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  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page1of17

    Alkanes Lecturers: Ms Joanne Low and Mrs Zhou (Mdm Tan Shuyun) Contents Alkanes (exemplified by ethane)

    (i) Free-radical reactions Hydrocarbons as fuels Learning Outcomes Candidates should be able to: (a) Recognise the general unreactivity of alkanes, including towards polar reagents

    (b) Describe the chemistry of alkanes as exemplified by the following reactions of ethane:

    (i) Combustion (ii) Substitution by chlorine and by bromine

    (c) Describe the mechanism of free-radical substitution at methyl groups with particular reference to the initiation, propagation and termination reactions

    (d) Recognise the environmental consequences of: (i) Carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the

    internal combustion engine and of their catalytic removal (ii) Gases that contribute to the enhanced greenhouse effect

    1. Introduction 1.1 Alkanes

    belong to a homologous series of hydrocarbons (contain C and H atoms only) are ____________ (only single bonds between atoms, hence they contain maximum

    number of hydrogens per carbon atoms) are combustible but unreactive There are two homologous series of alkanes:

    Aliphatic alkanes (open-chained alkanes) Alicyclic alkanes (closed-chained alkanes) General formula: CnH2n+2 General formula: CnH2n E.g. CH3CH2CH2CH2CH3 Pentane C5H12

    E.g. Cyclobutane C4H8 Cyclopentane C5H10

  • NH2 Chemistr

    1.2 Nomen No. of C atoms

    1

    2

    3

    4

    4

    5

    5

    6

    1.3 Alkyl g formed named

    Gen

    C

    ry 9647

    nclature

    IUPAC NMolecular

    MethaCH

    EthaC2H

    PropaC3H

    ButaC4H

    2-methylpC4H

    2,2-dimethyC5H

    cyclopenC5H

    cyclohexan

    groups

    d when ond by replac

    Aliphatic Aneral formu

    CH3 Metha

    CH3CHEthan

    CH3CH2CPropan

    CH3CH2CHButan

    Name/ formula

    ane 4

    ne H6

    ane H8

    ne 10

    propane 10

    ylpropane 12

    ntane 10

    ne C6H12

    e of the hycing ane b

    Alkane la: CnH2n+2

    H ne 2 H

    ne CH2 H ne 2CH2 H

    ne

    A

    Condeform

    CH

    CH3C

    CH3CH

    CH3 (CH

    CH(C

    C(CH

    H2C

    H2C

    HC

    H2C

    H2CCH

    HC

    ydrogen of by yl

    Ge

    Alkanes

    Page2of17

    ensed mula

    H4

    CH3

    H2CH3

    H2)2CH3

    CH3)3

    H3)4

    H2C

    CH2

    CH2

    CH2

    CH2

    CH2

    H2C

    the alkane

    Alkyl grneral formu

    CH3 Meth

    CH3CHEthy

    CH3CH2CProp

    CH3CH2CHButy

    Displayed

    CH

    C

    H

    H

    H

    C

    H

    H

    H

    es is remov

    roup ula: CnH2n+1 yl

    H2 yl CH2 yl

    H2CH2 yl

    C C

    H

    H

    H

    H

    H

    C

    CC

    H

    HH

    H

    H

    C

    CC

    CH

    H

    H

    H

    HH

    d formula

    C H

    H

    H

    C H

    H

    H

    C C

    H

    H H

    H

    H

    ved

    AGen

    C

    H

    C

    H

    H

    H

    H

    C

    C

    C

    HH

    H

    HH

    CC

    CC

    HH

    H

    H

    H

    H

    Melting point /oC

    182

    183

    190

    138

    160

    17

    94

    7

    Abbreviationeral symb

    Me

    Et

    Pr

    Bu

    NYJC 201

    C Boiling

    point /oC

    164

    88

    42

    0.5

    12

    10

    49

    81

    on ol: R

    4

    C

  • gH2 Chemistr

    2. Bondi Bonding a

    C has 4 C atom

    configu Shape

    2.1. Hybri

    W

    All the Hence Each c

    atomic

    C* (hyb

    How?

