hydrocarbons as & a level

Hydrocarbons AS & A Level

Model Answers 1

Time allowed: 86 minutes

Score: /64

Percentage: /100

Grade Boundaries:

Level A Level

Subject Chemistry

Exam Board OCR

Module Core Organic Chemistry

Topic Hydrocarbons

Paper AS & A Level

Booklet Model Answers 1

A* A B C D E

>85% 73% 60% 47% 34% 21%

1Dr. Asher Rana www.chemistryonlinetuition.com [email protected]

http://www.savemyexams.co.uk/

This question is about different alkanes present in crude oil.

(a) Compound A, shown below, is one of the structural isomers of C7H16.

Compound A

(i) What is meant by the term structural isomers? [1]

(ii) Name compound A. [1]

Structural isomers are compounds or molecules having

the same formula but different structural formulae

To name the hydrocarbon compound, we first analyse the primary C chain,

which is observed to have 4 carbon atoms (butane)

Next, it is observed that there are 3 methyl group branches (trimethyl) on

the 2nd and 3rd C atom

Hence, compound A would be called, 2,2,3-trimethylbutane (branches are

stated first and always aim for the lowest numbers, 2,2,3 rather than 3,3,2

for example).

[1]

b) The structural isomers of C5H12 have different boilingpoints.

Draw the skeletal formula of the structural isomer of C5H12 with the highest boilingpoint.

Skeletal formulae do not show atoms. C5H12 is a 5-carbon atom linear chain

2

Question 1

Dr. Asher Rana www.chemistryonlinetuition.com [email protected]


3

(c) A molecule of an alkane has 24 carbon atoms.

State the empirical formulae of this alkane. [1]

General formulae of alkanes are CnH2n+1

Hence, the empirical formulae (the lowest whole-number ratio of atoms of each

element) of the alkane C24H50, is C12H25

(d) Alkanes are used as fuels.

(i) Construct an equation for the complete combustion of octane C8H18. [1]

(ii) Combustion of 36.48 g of octane produced 2.50 mol of carbondioxide.

Show that this combustion was incomplete.[2]

Complete combustion processes of hydrocarbons produce carbon dioxide and

water as their only products. General formula for complete combustion is,

𝐶𝐶𝑥𝑥𝐻𝐻𝑦𝑦 + 𝑥𝑥 +𝑦𝑦4𝑂𝑂2 → 𝑥𝑥𝐶𝐶𝑂𝑂2 +

𝑦𝑦2𝐻𝐻2𝑂𝑂

For octane, x=8 and y=18. Thus,

𝐶𝐶8𝐻𝐻18 +252𝑂𝑂2 → 8 𝐶𝐶𝑂𝑂2 + 9 𝐻𝐻2𝑂𝑂

For the complete combustion,

𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑜𝑜𝑜𝑜 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑚𝑚 𝑜𝑜𝑜𝑜 𝐶𝐶8𝐻𝐻18 𝑁𝑁𝑁𝑁𝑁𝑁𝑏𝑏𝑁𝑁𝑏𝑏 =36.48 𝑔𝑔12𝑥𝑥𝑥 + 18

= 0. 32 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑚𝑚

Based on stoichiometry,

𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 𝑜𝑜𝑜𝑜 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑚𝑚 𝑜𝑜𝑜𝑜 𝐶𝐶𝑂𝑂2 𝑝𝑝𝑁𝑁𝑜𝑜𝑏𝑏𝑁𝑁𝑝𝑝𝑁𝑁𝑏𝑏 = 0.32 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑚𝑚 × 8 = 2.56 𝑁𝑁𝑜𝑜𝑚𝑚𝑁𝑁𝑚𝑚

Since the number of moles of CO2 produced was only 2.50 moles, (less than 2.56), the

combustion is proven to be incomplete.



(ii) Write an equation for the preparation of ethene from an alkane. [1]

(e) Alkanes in crude oil can be used to manufacture ethene. Two stages arerequired.

(i) Name the two stages. [1]

Fractional distillation is used to separate the different hydrocarbon molecules

in crude oil, based on chain length.

To manufacture ethene from long-chained alkane molecules, cracking is used

to break them into shorter chain lengths.

[Total 9 Marks]

Cracking of a long chain alkane produces an alkene and a shorter alkane

Hence, any starting alkane possessing more than 4 carbon atoms is accepted

An example would be heptane, C10H22

𝐶𝐶10𝐻𝐻22 → 𝐶𝐶8𝐻𝐻18 + 𝐶𝐶2𝐻𝐻4

Check for conservation of mass: do all the C and H atoms add up in the equation?



