CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY: COURSEBOOK
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Coursebook answers
Chapter 26
Self-assessment questions
1
a
CH3CCl2COOH, CH3CHClCOOH,
CH3CH2COOH
b
The electron-withdrawing carbonyl
group in the ethanoic acid molecule
weakens the O—H bond in the
—COOH group, making it more likely
for an ethanoic acid molecule to lose
an H+ ion than it is for an ethanol
molecule. Secondly, delocalisation of
electrons around the —COO− group
stabilises the ethanoate ion. This is not
possible in the ethoxide anion formed
when ethanol loses an H+ ion.
c
2
1
Methanoic acid would be the stronger
acid, as ethanoic acid has an electrondonating methyl group next to the
—COOH group, which does not aid
the breaking of the O—H bond. Also,
once the ethanoate anion is formed
the methyl group tends to increase the
concentration of the negative charge
on the —COO− end of the ion, making
it more attractive to H+ ions than a
methanoate anion. Therefore ethanoic
acid molecules are more likely to exist
in the undissociated form, whereas
methanoic acid molecules are less
likely to exist in the undissociated form.
a
because the methanoic acid formed
would be oxidised to carbon dioxide
(and water)
b
(COOH)2 → 2CO2 + 2H+ + 2e−
(COOH)2 + [O] → 2CO2 + H2O
c
i
to speed up the reaction because
the redox reaction is initially slow
before the Mn2+ ions form and act
as a catalyst
ii
25 cm3 of 0.0500 mol dm−3
ethanedioic acid, H2C2O4,
( 0.0500 × 25)
= 0.00125 moles
contains
1000
2 moles of KMnO4 will react with
5 moles of H2C2O4 so 0.00125
moles of H2C2O4 will react with
2
5
× 0.00125 moles of KMnO4
Therefore there are
2
5
× 0.00125
moles of KMnO4 in 8.65 cm3
of its solution so the number
of moles of KMnO4 in 1000 cm3
(i.e. its concentration) is:
(
2
5
)
moles dm−3
× 0.00125 × 1000
8.65
making the concentration of
KMnO4 solution 0.0578 mol dm−3
(to 3 significant figures)
3
4
a
CH3CH2COOH + SOCl2 →
CH3CH2COCl + SO2 + HCl
b
3HCOOH + PCl3 → 3HCOCl + H3PO3
(heat is required for this reaction to
occur)
c
CH3CH2CH2COOH + PCl5 →
CH3CH2CH2COCl + POCl3 + HCl
a
The carbonyl carbon in an acyl
chloride carries a greater partial
positive charge than the carbon atom
bonded to the oxygen atom in an
alcohol. This is because it has two
strongly electronegative atoms (oxygen
and chlorine) attached to the carbonyl
carbon, compared with just the oxygen
atom in an alcohol.
Cambridge International AS & A Level Chemistry © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY: COURSEBOOK
b
propanoic acid and hydrogen chloride
c
i
ii
The hydrolysis of CH3CH2COCl
is far more vigorous than the
hydrolysis of CH3CH2CH2Cl.
The hydrolysis of CH3CH2CH2Cl
needs a strong alkali and heating
under reflux to bring about a
reaction. The nucleophile is the
negatively charged hydroxide ion,
OH−, as opposed to the neutral
water molecule, which is sufficient
to hydrolyse CH3CH2COCl
quickly at room temperature.
That is because the carbon
bonded to the chlorine atom in
a CH3CH2CH2Cl molecule is
not as electron deficient as the
carbon atom in CH3CH2COCl.
In CH3CH2COCl the carbon
bonded to a chlorine atom is also
attached to an oxygen atom. It
has two strongly electronegative
atoms pulling electrons away
from it. Therefore the attack
by the nucleophile is much
more rapid. On the other hand,
C6H5Cl, an aryl chloride, will not
undergo hydrolysis. The p orbitals
from the Cl atom overlap with
the delocalised π electrons in
the benzene ring. This causes the
C—Cl bond to have some double
bond character, making it stronger
and more resistant to hydrolysis.
iii
steamy white fumes (of HCl)
5
ethanoyl chloride and ethanol
a
i
ii
butanoyl chloride and methanol
iii
benzoyl chloride and phenol
b
2
CH3CH2COCl, CH3CH2CH2Cl,
C6H5Cl
CH3CH2CONHCH2CH2CH3 + HCl
Cambridge International AS & A Level Chemistry © Cambridge University Press 2020