F322: Organic Chemistry - Hydrocarbons

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-Hydrocarbons are organic compounds that contain Hydrogen and Carbon only.
-Saturated means only single c-c bonds, whereas unsaturated includes c=c bonds.
-Aliphatic is straight or branched hydrocarbons.
-Alicyclic is a ring structure.
-Alkanes are a homologous series, CnH2n+2.
-Functional groups are responsible for chemical properties, the same ones act similarily. 
-A homologous series is the same functional group differing each time by a CH2. 

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Naming Hydrocarbons

Naming Hydrocarbons

-Nomenclature is a system of naming compounds.
-The stem indicates the number of carbon atoms.
-An alkyl group is an alkane with a hydrogen atom removed, usually shown as R.
-If there are double bonds then this indicates that the hydrocarbon is an alkene.
-If halogens are incorporated into the molecule then this means that the compound will be a halogenoalkane
-Alcohols are shown by the functional group -OH.
-Aldehydes are shown by -CHO, which contain a C-H and a C=O; producing a suffix of -al.
-Ketones are C-CO-C, with a C=O bond.
-Carboxylic acids contain the group -COOH.  

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Formulae Of Organic Compounds

Formulae Of Organic Compounds

-n = mass, m / molar mass, M

-Empirical formula is the simplest whole-number ratio of atoms of each element present in a compound.
-E.g. Benzene (C6H6) and ethyne (C2H2), have the same empirical formula of CH.?

-Molecular formula is the actual number of atoms present.
-E.g. Ethene (C2H4) contains two atoms of Carbon, and four of Hydrogen.

-General Formula
.Alkanes = CnH2n+2
.Alkenes = CnH2n
.Alcohols = CnH2n+1OH.

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Structural and Skeletal Formula

-A structural formula shows the minimal detail for the arrangement of atoms in a molecule.

-A skeletal formula is a simplified organic formula with hydrogen atoms removed from alkyl chains, leaving just a carbon skeleton and functional groups.

Skeletal Formula:
-No C or H atoms shown.
-There is a C atom at each end of the chain.
-There is a C atom where two lines meet.

-When drawing cyclic compounds, you usually represent them with skeletal formula rather than displayed. 
-For unsaturated hydrocarbons, the double bond is drawn between the associated C atoms.
-When drawing functional groups, they go simply at the end of the line on which C atom it lies.
-In exams you will come across more complex structures (i.e. Retinol/Paracetemol), which are usually medicinal drugs. 

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-Structural isomers are molecules with the same molecular formula but with different structural arrangements of atoms. 
-Happens in three ways:
.When chains are branched or unbranched. (i.e. Butane and 2-methylpropane - C4H10).
.When a functional group has been added along the chain.(i.e. Propan-1-ol and Propan-2-ol)
.Or when funtional groups are different. (i.e. Propanal and Propanone)

-Stereoisomers are compounds with the same structural formula but with a different arrangement of the atoms.
-A molecule must satisfy two criteria to have E/Z isomerism:
.A C=C bond must be present/
.Each C atom in the double bond must be attatched to two groups.
-E/Z isomerism is a type which different groups attatched to each carbon of the C=C bond mat be arranged differently due to restricted rotation of the C=C bond.
-cis-trans isomerism is a special type of E/Z, where there is a non-hydrogen group on each C of the C=C bond, the cis (Z) has the H atoms on each carbon on the same side; the trans (E) has the H atoms on different sides. 

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-Homolytic fission is the breaking of a covalent bond, with one of the bonded electrons going to each atom; forming two radicals.
-A radical is a species with an unpaired electron. (Shown as a single dot). X-Y -> X. + Y.
-Heterolytic fission is the breaking of a covalent bond with both of the electrons going to the atoms forming a cation and an anion. X-Y -> X+ + Y-
-An electrophile is an atom (or group of atoms) that is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.
-An nucelophile is an atom that is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond.

-An addition reaction is where a reactant is added to an unsaturated molecule to make a saturated molecule. 2 Reactants -> 1 Product
-A substitution reaction is where an atom or group of atoms is replaced with a different atom or group of atoms. 2 Reactants -> 2 Products.
-An elimination reaction refers to the removal of a molecule from a saturated molecule to make an unsaturated molecule. 1 Reactant -> 2 Products.

