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Halogenoalkanes - Elimination - Mark schemes

Q1.(a) Name of mechanism: nucleophilic substitution (1)

Mechanism:

Marks SN1 using same points

M2 requires

5

(b) Role of potassium hydroxide: Base (1)Mechanism:

5

[10]

Q2.C

[1]

Q3.(a) Identity of X; 2-methylpropene (1)

Absorption at 1650 cm–1 indicates an alkene present (1)OR a chemical answer e.g. Br2 (aq) brown to colourless

2

(b) ReagentsStep 1 KOH (allow NaOH) (1) alcoholic (1) warm (1)

Only allow solvent and warm if reagent correct

Step 2 HBr (1)

Mechanism: A → X

Or a carbocation mechanism

Mechanism X → B

11

(c) A gives three peaks (1)B gives one peak (1)

Allow one for “A has more peaks than B” when no number of peaks is given

2[15]

Q4.(a) (i)

If wrong carbocation, lose structure markIf wrong alkene, lose structure mark Can still score ¾ i.e. penalise M3

Penalise M2 if polarity included incorrectly no bond between H and Br bond is shown as or

(ii) credit secondary carbocation here if primary carbocation has been used in (i)

Ignore attack on this carbocation by 5

(b) (i) Structure:

No credit for propan-1-ol even when named correctlyCredit propane-2-ol

Name: propan-2-ol (1)Not 2-hydroxypropane

(ii) Name of mechanism: nucleophilic substitution (1) (both words)(NOT SN1 orSN2)

Mechanism:

penalise incorrect polarity on C-Br (M1)Credit the arrows even if incorrect haloalkaneIf SN1, both marks possible

5

(c) (i) elimination (1)Ignore nucleophylic eliminationPenalise electrophilic elimination

(ii) base (1)OR proton acceptorNOT nucleophile (base)

2[12]

Q5.

A[1]

Q6.(a) (i)

(ii) restricted rotation OR no rotation OR cannot rotate (1)3

(b) (i) Mechanism:

M1 and M2 independentCurly arrows must be from a bond or a lone pairDo not penalise sticks

Penalise M1 if precedes (penalise this once)Penalise incorrect δ+ δ– for M2Penalise + on C atom for M2Only allow M1 for incorrect haloalkane

Role of the hydroxide ion: nucleophile (1) electron pair donor lone pair donor

NOT nucleophilic substitution

(ii) Mechanism:

Only allow M1 and M2 for incorrect haloalkane unless RE on (i)+ charge on H on molecule, penalise M1M3 independentM2 must be to correct C–CM1 must be correct H atomCredit M1 and M2 via carbocation mechanism

No marks after any attack of C by OH–

Role of the hydroxide ion: base (1) proton acceptor accepts H+

7[10]

Q7.(a) Alcohol: Reaction = Substitution (/ hydrolysis) (1)

Ignore reference to nucleophilic, but electrophilic give zero

Alcohol: Role = nucleophile (/ lone pair donor) (1)

Alkene: reaction = elimination (1)Ignore ref to nucleophilic or electrophilic

Alkene: base (/ proton acceptor) (1)If no indication of order in (a) assume as in question.If order is wrong can still score 'role' mark.

4

(b) Alcohol: Role = butan-2-ol (1)Not 2-hydroxybutane or but-2-ol

Appropriate structure for CH3CH(OH) CH2CH3 (1)Brackets not essential

SN2 version SN1 version

– bond is polarC–Br bond is polar (1)

Lone pair of OH– C–Br bond breaks (1)

Attacks the Cδ+ forming carbocation / carbonium ion (1)

M1 can be scored from a diagram, M2 and M3 from written explanation only

5

(c)

If but-2-ene not given here it may be obtained from cis / trans isomer

H lost from different carbon atoms (1)H removes from C1 and C3 to give two isomers (1)Draws clear Cis and trans isomers for but-2-ene

Can score these marks from a diagram

8[17]

Q8.(a) M1 curly arrow from lone pair on oxygen of hydroxide ion to

H atom on C-H adjacent to C-Br1

M2 curly arrow from single bond of adjacent C-Hto adjacent single bond C-C

(only credit M2 if M1 is being attempted to correct H atom)1

M3 curly arrow from C-Br bond to side of Br atom(credit M3 independently)

1

(b) Ml credit a correct structure for either geometrical E-Z isomer and itsdesignation as either cis or trans.OR credit two correct geometrical E-Z isomer structures(ignore the names)OR credit two correct names for cis pent-2-ene and transpent-2-ene (ignore the structures)

1

M2 credit a second mark if all four parts of the required structures andnames are correct.

