Basic Concepts and Hydrocarbons

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  • Created by: helena
  • Created on: 27-05-13 12:20

Introduction to Organic Chemistry

A hydrocarbon is a compound of carbon and hydrogen only

An Unsaturated hydrocarbon contains carbon-to-carbon multiple bonds

A Saturated Hydrocarbon has single bonds only

Aliphatic hydrocarbons ~ straight of branched chains

Alicyclic Hydrocarbons ~ ring structure

Alkanes are saturated hydrocarbons

  • 109.5 degree bond angle
  • tetrahedral shape
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Formula of Organic Compounds

Remember n=m/Mr

Empirical Formula is the simplest whole number ratio of each element present in a compound

A molecular Formula is the actual number if atoms of each element in a molecule

General Formula is the simplest algebraic formula for a member of a homologous series

Displayed Formula shows the relative positioning of all the atoms in a molecule, and the bonds between them

Structural Formula shows the minimal detail for the arrangement of atoms in a molecule

A skeletal formula is a simplified organic formula with the hydrogen atoms removed, just leaving a carbon chain

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Structural Isomerism exists when molecules are composed of the same number and type of atoms, but these atoms are arranged in a different way

Stereoisomers are isomers with the same structural formula but a different arrangment of the atoms in space

A molecule must satisfy two criteria to have E/Z isomerism

  • a c=c bond must be present
  • Each carbon in the double bond must be attached to 2 different groups

Cis/trans isomerism is a special type of E/Z isomerism

  • cis = z = both major groups on the same side
  • trans = E = both major groups on different sides
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Organic Reagents and their Reactions

Homolytic Fission ~ each bonded atom takes one of the shared electrons, forming radicals, two species of the same type are formed

XY --> X· + Y·

Heterolytic Fission ~ one of the bonded atoms takes both of the shared pair of electrons, forming two ions. The positive ion is called a cation, and the negative ion is called an anion

XY --> X+ +Y-

Nucleophile ~ electron pair donors

Electrophile ~ An electron pair acceptor

In an Addition Reaction: 2 reactants --> 1 product

In a Substitution Reaction: 2 reactants --> 2 products

In an Elimination Reaction: 1 reactant --> 2 products

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Hydrocarbons from Crude Oil

Boiling point of Alkanes

  • As chain length increases, the boiling point increases
  • This is because there are more points of contact, meaning intermolecular forces get stronger
  • A branched isomer has a lower boiling point because there are fewer contact points

Fractional Distillation takes place in a fractionating column

  • Short chained hydrocarbons with lower boiling points condense near the top of the column
  • Long chained hydrocarbons with higher boiling points condense nearer the bottom 
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Hydrocarbons as Fuel

Alkanes burn in a plentiful supply of oxygen to form CO2

Alkanes will undergo incomplete combustion when there is a limited supply of oxygen. Carbon monoxide and water are formed

After fractional distillation there is a surplus of long chained hydrocarbons

Cracking breaks down long chained hydrocarbons to form a mixture of shorter chained hydrocarbons and alkenes

Most catalytic cracking uses a zeolite catalyst at around 450o

Unbranched hydrocarbons can be converted into branched alkanes in a process called isomerism

Aliphatic hydrocarbons can be converted into cycic or aromatic hydrocarbons in a process called reforming. Hydrogen gas is also formed

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Halogenation of Alkanes

Mechanism for Chlorination ~ radical substitution

Step 1 ~ Initiation 0 radicals --> 2 radicals

Cl2 --> 2Cl· (UV catalyst needed)

Step 2 ~ Propagation  A radicals --> A radicals

CH4 + Cl· --> CH3· + HCl

CH3· + Cl2 --> CH3Cl + Cl·

Step 3 ~ Termination  A radicals --> 0 radicals

2Cl· --> Cl2

CH3· + CH3· --> C2H6

CH3· + Cl· --> CH3Cl

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At least one C=C bond

More reactive than alkanes and usually take part in addition reactions

If they have one C=C bond then their general formula is CnH2n+2

Alkenes form E/Z isomers, usually cis/trans isomers

triagonal planar around the carbon

120bond angle around the carbon

double bond is formed by the overlap of the p-orbitals

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reactions of alkenes

Alkenes typically take part in addition reactions

It is added across the double bond, causing the π bond to break

Addition of Hydrogen

A mixture of hydrogen gas and the gaseous alkene is passed over a catalyst of nickel at a temperature of 150oC

Addition of Halogens

The halogen adds across the double bond to give a di-substituted halogenoalkane. Bromine goes from brown to colourless

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Further Addition reactions of Alkenes

H2C=CH2 + HBr --> CH3CH2Br 

H2C=CH2 + HI -->  CH3CH2I

H2C=CH2 + HCl --> CH3CH2Cl

H2C=CH2 + H2O --> CH3CH2OH (sulphuric Acid Catalyst)

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Alkenes and Bromine

Bromine reacts with alkenes at room temperature

the positively charged carbocation is unstable and quickly reacts with the bromide ion to form the organic product

alkenes with more than one double bond can be reacted with excess hydrogen in the preence of a nickel catalyst to form an alkane

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Making Polymers

Polymers are long chained molecule with large Molecular masses. They are made from short-chained molecules called monomers. If these monomers are alkenes and then many monomers can be added together to form a long polymer chain. This process is called addition polymerisation

Monomers are unsaturated

Addition polymers have saturated chains with no double bonds

Addition polymerisation i carried out in two important industrial processes

  • Radical polymerisation
  • The Ziegler-Natta process

A repeat unit is a way of representing polymers

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Polymers - Dealing with our Waste

Biodegradeable material is a material that is broken down naturally in the enviroment by living organisms

Recycling Polymers

  • Sorting
  • Reclamation

Other uses of polymer waste

  • Polymers as a fuel source
  • Feedstock recycling
  • Recycling PVC
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