(1) · 2020. 7. 21. · dna sct page 6 of 21 (4) (d) butan-1-ol reacts with acidified potassium...

DNA SCT Page 1 of 21

Q1. Use the Data Booklet to help you answer this question about DNA. The figure below shows a fragment of a DNA double helix. The letters A, C, G and T represent the four bases in one strand. The numbers 1, 2, 3, 4 and 5 represent the bases in the complementary strand.

(a) Complete Table 4 to show the correct sequence of bases in the complementary strand represented by the numbers 1 to 5

1 2 3 4 5

(1)

(b) Deduce the total number of hydrogen bonds formed between the five bases in each strand.

Tick (✔) one box.

10 12 13 15

(1)

(c) Base A is part of a nucleotide in the DNA strand shown in the figure above.

A nucleotide contains a 2-deoxyribose molecule.

An incomplete 2-deoxyribose molecule is shown.

Complete the structure to show the nucleotide that contains base A.

You should represent base A by the letter A.

(2)

(Total 4 marks)


Q2. Use the Data Booklet to help you answer these questions.

DNA exists as two strands of nucleotides in the form of a double helix with hydrogen bonding between the two strands.

(a) A deoxyribose molecule in a strand of DNA is shown.

Name the types of group attached to 2-deoxyribose at positions X and Y.

X _________________________________________________________________

Y _________________________________________________________________

(2)

(b) In the DNA double helix, adenine is linked by hydrogen bonds to a molecule in the other strand of DNA.

Complete the diagram below to show the other molecule and the hydrogen bonds between it and adenine.

(2)

(Total 4 marks)


Q3. This question is about the three amines, E, F and G.

(a) Amines E, F and G are weak bases.

Explain the difference in base strength of the three amines and give the order of increasing base strength.

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(6)

(b) Amine F can be prepared in a three-step synthesis starting from methylbenzene.

Suggest the structures of the two intermediate compounds.

For each step, give reagents and conditions only. Equations and mechanisms are not required.

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(5)

(Total 11 marks)

Q4. There are four isomeric alcohols with the molecular formula C4H10O

(a) Two of these are butan-l-ol (CH3CH2CH2CH2OH) and butan-2-ol. The other two isomers are alcohol X and alcohol Y.

Draw the displayed formula for butan-2-ol.

Alcohol X does not react with acidified potassium dichromate(VI) solution. Give the structure of alcohol X.

Name the fourth isomer, alcohol Y.

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(3)

(b) The infrared spectrum of one of these isomeric alcohols is given below.


Identify one feature of the infrared spectrum which supports the fact that this is an alcohol. You may find it helpful to refer to Table 1 on the Data Sheet.

Explain how infrared spectroscopy can be used to identify this isomeric alcohol.

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(3)

(c) British scientists have used bacteria to ferment glucose and produce the biofuel butan-1-ol.

Write an equation for the fermentation of glucose (C6H12O6) to form butan-1-ol, carbon dioxide and water only.

State one condition necessary to ensure the complete combustion of a fuel in air.

Write an equation for the complete combustion of butan-1-ol and state why it can be described as a biofuel.

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(4)

(d) Butan-1-ol reacts with acidified potassium dichromate(VI) solution to produce two organic compounds.

State the class of alcohols to which butan-1-ol belongs.

Draw the displayed formula for both of the organic products.

State the type of reaction that occurs and the change in colour of the potassium dichromate(VI) solution.

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(5)

(Total 15 marks)

Q5. Halogenoalkanes are useful compounds in synthesis. A reaction pathway is shown.

(a) Give the IUPAC name for CH2(OH)CH(CH3)CH2Br

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(1)

(b) Reaction 1 occurs via a nucleophilic substitution mechanism.

Explain why the halogenoalkane is attacked by the nucleophile in this reaction.

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(3)

(c) The infrared spectrum of Compound Y shows a significant absorption in the range 1680–1750 cm–1

Draw the displayed formula of Compound Y.

(d) Compound Z has the empirical formula C3H4NO

Give the structure of Compound Z.