(credit “linear” structures)(insist on the alkyl groups being attached clearly by C-C bonds)

1

(c) (i) Ml curly arrow from middle of C = C bond to H atom on H-Br(penalise M1 if partial negative charge or formal positive charge on H)(penalise Ml if pent-2-ene is used)

1

M2 curly arrow from H-Br bond to side of Br atom1

M3 correct structure for correct secondary carbocation1

M4 curly arrow from lone pair on bromide ion to the positivecarbon of carbocation, ensuring that bromide ion has anegative charge.

(with the exception of pent-2-ene, if the wrong alkene is used, only penalise the structure M3)(penalise the use of two dots in addition to a covalent bond, once only)

1

(ii) 1-bromopentane1

(iii) Ml 2-bromopentane is formed via the secondary (or 2°)carbocation

1

OR 1-bromopentane is formed via the primary (or 1°)carbocationM2 a secondary carbocation is more stable than a primarycarbocation -award this mark only if the quality of language justifiesthe award.

(the argument must involve clear statements about carbocations)

1[12]

Q9.(a) Reaction 2: NaOH OR KOH (1) M1 alcohol (ic) OR ethanol (ic)(1) M2

ignore heatCondition mark linked to correct reagent but award M2 if OH–

or base or alkali mentioned

Reaction 3: concentrated H2SO4 OR H3PO4 M1 (1) heat (1) M2OR 150°C - 200°C

Condition mark linked to correct reagent but award M2 if H2SO4 or H3PO4, but not concentratedPenalise reagent and condition if dilute H2SO4 / H3PO4

4

(b) Mechanism:

Award M3 independently M1 and M2 must be to / from correct places

E1 mechanism possible in which M2

Name: of mechanism = elimination (1)NOT dehydrohalogenationIgnore “base” OR “nucleophilic” before elimination

Reason: Reaction 2 has (very) low yield (1)5

QoL OR chloroethane has to be made (from ethane) OR chloroethane is expensive OR chloroethane is not redily available

(c) Mechanism:

Name of mechanism = elimination (1)NOT dehydration alone

Reason: Ethanol could come from (fermentation of) renewableQoL sugars / glucose / carbohydrates / sources (1)

6[15]

Q10.(a) (i) Nucleophilic substitution

1

2

M1 must show an arrow from the lone pair of electrons on theoxygen atom of the negatively charged hydroxide ion to thecentral C atom.M2 must show the movement of a pair of electrons from the

C-Br bond to the Br atom. Mark M2 independently.Penalise M1 if covalent KOH is usedPenalise M2 for formal charge on C or incorrect partial chargesPenalise once only for a line and two dots to show a bond.Max 1 mark for the mechanism for the wrong reactant and/or “sticks”Ignore product

Award full marks for an SN1 mechanism in which M1 is theattack of the hydroxide ion on the intermediate carbocation.

(ii) 2-bromopropane ONLY1

(iii) Polar C–Br OR polar carbon–bromine bond OR dipole on C–BrOR δ+ (δ–)C atom of carbon–bromine bond is δ+/electron deficient OR C―Br

(Credit carbon–halogen bond as an alternative tocarbon–bromine bond)

It must be clear that the discussion is about the carbon atom of the C–Br bond. NOT just reference to a polar molecule.Ignore X for halogen

1

(b) EliminationCredit “base elimination” but NOT “nucleophilic elimination”No other prefix.

1

3

M1 must show an arrow from the lone pair on oxygen of a negativelycharged hydroxide ion to the correct H atomM2 must show an arrow from the correct C-H bond to the C-C bondand should only be awarded if an attempt has been made at M1M3 is independent.

MechanismPenalise M1 if covalent KOHPenalise M3 for formal charge on C or incorrect partial chargesPenalise once only for a line and two dots to show a bond.Max 2 marks for the mechanism for wrong reactant and/or “sticks”Ignore product

Award full marks for an E1 mechanism in which M2 is on thecorrect carbocation.