    1. The2. Dur

    form3. The4. Bon

    atom

    groundstate

    ry 9647

    ing in Al

    and molec

    4 valence ms share euration. of the mol

    dization o

    What is hybThe comolecuHybridiview. Iorbital

    carbons inalkanes h

    carbon is aorbitals)

    bridised s

    e electron iring hybridm four sp3

    e four sp3 hnding will tms.

    eC heat

    kanes

    cular struc

    electrons aelectrons b

    lecule with

    of carbon

    bridizationncept of h

    ular orbitalsisation dest involves s.

    n alkanes aave carbonable to fo

    state):

    in the 2s odisation, on3 hybrid orhybrid orbitthen occur

    excitedst

    A

    cture of al

    and it has by forming

    respect to

    in alkanes

    n? ybridisatios for molecscribes the mixing of

    are sp3 hybn with tetrarm four sp

    rbital first gne 2s and rbitals. tals form ar when the

    1s fo

    tateC he

    Alkanes

    Page3of17

    lkanes

    electron co4 covalen

    o carbon: _

    s

    on is usefucules. bonding a

    f atomic o

    bridised. ahedral arrp3 hybrid o

    gets excitethree 2p

    a tetrahedre sp3 hybr

    our sp3 hyb

    hybreat

    onfigurationt bonds to

    _________

    ul in explai

    atoms fromrbitals to f

    rangementorbitals (by

    ed and it isatomic orb

    ral arrangerid orbitals

    brid orbitals

    ridisedstate

    n: _______o achieve

    ______

    ining the s

    m an atomsform new

    t of CC ay mixing o

    s promotedbitals are h

    ement. (Booverlap w

    s

    C

    _______. noble gas

    hape of

    s point of hybrid

    and CH one 2s an

    d to the 2p hybridised

    ond angle =with orbita

    NYJC 201

    s electronic

    bonds. d three 2p

    orbital. (mixed) to

    = 109o) als of othe

    4

    c

    p

    o

    r

  • H2 Chemistr Recall: Ov Sigma Pi () All sin Example: During

    form __ Each _ four C

    Example: The C The tet

    ___ orb The re

    ___ orb

    s orbital

    ry 9647

    verlap of oa () bondsbonds aregle bonds

    Methane

    hybridisat_______ h___ hybrid H ______

    Ethane (C

    atom formtrahedral cbital from emaining __bital of hyd

    l of H

    orbitals tos are forme formed bys are ____

    (CH4)

    tion, _____hybrid orborbital will___are for

    C2H6)

    s four sp3carbon atoeach C ato__ orbitals drogen ato

    A

    o form boned by ____y _______

    _________

    ____ and _itals. overlap __

    med.

    3 hybrid oroms form aom.

    of carbon m.

    non

    Alkanes

    Page4of17

    nds (Chem_________

    ______ove_ bonds.

    ________

    _______ w

    rbitals. a ________

    form ____

    ne sp3 orbitne sp3 orbi

    mical bond_ overlap o

    erlap of orb

    atomic orb

    with one of

    _____ by

    ______ by

    tal of C tal of C

    ding chaptof orbitals bitals.

    bitals are h

    f the __ orb

    _________

    ________

    ter)

    hybridised

    bitals of fou

    __ overlapp

    __ overlap

    NYJC 201

    (mixed) to

    ur H atoms

    ping of one

    ping with a

    4

    o

    s.

    e

    a

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page5of17

    3. Physical Properties 3.1 Boiling point and melting point Alkanes have relatively ______ boiling and melting points.

    Alkanes are non-polar: C H bond is considered non-polar as C and H only differ slightly in electronegativity. [electronegativity of C(2.5) and H(2.1) are similar]

    Have a ________________________ consisting of __________________ held together by ______________________________.

    Boiling or melting involves overcoming the weak van der Waals forces between the alkane molecules.

    Boiling point generally ____________ with an ____________ number of C atoms.

    C1 C4: gas C5 C17: liquid >C18: solid

    - When Mr increases, number of electrons ____________. - Larger electron cloud becomes __________________. - Strength of intermolecular van der Waals forces ____________. - __________________ is required to overcome the van der Waals forces.

    The greater the degree of branching (with the ______ number of C atoms), the

    ______ the boiling point.

    Name pentane 2-methylbutane 2,2-dimethylpropane

    Structure CH3CH2CH2CH2CH3

    CH3CHCH2CH3

    CH3 CH3CCH3

    CH3

    CH3 Boiling point /oC 36 28 10 Melting point /oC 130 160 17

    - Highly branched alkanes are more ____________ in shape. - ________________________ of contact between molecules. - Strength of intermolecular van der Waals forces decreases. - Less energy is required to overcome the weak intermolecular forces.