Suggest the reagent for Reaction 3.

Structure


Reagent for Reaction 3 _________________________________________

(2)

(Total 7 marks)

Q6. The alkene 3-methylpent-2-ene (CH3CH=C(CH3)CH2CH3) reacts with hydrogen bromide to form a mixture of 3-bromo-3-methylpentane and 2-bromo-3-methylpentane.

(a) The alkene 3-methylpent-2-ene (CH3CH=C(CH3)CH2CH3) exists as E and Z stereoisomers.

Draw the structure of Z-3-methylpent-2-ene.

(1)

(b) Name and outline the mechanism for the formation of 3-bromo-3-methylpentane from this reaction of 3-methylpent-2-ene with hydrogen bromide.

Explain why more 3-bromo-3-methylpentane is formed in this reaction than 2-bromo-3-methylpentane.


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(7)

(Total 8 marks)

Q7. (a) The reaction between aqueous persulphate ions, , and iodide ions, I–(aq), is catalysed by Fe2+(aq) ions. Suggest why this reaction has a high activation energy. Write equations to explain the catalytic action of Fe2+(aq) ions. Suggest why V3+(aq) ions will also act as a catalyst for this reaction but Mg2+(aq) ions will not.

(6)

(b) Outline a mechanism for the reaction between benzene and ethanoyl chloride and explain why AlCl3 acts as a Lewis acid catalyst for this reaction. Predict, with an explanation in each case, the suitability of FeCl3 and of NH4Cl to act as a catalyst for this reaction.

(9)

(Total 15 marks)


Q8. (a) Outline a mechanism for the reaction of CH3CH2CH2CHO with HCN and name the product.

Mechanism

Name of product ____________________________________________________

(5)

(b) Outline a mechanism for the reaction of CH3OH with CH3CH2COCl and name the organic product.


Mechanism

Name of organic product _______________________________________________

(5)

(c) An equation for the formation of phenylethanone is shown below. In this reaction a reactive intermediate is formed from ethanoyl chloride. This intermediate then reacts with benzene.

(i) Give the formula of the reactive intermediate.

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(ii) Outline a mechanism for the reaction of this intermediate with benzene to form phenylethanone.

(4)

(Total 14 marks)


Mark schemes

Q1. (a) 1 2 3 4 5

T G C A G 1

(b) 13 1

(c)

1 for completed 2-deoxyribose plus A

Allow either OH or trailing bonds

Don’t penalise ‘sticks’ in 2-deoxyribose. 1

1 for correct phosphate joined to CH2

If two phosphates shown CE=0

If CH2 missing award 1 if no further errors

If phosphate attached to oxygen on C3 award 1 if no further errors

1

[4]

Q2. (a) X – base

1

Y – phosphate (group) 1

Ignore organic

Any mention of sugar in either loses that mark

(b) If not Thymine CE=0


Correct structure scores 2, penalise by 1 each error in • structure of thymine • orientation of thymine • hydrogen bonding

Ignore lp on N and O

Don’t penalise non-linear H bonds

on RHS of thymine – allow with or without H or – [DNA strand]

2

[4]

Q3. (a) (Strength depends on availability of) lone pair on N (atom)

M1

E N (next to ring): (lp) delocalised into ring M2

(lp) less available (to donate to or to accept a H+) M3

F or G: N (next to alkyl): (positive) inductive effect/electrons pushed to N M4

(lp) more available (to donate to or to accept a H+) M5

order of increasing base strength E


Step 1 Cl2 & UV

Allow Br2 & UV

Step 2 KCN alcoholic & aq (both reqd)

Ignore temperature

Step 3 H2 / Ni or Pt or Pd

Allow LiAlH4 in (dry) ether – (with acid CE, followed by acid allow)

Not NaBH4 and not Sn/HCl or Fe/HCl 2

[11]

Q4. (a) M1

Displayed formula for butan-2-ol

M1 displayed formula must have all bonds drawn out, including the O―H but ignore angles

Penalise “sticks”

M2 Alcohol X is

M2 structure must be clearly identifiable as 2-methylpropan-2-ol and may be drawn in a variety of ways.