(c) Any one condition from this list to favour elimination;Apply the list principle

• alcohol(ic)/ethanol(ic) (solvent)

• high concentration of KOH/alkali/hydroxide OR concentratedKOH/hydroxide

Ignore “aqueous”

• high temperature or hot or heat under reflux or T = 78 to 100°CIgnore “excess”

1

(d) (i) Addition (polymerisation) ONLYPenalise “additional”

1

(ii) But-2-ene ONLY (hyphens not essential)Ignore references to cis and trans orE/ZIgnore butane

1[12]

Q11.B

[1]

Q12.(a) Electron pair donor

ORSpecies which uses a pair of electronsto form a co-ordinate / covalent bond.

QoLCredit “lone pair” as alternative wording

1

(b)

M1 Must show an arrow from the lone pair of electronson the carbon atom of the negatively chargedcyanide ion to the central C atom.

M2 Must show the movement of a pair of electrons fromthe C-Br bond to the Br atom. Mark M2 independently.

Award full marks for an SN1 mechanism in which M1 is theattack of the cyanide ion on the intermediate carbocation.

Penalise M1 if covalent KCN is usedPenalise M2 for formal charge on C or incorrect partial

chargesPenalise once only for a line and two dots to show a bond.Max 1 mark for the wrong reactant or “sticks”

2

(c) Ethylamine / CH3CH2NH2 is a nucleophileOREthylamine could react furtherOREthylamine could make secondary / tertiary aminesORTo make reaction with ammonia more likelyORTo minimise further substitutionORThe idea of releasing free amine from the saltORThe idea of removing a proton from the intermediatealkylammonium ionORThe idea that ammonia acts both initially as a nucleophile andthen as a base

Do not credit a simple reference to the equation or the mechanism requiring two moles of ammonia.

1

(d) EliminationCredit “base elimination” but NOT “nucleophilic elimination”No other prefix.

1

M1 Must show an arrow from the lone pair on oxygen of anegatively charged hydroxide ion to the correct H atom

M2 Must show an arrow from the correct C-H bond to theC-C bond and should only be awarded if an attempt hasbeen made at M1

M3 Is independent.


MechanismPenalise M1 if covalent KOHPenalise M3 for formal charge on C or incorrect partial chargesPenalise once only for a line and two dots to show a bond.Max 2 marks for the mechanism for wrong reactant or “sticks”

3[8]

Q13.(a) (i)

If wrong carbocation, lose structure markIf wrong alkene, lose structure markCan still score ¾ i.e. penalise M3Penalise M2 if polarity included incorrectlyno bond between H and Brbond is shown as or

4

(ii) CH3CH2CH2

credit secondary carbocation here if primary carbocation hasbeen used in (i)

Ignore attack on this carbocation by o1

(b) (i) Structure: 1

Name: propan-2-olNot 2-hydroxypropane

1

(ii) Name of mechanism: nucleophilic substitution (both words)(NOT SN1 or SN2)

1

Mechanism:

penalise incorrect polarity on C-Br (M1)Credit the arrows even if incorrect haloalkaneIf SN1, both marks possible

2

(c) (i) elimination1

(ii) baseOR proton acceptorNOT nucleophile

1[12]

Q14.A

[1]

Q15.(a) (base) elimination

(penalise other words before ‘elimination’ e.g. nucleophilic)1

M1: curly arrow from lone pair of electrons on oxygen of hydroxide ion(insist on a lone pair of electrons on the oxygen atom and a negative charge, but only credit this mark if the attack is to a correct H atom)

1

M2: curly arrow from the middle of the C-H bond to the middleof the C–C bond

1

(only credit this mark if the arrow originates from the correct C–H bond and if an attempt has been made at M1)

M3: curly arrow from the middle of the C–Br bond towards/alongsidethe Br atom

(credit M3 independently unless the bond breaking is contradicted by an additional arrow)(penalise curly arrow if the C–Br has a formal positive charge)(credit full marks for an E1 mechanism, with M2 awarded for a correct curly arrow on the correct carbocation)(award a maximum of two marks for either an incorrect haloalkane or an incorrect organic product)(maximum 2 marks for use of 'sticks' for the haloalkane, unless RE from 2(b), when credit can be given)