    The trend for melting point less regular than that of boiling point

    - Branched alkanes can have lower or higher melting points than the straight chain alkanes depending on the ____________ of the molecules in the solid

    - Highly symmetrical branched alkanes allow the molecules to be packed more efficiently in the solid and hence have unusually high melting point

    *at room temperature

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page6of17

    3.2 Solubility Alkanes are: soluble in non-polar (organic) solvents like CCl4, benzene and ether (R-O-R).

    - They can form van der Waals interactions with the non-polar solvent. - The energy released during the formation of van der Waals forces with the non-

    polar solvent is enough to overcome the van der Waals forces between the alkane molecules and the van der Waals forces between the solvent molecules.

    insoluble in polar solvents like water. - They can only interact with water molecules via weak van der Waals bonds - the energy released during the formation of weak van der Waals forces with

    water is not enough to overcome the strong hydrogen bonds between water molecules.

    and water molecules

    strong hydrogen bonding between water molecules

    water layeralkane layer

    weak interactions between alkane

    weak van der Waals between alkane molecules

    +

    +

    +

    + +

    +

    +

    +

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page7of17

    4. Chemical Properties Alkanes are saturated and are generally unreactive because:

    (i) The C-H bond is ____________ They have no centres of electrical charge to act as electrophiles or

    nucleophiles to attract polar reagents like H+, OH- or MnO4-. (ii) The C-C and C-H bonds are relatively ____________

    C C: 350 kJ mol-1 C H: 410 kJ mol-1

    Alkanes do, however, react with oxygen and halogens under appropriate conditions,

    like in the ________________________ or ____________.

    Alkanes undergo two main types of reactions: (i) Combustion (ii) Free Radical Substitution

    (i) Combustion Complete combustion

    Alkanes burn in ____________ oxygen to form ____________ and water. This reaction is very exothermic, which accounts for their use as fuels.

    Alkanes burn with a non-luminous blue flame with little or no soot if combustion is complete.

    General equation: CxHy + (x + 4

    y ) O2 x CO2 + 2y H2O

    e.g. Complete combustion of hexane:

    C6H14 (l) + 192

    O2 (g) 6CO2 (g) + 7H2O (l)

    Incomplete combustion

    In a ____________ supply of oxygen, alkanes burn to form ____________, water and soot (C).

    e.g. Incomplete combustion of methane:

    2CH4 + 3O2 2CO + 4H2O CH4 + O2 C + 2H2O (soot) 4CH4 + 5O2 2CO + 2C + 4H2O (soot)

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page8of17

    (ii) Substitution

    Alkanes react with halogen (e.g. Cl2 and Br2) to form halogenoalkanes (alkyl halides) when irradiated with ultraviolet light or heated.

    No reaction takes place in the dark. The mechanism for the halogenation reactions of alkanes is known as

    ______________________________.

    Type of reaction free radical substitution

    Equation CxHy (g) + Cl2 (g) CxHy-1Cl (g) + HCl (g) Reagents Cl2 Conditions

    Heat may be supplied to initiate the reaction. Reaction proceeds very slowly at room temperature.

    Product chloroalkanes Observations Yellowish-green colour of Cl2 decolourises.

    White fumes of HCl produced. Example: Write a balanced equation for the reaction of propane with bromine. Indicate clearly the conditions and observations in this reaction. Conditions: Observations: Note: - Solvent is CCl4 (used when alkane and halogen is present in different phases)

    Example: C3H8 (l) + Br2 (g) - Bromination takes place less readily than chlorination because bromine is less

    reactive than chlorine. (Refer to Group VII) - Fluorination is dangerously exothermic while iodination is slow and reversible.

  • H2 Chemistr

    5. React

    A ccrea

    Bon

    The

    i)

    ii) 5.1 Homo

    5.2 Hetero

    ry 9647

    ion mec

    chemical rated.