M3 Alcohol Y is named (2)-methylpropan-1-ol ONLY

M3 must be correct name, but ignore structures 3

(b) M1 The infrared spectrum shows an absorption/peak in the range 3230 to 3550 (cm–1)(which supports the idea that an alcohol is present)

In M1, allow the words “dip”, “spike”, “low transmittance” and “trough” as alternatives for absorption.

M2 Reference to the ‘fingerprint region’ or below 1500 (cm–1)

M3 Match with or same as known sample/database spectra

Check the spectrum to see if alcohol OH is labelled and


credit.

OR

M2 Run infrared spectra (of the alcohols)

M3 Find which one matches or is the same as this spectrum. 3

(c) M1 balanced equation C6H12O6 → CH3CH2CH2CH2OH + 2CO2 + H2O or C4H9OH

Or multiples for M1 and M3 In M1 and M3 penalise use of C4H10O or butan-2-ol once only

M2 Any one from

• excess/adequate/sufficient/correct amount of/enough/plenty/ a good supply of oxygen or air

• good mixing of the fuel and air/oxygen

For M2, do not accept simply “oxygen” or “air” alone Ignore reference to “temperature”

M3 CH3CH2CH2CH2OH + 6O2 → 4CO2 + 5H2O or C4H9OH

M4 A biofuel is a fuel produced from (renewable) biological (re)source(s)

OR

(renewable) (re)source(s) from (a specified) plant(s)/fruit(s)/tree(s)

In M4 Ignore references to “carbon neutral” Ignore “sugar” and “glucose”

4

(d) M1 butan-1-ol is a primary or 1° (alcohol)

M2 Displayed formula (ONLY) for butanal CH3CH2CH2CHO

M3 Displayed formula (ONLY) for butanoic acid CH3CH2CH2COOH

M2 and M3 displayed formula must have all bonds drawn out including the O―H but ignore angles.

If butanal and butanoic acid formulae are both correctly given but not displayed, credit one mark out of two.

M4 Oxidation (oxidised) OR Redox

M5 orange to green

Both colours required for M5 Ignore states

5

[15]


Q5. (a) 3-bromo-(2)-methylpropan-1-ol ONLY

3 and 1 are essential, 2 may be omitted, but any other number here is wrong

Ignore hyphens and commas 1

(b) Bromine is more electronegative than carbon

Allow difference in electronegativity if polarity of bond shown M1

C is partially positive / electron deficient

M2 and M3 can be awarded from diagram that shows nucleophilic attack

M2

Lone/electron pair (on the nucleophile) donated to the partially positive carbon

Allow lone pair attracted to / attacks the partially positive carbon

M3

(c)

Must be displayed with all bonds shown 1

(d)

Not need be displayed

See General Marking instructions section 3.12 for penalties for incorrectly drawn bonds such as C–HO or C–NC etc.

1

KCN & (dil) acid

Allow

Allow HCN

Ignore alcoholic solvents

Penalise conc. HCl, H2SO4 or any HNO3 1

[7]


Q6.

(a)

Must show all 4 groups bonded to C=C

Allow CH3− for methyl group; allow C2H5 for ethyl group

Allow correct structure of the style

Allow correct skeletal structure

1

(b) M1 electrophilic addition

NB the arrows here are double-headed 1

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

1

M3 must show the breaking of the H-Br bond 1

M4 is for the structure of the tertiary carbocation 1

M5 must show an arrow from the lone pair of electrons on the negatively charged bromide ion towards the positively charged atom (of either a secondary or) of a tertiary carbocation

1

M6 3-bromo-3-methylpentane is formed from 3y carbocation OR


2-bromo-3-methylpentane is formed from 2y carbocation 1

M7 3y carbocation more stable than 2y

1

M2-M5 Penalise one mark from their total if half-headed arrows are used

M2 Ignore partial negative charge on the double bond

M3 Penalise incorrect partial charges on H-Br bond and penalise formal charges

Penalise M4 if there is a bond drawn to the positive charge

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

Max 3 of any 4 marks (M2-5) for wrong organic reactant or wrong organic product (if shown) or secondary carbocation

