Alkanes- alkenes and stereoisomers

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94 Terms

1
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what is the general formula of alkanes?

CnH2n+2

2
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are alkanes saturated or unsaturated?

saturated

3
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what structure do the covalent bonds around each carbon atom of alkanes form?

  • a tetrahedral structure

4
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what is the name of the covalent bond within alkanes?

  • sigma bonds

5
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what are sigma bonds and can they rotate?

overlap of orbitals directly between the bonding atoms, free rotation of the σ-bond

6
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why are alkanes non polar?

because their carbon and hydrogen atoms have similar electronegativities

7
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are alkanes soluble or insoluble within water?

they are insoluble within water

8
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why are alkanes insoluble in water?

  • they have no permanent dipoles so cannot form hydrogen bonds

9
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why are alkanes generally unreactive?

because of the strong covalent bonds within alkane molecules

10
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what does a longer carbon chain result in?

  • the stronger the induced dipole dipole interactions

  • as there is more surface contact

  • more energy needed to overcome induced dipole dipole interactions

11
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why do branched chain alkanes have lower boiling points than their straight chain isomers?

  • they have smaller molecular surface areas so induced dipole dipole forces are reduces

12
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what type or reaction is the burning of alkanes?

  • a combustion reactipn

13
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why must liquid alkanes be vaporised first before they undergo combustion?

  • combustion reactions happen between gases

  • so liquid alkanes must be vaporised first

  • smaller alkanes are more volatile so they burn more easier

14
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define volatile?

turns into a gas more easily

15
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why do larger alkanes make good fuels?

  • larger alkanes release heaps more energy per mole as they have more bonds to react

16
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what happens when alkanes react fully with oxygen?

  • they undergo complete combustion to produce carbon dioxide and water

17
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what happens when alkanes react partially with oxygen?

  • they undergo incomplete combustion producing carbon monoxide and water or carbon particulates and water

18
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what can carbon monoxide (CO) bind to, then prevent?

  • it binds to haemoglobin before oxygen therefore prevents oxygen from being carried around the body

19
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what is breaking covalent bonds called?

  • bond fission

20
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state the 2 ways in which covalent bonds can break?

  • homolytic fission

  • heterolytic fission

21
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what occurs in heterolytic fission?

  • the bond breaks unevenly with one of the bonding atoms recieving both electrons from the bonded pair

  • two different substances are formed, a positively charged anion and a negatively charged cation

22
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what occurs in homolytic fission?

  • the bond breaks evenly and each bonding atom recieves one electron from the bonded pair

  • 2 electrically uncharged radicals are formed

23
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define radical?

  • particles that have an unpaired electron

24
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why are radicals very reactive?

  • because of their unpaired electron

25
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in what reactions do halogens react with alkanes, using what light?

in photochemical reactions using ultraviolet light

26
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what occurs in a free radical substitution reaction?

  • a hydrogen atom is reolaced by chlorine or bromine

27
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what are the 3 reaction mechanism stages?

  • initiation reactions

  • propagation reactions

  • termination reactions

28
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state the steps of initiation?

  • uv light provides enough energy to break the covalent bond

  • homolytic fission occurs

  • therefore the atom becomes a highly reactive free radical due to its unpaired electron

29
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state the steps of propagation?

  • a bromine radical reacts with a C-H bond in the alkane

  • this forms an alkyl radical and a molecule of hydrogen bromide HBr

  • each alkyl reacts with another bromine molecule

  • this forms a halogenalkane and a new bromine radical

30
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state the steps in termination?

  • when two free radicals join together they make a stable molecule

  • there are any possible termination reactions

31
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what is the general formula of alkenes?

C nH2n

32
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are alkenes saturated or unsaturated?

unsaturated

33
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what 2 bonds is a double bond made from?

  • a sigma and a pi bond

34
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when is a sigma bond formed?

  • when 2 s orbitals overlap in a straight line

35
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when is a pi bond formed?

  • by the sideways overlap of two adjacent p orbitals

36
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why are pi bonds weaker than sigma bonds?

  • because the electron density is spread about above and below the nuclei so the electrostatic attraction between the nuclei and the shared pair of electrons is weaker

37
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do pi or sigma bonds have relatively low enthalpy?

pi bonds’

38
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what is the shape around the c=c bond in alkenes?

trigonal planar

39
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can c=c double bonds rotate?

no

40
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define stereoisomers?

  • compounds with the same structural formula but with a different arrangement in space

41
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what are the 2 types of stereoisomers?

E and Z

42
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how are groups positioned in a Z isomer?

has both groups has the same groups either above or below the double bond

43
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how are groups positioned in an E isomer?

has the same groups positioned across the double bond

44
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  • if the groups of the higher priority are on the same side of the double bond what is the name of the isomer?

