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Edexcel | Chemistry WCH0 | S 16 5 1 | 1 hour 45 minutes
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19
(a)
Add To Classified
3 Mark s

This question is about benzene and some of its compounds.

 In 1865, Kekulé suggested that the benzene molecule was a six-membered 

carbon ring with alternate double and single bonds.

  However, this suggestion did not fully explain the structure and stability of benzene.

  State and explain three different types of evidence that led to this structure of benzene 

being rejected, in favour of benzene with a ring of delocalised electrons.

ALLOW reverse arguments for Kekule

structure Any three points from

Type of reaction

Benzene reacts by (mostly electrophilic)

substitution OR does not react by

(electrophilic) addition

OR

Benzene does not react like alkenes /

does not decolourise bromine water

ALLOW

Other suitable reactions /

benzene needs a catalyst /halogen carrier

to react with bromine (1)

Di-substitution

There are only 3 isomers of di-substituted

compounds (not 4)

OR

Some di-substituted compounds are the

same,

e.g. 1,2 and 1,6 (1)

Thermochemical

Benzene’s (standard) enthalpy (change) of

hydrogenation is less exothermic than if

it had (three localised C=C) double bonds /

is not three times the value for three

(localised C=C) double bonds

ALLOW

Benzene is more stable by ~150 kJ mol-1

OR

stated enthalpies (of hydrogenation)

−205 to −210 kJ mol-1 for benzene and

−360 kJ mol-1 for 3 (localised C=C)

double bonds

OR

(Standard) enthalpy (change) of

combustion is less exothermic than if it

had three (localised C=C) double bonds

ALLOW

(Standard) enthalpy (change) of formation of

benzene is less endothermic than that of

“cyclohexa-1,3,5-triene”

(1)

IGNORE

Just ‘thermodynamically more stable’

X-ray diffraction – does not need to be

mentioned

The C-C bond lengths in benzene are midway

between that of a single bond and a

double bond / are all the same length

OR

Benzene is a regular hexagon (and Kekule

structure is not) (1)

IGNORE

Bond angles are the same

Infrared

The infrared spectrum for benzene has a

peak for an aromatic C=C at a different

wavenumber

/ absorption / frequency to an alkene C=C

OR

Benzene has peaks at 1600, 1580, 1500,

1450 (cm−1) rather than 1669 – 1645

(cm−1)

ALLOW

Benzene has no peak for alkene C=C / 1669

– 1645 (cm−1) (1)

IGNORE

different C-H absorptions / just ‘different

peaks to alkenes’

IGNORE

References to NMR

Electron density map

Benzene shows an even spread of electrons

19
(b)
Add To Classified
4 Mark s

Benzene reacts with a mixture of concentrated nitric acid and

concentrated sulfuric acid to form nitrobenzene.

   Give the mechanism for this reaction, including an equation for the formation

of the electrophile.

19
(b) (I)
Add To Classified

Intermediate structure including charge with

horseshoe covering at least 3 carbon atoms

and facing the tetrahedral carbon

and some part of the positive charge must be

within the horseshoe (1)

Curly arrow from C-H bond to anywhere in the

hexagon reforming the delocalised structure (1)

Correct Kekulé structures score full marks

19
(b) (II)
Add To Classified
2 Mark s

The preparation of nitrobenzene is usually carried out between 50 °C and 60 °C.

   Explain why the temperature used should not be higher or lower.

Higher temperature causes multiple

substitution of NO2 groups / formation of

dinitrobenzene / formation of

trinitrobenzene

ALLOW further nitration / substitution (1)

IGNORE decomposition of benzene /

nitrobenzene / addition of NO2

groups

At lower temperature reaction is (too) slow(1)

IGNORE

References to activation energy / reaction

does not occur at low temperature

19
(c) (I)
Add To Classified
2 Mark s

Benzene reacts with pure bromine when heated under reflux in the presence of a 

Friedel-Crafts catalyst.  Phenol reacts with bromine water at room temperature.

   Write the equation for the reaction between phenol and excess bromine water.

Formula of organic product (1)

Rest of equation correct (1)

ALLOW Br on any 3 carbon atoms

ALLOW C6H5OH + 3Br2 → C6H2(OH)Br3 +

3HBr for

both marks, allow C6H2(Br3)OH, ignore

missing brackets

ALLOW correct balanced equations to form

mono or di substituted product for 1 mark

ALLOW Kekulé structures

IGNORE position of bond to OH if vertically

above or below the ring / name of product /

state symbol

19
(c) (II)
Add To Classified
2 Mark s

Explain why phenol reacts with bromine under much milder conditions than

those required for the reaction between benzene and bromine.

MP1

Lone pair of electrons on oxygen

(may be shown on a diagram)

and

EITHER

Overlaps with pi cloud /delocalised

electrons / delocalised system

OR

Feeds into / donates into / interacts

with (benzene) ring /delocalised

electrons / delocalised system

OR

Increases the electron density of the

(benzene) ring (1)

MP2

(Increased electron density) makes the ring

more susceptible to electrophilic attack

ALLOW phenol is a better nucleophile (1)

19
(d) (I)
Add To Classified
2 Mark s

1-chloro-1-phenylethane is found in some fruits, such as apples, and is used to

make fragrances.

  It can be synthesised from benzene in three steps.

 

Identify, by name or formula, the reagent and catalyst used in Step 1.

 

If name and formula are given, both

must be correct

Ethanoyl chloride / CH3COCl (1)

aluminium chloride /

AlCl3 / iron(III) chloride

/ FeCl3

Conditional on correct reagent or a ‘near

miss’ eg acyl chloride

ALLOW

corresponding bromides (1)

NOTE

Reagent and catalyst in either order and

they do not need to state which they are

IGNORE

Friedel-Crafts catalyst / Lewis acid catalyst /

any solvent mentioned

19
(d) (II)
Add To Classified
1 Mark

Draw the structure for Compound X

19
(d) (III)
Add To Classified
1 Mark

 Identify, by name or formula, the reagent used in Step 2.

 

If name and formula are given both

must be correct

Lithium aluminium hydride / LiAlH4 /

lithium tetrahydridoaluminate((III)) /

sodium borohydride /

sodium tetrahydridoborate / NaBH4

IGNORE solvents / temperature, even if

incorrect

19
(d) (IV)
Add To Classified
1 Mark

Identify, by name or formula, the reagent used in Step 3.

 

If name and formula are given, both must be

correct

Phosphorus(V) chloride / phosphorus

pentachloride / PCl5 / phosphorus(III)

chloride / phosphorus trichloride /

PCl3 / phosphorus and chlorine / P

and Cl2 thionyl chloride /SOCl2 /

conc hydrochloric acid / HCl and zinc

chloride / ZnCl2 / zinc / Zn

No TE on 19(d)(ii)

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