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Chemistry OCR A-Level > Chapter 25 > Flashcards

Chapter 25 Flashcards

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1
Q

What is the molecular formula of benzene?

A

C₆H₆

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2
Q

What colour is benzene?

A

Colourlesss

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3
Q

What type of smell does benzene have?

A

Sweet

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4
Q

How flammable is benzene?

A

Highly flammable

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5
Q

What is a carcinogen?

A

Can cause cancer

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6
Q

What is the structure of benzene?

A

1) hexagonal ring of six carbon atoms

2) carbon atoms joined to two other carbon atoms and to one hydrogen atom

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7
Q

What is benzene classed as?

A

1) aromatic hydrocarbon

2) arene

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8
Q

What is the Kekulé model?

A

1) six membered ring of carbon atoms

2) joined by alternate single and double bonds

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9
Q

What is the evidence to disprove Kekulé’s model?

A

1) the lack of reactivity of benzene
2) the lengths of the carbon-carbon bonds in benzene
3) hydrogenation enthalpies

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10
Q

What do alkenes decolourise?

A

Bromine water

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11
Q

How does the lack of reactivity of benzene disprove Kekulé’s model?

A

1) benzene does not undergo electrophilic addition reactions

2) benzene does not decolourise bromine under normal conditions

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12
Q

How do the lengths of the carbon-carbon bonds in benzene disprove Kekulé’s model?

A

All the bonds in benzene are 0.139nm in length, between the length of a single bond, 0.153nm, and a double bond, 0.134nm

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13
Q

What is hydrogenation?

A

The addition of hydrogen to an unsaturated compound

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14
Q

What is the IUPAC name of the Kekulé model?

A

cyclohexa-1,3,5-triene

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15
Q

When cyclohexene is hydrogenated, how many double bonds react with hydrogen?

A

One

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16
Q

What is the structure of the delocalised model of benzene?

A

1) planar
2) cyclic
3) hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms

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17
Q

What is the bonding in the delocalised model of benzene?

A

1) each carbon atom uses three of its available four electrons i bonding to two other carbon atoms
2) each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
3) adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density
4) overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure
5) six electrons occupying system of pi-bonds are delocalised

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18
Q

What groups are prefixes to benzene?

A

1) short alkyl chains
2) halogens
3) nitro groups

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19
Q

What are aromatic compounds with one substituent group?

A

Monsubstituted

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20
Q

When is benzene considered to be substituent?

A

When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms

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21
Q

When is the prefix phenyl used instead of benzene?

A

When benzene is considered to be a substituent

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22
Q

What are the three exceptions when naming aromatic compounds?

A

1) Benzoic acid (benzenecarboxylic acid)
2) phenylamine
3) benzaldehyde (benzenecarbaldehyde)

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23
Q

How many substituent groups do disubstituted compounds have?

A

Two

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24
Q

How are substituent groups listed?

A

1) alphabetical order

2) using the smallest numbers possible

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25
Q

What are the electrophilic substitution reactions of benzene?

A

1) nitration
2) halogenation
3) alkylation
4) acylation

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26
Q

What is nitrobenzene formed from?

A

Benzene + nitric acid

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27
Q

What are the conditions needed for the nitration of benzene?

A

1) sulfuric acid catalyst

2) 50°C

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28
Q

What piece of equipment is used to maintain a steady temperature?

A

Water bath

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29
Q

What could happen if the temperature of the nitration of benzene rises above 50°C?

A

Further substitution reactions may occur leading to the production of dinitrobenzene

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30
Q

What is an electrophile?

A

An atom (or group of atoms) which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond

31
Q

What is nitrobenzene an important starting material of?

A

1) dyes
2) pharmaceuticals
3) pesticides

32
Q

What is the electrophile involved in the mechanism for the nitration of benzene?

A

The nitronium ion, NO₂+, produced by the reaction of concentrated nitric acid with concentrated sulfuric acid

33
Q

What is the equation where concentrated nitric acid and concentrated sulfuric acid react together to produce nitronium ions?

A

HNO₃ + H₂SO₄ –> NO₂+ + HSO₄- + H₂O

34
Q

What happens to the H+ ion formed from the nitration of benzene?

A

It reacts with the HSO₄- ion to regenerate the catalyst H₂SO₄
H+ + HSO₄- –> H₂SO₄

35
Q

When do halogens react with benzene?

A

When a halogen carrier is present

36
Q

Give some examples of common halogen carriers.

A

1) AlCl₃
2) AlBr₃
3) FeCl₃
4) FeBr₃

37
Q

What are the conditions needed for the bromination of benzene?

A

1) room temperature and pressure

2) halogen carrier

38
Q

What is the electrophile involved in the mechanism for the bromination of benzene?

A

Br+

39
Q

What is the equation where bromine and a halogen carrier react together to produce bromonium ions?

