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A Level Chemistry > Module 6 - chapter 25 P2 > Flashcards

Module 6 - chapter 25 P2 Flashcards

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1
Q

What evidence is there suggesting that the kekulé model is incorrect?

A
  • no rapid decolourisation of bromine water observed under standard conditions (benzene should undergo electrophilic addition due to the c=c bonds, suggesting that benzene doesn’t have c=c bonds in its structure). benzene is also resistant to reaction
  • ALL c-c bonds in benzene are the same length of 0.139nm - intermediate between the length of a single bond and a double bond (don’t say sigma/pi). Concluding that benzene doesn’t have single and double bonds but bonds intermediate in length. This was found by X-ray diffraction which is used to measure the length of bonds in a molecule.
  • the enthalpy of hydrogenation is less exothermic than expected from studies of cyclohexene - if following the kekulé model the enthalpy would be expected to be x3 the enthalpy of cyclohexene bc of the 3 double bonds (-120KJmol-1 x 3 = -360) but the actual enthalpy is -208. The actual benzene structure is more stable than expected.
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2
Q

What is the kekulé model?

A

Benzene is a ring of 6 carbon atoms joined by alternate single and double bonds

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3
Q

What is the delocalised model of benzene?

A
  • benzene is planar, cyclic, hexagonal hydrocarbon, C6H6
  • only three electrons are used by each carbon atom for bonding (bonding to 2Cs and 1H) so each C has one electron in a p-orbital at right angles to the plane of the sigma bonds.
  • adjacent p-orbital electrons overlap sideways in both directions, 3 above and 3 below the plane of the carbon atoms in the ring structure to give electron density above and below the ring. 6e- delocalised in total bc 3e- used by each c for bonding so only 1e- delocalised per c.
  • the six electrons occupying this DELOCALISED PI SYSTEM (not pi bonds bc pi bonds only contain 2e-) are said to be delocalised (spread over more than 2 atoms = over 6 atoms) and in the p orbital overlap ring
  • bond angles = 120 (trigonal planar)
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4
Q

Properties of benzene

A
  • benzene is the simplest aromatic hydrocarbon (an arene)
  • colourless, sweet smelling, highly flammable liquid
  • found naturally in crude oil, is a component of petrol and also found in cigarette smoke
  • classified as a carcinogen (can cause cancer)
  • liquid at room temperature, immiscible in water (only London forces between molecules)
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5
Q

Nitro group

A

NO2

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6
Q

OH group

A

Phenol

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7
Q

Amino

A

NH2

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8
Q

Rule when drawing groups on benzene

A

Phenols and methyl groups always position 1

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9
Q

Uses of benzene

A

Polystyrene, dyes and fibres (polyesters), explosives (TNT = 2,4,6 trinitrotoluene), pharmaceuticals

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10
Q

How does electrophilic addition occur with benzene?

A

Electrophiles (electron pair acceptors) attack a benzene ring, a catalyst is required (H2SO4)

When showing mechanism show curly arrow from the centre circle to the electrophile as the electrons are attracted to the electrophile

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11
Q

Reagents for nitration of benzene (nitrating mixture) and why

A

HNO3 and H2SO4 both CONCENTRATED, H2SO4 to catalyse bc reaction is slow

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12
Q

Equation for the nitration of benzene

A

Benzene + HNO3 —(H2SO4 50C)—> nitrobenzene + H2O

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13
Q

Equation for generation of electrophile for nitration of benzene

A

HNO3 + H2SO4 -> NO2+ + HSO4- + H2O

Electrophile = NO2+ (nitronium ion) NOT HNO3

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14
Q

Equation for regeneration of catalyst for nitration of benzene

A

H+ + HSO4- -> H2SO4

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15
Q

What type of bond is formed in the nitration of benzene?

A

Daitive covalent bond

OR

Coordinate bond

Not “daitive coordinate”

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16
Q

Why is the temperature key for electrophilic substitution of benzene?

A

To maintain a good rate of reaction and if the temperature rises above 50C, further substitution reactions may occur to form dinitrobenzene

Reaction with excess nitric acid (2HNO3) above 50C (70C) -> 1,3 dinitrobenzene

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17
Q

How does halogenation of benzene occur?

A

A halogen carrier is used (something with 3 halogens) as a catalyst.

E.g. AlCl3, FeCl3, AlBr3, FeBr3

The halogen carrier is generated in situ (in the reaction vessel) from the metal and halogen

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18
Q

Equation for the generation of electrophile for halogenation of benzene

A

Br2 + FeBr3 -> FeBr4- + Br+

Br-Br bond broken by heterolytic fission

(Bromination)

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19
Q

Equation for the halogenation of benzene

A

Benzene + halogen -> halogenated benzene + H+

Room temp and pressure

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20
Q

Equation for regeneration of catalyst with halogenation of benzene

A

FeBr4- + H+ -> FeBr3 + HBr

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21
Q

What is alkylation?