(b) (i) M1: compounds with the same structural formula1

M2: but the bonds/groups/atoms have different spatialarrangements or orientation or configuration/are arrangeddifferently in space/3D

(ignore reference to the same molecular formula for M1)1

(ii) M1: correct structural representation for cis-but-2-ene andits name or its identification as the cis isomer

1

M2: correct structural representation for trans-but-2-eneand its name or its identification as the trans isomer

(accept representations which are 90° to linear)(award one mark for two correct structures but either wrong/no names)(maximum 1 mark for an incorrect alkene)

1

(iii) geometric(al) or cis-trans1

(c) nucleophile or electron pair donor(penalise ‘base’)

1

(d) CH3CH2CH2CH2Br + 2NH3 → CH3CH2CH2CH2NH2 + NH4Br(M1 correct product)(M2 balanced equation using 2NH3 and leading to NH4Br)(penalise M1 for use of C4H9NH2 or for incorrect haloalkane, but allow consequent correct balancing of equation with 2 moles of ammonia)

2

(1–)butylamine(credit 1–aminobutane and butyl–1–amine)(award QoL mark for correct spelling)

1[13]

Q16.(a) 2-bromobutane;

1

(b) Elimination;(penalise “nucleophilic” OR “electrophilic” before the word “elimination”)

1

M1: curly arrow from lone pair on oxygen of hydroxide ion to H atomon correct C-H adjacent to C-Br;

(penalise M1 if KOH shown as covalent with an arrow breaking the bond)

1

M2: curly arrow from single bond of adjacent C-H to adjacentsingle bond C-C;

(only credit M2 if M1 is being attempted to correct H atom)1

M3: curly arrow from C-Br bond to side of Br atom;(credit M3 independently unless arrows contradict)(Credit possible repeat error from 2(c)(iii) for M3)(If the wrong haloalkane is used OR but-1-ene is produced, award MAX. 2 marks for the mechanism)(If E1 mechanism is used, give full credit in which M1 and M2 are for correct curly arrows on the correct carbocation)

(c) (i) (structural) isomers/hydrocarbons/compounds/they have the samemolecular formula, but different structural formulas/different structures; 1

(penalise statements which are not expressed in good English and which do not refer clearly to structural isomers i.e. plural)(penalise statements which refer to “different (spatial) arrangements”)(credit” different displayed formulas”)(Q of L mark)

(ii) Correct structure for but-1-ene;1

[7]

Q17.A

[1]

Q18.(a) M1 electrophilic addition

For M1, both words requiredAccept phonetic spelling

For the mechanismM2 Ignore partial negative charge on the double bond

M2 must show an arrow from the double bond towards the H atom of the H–Br molecule

M3 Penalise partial charges on H–Br bond if wrong way and penalise formal charges

M3 must show the breaking of the H–Br bondPenalise once only in any part of the mechanism for a line and two dots to show a bond

M5 must show an arrow from the lone pair of electrons on the negatively charged bromide ion towards the correct (positively charged) carbon atom

Maximum any 3 of 4 marks for the mechanism for wrong (organic) reactant OR wrong organic product (if shown) OR primary carbocationAccept the correct use of sticks

NB These are double-headed arrows5

(b) M1 Nucleophilic substitutionFor M1, both words required

Accept phonetic spelling

For the mechanismPenalise M2 if NH3 is negatively charged

M2 must show an arrow from the lone pair of electrons on the nitrogen atom of an ammonia molecule to the correct C atom

Penalise M3 for formal charge on C of the C−Br or incorrect partial charges on C−BrPenalise M3 for an additional arrow from the Br to something else

M3 must show the movement of a pair of electrons from the C–Br bond to the Br atom. Mark M3 independently provided it is from their original molecule

The second mole of ammonia is not essential for M5; therefore ignore any species here

M4 is for the structure of the alkylammonium ion, which could be a condensed formula. A positive charge must be shown on / or close to, the N atom

Penalise once only for a line and two dots to show a bond

M5 is for an arrow from the N–H bond to the N atomMaximum any 3 of 4 marks for the mechanism for wrong organic reactant OR wrong organic product if shown

Award full marks for an SN1 mechanism in which M2 is the attack of the ammonia on the intermediate carbocation

Accept the correct use of “sticks”


(c) M1 (addition) polymerisation OR poly-additionIgnore “additional”Credit polyprop-1-ene and polypropylene

M2 poly(propene) / polypropenePenalise “condensation polymerisation”

2

(d)

Penalise M1 if covalent KOH

M1 must show an arrow from the lone pair on the oxygen of a negatively charged hydroxide ion to a correct H atom

Penalise M3 for formal charge on C of C–Br or incorrect partial charges on C−Br.