A

Br₂ + FeBr₃ –> FeBr₄- + Br+

Br₂ + AlBr₃ –> AlBr₄- + Br+

40
Q

What happens to the H+ ion formed from the bromination of benzene?

A

It reacts with the FeBr₄-/AlBr₄- ion to regenerate the halogen carrier catalyst
H+ + FeBr₄- –> FeBr₃ + HBr
H+ + AlBr₄- –> AlBr₃ + HBr

41
Q

What is alkylation?

A

Can be used to add a carbon chain to the benzene ring

42
Q

What is the alkylation of benzene?

A

The substitution of a hydrogen atom in the benzene ring by an alkyl group

43
Q

How is the alkylation of benzene carried out?

A

By reacting benzene with a haloalkane in the presence of AlCl₃

44
Q

What are alkylation reactions sometimes called?

A

Friedel-Crafts alkylation reactions after the two chemists who fist carried out the reaction

45
Q

What is the acylation of benzene?

A

When benzene reacts with an acyl chloride in the presence of an AlCl₃ catalyst to form am aromatic ketone

46
Q

What is the first member of the acyl chloride homologous series?

A

Ethanoyl chloride, CH₃COCl

47
Q

What is produced when benzene and ethanoyl chloride react together?

A

Phenylethanone + hydrochloric acid

48
Q

What do alkenes decolourise?

A

Bromine water by an electrophilic addition reaction

49
Q

What is produced when cyclohexene and bromine react together?

A

1,2-dibromocyclohexane

50
Q

Describe the electrophilic addition mechanism reaction between cyclohexene and bromine

A

1) π-bond in alkene contains delocalised e- above + below plane of 2 C-atoms in double bond. This produces an area of high electron density
2) localised e- in π-bond induce dipole in non-polar Br molecule making one Br atom slightly positive and the other slightly negative
3) slightly positive Br atom enables Br molecule to act like an electrophile

51
Q

Why does benzene not react with bromine unless a halogen carrier catalyst is present?

A

1) benzene has delocalised π-electrons above + below the plane of C-atoms in the ring structure
2) the e- density around any two C-atoms in the benzene ring is less than that in a C=C double bond in an alkene
3) when a non-polar molecule approaches the benzene ring there is insufficient π-electron density around any two C-atoms to polarise the molecule, preventing any reaction taking place

52
Q

What are phenols?

A

A type of organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring

53
Q

What is the simplest member of the phenols?

A

Phenol, C₆H₅OH

54
Q

What are compounds containing an -OH group bonded to a carbon side chain, rather than the aromatic ring, classified as?

A

Alcohols

55
Q

Is phenol more or less soluble in water than alcohols?

A

Less soluble

56
Q

Why is phenol less soluble in water than alcohols?

A

Due to the presence of the benzene ring

57
Q

Is phenol a strong or weak acid?

A

Weak acid

58
Q

Why is phenol classified as a weak acid?

A

When dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton

59
Q

Is phenol more or less acidic than alcohols?

A

More acidic

60
Q

Is phenol more or less acidic than carboxylic acids?

A

Less acidic

61
Q

What is Kₐ?

A

The acid dissociation constant

62
Q

How can you tell the difference between a phenol and a carboxylic acid?

A

Carboxylic acids react with sodium carbonate to produce carbon dioxide

63
Q

What does phenol react with sodium hydroxide to form?

A

Sodium phenoxide + water

64
Q

What mechanism do phenols undergo?

A

Electrophilic substitution

65
Q

What does phenol react with bromine water to form?

A

2,4,6-tribromophenol + 3HBr

66
Q

What observation is made when bromine water is reacted with phenol?

A

1) bromine water is decolourised

2) white precipitate of 2,4,6-tribromophenol is formed

67
Q

What is formed when phenol reacts with dilute nitric acid?

A

2-nitrophenol + H₂0
OR
4-nitrophenol + H₂O

68
Q

Why do bromine and nitric acid react more readily with phenol than they do with benzene?

A

1) a lone pair of e- from the oxygen p-orbital of the -OH group is donated to the π-system of phenol
2) e- density of benzene ring in phenol is increased
3) increased e- density attracts electrophiles more strongly than with benzene

69
Q

How would you carry out a test to distinguish between solutions of benzene and phenol?

A

Add bromine water, both would decolourise the bromine water but phenol would produce a white precipitate

70
Q

Are 2- and 4- directing groups activating groups or deactivating groups?

A

Activating groups (except the halogens)

71
Q

Are 3- directing groups activating groups or deactivating groups?

A

Deactivating groups

72
Q

What groups are 2- and 4- directing?

A

1) -NH₂ or -NHR
2) -OH
3) -OR
4) -R or C₆H₅
5) -F, -Cl, -Br, -I

73
Q

What groups are 3- directing?

A

1) RCOR
2) -COOR
3) SO₃H
4) -CHO
5) -COOH
6) -CN
7) -NO₂
8) -NR₃+

Chemistry OCR A-Level (28 decks)

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