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Hydrocarbons From Crude Oil

-Crude oil is a mixture of over 150 different hydrocarbons, and needs to be broken down to create useful chemicals.
-Fractional distillation is the seperation of the components in a liquid mixture into fractions which differ in boiling point by means of distillation, typically using a fractional column.
-In a fractionating column:
.Short-chained hydrocarbons with low boiling points condense near the top.
.Longer-chained hydrocarbons with high boiling points condense near the bottom.
.Petroleum gas does not condense and flows out.

-As chain length increases the boiling point increases because the intermolecular forces become stronger, there are more points of contact and therefore more van der Waals' forces; needing a higher temperature/energy.

-As branching increases the boiling point decreases as there are fewer points of contact between the molecules, leading to few van der Waals' forces and less energy is needed. 

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Hydrocarbons As Fuels

-Short-chained alkanes are valuable as fuels, for example methane burns cleanly in air to produce CO2 and H2O; however some of the alkane burnt in a car engine undergoes incomplete combustion which produces CO. 

-Cracking refers to the breaking down of long-chained saturated hydrocarbons to form a mixture of shorter-chained alkanes and alkenes. 

-Producing branched alkanes - Unbranched can be converted to branched alkanes in isomerisation. (Pentane -> 2,2-dimethylpropane)
-Producing cyclic alkanes - Aliphatic hydrocarbons can be converted into cyclic via reforming, H gas also produced. (Pentane -> Cyclopentane + H2)

Improving fuels - branched and cyclic alkanes are important additives in increasing the burn efficiency.  

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Fuels Of The Future

-Overall crude oil is being consumed too quickly, all the petrochemicals and fuels are limited; they also produce atmospheric pollution (i.e. CO and SO2).
-As well as this they increase the amount of greenhouse gases and increase the rate of Global Warming.

-However a biofuel is derived from recently living material such as plants/animal waste (i.e. Ethanol can be made by fermenting sugar).
-In the UK bioethanol is being endorsed as a good solution.

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Substitution Reactions Of Alkanes

-Radical substitution is a type of reaction in which a radical replaces a different atom or group of atoms. (CH4 + Cl2 -> CH3Cl + HCl)
-Covalent bonds are broken by homolytic fission to form radicals, and a H atom in the alkane is substituted by a halogen atom.

-Mechanism is a sequence of steps showing the path taken by electrons in a reaction.
Step 1 - Initiation
The first step in radical substitution in which the free radicals are generated by UV.
Cl-Cl -> Cl. + Cl.

Step 2 - Propogation
The two repeated steps that build up the products in chain reaction.  
1st) CH4 + Cl. -> .CH3 + HCl                      2nd) .CH3 + Cl2 -> CH3Cl + Cl.

Step 3 - Termination 
The step at the end when two radical combine to form a molecule.
Cl. + Cl. -> Cl2               .CH3 + .CH3 -> C2H6              .CH3 + .Cl -> CH3Cl

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-Alkenes are unsaturated hydrocarbons with at least one C=C bond.
.Aliphatic with one double bond = CnH2n.
.Alkenes are more reactive than alkanes, and typically take part in addition reactions.
.Alkenes can form E/Z isomers.

-The C=C bond is made up of two parts, a sigma bond and a pi bond.(Sigma also in alkanes)
-A pi-bond is the reactive part of a double bond formed above and below the plane of the bonded atoms by sideways overlap of p-orbitals. It fixes the C atoms in position, at either end; reducing rotation.
-In alkenes all carbons uses three of its electrons to form three sigma bonds, and one to form a pi-bond.

-Three regions of electron density surround each C atom in the double bond.
- Pairs repel as much as possible, forming a triginal planar molecule with bond angle 120.

-Cyclic alkenes have closes rings (i.e. cyclohexene), that do not follow the same pattern of general formula. 

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Reactions Of Alkenes (Part 1)

-The C=C increases reactivity (i.e. from C-C being 347, to say 620), this is because a double bond is stronger than a single bond and a pi-bond is weaker than a sigma bond.
-When an alkene reacts the pi-bond breaks and the sigma remains intact.

-Alkenes typically take part in addition reactions.
.A small molecule is added across the double bond causing the pi-bond to be broken.
.Two reactant molecules react together to form one product molecule.
.Forming a saturated product.