M2 must show an arrow from a correct C–H bond adjacent to the C–Br bond to the appropriate C–C bond. Only award if an arrow is shown attacking the H atom of a correct C−H bond in M1

Ignore other partial chargesPenalise once only in any part of the mechanism for a line and two dots to show a bond

M3 is independent provided it is from their original molecule, but CE=0 if nucleophilic substitution

Maximum any 2 of 3 marks for wrong organic reactant

Award full marks for an E1 mechanism in which M3 is on the correct carbocation.

Accept the correct use of “sticks” for the molecule except for the C–H being attacked


[15]

Q19.(a) (i) Electron pair donor

OR

Species which uses a pair of electrons to form aco-ordinate/covalent bond.

Credit “lone pair” as alternative wordingCredit “electron pair donator”

1

(ii) Replacement of the halogen (atom) (by the nucleophile)

OR

The carbon-halogen bond/C-X breaks and a bond formswith the nucleophile or between the carbon and the nucleophile

They must describe the idea of substitution in a haloalkane.Accept the idea that a nucleophile replaces the halogen which becomes a halide ionPenalise reference to “halogen molecule” and penalise the

idea that the haloalkane contains a halide1

(iii) Splitting molecules using/by water

OR

breaking/splitting/dissociating (C¡VX) bond(s)/using/by waterNOT simply the reaction with water or simply the addition of water.Ignore “compound”

1

(iv) (Heat) energy/enthalpy required/needed/absorbed (at constantpressure) to break/split it/the (carbon-halogen) bond

OR

(Heat) energy/enthalpy required/needed/absorbed (at constantpressure) for homolysis of the (C–X/the carbon-halogen) bond

Ignore bond formationIgnore “average”

1

(b)

M1 must show an arrow from the lone pair of electrons on theoxygen atom of the negatively charged hydroxide ion to thecentral C atom.

M2 must show the movement of a pair of electrons from theC-Br bond to the Br atom. Mark M2 independently.

Award full marks for an SN1 mechanism in which M1 is the attack of thehydroxide ion on the intermediate carbocation.

Penalise M1 if covalent KOH is usedPenalise M2 for formal charge on C or incorrect partial chargesPenalise once only for a line and two dots to show a bond.Max 1 mark for the wrong reactantAccept the correct use of “sticks”

2

(c) (i)

M1 must show an arrow from the lone pair on oxygen of anegatively charged hydroxide ion to the correct H atom

M2 must show an arrow from the correct C-H bond to the C-Cbond and should only be awarded if an attempt has beenmade at M1

M3 is independent provided it is from the original molecule


Penalise M1 if covalent KOHPenalise M3 for formal charge on C or incorrect partial chargesPenalise once only for a line and two dots to show a bond.Max 2 marks for wrong reactantAccept the correct use of “sticks” for the molecule except for the C-H being attacked

3

(ii) M1 Stated that the spectrum has an absorption/absorbance/peak in the range 1620 cm –1 to 1680 (cm–1) or specifiedcorrectly in this range from the spectrum

M2 depends on correct range or wavenumber being specified

M2 (Infrared absorption) due to C=C OR carbon-carbon double bondQoL for correct M1 statement which includes both the word absorption (or alternative) and the correct range or wavenumberAllow “peak” OR “dip” OR “spike” OR “trough”OR “low transmittance” as alternatives for absorption.For M2 it is not sufficient simply to state that an alkene has C=CM2 could be on the spectrumIgnore reference to other absorptions

2[11]

Q20.(a) (i) M1 Elimination

M1 Credit “base elimination” but no other prefix.

