H2 Chemistry 9647 Alkanes NYJC 2014

Page1of18

Alkanes Lecturers: Ms Joanne Low and Mrs Zhuo (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 saturated (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

Page2of18

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

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C has 4 C atom

configu Shape

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Alkanes

Page3of18

lkanes

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NYJC 201

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four sp Each s four C

Example: The C The tet

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s orbital

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verlap of oa () bondsbonds aregle bonds

Methane

hybridisatp3 hybrid osp3 hybrid oH bond

Ethane (C

atom formtrahedral cfrom each maining spital of hydr

l of H

orbitals tos are forme formed bys are sigm

(CH4)

tion, one 2orbitals. orbital will ds are form

C2H6)

s four sp3carbon atom C atom. p3 orbitals rogen atom

A

o form boned by heady side-on

ma bonds.

2s and thre

overlap hemed.

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of carbon m.

non

Alkanes

Page4of18

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NYJC 201

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H2 Chemistry 9647 Alkanes NYJC 2014

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3. Physical Properties 3.1 Boiling point and melting point Alkanes have relatively low 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 simple molecular structure consisting of alkane molecules held together by weak van der Waals forces.

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

Boiling point generally increase with an increasing number of C atoms.

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

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

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

lower 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 spherical in shape. - Smaller surface area 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 packing 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

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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 van der Waals interactions between alkane

weak van der Waals between alkane molecules

+

+

+

+ +

+

+

+

H2 Chemistry 9647 Alkanes NYJC 2014

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

(i) The C-H bond is non-polar 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 strong

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 presence of ultraviolet light or heat.

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

(i) Combustion Complete combustion

Alkanes burn in excess oxygen to form carbon dioxide 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 limited supply of oxygen, alkanes burn to form carbon monoxide, 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

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(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 free radical

substitution.

Type of reaction free radical substitution

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

Ultra-violet light sunlight 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.

C3H8 (g) + Br2 (l) C3H9Br (l) + HBr (g) Conditions: UV light or heat Observations: Reddish-brown Br2 turns colourless. White fumes of HBr formed. Note: - Solvent is CCl4 (used when alkane and halogen is present in different phases)

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

because weaker C-Br and H-Br bonds are formed. (Refer to Group VII) - Fluorination is dangerously exothermic while iodination is slow and reversible.

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Alkanes

Page9of18

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NYJC 201

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CH3CH2H

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H2 + Cl

Cl

H2 + CH3CH2

uv

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Mechanism

Cl Cl

CH3CH2

CH3C

CH3CH2

ClCl

2

Alkanes

RS) Mechanis

2 + HCl

CH2Cl + Cl

2Cl

CH3CH2CH2CH

sm

Homoly

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Page10of18

ytic fission of Cl

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

chlorine radical rde molecule. hly reactive ethy

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

BE(H-Cl) = 431

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NYJC 2014

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Cl bond under

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cts with a chlor

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4

Description

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

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For initiation sof a question fo

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v. Cl

Cl

Cl2

Cl

Cl

step uv light ior free radical s

Alkanes

presence of a possible pro

Cl

Cl

Cl2

Cl

is to be written substitution.

limited amounoduct. Sugge

HCl

Cl

in your answe

Page11of18

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NYJC 2014

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Page12of1

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NYJC 201

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H2 Chemistry 9647 Alkanes NYJC 2014

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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

Br

Br

Br

= 1 : 2 : 3 : 6

C C

Hd

Hd

Hd

Ha

C

Hb

C

Hb

Hc

Hc

Hc

C HdHd

Hd

H2 Chemistry 9647 Alkanes NYJC 2014

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Example: Draw all the possible monosubstituted products when a mixture of pentane 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 = 6 : 4 : 2 = 3 : 2 : 1

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

Ans:

C

CH

H

H

C C

CH

H

HH

H

H

H

H

H

H2 Chemistry 9647 Alkanes NYJC 2014

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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

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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

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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

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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]