MODULE 1: General Chemistry

 

7.3.2  Alkanes

 

 

ALKANES

General formula

CnH2n+2

 

Functional group

 (C-H)

FORMULA

NAME

melt. pt.

boil. pt.

density

CH4

methane

-183

-162

0.42

C2H6

ethane

-183

-89

0.55

C3H8

propane

-188

-42

0.58

C4H10

butane

-138

-1

0.58

C5H12

pentane

-130

36

0.63

C6H14

hexane

-95

69

0.66

 

Associated with these alkanes we have alkyl radicals of general formula CnH2n+1 (symbol R)

e.g.       CH3-  methyl, C2H5-  ethyl, C3H7- propyl

This family of alkanes forms an homologous series.

 

 

Fractional distillation as a source of hydrocarbon mixtures.

 

 

HYDROCARBONS

alkanes, alkenes, benzene.

Sources

1. Petroleum.

                           Petroleum deposits were formed by the action of pressure and temperature on marine life sediments, mainly from the fatty acid constituents, under the catalytic action of various rocks and acid clays.

Crude petroleum is a complex mixture of gaseous, liquid and solid hydrocarbons such as alkanes , cycloalkanes, aromatics (benzene) and some alkenes.  Also present are some compounds of oxygen, nitrogen and sulphur.

It has no uses in its raw form so to provide useful production has its components must be part separated and unnecessary modified. The fundamental process of refining and when is primary distillation

Refining

            The function of an oil refinery is to manufacture from crude oil those quantities of the oil products required by consumers. This is carried out by the use of various physical and chemical processes.

 

Refinery gas ( 1 - 2 per cent of crude oil) contains hydrocarbons that a gases at normal temperatures. It includes the L canes with one to fall carbon atoms in their molecules, with methane as a major components. The main use of refinery Gas is as against this fuel.

Gasoline (us 15 to 30 per cent) has is a complex liquid mixture of hydrocarbons containing mainly c5 - C 10, pounds whose point points range from 40 to greasy 2180 degrees C. The major use of gasoline is as a fuel were in internal combustion engines. A considerable part proportion of this fraction is used to produce chemicals by cracking. The part of the gasoline fraction used is called a snap offer.

Kerosene (Aids 10 - 15 per cent) consists mainly of C4 and C 12 hydrocarbons, with boiling point from 162 to 50 degrees C. It is used as a fuel in jet engines and four domestic heating. It can be cracked up to produce extra gasoline.

The diesel oil or gas or oil (15 - 20 per cent) adds containing C 13 - C 25 compounds, boils and arranged to 20 - 350 degrees C. It is used in diesel engines where the fuel is ignited by compression instead of by a spark. And also for industrial heating purposes. It can also be cracked to produce extra gasoline.

Residue (40 - 50 per cent) adds boils above 350 degrees C and is a highly complex mixture of them volatile hydrocarbons. Most of it is used as fuel oil in large furnaces such as those in power stations or big ships. Proportion of it is used to make a lubricating oils and waxes. Both these contain C 26 - C 20 a time carbon as

 

Distillation

Crude oil is fractionally distilled to give 4 main fractions.

C1-C4

refinery gases

methane, ethane, propane, butane

C8-C16

light distillates

petrol, aviation fuel, kerosene, benzene

C17-C20

middle distillates

heating oil, diesel, feedstock for cracking

C21+

residue

paraffin wax, lubricating oil, petroleum jelly, bitumen

 

2.  Coal Tar

When coal is carbonised (burnt in the absence of air) one of the products is a viscous black liquid called coal tar.  This is distilled into five main fractions.  The light oil fraction (boil pt. up to 170oC) is a source of benzene.

 

 

Cracking as a source of alkenes and shorter chain alkanes.

 

 

Cracking

In this process larger molecules are broken down into smaller ones, either by high temperature and pressure (thermal cracking) or by a catalyst (catalytic cracking).