    nd cleavag

    ere are two

    Homol Hetero

    lytic FissiHomolytic _________When the electrons. Free radic[Recall: A A free radtendency t

    olytic fissiHeterolytic_________When the bonding e

    hanism

    reaction ta

    ge or bond

    o types of b

    lytic fissioolytic fissi

    ion fission ten

    _____ electbond brea

    cals are fofree radica

    dical is extto pair up w

    ion c fission te

    _____ electbond brea

    lectrons to

    A

    akes place

    fission is t

    bond fissio

    on on

    nds to occutronegativaks, each

    ormed in hoal is an atoremely reawith anothe

    ends to octronegativaks, the mo form an a

    Alkanes

    Page9of17

    e when old

    the breakin

    on required

    ur when thvities. of the bon

    omolytic fisom or moleactive becer electron

    ccur whenve than the

    more electroanion. The

    +

    d bonds a

    ng of chem

    d for organ

    e two bond

    nded atom

    ssion. ecule with aause the u

    n from anot

    one of the other. (e.onegative counterpa

    are broken

    mical bonds

    ic chemistr

    ded atoms

    s takes __

    an unpaireunpaired ether specie

    he two bong. polar boatom take

    art become

    Half arr

    Depictsof a sin

    Full arrow

    Depicts thof an elec

    n and new

    s.

    ry mechan

    are identic

    _____ of th

    ed electronelectron haes.

    nded atomonds)

    es _______es a cation

    row :

    s the movengle electr

    w :

    he movemctron pair.

    NYJC 201

    w ones are

    nisms:

    cal or have

    he bonding

    . E.g. Cl]as a strong

    ms is much

    ______ then.

    ement ron.

    ment .

    4

    e

    e

    g

    ] g

    h

    e

  • H2 Ch

    5.3 F

    Initiat

    Cl

    Propa Cl + CH3C

    Termi CH3C Cl + CH3C

    emistry 9647

    ree-Radical S

    Mtion

    Clu.v.

    agation

    CH3CH2H

    H2 + ClCl

    ination

    H2 + Cl

    Cl

    H2 + CH3CH2

    uv

    ubstitution (F

    Mechanism

    Cl Cl

    CH3CH2

    CH3C

    CH3CH2

    ClCl

    2

    Alkanes

    RS) Mechanis

    2 + HCl

    CH2Cl + Cl

    2Cl

    CH3CH2CH2CH

    sm

    Homoly

    Why is CH3CH C-H bo Note: B The c

    hydro A high

    Why i Cl + The f

    forma Note: The e

    radica The tw

    destro

    H3

    The c

    Page10of17

    ytic fission of Cl

    this step not favH3 CHonds in alkanes aBE(Cl-Cl) = 242

    chlorine radical gen chloride mohly reactive meth

    s this step not fa CH3CH2H formation of a ation of a H-Cl bo

    BE(H-Cl) = 431

    ethyl radical reaal. wo steps are reoyed i.e. termina

    chain reaction te

    NYJC 2014

    D

    Cl bond under

    vourable? H3CH2 + H are not broken akJmol-1, BE(C-H

    removes a hyolecule. hyl radical is pro

    avourable? CH3C-Cl bond is leond in the above kJmol-1, BE(C-C

    cts with a chlor

    epeated till all thation step.

    rminates when t

    4

    Description

    UV radiation to f

    as the C-H bond H) = 435 kJmol-1

    ydrogen atom f

    oduced.

    CH2Cl + Hess exothermic e mechanism. Cl) = 340 kJmol-

    rine molecule to

    he reactants hav

    two free radicals

    form free Cl rad

    is much stronge

    from a methan

    and hence les

    -1

    o produce chloro

    ve been used u

    s collide with eac

    dicals.

    er than the Cl-C

    e molecule to

    ss favourable th

    oethane and a c

    p or when radic

    ch other.

    l bond.

    form a

    han the

    chlorine

    cals are

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page11of17

    Example The chlorination of propane in the presence of a limited amount of Cl2 forms 2-chloropropane as a possible product. Suggest the mechanism of the reaction.

    Example 1-cholorocyclohexane can be made from cyclohexane. Describe the reaction mechanism of its formation.

  • H2 Chemistr

    5.4. ProblIn reality, aof the follo 5.4.1 Mult Depen

    for rea The re

    CH3Cl CCl4 p To avo

    (This m

    ExamplA possi1,1-dich Mechan Initiatio

    Cl CPropag

    Termina

    (**Write a

    ry 9647

    ems with a pure sam

    owing prob

    ti-substitunding on thaction, a meaction of C

    predominpredominatoid multisumaximises

    le ble produchloroethan

    nism: Free

    on

    Cl Cuv/ heat

    ation

    ation

    at least 3, in

    Free-Radmple of thelems:

    ution he relative ixture of pCH4 and C

    ates whentes when t

    ubstitution, s the proba

    ct for the cne. Sugges

    Radical S

    Cl + Cl

    ncluding for

    P

    ical Subst chloroalka

    amounts oroducts wi

    Cl2 is able to

    n there is ehere is excuse an __

    ability that C

    chlorinatiost the mec

    Substitutio

    rmation of t

    Alkanes

    Page12of1

    titution ane (or bro

    of halogen ll be obtaino yield a m

    xcess CH4cess Cl2. _________Cl attacks

    n of ethanchanism of

    on

    he halogen

    7

    omoalkane

    and alkanned. mixture of C

    4.