A

The substitution of a hydrogen into the benzene ring by an alkyl group.

A haloalkane reacts in the presence of halogen carrier

It increases the number of c-c bonds in a compound.

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22
Q

What is used as the catalyst for alkylation?

A

Halogen carrier, AlCl3

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23
Q

What is the equation for the generation of the electrophile for alkylation?

A

C2H5Cl + AlCl3 -> AlCl4- + C2H5+

Could be any haloalkane

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24
Q

Equation for alkylation

A

Benzene + haloalkane -> alkylbenzene + H+(hydrogen halide)

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25
Q

Equation for regeneration of catalyst for alkylation

A

H+ + AlCl4- -> AlCl3 + HCl

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26
Q

What is acylation?

A

Addition reaction of benzene with an acyl group (could be halide or oxide) in the presence of a halogen carrier (AlCl3) as a catalyst. An aromatic ketone is formed

This is benzene with a ketone attached to the ring with the carbon chain from the acyl chloride attached where the Cl was

Benzene ring
Carbon with double bonded oxygen bonds to ring
Carbon chain attached to ketone in place of Cl

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27
Q

What is an acyl chloride?

A

COCl, eg ethanoyl chloride = CH3COCl

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28
Q

Naming benzene with a carboxylic acid attached

A

… carboxylic acid

Benzoic acid

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28
Q

Naming benzene with an aldehyde attached

A

Benzaldehyde

Benzenecarbaldehyde

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28
Q

Carboxylic acid group

A

C=O

-OH

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28
Q

Aldehyde group

A

C=O

-H

28
Q

What type of reactions are nitration, halogenation, alkylation and acylation?

A

Electrophilic substitution

28
Q

Cyclohexene v benzene

A

Cyclohexene only one double bond, benzene alternating

Benzene has lower electron density than cyclohexene as the electrons are delocalised

Benzene is more stable so requires more energy to bond with halogens

30
Q

What does a curly arrow represent?

A

Movement of a pair of electrons

31
Q

In an intermediate benzene how many electrons are involved in the pi bonding?

A

4 involved in the pi bonding as 2 are used in the daitive coordinate bond with the substituent

32
Q

Differences between cyclohexene and benzene reacting with bromine/a halogen

A

Cyclohexene reacts by regular electrophilic addition

Benzene requires a halogen carrier, bromine is not decolourised without one

33
Q

What is phenol?

A

Benzene with OH group

34
Q

Cyclohexene v benzene

A

Cyclohexene only one double bond, benzene alternating/ delocalised ring

Benzene has lower electron density than cyclohexene as the electrons are delocalised

Benzene is more stable so requires more energy to bond with halogens

35
Q

Why does benzene only react with bromine if a halogen carrier is present?

A

Benzene is too stable to react with non-polar molecules.

Because benzene has delocalised electrons above and below the plane of the carbon atoms in the benzene ring, resulting in insufficient electron density around any two carbon atoms to polarise the bromine molecule, therefore the reaction can’t take place so the electrophile must be generated

In alkenes the reaction can occur because the electrons are localised

37
Q

Is NaOH a strong or weak base?

A

Strong, metal hydroxides are strong bases

39
Q

Why are phenols stronger acids than alcohols but less acidic than carboxylic acids?

A

alcohols can’t react with strong or weak bases, phenol can react with strong bases, carboxylic acids are stronger acids because they can react with weak bases but the others can’t

there is a positive inductive effect with alcohols because with ethanol for example, the ethoxide ion accepts a proton due to the oxygen being more electronegative than the carbons and hydrogens so the lone pair is attracted. the electron density of the oxygen increases so the ethoxide ion accepts protons more easily

41
Q

Is sodium carbonate a strong or weak base?

A

Weak, metal carbonates are weak bases, only carboxylic acids are strong enough to react with sodium carbonate

sodium carbonate dissociates into carbonic acid which is a stronger acid than phenol

42
Q

Does phenol react with strong or weak bases?

A

Only strong bases because it is a weak acid

43
Q

examples of halogen carriers?

A

AlCl3, FeBr3, AlBr3

44
Q

what does benzene reacting with ethanoyl chloride form? what type of reaction is this?

A

Phenylethanone, electrophilic substitution acylation

45
Q

how to distinguish between phenol and carboxylic acids?

