Alkanes

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Least reactive Acids have no effects Strong oxidizing and reducing agents have

little effect on alkanes Two important reactions Combustion Halogenation

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It’s a reaction between a substance and oxygen with the evolution of heat and light(flame)

Alkanes undergo combustion when ignited

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Combustion exothermic reaction alkanes used as fuel source

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C4H10 + ___ O2 ___ CO2 + ___ H2O4 513/2

Incomplete Combustion with insufficient O2 produces CO• Poor ventilation, cigarettes

C4H10 + ___ O2 ___ CO + ___ H2O4 59/2

CO is poisonous because it binds to the hemoglobin in the blood, preventing the absorption of O2

In combustion reactions, alkanes react with

oxygen. CO2, H2O and energy

are produced.Alkane + O2 CO2 +

H2O + heat

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Reaction between a substance and a halogen in which one or more halogens atoms are incorporated into molecules of the substance

Product is hydrocarbon derivative Halogenation is an example of Substitution

reactionIt is a reaction in which part of a small

reacting molecule replaces an atom or group of atoms on a hydrocarbons

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Simplest reaction to study is methane and chlorine

All other alkanes will react the same way That’s why we study homologous series Chlorine + methane chloromethane +

hydrogen chloride Cl 2 + CH 4 CH 3 Cl + HCl

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heat or R-H + X2 R-X + H-X lightR= Alkyl groupHalogenation means: X2 where X = F, Br,

Cl,I

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Mechanism◦How the reaction occurs through

multiple steps (most reactions actually occur in many steps)

Chain Reaction◦Reactions that occur on their own after

some initiating event Free Radicals

◦Atoms that have one free electron—highly reactive

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Initiation Step◦ Step where a bond is split by heat/light, producing

free radicals Propagation Step

◦ Step where free radicals react with non-radicals, producing more free radicals and continuing the “chain reaction”

Termination Step◦ Step where free radicals react with each other,

producing non-radicals and terminating the “chain reaction”

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Reaction starts with one photon of light. Must be UV Wavelength less than 400nm Energy less than 300kJ per mole Reaction proceeds to completion after this. Reason is the chlorine molecule Light breaks the Cl-Cl bond • Homolytic fission Each atom takes an electron Forms radicals.

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Homolytic fission forms radicals Radicals are very reactive atoms or groups. Cl 2 2Cl . Radicals react quickly with surrounding

molecules.

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Initiation

Propagation

Propagation

Termination

Radical Halogenation of Alkanes

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Mechanism (chain reaction):

Step 1 Cl2 ⇌ Cl· + Cl ·

Step 2 Cl· + CH4 CH3· + HCl

Step 3 CH3· + Cl2 CH3Cl + Cl·

Step 4 Cl· + Cl· Cl2Overall reaction: CH4 + Cl2 CH3Cl + HCl

Halogenated product is a haloalkaneNaming: halogen atom is a substituent, replace –ine ending with –o

-F fluoro -Cl chloro -Br bromo -I iodo

chloromethane

h(Free Radicals)

Why not 1-chloromethane?

Type of Step

CH4 + Cl2 CH3Cl + HCl

Halogen substitutes for hydrogen in alkane →multiple results

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CH4 CH3Cl CH2Cl2 CHCl3 CCl4

Cl2

Cl2

Cl2

Cl2

CompoundCH3Cl

CH2Cl2CHCl3CCl4

IUPAC NameChloromethaneDichloromethaneTrichloromethaneTetrachloromethane

Common NameMethyl chlorideMethylene chlorideChloroformCarbon tetrachloride

All are liquids at room temperature• Heavy Cl atoms increase LDF• Polar C-Cl bonds – can have polar molecules

Furtherpropagation If excess chlorine is present, further substitution takes place

The equations show the propagation steps for the formation of...

dichloromethane Cl• + CH3Cl ——> CH2Cl• + HCl Cl2 + CH2Cl• ——> CH2Cl2 + Cl•

trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl Cl2 + CHCl2• ——> CHCl3 + Cl•

tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl Cl2 + CCl3• ——> CCl4 + Cl•

Mixtures Because of the many possible reactions there will be a mixture of products. Individual haloalkanes can be separated by fractional distillation.

CHLORINATION OF METHANECHLORINATION OF METHANE

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Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

THERMAL

HIGH PRESSURE ... 7000 kPa HIGH TEMPERATURE ... 400°C to 900°C FREE RADICAL MECHANISM HOMOLYTIC FISSION PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making polymers and ethanol PRODUCES HYDROGEN ... used in the Haber Process and in margarine manufacture

Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission

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Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products

THERMAL proceeds via a free radical mechanismCATALYTIC proceeds via a carbocation (carbonium ion) mechanism

CRACKINGCRACKING

CATALYTIC

SLIGHT PRESSURE HIGH TEMPERATURE ... 450°C ZEOLITE CATALYST CARBOCATION (IONIC) MECHANISM HETEROLYTIC FISSION PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS USED FOR MOTOR FUELS

ZEOLITES are crystalline aluminosilicates; clay like substances

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• At each level a mixture of compounds in a similar boiling range is taken off

• Rough fractions can then be distilled further to obtain narrower boiling ranges

• Some fractions are more important - usually the lower boiling point ones, high boiling fractions may be broken down into useful lower boiling ones - CRACKING

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