    _________s CH4.)

    ne in the pthe reactio

    , product an

    e) is seldom

    e present

    CH3Cl, CH2

    _____ or a

    presence oon.

    nd one poss

    m obtained

    and the tim

    2Cl2, CHC

    ________

    of excess

    sible side p

    NYJC 201

    d because

    me allowed

    l3 and CCl4

    _________

    chlorine is

    roduct)

    4

    d

    4:

    _.

    s

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page13of17

    5.4.2 Isomeric Products In the case of alkanes with 3C, isomeric products are formed depending upon which

    H atom is replaced.

    Expected Ratio: 10% 90% A simplified way to obtain the expected ratios is to count the number of hydrogens that

    have the same chemical environment which will lead to the formation of a specific product. (probability)

    Example: Draw all the possible monosubstituted products when a mixture of 2-methylbutane and bromine is allowed to react in the presence of sunlight. State the ratio in which they are formed.

    Br

    C C

    H

    H

    H

    C

    H

    H

    C

    H

    H

    C

    H

    H

    H

    H :

    H

    C C

    H

    H

    Br

    C

    H

    H

    C

    H

    H

    C

    H

    H

    H

    H :

    H

    C C

    H

    H

    H

    C

    H

    Br

    C

    H

    H

    C

    H

    H

    H

    H

    a) Draw the displayed formula of another isomer of C5H12 which reacts with bromine to form only one monobrominated product. Ans:

    C C

    Hd

    Hd

    Hd

    Ha

    C

    Hb

    C

    Hb

    Hc

    Hc

    Hc

    C HdHd

    Hd

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page14of17

    6. Environmental Concern (Refer to Heterogenous catalysis in Kinetics, Page 30) There are several environmental concerns regarding the use of hydrocarbons as fuels.

    (i) Pollutants such as carbon monoxide, oxides of nitrogen and unburnt hydrocarbons (soot) found in car exhaust can be removed from the car engines using catalytic converter.

    Pollutants Formation in car engine Environmental/ Health Impact

    Reaction for removal in catalytic converter

    Carbon monoxide

    Incomplete combustion of fuel

    Combines irreversibly with haemoglobin and makes it ineffective as oxygen carrier in the human body suffocation or blood poisoning

    Conversion of CO to CO2 2CO(g) + O2(g) Pt 2CO2(g)

    Oxides of nitrogen (NO2, NO)

    Reaction of N2 with O2 at high temperatures

    Catalyzes formation of acid rain Forms smog

    Conversion of NO to N2 2NO(g) Pt N2(g) + O2(g) 2NO(g) + 2CO(g) N2(g) + 2CO2(g)

    Unburnt hydrocarbon

    Incomplete combustion of fuel

    Forms smog Oxidation of unburnt hydrocarbon to CO2 and H2O e.g. 2C8H18(l) + 25O2(g) Pt 16CO2(g) + 18H2O(l)

    Note:

    - The catalyst in the converters would be poisoned by the lead present in petrol. Therefore, cars fitted with catalytic converters must use unleaded petrol.

    - A honeycomb structure is used to maximise the surface area on which catalysed reactions can take place.

    (ii) Carbon dioxide and methane are greenhouse gases responsible for global warming

    and climate changes. One of the major sources of carbon dioxide comes from burning of fossil fuels for energy production; hence there is a need to find alternative fuels for mankind.

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page15of17

    Summary of important concepts in Alkanes

    Physical Properties of Alkanes: Non-polar Consisting of molecules held together by weak intermolecular van der Waals forces. Soluble in non-polar (organic) solvents Insoluble in polar solvents Boiling point increases with number of carbons

    o Number of electrons in the molecule increases. o Polarisability increases, o Strength of intermolecular van der Waals forces increases. o More energy is required.