Penalise M2 if covalent KOHPenalise M4 for formal charge on C or Br of C−Br or incorrect partial charges on C−Br


Ignore other partial charges

M3 must show an arrow from a correct C−H bond adjacent to the C−Br bond to a correct C-C bond. Only award if an arrow is shown attacking the H atom of a correct adjacent C−H bond in M2

Penalise once only in any part of the mechanism for a line and two dots to show a bond

M4 is independent provided it is from their original molecule, BUT CE=0 for the mechanism (penalise M2, M3 and M4 only) if nucleophilic substitution mechanism is shown

Maximum any 2 of 3 marks for the mechanism for wrong organic reactant or wrong organic product (if shown).Credit the correct use of “sticks” for the molecule except for the C−H being attacked

Award full marks for an E1 mechanism in which M4 is on the correct carbocation

Penalise M4, if an additional arrow is drawn from Br eg to K+


(ii) Displayed formula for 3-methylbut-1-ene

All bonds and atoms must be drawn out, but ignore bond angles

1

(iii) Position(al) (isomerism or isomer)Penalise any other words that are written in addition to these.

1

(b) (i) Displayed formula for 3-methylbutan-2-ol

All bonds and atoms must be drawn out, but ignore bond angles.

1

(ii) Any one from

• Lower / decreased temperature OR cold

• Less concentrated (comparative) OR dilute KOH

• Water (as a solvent) / (aqueous conditions)Ignore “pressure”.

1

(iii) Nucleophilic substitutionBoth words needed - credit phonetic spelling.

1

(iv) (Strong / broad) absorption / peak in the range 3230 to 3550 cm−1 or specified value in this range or marked correctly on spectrum

Allow the words “dip” OR “spike” OR “trough” OR “low transmittance” as alternatives for absorption.

1[10]

Q21.(a) (i)

Penalise one mark from their total if half-headed arrows are usedPenalise M3 for formal charge on C of the C-Br or incorrect partial charges on C-BrIgnore other partial charges

M1 must show an arrow from the lone pair on oxygen of a negatively charged hydroxide ion to the correct H atom

Penalise once only in any part of the mechanism for a line and two dots to show a bond.

M2 must show an arrow from the correct C–H bond to the correct C–C bond.Only award if an arrow is shown attacking the H atom of the correct C–H

bond in M1

M3 is independent but CE=0 if nucleophilic substitution

N.B these are double-headed arrows3

(ii)

Award 1 mark if both correct stereoisomers but in the wrong placesAccept no other alkenes.Be reasonably lenient on the bonds to ethyl (or to CH2CH3) since the question is about E and Z positions but penalise once only if connection is clearly to the CH3 of CH2CH3

Accept linear structures2

(iii) M1 (Compounds / molecules with) the same structural formulaPenalise M1 if “same structure”

M2 with atoms/bonds/groups arranged differently in spaceIgnore references to “same molecular formula” or “same empirical formula” or any reference to “displayed formula”

ORatoms/bonds/groups that have different spatial arrangements / different orientation.Mark independently

2

(b)

M1must show an arrow from the double bond towards the H atom of the H – O bond

OR HO on a compound with molecular formula for H2SO4

M1 could be to an H+ ion and M2 an independent O – H bond break on a compound with molecular formula for H2SO4

M1 Ignore partial negative charge on the double bond.

M2 must show the breaking of the O – H bond.M2 Penalise partial charges on O – H bond if wrong way and penalise formal chargesIn M2 do not penalise incorrect structures for H2SO4

M3 is for the structure of the carbocation.

M4 must show an arrow from the lone pair of electrons on the correct oxygen of the negatively charged ion towards a correct (positively charged) carbon atom.

M4 NOT HSO4–

For M4, credit as shown or –: OSO 3H ONLY with the negative charge anywhere on this ionOR correctly drawn out with the negative charge placed correctly on oxygenPenalise once only in any part of the mechanism for a line and two dots to show a bond

NB The arrows here are double-headedMax 3 of any 4 marks for wrong organic reactant or wrong organic product (if shown)Accept the correct use of “sticks”

4[11]

Q22.(a) P 3,3−dimethylbut−1−ene

ORaccept 3,3−dimethylbutene

Ignore absence of commas, hyphens and gapsRequire correct spelling

Q 3−chloro−2,2−dimethylbutaneOR

accept 2−chloro−3,3−dimethylbutaneIn Q, “chloro” must come before “dimethyl”

2

(b) M1 Electrophilic addition

M2 must show an arrow from the double bond towards the H atom of HClM3 must show the breaking of the H−Cl bondM4 is for the structure of the carbocationM5 must show an arrow from the lone pair of electrons on the negatively charged chloride ion towards the positively charged carbon atom on their carbocation.