Max 2 of any 4 marks in the mechanism for use of bromine

Do not penalise the “correct” use of “sticks”

For M5, credit attack on a partially positively charged carbocation structure but penalise M4

M6 is high demand and must refer to product being formed from/via correct class of carbocation

M7 is high demand and must be clear answer refers to stability of carbocations (intermediates) not products

Candidate that states that products are carbocations would lose M6 and M7

M6,7 allow carbonium ion in place of carbocation; or a description of carbocation in terms of alkyl groups/ number of carbon atoms joined to a positive C

When asked to outline a mechanism, candidates are expected to draw a mechanism with curly arrows (specification 3.3.1.2). On this occasion only we would allow a detailed description as shown.

M2 must describe the movement of a pair of electrons / curly arrow from the C=C towards the H atom of the H-Br molecule

M3 must describe the breaking of the H-Br bond with the bonding pair of electrons moving to the Br / curly arrow from H-Br bond to Br

M4 is for the structure of the tertiary carbocation (i.e. positive C bonded to one methyl and two ethyl groups)

M5 must describe the movement of a pair of electrons from the Br− ion to the positive C atom of the carbocation / curly arrow from the lone pair of electrons on the negatively charged bromide ion towards the positively charged C atom (of either a secondary or) of a tertiary carbocation

[8]

Q7. (a) High Ea: S2O82– repels I– or both ions negative (1)


2Fe2+ + S2O82– → 2Fe3+ + 2SO42– (1) 2Fe3+ + 2I– → 2Fe2+ + I2 (1)

N.B. Ignore additional incorrect equations

Vanadium is a transition element or Magnesium is not a transition element (1)

Vanadium has variable oxidation states (1)

Magnesium only forms Mg2+, or has only one oxidation state (1)

N.B. Score two marks for “Only vanadium has variable oxidation states”

6

(b) AlCl3 + Cl-COCH3 → AlCl4– + CH3CO+ (1)

H+ + AlCl4– → AlCl3 + HCl (1) Lewis acid: AlCl3 accepts electron pair

N.B. penalise incorrect acyl chloride by one N.B. penalise chloroethane by two marks i.e. first equation mark, attack on benzene mark

NH4Cl: Not a catalyst (1)

FeCl3: A catalyst (1) has a low energy vacant shell or has spaces or vacancies in d shell or has a partially filled d shell or able to accept an electron pair or can form FeCl4– (1)

9

[15]

Q8. (a) Mechanism


Allow C3H7 if structure shown elsewhere

penalise HCN splitting if wrong

Name of product: 2-hydroxypenta(neo)nitrile (1)

or 1-cyanobutan-1-ol 5

(b) Mechanism

Name of organic product: methylpropanoate (1) 5

(c) (i) ([) CH3CO (])+ (1)

(ii)

4

Notes

(abc) extra curly arrows are penalised

(a) be lenient on position of negative sign on : CN– but arrow must come from lp

(a)/(b) alone loses M2 but can score M1 for attack on C+, similarly

(a) allow 2-hydroxypentanonitrile or 2-hydroxypenta(ne)nitrile ... pentylnitrile

(b) in M4, allow extra: Cl– attack on H, showing loss of H+

(c) (i) allow formula in an “equation”(balanced or not) be lenient on the position of the + on the formula

(ii) for M1 the arrow must go to the C or the + on the C don’t be too harsh about the horseshoe, but + must not be close to the saturated C M3 must be final step not earlier; allow M3 even if structure (M2) is wrong


[14]

Organic points

(1) Curly arrows: must show movement of a pair of electrons, i.e. from bond to atom or from lp to atom / space e.g.

(2) Structures

penalise sticks (i.e. ) once per paper

Penalise once per paper

allow CH3– or –CH3 or or CH3 or H3C–

(1) · 2020. 7. 21. · dna sct page 6 of 21 (4) (d) butan-1-ol reacts with acidified potassium...

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