    Branching decreases the boiling point. o More spherical in shape o Smaller surface areas of contact between molecules o Strength of intermolecular van der Waals forces decreases o Less energy is required

    Chemical Properties of Alkanes: Alkanes are unreactive because the C-H bond is non polar and relatively strong. Carbon atoms in alkanes form four sp3 hybrid orbitals from hybridization of one 2s and three 2p

    orbitals. They form 4 sigma bonds () via head-on overlap of orbitals. Complete combustion of alkanes produces carbon dioxide(CO2) and water(H2O). Alkanes undergo reactions by a mechanism called free-radical substitution in the presence of

    ultraviolet light or heat.

    o In reaction of ethane with bromine, the 3 stages in the mechanism are (1) Initiation

    Equation: Cl Clu.v. Cl Cl

    (2) Propagation Equations:1st step - Cl 2CH3 CH2H H HClCH3CH2

    2nd step - Cl Cl ClCH3CH2 CH3CH2 Cl

    (3) Termination Equations:

    2 Types of Bond Fission

    Occurs between atoms of _______ electronegativities

    Electrons in bond broken goes to Arrows Products

    (1) Homolytic fission Similar One to each atom

    Radicals

    (2) Heterolytic fission Different

    Both electrons go to more EN atom

    Cation + Anion

    CH3CH2 Cl CH3CH2 Cl

    Cl Cl Cl ClCH3CH2 CH3CH2 CH2CH3CH3CH2

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page16of17

    Annex (For additional reading) A1. Cracking Cracking is a process of breaking down large alkane molecules into smaller alkanes and alkenes. There are two processes we can split alkane chains: By high temperature (about 800oC) and pressure, known as thermal cracking. By catalyst (Al2O3 and SiO2, 450 oC), known as catalytic cracking. For example, C10H22 C4H8 + C6H14 C14H30 C2H4 + C12H26

    [Note TYS Pg 118, Qn 5a (N2005/III/8 Either) is no longer in syllabus]

    A2. Hydrocarbons as Crude Oil Fractional Distillation Petroleum is a mixture of a very large number of different hydrocarbons; the most commonly found molecules are alkanes (linear or branched), cycloalkanes and aromatic hydrocarbons. Raw oil or unprocessed ("crude") oil is not useful in the form it comes in out of the ground. To make it useful, it must be separated into its components by fractional distillation as shown in Figure 1.

    Figure 1. Fractional distillation of crude oil. The fractionating column is cooler at the top than at the bottom because the fractions at the top have lower boiling points than the fractions at the bottom.

  • H2 Chemistry 9647 Alkanes NYJC 2014

    Page17of17

    Crude oil is fractionally distilled to give the following fractions. Fraction Length of Carbon

    chain Uses

    Refinery

    gases

    C1-C4 Fuel; domestic heating, gas cookers

    Gasoline C5-C12 Fuel in internal combustion engines

    Kerosene C12-C18 Fuel for jet engines

    Diesel Oil C18-C25 Fuel for transport and industrial

    heating

    Residue > C25 paraffin wax, lubricating oil, petroleum

    jelly, bitumen

    A3. Octane number (octane rating)

    In a cylinder of a motor car engine, a mixture of petrol vapour (mostly C5 to C10 alkanes) and the air is ignited by an electric spark, producing an explosive reaction which drives the piston down.

    Petrol rich in straight-chain alkanes (e.g. heptane) ignites very readily and explodes rapidly, causing knocking of the engine and inefficient combustion.

    Combustion of branch-chain alkanes (e.g. 2,2,4-trimethylpentane) is much smoother and more controlled. It is a more efficient fuel and less likely to cause knocking.

    A numerical representation of the antiknock properties of motor fuel, compared with a standard reference fuel.

    Heptane is assigned an octane number of 0. 2,2,4-trimethylpentane (iso-octane) is assigned an octane number of 100. The octane number of a petrol is found by comparing its performance with a mixture of

    heptane and 2,2,4-trimethylpentane. The octane number of a sample of fuel is determined by burning the gasoline in an

    engine under controlled conditions, e.g., of spark timing, compression, engine speed, and load, until a standard level of knock occurs.

    Gasoline that have high octane numbers denotes that they have good anti-knock properties

    Reference texts 1. Understanding Chemistry for Advanced level, Ted Lester, Janet Renshaw, Chapter 19 [540LIS] 2. Chemistry for Advanced Level. Peter Cann, Peter Hughes, Chapter 23 [540CAN]