NB The arrows here are double−headedM1 both words requiredFor the mechanismM3 Penalise incorrect partial charge on H−Cl bond and penalise formal chargesIgnore partial negative charge on the double bond.

Maximum 3 of 4 marks for a correct mechanism using HBr or the wrong organic reactant or wrong organic product (if shown) or a primary carbocationPenalise once only in any part of the mechanism for a line and two dots to show a bondCredit the correct use of “sticks”For M5, credit attack on a partially positively charged carbocation structure, but penalise M4

5

(c) M1 Nucleophilic substitutionFor M1, both words required.Accept phonetic spelling

M2 must show an arrow from the lone pair of electrons on the nitrogen atom of an ammonia molecule to the correct C atomM3 must show the movement of a pair of electrons from the C− Cl bond to the Cl atom. Mark M3 independently provided it is from their original moleculeM4 is for the structure of the alkylammonium ion, which could be a condensed formula. A positive charge must be shown on, or close to, the N atom.M5 is for an arrow from the N−H bond to the N atomAward full marks for an SN1 mechanism in which M2 is the attack of the ammonia on the intermediate carbocation

NB These are double-headed arrowsFor the mechanismPenalise M2 if NH3 is negatively charged.Penalise M3 for formal charge on C of the C−Cl or incorrect partial charges on C−ClPenalise M3 for an additional arrow from the Cl to something elseThe second mole of ammonia is not essential for M5; therefore ignore any species herePenalise once only for a line and two dots to show a bondMaximum 3 of 4 marks for the mechanism for wrong organic reactant OR wrong organic product if shownAccept the correct use of “sticks”

5

(d) M1 (base) eliminationM1 Dehydrohalogenation

M2 KOH OR NaOHM3 Must be consequential on a correct reagent in M2, but if incomplete or inaccurate attempt at reagent (e.g. hydroxide ion), penalise M2 only and mark on

Any one from• high temperature OR hot OR heat / boil under reflux• concentrated• alcohol / ethanol (as a solvent) / (ethanolic conditions)

M3 not “reflux” aloneM3 if a temperature is stated it must be in the range 78C to 200 °CIgnore “pressure”

3

(e) M13NaBr + H3PO4 3HBr + Na3PO4

M1 Credit correct ionic species in the equation

M2 and M3SO2 and Br2 identifiedM4Concentrated sulfuric acid• is an oxidising agent• oxidises the bromide (ion) or Br − or NaBr or HBr • is an electron acceptor

In M2 and M3 the two gases need to be identified. If equations are used using sulfuric acid and the toxic gases are not identified clearly, allow one mark for the formulas of SO2 and Br2

• apply the list principle as appropriate but ignore any reference to HBr• the marks are for identifying the two gases either by name or formula

4[19]

Q23.B

[1]

Q24.(a) (i) M1 Elimination

M2 must show an arrow from the lone pair on the oxygenof a negatively charged hydroxide ion to a correct H atom

M3 must show an arrow from a C-H bond adjacent tothe C-Br bond towards the appropriate C-C bond.Only award if a reasonable attempt has been madeat the attack on the H atom of the appropriate adjacent C-H

M4 is independent provided it is from their original molecule

Award full marks for an E1 mechanism in which M3 is on the correct carbocation.

N.B. These are double-headed arrowsFor M1, accept “Base elimination” but no other prefix.Penalise M2 if covalent KOHPenalise M4 for formal charge on C of C-Br or incorrect partial charges on C-BrIgnore other partial chargesPenalise once only in any part of the mechanism for a line and two dots to show a bond.Max any 2 of 3 marks for the mechanism for wrong reactant (or wrong product if shown).Accept the correct use of “sticks” for the molecule except for the C-H being attacked

4

(ii) Structure for pent-1-ene

CH3CH2CH2CH=CH2

Penalise C3H7

Accept correct “sticks”1

(b) M1 Electrophilic addition

M2 must show an arrow from the double bond towards the Br atom of the Br-Br molecule

M3 must show the breaking of the Br-Br bond.

