Alkanes 2/17/2016Dr Seemal Jelani Chem-1601

 Least reactive  Acids have no effects  Strong oxidizing and reducing agents have little effect on alkanes  Two important reactions Combustion Halogenation 2/17/2016Dr Seemal Jelani Chem-1602

 It’s a reaction between a substance and oxygen with the evolution of heat and light(flame)  Alkanes undergo combustion when ignited 2/17/2016Dr Seemal Jelani Chem-1603

Combustion  exothermic reaction  alkanes used as fuel source 2/17/2016Dr Seemal Jelani Chem-1604 C 4 H 10 + ___ O 2  ___ CO 2 + ___ H 2 O / 2 Incomplete Combustion with insufficient O 2 produces CO Poor ventilation, cigarettes C 4 H 10 + ___ O 2  ___ CO + ___ H 2 O 4 5 9/29/2 CO is poisonous because it binds to the hemoglobin in the blood, preventing the absorption of O 2

In combustion reactions,  alkanes react with oxygen.  CO 2, H 2 O and energy are produced. Alkane + O 2 CO 2 + H 2 O + heat 2/17/2016Dr Seemal Jelani Chem-1605

2/17/2016 Dr Seemal Jelani Chem-160 6

 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 reaction It is a reaction in which part of a small reacting molecule replaces an atom or group of atoms on a hydrocarbons 2/17/2016Dr Seemal Jelani Chem-1607

 ​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 2/17/2016Dr Seemal Jelani Chem-1608

heat or  R-H + X 2 R-X + H-X light R= Alkyl group Halogenation means: X2 where X = F, Br, Cl,I 2/17/2016Dr Seemal Jelani Chem-1609

 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 2/17/2016Dr Seemal Jelani Chem-16010

 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” 2/17/2016Dr Seemal Jelani Chem-16011

 ​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. 2/17/2016Dr Seemal Jelani Chem-16012

 ​Homolytic fission forms radicals​  Radicals are very reactive atoms or groups.  Cl​ 2 ​ ​2Cl​.  Radicals react quickly with surrounding molecules. 2/17/2016Dr Seemal Jelani Chem-16013

Initiation Propagation Termination Radical Halogenation of Alkanes 2/17/2016Dr Seemal Jelani Chem Mechanism (chain reaction): Step 1Cl 2 ⇌ Cl· + Cl · Step 2Cl· + CH 4  CH 3 · + HCl Step 3CH 3 · + Cl 2  CH 3 Cl + Cl· Step 4Cl· + Cl·  Cl 2 Overall reaction: CH 4 + Cl 2  CH 3 Cl + HCl Halogenated product is a haloalkane Naming: halogen atom is a substituent, replace –ine ending with –o -F fluoro-Cl chloro-Br bromo-I iodo  chloromethane hh (Free Radicals) Why not 1-chloromethane? Type of Step CH 4 + Cl 2  CH 3 Cl + HCl

Halogen substitutes for hydrogen in alkane →multiple results 2/17/2016Dr Seemal Jelani Chem CH 4 CH 3 ClCH 2 Cl 2 CHCl 3 CCl 4 Cl 2  Cl 2  Cl 2  Cl 2  Compound CH 3 Cl CH 2 Cl 2 CHCl 3 CCl 4 IUPAC Name Chloromethane Dichloromethane Trichloromethane Tetrachloromethane Common Name Methyl chloride Methylene chloride Chloroform Carbon tetrachloride All are liquids at room temperature Heavy Cl atoms increase LDF Polar C-Cl bonds – can have polar molecules

Further propagationIf excess chlorine is present, further substitution takes place The equations show the propagation steps for the formation of... dichloromethane Cl + CH 3 Cl ——> CH 2 Cl + HCl Cl 2 + CH 2 Cl ——> CH 2 Cl 2 + Cl trichloromethane Cl + CH 2 Cl 2 ——> CHCl 2 + HCl Cl 2 + CHCl 2 ——> CHCl 3 + Cl tetrachloromethane Cl + CHCl 3 ——> CCl 3 + HCl Cl 2 + CCl 3 ——> CCl 4 + 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 METHANE 2/17/2016Dr Seemal Jelani Chem-16016

Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products THERMALproceeds via a free radical mechanism CATALYTICproceeds via a carbocation (carbonium ion) mechanism CRACKING THERMAL HIGH PRESSURE kPa HIGH TEMPERATURE °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 2/17/2016Dr Seemal Jelani Chem-16017

Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products THERMALproceeds via a free radical mechanism CATALYTICproceeds via a carbocation (carbonium ion) mechanism CRACKING CATALYTIC SLIGHT PRESSURE HIGH TEMPERATURE °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 2/17/2016Dr Seemal Jelani Chem-16018

In the past, most important organic chemicals were derived from coal. Nowadays, natural gas and crude petroleum provide an alternative source. The composition of crude petroleum varies according to its source it is a dark coloured, viscous liquid consists mostly of alkanes with up to 40 carbon atoms, plus water, sulphur and sand Can be split up into fractions by fractional distillation. Distillation separates the compounds according to their boiling point CONTENTS PETROCHEMICALS 2/17/2016Dr Seemal Jelani Chem-16019

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 2/17/2016Dr Seemal Jelani Chem-16020

Boiling C’s per Name of Use(s) range / °C molecule fraction < LPG (LiquefiedCalor Gas Petroleum Gas)Camping Gas GASOLINEPetrol NAPHTHAPetrochemicals KEROSINEAviation Fuel GAS OILCentral Heating Fuel > LUBRICATING OIL Lubrication Oil > FUEL OILPower Station Fuel Ship Fuel > WAX, GREASECandles Grease for bearings > 400 > 50 BITUMENRoad surfaces Roofing CONTENTS PETROCHEMICALS 2/17/2016Dr Seemal Jelani Chem-16021