 

Thermal Cracking

When alkanes are heated to high temperatures their molecules vibrate strongly enough to break and form smaller molecules. One of these molecules is usually an alkane. Reducing chain length generally results in unsaturation. Such reactions are known as cracking

e.g.       C8H18    C5H12  +  CH3CH=CH2

            octane                          pentane            propene

Thermal cracking is generally used for cracking residues to middle distillates.

 

Catalytic Cracking

By using a catalyst, cracking can be made to occur at fairly low temperature. This is known as catalytic cracking.

Catalytic cracking is the most important source of petrol and raw materials for the chemical industry. Heavier fractions can be cracked to produce extra gasoline. Cracking tends to produce branched-chain rather than straight-chain alkanes, so the gasoline produced this way has a high octane rating. Processes similar to cracking can be used to convert low-grade gasoline to high grade fuel.

The catalysts are usually natural clays and synthetic alumina/ silica mixtures (Al2O3/SiO2).

 

Isomerisation

This involves breaking up straight chainalkanes and reassembling them as branched chain isomers.

 Both of these processes are important in the production of unleaded gasoline.

 

 

 

Catalytic Reforming

Reforming involves converting straight chain alkanes into ring molecules such as arenes and cycloalkanes.

Benzene C6H6 and other aromatic compounds can be made by passing petrol vapour over a heated platinum catalyst.

                           500oC/15 atm

            C6H14    C6H6  +  4H2

            hexane  Pt catalyst         benzene

 

The u.s.a. obtains about half its benzene in this way.

 

 

 

 

Combustion in a limited and plentiful supply of air.

 

 

 

Combustion

The alkanes are used as fuels and burn in excess air or oxygen producing carbon dioxide and water.

            2C2H6  + 7O2    4CO2  + 6H2O

            C3H8  + 5O2    3CO2  + 4H2O

 

In general

CnH2n+2  +   (3n+1)   O2    nCO2  +   (n+1) H2O

            2

 

In a limited air supply carbon monoxide is produced

                        2C2H6  + 5O2    4CO  + 6H2O

 

 

Reaction with chlorine or bromine , monohalogenation only (except for methane with chlorine.

 

 

Alkanes are generally unreactive.  They are saturated and react by substitution.

 

Halogenation

Chlorine and bromine react with alkanes in the presence of strong sunlight or u.v. light giving a series of products formed by successive replacement of a hydrogen atom by a halogen atom. 

            R-H  +  X2    R-X  +  HX

 

 

Mechanism of the photochemical reaction between chlorine and methane viewed as a free radical substitution.

 

 

Chlorine reacts explosively with methane in the presence of strong sunlight or u.v. light to give a mixture of products.

CH4  +  Cl2     CH3Cl  +  HCl

                                                       CH2Cl2

                                                       CHCl3

                                                       CCl4

This is photochemical chlorination.

 

Mechanism

The mechanism of a reaction is the course believed to be followed by the reactants in combining together and the various stages inv olved in reaching the final products.This is a free radical chain reaction.

Initiation

Cl2    2Cl.  (a few)

 

Propogation

Cl.  +  CH4     CH3.  +  HCl

CH3.  +  Cl2     CH3Cl  +  Cl.

CH3Cl  +  Cl.  CH2Cl.  +  HCl

CH2Cl.  +  Cl2    CH2Cl2  +  Cl.

Termination

2Cl.   Cl2

CH3.  +  Cl.   CH3Cl

2CH3.   C2H6 

 

 

7.3.1    Isomerism in Organic Compounds

 

Structural isomerism for aliphatic compounds containing up to six carbon atoms, to include branched structures. (Cyclic structures excluded)

Isomerism 

Strutural  isomerism  occurs when 2  or more  compounds have the same molecular formula but different structural formula. (ie same number of atoms but bonded together differently).

Draw and name all the structural isomers of C4H10, C5H12 and C6H14.

 

 

Environmental problems associated with spillage and combustion of alkane fuels.

 

 

 

 

 

Unreactive nature of alkanes towards electrophiles and nucleophiles.