M4 is for the structure of the tertiary carbocation with Br on the correct carbon atom.

M5 must show an arrow from the lone pair of electrons on the negatively charged bromide ion towards the positively charged carbon atom.

N.B. These are double-headed arrowsFor M1, both words required.For the mechanismM2 Ignore partial negative charge on the double bond.M3 Penalise partial charges on Br-Br bond if wrong way and penalise formal chargesPenalise once only in any part of the mechanism for a line and two dots to show a bondMax any 3 of 4 marks for the mechanism for

wrong organic reactant or wrong organic product (if shown) or primary carbocation.If HBr is used, max 2 marks for their mechanismAccept the correct use of “sticks”

5

(c) M1 Nucleophilic substitution

M2 must show an arrow from the lone pair of electrons on the nitrogenatom of an ammonia molecule to the C atom.

M3 must show the movement of a pair of electrons from the C-Br bondto the Br atom. M3 is independent provided it is from their original molecule

M4 is for the structure of the alkylammonium ion, which could be acondensed formula. A positive charge must be shown on/or close to,the N atom.

M5 is for an arrow from the N-H bond to the N atom.

Award full marks for an SN1 mechanism in which M2 is the attackof the ammonia on the intermediate carbocation.

N.B. These are double-headed arrowsFor M1, both words required.Penalise M2 if NH3 is negatively charged.Penalise M3 for formal charge on C or incorrect partial chargesThe second mole of ammonia is not essential for M5; therefore ignore any species here.Penalise once only for a line and two dots to show a bond.Max any 3 of 4 marks for the mechanism for wrong organic reactant (or wrong organic product if shown)Accept the correct use of “sticks”

5[15]

Q25.(a) (Compounds with the) same molecular formula but different structural / displayed /

skeletal formula1

(b) (basic) elimination1

Mechanism points:

Correct arrow from lone pair on :OH– to H on C adjacent to C–Br1

Correct arrow from C–H bond to C–C1

Correct arrow from C–Br bond to Br1

Structure of chosen product1

OR

[6]

Q26.(a) NaOH/KOH

IGNORE OH-NOT M1 if any mention of acidified/H+ in reagents or conditions

1

Reaction 1 = ethanolic/alcoholic AND reaction 2 = aqueousIGNORE tempNOT ethanoic

1

rxn 1 = base/proton acceptor1

rxn 2 = nucleophile/lone pair donor/electron pair donor1

(Base) EliminationNOT nucleophilic

1


M7 must show an arrow from a correct C–H bond on C adjacent to the C of the C–Br bond to a correct C–C bond. Only award if an arrow is shown attacking the H atom of a correct adjacent C–H bond in M6

M8 is independent provided it is from their original molecule and shows curly arrow from C-Br to Br

ALLOW correct E1 mechanismIGNORE incorrect inorganic productsIf forming pent-2-ene can award M8 only even if arrows in mechanism correctIf C chain length or halogen wrong in reactant or product max 2/3

111

(b) 1

1

1

If no M2 and M3 ALLOW 1 mark if both structures OR both names correctNOT cis and trans

No free rotation around C=CALLOW no rotation of C=C

1

2 different atoms/groups on each of the C=C Cs owtteIGNORE ‘functional’

1

(c) Same volume/amount of AgNO3(aq) added to same volume/amount/no. of drops of haloalkane (in beaker/flask) in each experiment

Both volume references neededIGNORE inappropriate volumes

1

Same temp OR same [AgNO3] each time1

record time to measure sensible observation about the amount of AgCl ppt

e.g. first appearance of ppt / ppt obscures mark / reading on a colorimeterIGNORE colour of pptALLOW silver mirrorNOT reference to same time if describing method based on timing how long (for ppt to form)ALLOW gravimetric method based on same time for each experiment

1

Rate = amount/time OR proportional to 1/time OR reference to shorter time = higher rate/longer time = lower rate

ALLOW greater mass = higher rate if gravimetric method1

[17]

gcseprep.com€¦ · web viewpenalise m2 if polarity included incorrectly. no bond between h and...

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