Describe the steps of electrophilic addition when alkenes react with H, halogens and hydrogen halides

A
  1. Pair of electrons from pi bond is attracted the electropositive bromine atom and foems a dative covalent bond with bromine atom. Pi bond breaks
  2. Carbocation is attacked by bromide ion. Lone pair of electrons on Br- ion forms a dative covalent bond with the positively charged carbon atom
47
Q

How does the stability of carbocations change with number of alkyl groups

A

Stability increases with number of alkyl groups because alkyl groups are electron-donating and therefore push the shared pair of electrons towards the carbon atom with the positive charge so positive charge on C atom reduces and carbocation is more stable.

48
Q

Why is a tertiary carbocation the most stable

A

Due to the electron-donating character of three alkyl groups as opposed to 2 or 1

49
Q

What is addition polymerisation

A

Many monomers react to form a large molecule (polymer) by an addition reaction.
The double bond breaks and only one product forms

50
Q

What is the empirical formula of a poly(alkene)

A

The same as that of the monomer from which it is produced since no atoms are lost in the addition reaction

51
Q

Uses of poly(chloroethene) / PVC

A

water pipes, electrical insulation and guttering

52
Q

Uses of PTFE

A

non-stick coating for saucepans and burette taps

53
Q

Are addition polymers reactive or unreactive

A

Unreactive

54
Q

Problems with disposal of polymers

A

Addition polymers are mostly non-biodegradable since they are chemically inert and burning them produces toxic gases like hydrogen chloride. FIll up landfull sites

55
Q

Conditions for reaction between alkenes and halogens

A

Room temperature

56
Q

Why is the major product 2-chloropropane when propene reacts with hydrogen chloride?

A

The carbocation intermediate in the formation of 2-chloropropane is more stable than the carbocation in the formation of 1-chloropropane.

A secondary carbocation is more stable than a primary carbocation due to the electron donating character of 2 alkyl groups as opposed to one.

57
Q

Suggest why it is not possible to quote an exact value for the molar mass of any addition polymer but only an average value

A

The polymer is made up of molecules of different chain lengths

58
Q

Why does any polymer not have a sharp melting point

A

It is made up of molecules of different chain lengths and therefore melts over a range of temperatures

59
Q

Why does poly(chloroethene) have a higher MP than poly(ethene)

A

There are permanent dipole-dipole attractions between the polymer chains in poly(chloroethene).

There are Van der Waals forces between the polymer chains in poly(ethene), which are weaker.

More energy is needed to break the stronger intermolecular forces between the polymer chains in poly(chloroethene).

AQA A Level Chemistry (30 decks)

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