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A - Level Chemistry > Chapter 25 > Flashcards

Chapter 25 Flashcards

1
Q

describe, in terms if orbital overlap, the similarities and differences between the bonding in the Kekules model and the delocalised model of benzene.

A

similarities
- orbital overlap (sideways overlap of p orbitals)
- pi bond (pi bond/system/ring above and below atoms/ring/plane)

differences
- kekule = alternating pi bonds, localised pi electrons, 2 electrons in pi bond, 3 pi bonds
- delocalised = all p orbital overlap/ pi electrons spread around the ring/ 6 electrons in pi bond

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2
Q

explain the relative resistance of chlorination in benzene compared with an alkene.

A

reactivity of alkene
- pi electrons are localised / pi bond is localised

reactivity of benzene
- electrons are delocalised/ pi ring is delocalised

in alkene
- higher electron density
- more susceptible to electrophilic attack

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3
Q

what is an example of a redox reaction of phenol with a metal?

A

e.g. phenol + Na => sodium phenoxide (salt) + Hydrogen

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4
Q

What are the conditions and reagents for the electrophilic substitution of Benzene with Alkyl groups?

A
  • Haloalkane e.g. C2H5Cl or C2H5Br
  • presence of AlCl3 or FeBr3 (acts as a halogen carrier catalyst)
  • Benzene
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5
Q

what is the functional groups on a phenol?

A
  • phenol

-OH group

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6
Q

what is the pH of an alcohol?

A

7 (it is neutral)

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7
Q

what happens to phenol in water?

A
  • when dissolved in water phenol partially dissociates forming phenxide ion and proton.
  • phenol is less soluble in water than alcohols, because of the non-polar benzene ring
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8
Q

what does delocalised mean?

A

electrons are shared between more than 2 atoms

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9
Q

explain what the curly arrow in an electrophilic substitution represents.

A

the movement of a pair of electrons from a covalent bond breaking.

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10
Q

Explain the relative resistance to bromination of benzene?

A

Benzene:

  • contains delocalised pi electron ring
  • low electron density
  • so its unable to induce a dipole on Br-Br and attract electrophiles

needs halogen carrier to induce dipole on Br2

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11
Q

Explain the relative resistance bromination of Phenol.

A

Phenol:

  • 1 of lone pairs of electrons from oxygen atom is partially donated into the pi electron ring
  • high electron density
  • is able to induce a dipole in Br-Br and attract it as an electrophile
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12
Q

why is it easier to brominate phenol than Benzene?

A

phenol:

  • delocalised pi electron ring
  • 1 lone pair of electron from oxygen atom is partially donated into pi electron ring
  • higher electron density than Benzene
  • so it can induce a dipole in Br2 and attract it as an electrophile
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13
Q

What are the conditions and reagents for the electrophilic substitution of Benzene with NO2?

A

Concentrated sulphuric and nitric acid

50*C for Monosubstitution

70*C for disubstitution

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14
Q

why is phenol a weak acid?

A

it partially dissociates to produce a p+

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15
Q

Explain the relative resistance to bromination of cyclohexene.

A

cyclohexene:

  • has localised pi electrons
  • high electron density
  • so its able to induce a dipole in Br-Br and attract it as an electrophile
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16
Q

what colour does phenol turn litmus paper?

A

Red

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17
Q

how does the nitration of benzene show that sulphuric acid is acting as a catalyst?

A

the sulphuric acid is regenerated at the end.

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18
Q

What are the conditions and reagents for the electrophilic substitution of Benzene with Halogens?

A
  • Halogen carrier e.g. FeBr3 and AlCl3
  • room temperature and pressure
  • Benzene
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19
Q

why is the mechanism for the nitration of benzene described as substitution?

A

H+ substituted for NO2+

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20
Q

what does localised mean?

A

electrons are shared between 2 atoms.

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21
Q

Are phenols weaker or stronger acids than carboxylic acids?

A

they are weaker than carboxylic acids and they only react with strong bases e.g. NaOH.

phenols don’t react with weak bases like metal carbonates

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22
Q

what is an example of the neutralisation of phenol with a base?

A

e.g. acid + base => salt + water

phenol + KOH => potassium phenoxide + water

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23
Q

what is the Qualitative test for the identification of phenol, Benzene, and Cyclohexene?

A

add Bromine

  • Bromine is decolourised by cyclohexene
  • Bromine decolourises and a white precipitate is formed with Phenol
  • there is no observed changed with Benzene
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24
Q

write an equation for the generation of the electrophile NO2+.

A

HNO3 + H2SO4 —> NO2+ + HSO4- + H2O

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25
Q

describe what happens to the p - orbital electrons in the C atoms in the benzene.

A

they’re at right angles to plane of ring and overlap sideways, above and below the ring.

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26
Q

what is benzene?

A

colourless sweet smelling, highly flammable

  • found naturally in crude oil, is a component of petrol
    • a carcinogen, can cause cancer
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27
Q

why is benzene not used in schools?

A

it is a carcinogen

28
Q

what is the IUPAC name of Benzene?

A

Cyclohexa-1,3,5-triene

29
Q

compared to alkenes how does benzene react with water when in the kekule structure?

A

it is less reactive

30
Q

what is the geometry and bond angle around each carbon in Benzene?