A

react with sodium carbonate, only carboxylic acids will produce CO2 gas in reaction as phenol is not acidic enough to react

46
Q

reaction of phenol with sodium hydroxide

A

forms sodium phenoxide and water, the proton is removed from the phenol and replaced with the sodium in ionic bonding

47
Q

uses of phenyl esters?

A
  • polyesters

- some types of liquid crystal displays (LCDs)

48
Q

bromination of phenol

A

doesn’t require halogen carrier and can occur at room temp because high electron density ring induces dipole on bromine.

RAPIDLY decolourises bromine water because bromine substituted into ring.

white precipitate of 2,4,6 tribromophenol formed and HBr

49
Q

nitration of phenol

A

dilute nitric acid at room temp and no sulfuric acid catalyst required

forms isomers of 2-nitrophenol and 4-nitrophenol and water

50
Q

why is phenol more reactive than benzene?

A

lone pair of electrons from oxygen in pi system so increased electron density in the delocalised ring and electrophiles are more attracted to the ring.

higher electron density can polarise molecules e.g. bromine so no halogen carrier is required

51
Q

what is activating the delocalised ring of electrons and how does this affect reaction?

A

when a lone pair of electrons is added to the delocalised ring of electrons, makes the reaction occur more readily due to increased electron density

e.g. NH2 activates the ring because it isn’t electronegative enough to attract electrons

52
Q

what is second substitution called?

A

disubstitution

53
Q

what is deactivating the delocalised ring of electrons and how does this affect reaction?

A

electrons are removed from the delocalised ring by being attracted by an electronegative group. decreased electron density in the ring so reactions occur less readily and more slowly - catalyst, high temp, high conc and/or halogen carrier required

e.g. NO2 very electronegative so deactivates ring

54
Q

directing effect of NO2

A

positions 3 and 5

55
Q

directing effect of NH2

A

positions 2 and/or 4

56
Q

how does the directing effect link to activating/deactivating groups?

A

all activating groups (and halogens) direct to positions 2 and 4

all deactivating groups direct to positions 3 and 5

57
Q

property of phenol

A

bactericidal

58
Q

Acid definition

A

Proton donor

59
Q

Base definition

A

Proton receiver

60
Q

Properties of phenol

A

Colourless solid at room temp whereas benzene is liquid at room temp. This is because phenol can hydrogen bond so the mpt is higher

Dissolves in water due to hydrogen bonds but less soluble than alcohols due to non-polar benzene ring

Dissociates in water slightly as behaves as a weak acid
Equation :
C6H5OH(aq) + H2O C6H5O-(aq) + H3O+(aq)

Produces proton and phenoxide ion

61
Q

What are alkylation and acylation also known as?

A

Friedel-Crafts alkylation/acylation

63
Q

Which homologous series is benzene part of?

A

Arene

64
Q

what does aromatic mean?

A

contains a benzene ring : historically was used to classify derivatives of benzene because many pleasant-smelling compounds have a benzene ring, despite many odourless compounds containing a benzene ring the term is still used

65
Q

what are compounds with one substituted group called?

A

monosubstituted

66
Q

naming as benzene/phenyl

A

in monosubstituted aromatic compounds the benzene ring is considered the parent chain so the compound is named “alkyl/halo/nitro benzene”. When the benzene ring is connected to an alkyl chain with a functional group or with seven or more carbon atoms benzene is the substituent so phenyl is used as the prefix instead, e.g. phenylethanone or phenyloctane

67
Q

what are the exceptions to the phenyl/benzene naming rule?

A

BENZOIC ACID (benzenecarboxylicacid)

PHENYLAMINE

BENZALDEHYDE (benzenecarbaldehyde)

68
Q

how do you list substituent groups in a benzene ring?

A

alphabetical order and using the lowest numbers possible

69
Q

how does the delocalised model of benzene account for its observed stability?

A

the pi-bond electron density is spread out above and below the ring rather than concentrated areas of electron density in the separate bonds as in the kekule model. the presence of delocalised electrons makes it more stable (by resonance and loss of internal energy)

70
Q

use of nitrobenzene

A

starting material for dyes, pharmaceuticals (paracetamol) and pesticides

71
Q

how to show the role of a catalyst/halogen carrier in reactions

A

show stages of formation of electrophile and regeneration of catalyst

72
Q

how can halogen carriers be generated in situ?

A

by the reaction of the metal and the halogen in the reaction vessel, e.g. Al or Fe as reacting metal

73
Q

what exception is there for the reaction conditions when halogenating benzene

A

bromination of benzene can occur at rtp in the presence of a halogen carrier

74
Q

official name for alkylation

A

Friedel-Crafts alkylation

75
Q

which direction does the arrow go in mechanisms in relation to the electrophile?

A

towards electrophile from inner ring

A Level Chemistry (21 decks)

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