F322: Organic Chemistry - Hydrocarbons

HideShow resource information

Introduction

Introduction

-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. 

1 of 19

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.  

2 of 19

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.

3 of 19

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. 

4 of 19

Isomerism

-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. 

5 of 19

Reactions

-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.

6 of 19

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. 

7 of 19

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.  

8 of 19

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.

9 of 19

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
 

10 of 19

Alkenes

-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. 

11 of 19

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

12 of 19

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 

13 of 19

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.

14 of 19

Reactions Of Alkenes (Part 4: Bromine)

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

Mechanism
-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. 

15 of 19

Chemistry Of Alkenes

Propene

Propene + Br2 -> 1,2-dibromopropane

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

Propene + HBr(g) -> 1-bromopropane + 2-bromopropane
A MIXTURE OF STRUCTURAL ISOMERS.

Propene + H2O(g)--H3PO4 catalyst, high temperature/pressure -> Propan-1-ol + Propan-2-ol
A MIXTURE OF STRUCTURAL ISOMERS.

16 of 19

Industrial Importance Of Alkenes

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

-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.

17 of 19

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. 

18 of 19

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.  

19 of 19

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Functional Groups resources »