  • if the groups of the higher priority are on the same side of the double bond the compound is the z isomer

45
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  • if the groups of higher priority are diagonally placed across the double bond what is the name of the isomer?

  • if the groups of higher priority are diagonally placed across the double bond the compound is the E isomer

46
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what other isomers can E/Z isomers also be called?

  • cis- trans isomers

47
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define the cis isomer?

has the hydrogen atoms on the same side

48
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define the trans isomer?

has the hydrogen atoms on opposite sides

49
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define electrophile?

electron pair acceptors

50
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what do alkenes react with hydrogen to form?

alkanes

51
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state the conditions needed for alkenes reacting with hydrogen to form alkanes?

Nickel catalyst , hydrogen and high temperatures

52
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what do halogens react with alkenes to form?

dihaloalkanes

53
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what is the name of the reaction when halogens react with alkenes to form dihaloalkanes?

electrophilic addition

54
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what do alkenes react with hydrogen halides to form?

haloalkanes

55
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what can alkenes be hydrated with steam to form?

alcohols

56
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what conditions are needed for alkenes to react with steam to form alcohols?

  • high temperatures

  • high pressures

  • acid catalyst, H3PO4

57
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what can be used to test for alkenes?

bromine water, turns colourless in the presence of alkenes

58
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what is the general formula of alcohols?

CnH2n+1OH

59
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what are the types of alcohols?

  • primary

  • secondary

  • tertiary

60
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what makes alcohols polar?

the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom

61
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as alcohols get larger, does solubility increase or decrease ?

their solubility in water decreases

62
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why does solubility decrease when alcohols increase in size?

in larger alcohols most of the molecule is a non polar carbon chain so there’s less attraction for the polar H2O molecules

63
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what bonds do alcohols form with each other?

hydrogen bonds

64
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why do alcohols have low volatility?

due to their strong hydrogen bonds

65
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in substitution reactions involving alcohols what is the hydroxyl group replaced by, transforming the alcohol into what?

the hydroxyl group is replaced by the halide so the alcohol is transformed into a haloalkane

66
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what takes place in eleimintaion reactions?

water is eliminated from alcohols

67
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what is the oxidising agent which can oxidise alcohols?

acidified dichromate

Cr2O7 2–/H+ (i.e. K2Cr2O7/H2SO

68
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what can primary alcohols be oxidised to form?

  • aldehydes and then to carboxyllic acids

69
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what can secondary alcohols be oxidised to form?

ketones

70
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what can tertiary alcohols be oxidised to form?

tertiary alcohols cannot be oxidised

71
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under what conditions will a primary alcohol completely oxidise to form a carboxylic acid?

when heated strongly under reflux with excess acidified dichromate

72
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what is the general formula of aldehydes and ketones?

CnH2nO

73
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what functional group do aldehydes and ketones have?

C=O

74
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what do aldehydes have attached to their carbonyl carbon atom?

  • a hydrogen and one alkyl group

75
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what do ketones have attached to their carbonyl carbon atom?

  • 2 alkyl groups

76
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are electrophiles positively or negatively charged ions?

positively charged ions

77
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define addition polymerisation?

  • when the double bonds in alkenes open up and join together to make long chains called polymers

78
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why are many polymers difficult to dispose of?

they are not biodegradable

79
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where can waste plastics be buried?

in landfills

80
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why is there a need to reduce landfill?

  • because the amount of waste we generate is increasing there is a need to reduce landfills a much as possible

81
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state 2 ways in which plastics derived from non renewable oil fractions can be reused?

  • can be recycled be melting and remoulding them

  • can be cracked into monomers and used as organic feedstock to make more plastics or other chemicals

82
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why should we be cautious when burning waste plastics?

  • polymers that contain chlorine e.g PVC produce HCL when burned which needs to be removed

83
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define biodegradable polymers?

polymers which naturally decompose

84
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what can biodegradable polymers be made from?

renewable raw materials such as starch or oil fractions

85
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define photodegradable polymers?

polymers which decompose when exposed to light

86
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define haloalknes?

alkanes with at least one halogen atom in place of a hydrogen

87
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is the carbon halogen bond in haloalkanes polar?

yes

88
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why is the carbon halogen bond in haloalkanes polar?

because halogens are more electronegative than carbon

89
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define a nucleophile?

  • an electron pair donor, negative ions

90
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what is the name of the reaction when halogens are hydrolysed to alchohols?

  • nucleophilic substitution reaction

91
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what are the conditions needed to hydrolyse a haloalkane to an alcohol?

  • a warm aqueous alkali

  • reflux

92
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what does the speed at which different haloalkanes are hydrolysed depend on?

bond enthalpy

93
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which haloalkanes have the weakest bonds therefore hydrolyse quickest?

iodoalkanes

94
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which haloalkanes have the strongest bonds therefore hydrolyse slowest?

fluoroalkanes