Addition of Hydrogen
A mixture of H gas and alkene is passed over a nickel catalyst at 150 degrees.
-The H adds across the double bond, and forms an alkane.
-The reactions is hydrogenation.

Ethene + H2           --Ni 150-->            Ethane
UNSATURATED                                SATURATED

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Reactions Of Alkenes (Part 2)

Addition of Halogens
Alkenes react rapidly with halogens such as Cl, I, and Br.
-The halogen adds across the double bond to give a di-substituted halogenealkane.
-The reaction is known as halogenation.
Ethene + Br2 -> 1,2-dibromoethane
If bromine is added to an alkene the colour goes from orange to colourless; as it reacts.

Addition of Hydrogen Halides
-HCl, HBr, or HI add across the double bond to produce a halogenealkane.
-Are gasses at room temperature and are bubbled into the alkene.
Ethene + HBr -> Bromoethane

Addition of Steam
A way of preparing alcohols in industry.
-Steam and gaseous alkene are heated to high temperature/pressure, in presence of a phosphoric catalyst.
-Known as hydration reaction.
Ethene + Steam  ---H3PO4---> Ethanol 

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Reactions Of Alkenes (Part 3)

Addition to Unsymmetrical Alkenes
These form a mixture of organic products.
Propene + HBr ---> 2-bromopropane ---> 1-bromopropane
                                   MAJOR                   MINOR

-Electrophilic addition is a type of addition in which an electrophile is attracted to an electron rich centre or atom, to accept a pair of electrons and form a new covalent bond.
-HBr is a polar molecule, the electron pair in the pi-bond is attracted to the slightly postive H atom, causing it to break.
-A new bond forms between one of the C atoms and the H atom, then the H-Br breaks by heterolytic fission, with the electron pair going to Bromine.
-A Br- and a carbocation is formed - an organic ion in which a C atom has a positive charge.
-The curly arrow represents the movement of an electron pair in the breaking/formation of a covalent bond, the Br quickly reacts with the unstable carbocation.

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Reactions Of Alkenes (Part 4: Bromine)

Addition of Bromine
-Reacts with alkenes at room temperature, used as a test for unsaturation.

-Bromine is non-polar, the pi-bond in the alkene repels it, inducing a dipole in Br2.
-The electron pair in the pi-bond is now attracted the the slighly positive Br atom.
-This causes the bond to break and a new bond between C and Br to form.
-A Br- and a carbocation are formed, reacting quickly; all bonds are broken by heterolytic fission and ions are formed in the intermediates. 

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Chemistry Of Alkenes


Propene + Br2 -> 1,2-dibromopropane

Propene + H2 ---Ni catalyst, 150 degrees---> Propane

Propene + HBr(g) -> 1-bromopropane + 2-bromopropane

Propene + H2O(g)--H3PO4 catalyst, high temperature/pressure -> Propan-1-ol + Propan-2-ol

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Industrial Importance Of Alkenes

-Ethene can be uses to make other important chemicals:
.Paint remover

-Polymers are long-chained molecules with large molecular masses, made up of monomers; creating a long chain of monomers is known as addition polymerisation.
-Addition polymerisation is the process in which unsaturated alkene molecules add on to a growing polymer chain one at a time to form a long saturated polymer. 

-Radical polymerisation requires 200 degrees, and very high pressures. 
-This leads to branching and a production of polyphenylethene and polystyrene.

-The Ziegler-Natta process involes a TiCl3 or Al(C2H5)2Cl catalyst at a temperature of 60.
-This is the most common way to manifacture polyethene.

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Polymer Chemistry

Addition Polymerisation

Common examples include:
-Ethene -> Polyethene
-Propene -> Polypropene
-Phenylethene -> Polystyrene

To identify the monomer just take one section and evaluate the atoms, and re-include a C=C bond. 

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Polymers And Waste

-Different polymers can be used for packaging, however they are currently being replaced by biodegradable materials; due to the increase of landfill sites and litter. 
-To protect the environment, more polymer waste is being recylced - by being sored and reclamated; using the identification codes and then being processed. (i.e. PET bottles are now being converted into a large selection of new materials. 

-Polymers can be used as a fuel source under controlled conditions, to harness electricity.

-As well as this you can use them in feedstock recycling, to convert polymers into a gas that can be used to fuel other reactions.

-Finally the recylcing of PVC is problematic due to its high Chlorine content, however new technological advances have led to the first recycling plant.  

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