Allyl bromide, CH2=CHCH2Br, is used in the production of polymers.

(a) Part of the C=C double bond in allyl bromide is called a π-bond.

Draw a labelled diagram to show the formation of the π-bond.

(b) Allyl bromide is a member of a homologous series. Compounds in this series have the samegeneral formula.

(i) What is meant by the term homologous series? [2]

(ii) What is the general formula of the homologous series that has allyl bromide as amember? [1]

(iii) Give the systematic name for allyl bromide. [1]

[2]

Pi bonds are sideways overlapping atomic (p) orbitals which lie above and below the plane

of a molecule. A pi bond, in addition to a sigma bond (formed from overlapping of

s orbitals), forms a double covalent bond.

Formation of the pi bond is shown below,

Homologous series:

• Series of compounds with the same functional group/chemical

properties/reactions

• With each successive member differing by CH2

Allyl bromide, given in the question: CH2=CHCH2Br or C3H5Br

General formula based on allyl bromide: CnH2n-1Br

The systematic name of allyl bromide would be 3-bromoprop-1-ene

This is because the bromine atom is on the 3rd C atom in the linear chain and

double bond between the 1st and 2nd C atom

5

Question 2



6

(c) Reaction mechanisms use curly arrows and can involve electrophiles and nucleophiles.

(i) What does a curly arrow represent in mechanisms? [1]

(ii) What is meant by the term nucleophile? [1]

A curly arrow in mechanism diagrams represents the movement of an electron

pair

Nucleophiles are chemical species that donates an electron pair to an

electrophile to form a chemical bond in a reaction. They have regions of high

electron density and are therefore attracted to partially positive regions on other

molecules.

(d) Allyl bromide, CH2=CHCH2Br, reacts with aqueous sodium hydroxide.

(i) Outline the mechanism of this reaction.

Include curly arrows, relevant dipoles and final product(s). [3]

OH- from NaOH acts as the nucleophile, donating its electron pair to the partially

positive carbon atom in allyl bromide (induced dipole between C-Br)

It can be seen that the curly arrow is from the electron pair to the carbocation

and this forces the electrons in the C-Br bond to the Br atom, forming Br-.

A primary alcohol is then produced with Br- being displaced by the OH-.



(e) Allyl bromide, CH2=CHCH2Br, reacts with bromine, Br2.

(i) Outline the mechanism of this reaction.

Include curly arrows, relevant dipoles and the structures of the intermediate and finalproduct(s). [4]

(ii) Name the type of mechanism. [1]

The reaction above is a nucleophilic substitution reaction

Reaction between allyl bromide and bromine starts with a curly arrow from the

double bond in allyl bromide (the electron dense area) to form a bond with the

delta positive Br (this partial charge has been induced due to the proximity of Br2 to

the negative charge in the double bond). This repels the electrons in the Br-Br bond

to form Br-

A carbocation intermediate with 3 single bonds is formed and subsequently

attacked by the leftover Br- electrophile. The curly arrow represents the movement

of the lone pair of electrons from the Br- ion



8

(ii) Name the type of mechanism. [1]

Forming a product of 1,2,3-tribromopropane

Reaction is an electrophilic addition

(f) Allyl bromide is reacted as shown below.

CH2=CHCH2Brstep 1 step 2

CH3CH2CH2Br

1-bromopropane

mixture of organic productsCl2

(i) State the reagents and conditions for step 1 [1]

Conversion of allyl bromide to 1-bromopropane is a hydrogenation reaction,

requiring a nickel catalyst and H2 gas

(ii) In step 2, 1-bromopropane reacts with chlorine by radical substitution.

Outline the mechanism for the monochlorination of 1-bromopropane.In your mechanism, you can show the formula of 1-bromopropane as C3H7Br.

Include the names of the three stages in this mechanism, state the essential conditionsand all termination steps. [5]

Radical substitution reaction proceeds from the formation of chlorine free radicals

Initiation step:

𝐶𝐶𝑚𝑚2𝑈𝑈.𝑉𝑉 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙

2 𝐶𝐶𝑚𝑚 ∙

Propagation step:

𝐶𝐶𝑚𝑚 ∙ + 𝐶𝐶3𝐻𝐻7𝐵𝐵𝑁𝑁 → 𝐻𝐻𝐶𝐶𝑚𝑚 + 𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁 ∙

𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁 ∙ +𝐶𝐶𝑚𝑚2 → 𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁𝐶𝐶𝑚𝑚 + 𝐶𝐶𝑚𝑚 ∙



(iii) Radical substitution produces a mixture of organic products.