A

120*, trigonal planar, has 3 bonding regions and no lone pairs.

31
Q

what is the 1st piece of evidence to disapprove Kekules model?

A

1 - lack of reactivity of benzene

benzene doesn’t undergo electrophilic addition reaction to decolourise bromine water from orange to colourless under normal conditions.

32
Q

what is the benzene ring called when its attached to an alkyl chain with a functional group?

A

the prefix phenyl is used in the name.

e.g. phenylethanone and 2-phenyloctane

33
Q

is Benzene soluble in water? explain why.

A

no because its non-polar

34
Q

what are the main features of the delocalised model of Benzene?

A
  • Benzne is planar, cyclic, hexagonal Hydrocarbon containing 6 carbon and 6 hydrogen atoms.
    • each carbon atom uses 3 of its available 4 electrons in bonding to 2 other Carbon atoms and to 1 hydrogen atom.
  • each carbon atom has 1 electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms.
  • 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.
  • this overlapping of the p-orbitals creates a system of pi-bonds, which spread over all 6 of the carbon atoms in the ring structure.
  • the 6 electrons occupying this system of pi-bonds are said to be delocalised.
35
Q

what is the empirical formula of Benzene?

A

CH

36
Q

in benzene, 3 of electrons in outer shell of each carbon form bonds with 2 C and H. the 4th electron is in what type of orbital?

A

P-orbital

37
Q

what is the name of the electrophile NO2+?

A

nitronium ion

38
Q

what is the 3rd piece of evidence to disapprove Kekules model

A

3 - hydrogenation enthapies

if benzene did have Kekules structure then it’d be expected to have an enthalpy of hydrogenation that is x3 of cyclohexene.

enthalpy of hydrogenation of cyclohexene = 120KJmol^-1

the expected enthalpy of hydrogenation of Kekules structure of benzne = 3 x -120 = -360KJ mol^-1.

however, the actual enthalpy of hydrogenation of benzene is -208 KJ mol^-1, meaning 152 KJ mol^-1 less energy is produced.

therefore, the actual Benzne structure is more stable than the theoretical Kekule model of benzene.

this shows that benzene is less exothermic than expected.

39
Q

what is the 2nd piece of evidence to disapprove Kekules model of Benzne?

A

2 - the lengths of the carbon-carbon bonds in benzene

using X-ray diffraction to measure the length fo the bonds in a molecule.

when benzene was examined in 1929, it was found that all the bonds in benzene were 0.139nm in length. this is between the bond length of a single bond (0.153nm) and a double bond (0.134nm)

40
Q

what are the carbon - carbon bonds like if the benzene adopt the kekule structure?

A

alternating long C-C and short C=C bonds

41
Q

what is the 3 main pieces of evidence that disapprove Kekules model of benzene?

A

1 - lack of reactivity of benzene

2 - the length of carbon-carbopn bonds in benzene

3 - hydrogenation enthalpies

42
Q

what is the basis for benzene compounds being described as aromatic?

A

many benzene compounds have pleasant smells.

43
Q

how many electrons in the outer shell of a carbon atom?

A

4

44
Q

what is an electrophile?

A

an electron pair acceptor (species attracted to areas of high electron density)

45
Q

what is the molecular formula of Benzene?

A

C6H6

46
Q

what is another term for an aromatic hydrocarbon?

A

Arene

47
Q

summarise the unusual bonding in benzene.

A

p - orbitals overlap forming a delocalised pi system

48
Q

What’s the state of benzene at room temp

A

Liquid

49
Q

What’s the Bond angle of benzene

A

120*

50
Q

What technique was used to find bond lengths of benzene

A

X ray diffraction

51
Q

What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

A

It delocalises to from rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

52
Q

How do rings of electron density affect the stability of benzene

A

Makes benzene very unstable, even though its unsaturated (aromatic stability)

53
Q

Why does benzene have a relatively high MP

A

Close packing of flat hexagonal molecules when solid

54
Q

Why’s benzene attacked by electrophiles

A

High electron density above/below ring due to delocalised electrons

55
Q

Nitration of benzene is what type of reaction

A

Electrophilic substitution reaction

56
Q

What ion’s used to nitrate benzene

A

NO2+

57
Q

What’s the cataclysm in nitration of benzene

A

Sulfuric acid

58
Q

What type of catalyst is used for a friedel-crafts reactions

A

Halogen carrier

E.g. AlCl3

59
Q

Why does benzene not react directly with halogens

A

Aromatic ring is too stable

60
Q

What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

A

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

61
Q

What is happening when AlCl is formed in terms of electrons?

A

The lone pair of electrons on the chlorine atom is forming a coordinate bond to Al

62
Q

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

A

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene

63
Q

What reactions can you carry out to show the weak acidity of phenol?

A

A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates

64
Q

What is the relative ease of electrophilic substitution of phenol compared to benzene and why?

A

It is easier for electrophilic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into ring therefore this increases the electron density of the ring thus electrophiles are more attracted phenol

65
Q

What is the directing effect of electron donating groups OH and NH2?

A

They direct group to the 2 and 4 position of the ring in electrophilic substitution of aromatic compounds

66
Q

What is the directing effect of electron withdrawing group NO2?

A

NO2 directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds

A - Level Chemistry (11 decks)

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