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Physical Properties

Solubility: hydrophobic Density: less than 1 g/mL Boiling points increase with increasing

carbons (little less for branched chains).

Melting points increase with increasing carbons (less for odd- number of carbons).

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Chemical properties Properties of Alkanes Called paraffins (low affinity compounds) because

they do not react as most chemicals

They will burn in a flame, producing carbon dioxide, water, and heat

They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

They can cracked under high temperature and pressurre

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Reactions of Alkanes

Reactions of Alkanes

Combustion Halogenation Cracking

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Combustion

Burning of hydrocarbons in the presence of oxygen (high temperature oxidation of hydrocarbon fuels) yields a large amount of energy as heat and products which are stable oxides

CH4 + 2O2 → CO2 + 2H2O

(bond energies → energy release ~810 kJ)

Heats of Combustion Alkane + O2 CO2 + H2O

Long-chain

157.4 157.4

166.6 164.0158.7 158.6

=>

158.3

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Bond Making and Bond BreakingDuring chemical reactions, energy is either released to the environment (exothermic reaction) or absorbed from the environment (endothermic reaction). During chemical reactions, bonds are broken in the reactants and new ones are made in the products. Bond-breaking is an endothermic process and bond-making is an exothermic process. The average bond dissociation energies of some chemical bonds are shown in the following table.

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Selected Bond Energies

BondBond Energy(kJ/mole)

BondBond Energy(kJ/mole)

H-H 432 C=O 799

O=O 494 C-C 347

O-H 460 C=C 611

C-H 410C=C (aromatic)

519

C-O 360 N=O 623

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The negative value of the Enthalpy of Reaction indicates that the reaction is a heat producing or exothermic.

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Oxidation is easy to recognize when an element changes changes oxidation state to become an ion. However when dealing with organic molecules, atoms are joined by covalent bonds rather than ionic ones so oxidation is not as easily recognized.

In organic molecules, an atom is oxidized if during a reaction it becomes bonded to a more electronegative element which pulls electron density away from it.

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In general, oxidation occurs if the oxygen content of a covalently bonded molecule increases or if the hydrogen content decreases.The carbon atom in methane (CH4) is more reduced than the carbon atom in carbon dioxide (CO2)

Cycloalkane Stability

5- and 6-membered rings most stable Bond angle closest to 109.5 Angle (Baeyer) strain Measured by heats of combustion

per -CH2 - =>

FREE RADICAL HALOGENATION

Cl2 h

H— C lH C

H

HH

H C

Cl

HH

Example: methane + chlorine

• multistep process involving radicals

CH4 + Cl2 CH3Cl + HCl

Halogenation = SubstitutionHalogenation = Substitution

very reactive species with an odd

(unpaired) electron (a total of only 7

electrons)

• sp2 hybridised

• trigonal planar

CH

H

H

Free Radicals

ALKANES - HALOGENATIONALKANES - HALOGENATION

• Mechanism involves three stages

Step 1. InitiationStep 1. Initiation

• the initial production of free radicals

Cl Cl Cl Clh

reactive species

Step 2. PropagationStep 2. Propagation

• radicals undergo substitution reactions

• chain reaction

H3C H Cl H3C + H Cl

Cl ClH3C H3C Cl + Cl

Step 3. TerminationStep 3. Termination

• radicals collide and form stable products

• destruction / removal of free radicals

• chain reaction is broken

Cl Cl Cl Cl

CH3H3C H3C CH3

H3C ClH3C Cl

Cracking= PyrolysisCracking= Pyrolysis

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C3H8(g) => C2H4(g) + CH4(g)

C2H6(g) => C2H4(g) + H2(g)

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Cracking is the process by which hydrocarbons with relatively high molecular mass are chemically converted to hydrocarbons with lower molecular mass. This process makes greater use of the saturated hydrocarbons found in crude oil. Unsaturated hydrocarbons, with small molecular mass, such as ethene, propene and butadiene are of greater use to modern society as fuels and as raw materials for the production of plastics.

large molecule smaller moleculesCatalyst + heat

Cracking

Cracking is the name given to the process by which large hydrocarbon molecules are broken into shorter chains or monomers. This is done under high pressures and temperatures, but a catalyst allows slightly lower pressures and temperatures to be used.

Cracking

http://www.doitpoms.ac.uk/tlplib/recycling-polymers/origin.php

An example of possible fractions naphtha might split into is shown below.

Once separated the hydrocarbons are filtered to purify them.

THERMAL CRACKING, WITHOUT CATALYSTS.PYROLYSIS

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The first step in the process is the homolytic cleavage of a hydrocarbon.

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THE RADICALS RECOMBINATION

potential products:

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DISPROPORTIONATION REACTION

During this process, a hydrogen atom is transferred from one hydrocarbon radical to another

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Catalytic Cracking In Petroleum industry

C(m+n)H2(m+n)+2 → CmH2m + CnH2n+2

alkane alkene alkane(kerosene (gasoline size)gas/oil size)

The industrially used catalysts are zeolites (finely-porous aluminosilicates), which are mixed with the naphtha (gasoline range) as a fine powder. This is blown through a chamber at 500ºC.

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Cracking In Petroleum industry

The introduction of catalytic cracking and polymerization processes in the mid- to late 1930's met the demand by providing improved gasoline yields and higher octane numbers.

. Higher-compression gasoline engines required higher-octane gasoline with better antiknock characteristics.

Cracking In Petroleum industry

Catalytic - uses a catalyst to speed up the cracking reaction. Catalysts include zeolite, aluminum hydrosilicate, bauxite and silica-alumina.

fluid catalytic cracking - a hot, fluid catalyst (1000 degrees Fahrenheit / 538 degrees Celsius) cracks heavy gas oil into diesel oils and gasoline.

hydrocracking - similar to fluid catalytic cracking, but uses a different catalyst, lower temperatures, higher pressure, and hydrogen gas. It takes heavy oil and cracks it into gasoline and kerosene (jet fuel).

http://science.howstuffworks.com/oil-refining5.htm

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