Suggest two reasons why. [2]

Termination step:

𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁 ∙ +𝐶𝐶𝑚𝑚 ∙ → 𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁𝐶𝐶𝑚𝑚

2 𝐶𝐶3𝐻𝐻6𝐵𝐵𝑁𝑁 ∙ → 𝐶𝐶6𝐻𝐻12𝐵𝐵𝑁𝑁2

2 𝐶𝐶𝑚𝑚 ∙ → 𝐶𝐶𝑚𝑚2

[Total 25 Marks]

Radical substitution produces a mixture of organic products due to:

• further substitution seen in the termination steps.

• having more than one termination step

• different termination products

• substitutions can take place at different positions along the chain



This question is about cyclic organic compounds.

The table shows some information about cycloalkanes.

Cycloalkane Skeletal formula Boiling point/ °C

Cyclopropane –33

Cyclopentane 49

Cyclohexane 81

(a) These cycloalkanes are members of the same homologous series and have the same generalformula.

(i) What is meant by the term homologous series? [2]

(ii) State the general formula for these cycloalkanes. [1]

(iii) Explain the increase in boiling points of the cycloalkanes shown in the table. [2]

Homologous series are compounds with the same functional group with

similar chemical reactions, each successive member differing by CH2

Cycloalkanes have the same general formula as alkenes which is CnH2n

To check, add Cs and Hs to the skeletal formula given in the question and count

them up.

A general observation is that the more carbon atoms there is, the higher the boiling

point of the cycloalkane.

This is due to increasing contact surface area which leads to more van der Waals

forces. Hence, more energy is required to break the stronger intermolecular forces in

the larger carbon rings.

10

Question 3



(b) The C–C–C bond angles in cyclohexane are 109.5°.

State and explain the shape around each carbon atom in cyclohexane.

[1]

(d) Petrol contains both cyclohexane, C6H12, and hexane.

Cyclohexane can be prepared from hexane.

(i) Construct the equation for this reaction

[2]

(ii) Suggest one advantage of adding cyclohexane to hexane in petrol. [1]

(c) In the absence of ultraviolet radiation, cyclopropane undergoes an addition reactionwith bromine.

Suggest the structure of the organic product formed in this reaction.

The shape would be a tetrahedral.

This is because an angle of 109.5⁰ results from repulsion of four bonding pairs.

The formula for cyclopropane is C3H6 and Br2 is added in the reaction, resulting in:

𝐶𝐶6𝐻𝐻14 → 𝐶𝐶6𝐻𝐻12 + 𝐻𝐻2

Cyclohexane burns more efficiently as it increases the octane number in petrols

which reduces knocking.

(e) Cyclobutane is another cycloalkane.

There are several unsaturated isomers of cyclobutane that

are alkenes. Two of these isomers are stereoisomers.

(i) Explain what is meant by the term stereoisomers. [1]

Stereoisomers are compounds with the same structural formula but different

arrangement of atoms in space

. [1]



12

[2](ii) Draw these two stereoisomers.

Butene exhibits E and Z stereoisomerism

(f) In the presence of ultraviolet radiation, cyclohexane reacts with bromine.

A mixture of cyclic organic compounds is formed, including C6H11Br.

(i) Complete the table below to show the mechanism of the reaction between bromine andcyclohexane to form C6H11Br.

Include all possible termination steps in your answer. [5]

A free radical substitution mechanism is needed:

Initiation step: UV radiation forms free radicals

𝐵𝐵𝑁𝑁2 → 2 𝐵𝐵𝑁𝑁 ∙

Propagation step: Free radicals from initiation form more free radicals

𝐵𝐵𝑁𝑁 ∙ + 𝐶𝐶6𝐻𝐻12 → 𝐻𝐻𝐵𝐵𝑁𝑁 + 𝐶𝐶6𝐻𝐻11 ∙

𝐶𝐶6𝐻𝐻11 ∙ + 𝐵𝐵𝑁𝑁2 → 𝐶𝐶6𝐻𝐻11𝐵𝐵𝑁𝑁 + 𝐵𝐵𝑁𝑁 ∙

Termination step: Free radicals combine

𝐶𝐶6𝐻𝐻11 ∙ +𝐵𝐵𝑁𝑁 ∙ → 𝐶𝐶6𝐻𝐻11𝐵𝐵𝑁𝑁

𝐶𝐶6𝐻𝐻11 ∙ +𝐶𝐶6𝐻𝐻11 ∙ → 𝐶𝐶12𝐻𝐻22

𝐵𝐵𝑁𝑁 ∙ + 𝐵𝐵𝑁𝑁 ∙ → 𝐵𝐵𝑁𝑁2



(ii) The initiation step involves homolytic fission.

(g) The reaction between cyclohexane and bromine in (f) also forms C6H10Br2.

(i) Write an equation, using molecular formulae, for the reaction of cyclohexane and brominein the presence of ultraviolet radiation to form C6H10Br2. [1]

(ii) Name one of the structural isomers of C6H10Br2 formed in the reaction betweencyclohexane and bromine.

[Total 21 Marks]

[1]

Explain why the initiation step is an example of homolytic fission. [1]

Homolytic fission is the breaking of covalent bonds which forms two radicals with one

electron from the bond pair going to each atom.

𝐶𝐶6𝐻𝐻12 + 2 𝐵𝐵𝑁𝑁2 → 𝐶𝐶6𝐻𝐻1𝑜𝑜𝐵𝐵𝑁𝑁2 + 2 𝐻𝐻𝐵𝐵𝑁𝑁

The product would consist of 2 bromine atoms, hence dibromo.

Possible products formed would be:

• 1,1-dibromocyclohexane




Check that the location numbers are the lowest possible, e.g. 2,4 – should be 1,3 -



Crude oil is a complex mixture of many hydrocarbons.

Crude oil is processed by the petroleum industry to make fuels and petrochemicals.

(a) The straight-chain alkane, A, is present in crude oil.A has molecules with ten carbon atoms.

(i) What is the molecular formula of A? . [1]

(ii) B is a branched-chain isomer of A.

Draw the skeletal formula of a possible structure for B.

Name your structure. [2]

(iii) The branched-chain isomer B has a lower boiling point than the straight chain alkane A.

Explain why. [2]

The general formula of alkanes is CnH2n.

Therefore, the molecular formula is C10H22 which is decane.

Skeletal formulae do not show the C and H atoms in a hydrocarbon chain.

Each line represents a bond between two C atoms.

Example of branched isomers of A would be 2-methylnonane and 3-

methylnonane

There are 75 isomers of decane in total and the whole extensive list can be

found at http://www.3rd1000.com/chem301/decane.htm

Branching lowers the boiling point of a compound because it has less surface

area of contact for intermolecular forces to act between. In this case, B has

fewer points of contact between its molecules than A, resulting in fewer van

der Waals’ forces. Hence, less energy is needed to overcome the intermolecular

forces in B than in A.

14

Question 4


http://www.3rd1000.com/chem301/decane.htm


15

(b) A chemist heats a pure sample of C15H32 in the presence of a catalyst.

A reaction called cracking happens.

(i) Construct an equation to show the cracking of C15H32. [1]

(ii) When cracking takes place, a large number of products are formed.

Suggest why a large number of products are formed. [1]

Cracking produces an alkene and a shorter chained alkane. Since the question does

not specify a particular hydrocarbon produced in the reaction, there are multiple

answers, such as:

𝐶𝐶15𝐻𝐻32 → 𝐶𝐶13𝐻𝐻28 + 𝐶𝐶2𝐻𝐻4

Check that the number of C atoms in the products totals 15 and that the number

of H atoms total 32.

Diverse products can be formed from cracking as any C-C bond in the alkane

chain can break.

(c) The petroleum industry processes straight-chain alkanes into cyclic hydrocarbons.

For example, octane can be processed into a cyclic hydrocarbon and hydrogen.

(i) Suggest the structure of this cyclic hydrocarbon. [1]

Any cyclic hydrocarbon with 8 carbon atoms, e.g.:

or:



(ii) Why does the petroleum industry process straight-chain alkanes into cyclic hydrocarbons?

[1]

[Total 9 Marks]

Cyclic hydrocarbons promote efficient combustion by reducing knocking and

increasing the octane number.



hydrocarbons as & a level

Documents