Hydrocarbons Flashcards
Fossil fuel
one that is derived from organisms that lived long ago
Non-renewable resources
cannot be reformed in a reasonable timescale
Greenhouse gas
causes increase in earths temp
Γ³ bonds
made by end to end overlap of s or p orbitals
Complete combustion
combustion that occurs with excess oxygen
Incomplete combustion
Combustion that occurs with insufficient oxygen
halogenation
Reaction with any halogen and an organic compound
Initiation
Reaction that starts a process. Molecule is turned into 2 radicals
homolytic bond fission
When a bond is broken and each of the bonded atoms receives one of the bond electrons
Radical
Species with an unpaired electron
Highly reactive and they react indiscriminately to gain another electron to form an electron pair
Propagation
Reaction by which the process continues
Molecule reacts with a radical to make a new molecule and a new radical
Chain reaction
One that involves a series of steps and, once started, continues
Termination
Reaction that ends the process
Two radicals react to make a molecule
Reaction mechanism
Shows the stages by which a reaction proceeds
Substitution reaction
One in which one atom/group is replaced by another atom/group
Pi bond
One formed by sideways overlap of P orbitals
E-Z isomers
Isomerism that occurs in alkenes due to restricted rotation about the double bond
Electrophile
Electron deficient species that can accept an lone pair of electron
heterolytic bond fission
When a bond is broken and one of the bonded atoms receives both electrons from the covalent bond. Ions are formed
Addition reaction
A reaction in which reagents combine to give only one product
Carboncation
An ion with a positively charged carbon atom
Polymerisation
Joining of a very large number of monomer molecules to make a long chain polymer molecule
Monomer
Small molecule that can be made into a polymer
Repeat unit
Section of a polymer that is repeated to make a whole structure
Advantages of fossil fuels
- Available in a variety of forms so type of feel can be matched with its use
- Available at all times
- Currently widely available and current infrastructure is generally set up around the use of them
Disadvantages of fossil fuels
- Non renewable
- Combustion of fossil fuels produces carbon dioxide: greenhouse gas- Absorb infrared radiation from the Earthβs surface then emitted in all directions
- acid rain
- carbon monoxide
alkanes structure
And properties
- General formula CnH2n+2
- Saturated hydrocarbons
- Properties vary as the relative molecular mass increases
Small alkanes or gases in room temperature, larger are liquids - Burn by reaction with oxygen, reaction is exothermic
Products when hydrocarbons undergo complete combustion
Water and carbon dioxide ( If sufficient oxygen is present)
Write a balanced chemical equation for the complete combustion of ethane
2C(2)H(6) + 7O(2) β 4CO(2) + 6H(2)O
When does incomplete combustion occur? What is required to ensure complete combustion occurs?
Incomplete combustion occurs when there is an insufficient supply of oxygen.
To ensure complete combustion occurs, the reaction should be carried out with excess oxygen.
Compare the difference in reactivity of alkanes and alkenes
Alkenes have a high electron density in the carbon-carbon double bond, C=C. This makes them more susceptible to attacks from electrophiles, compared to alkanes, making alkenes more reactive.
Halogenation process
Alkanes do not react with halogens in the dark but if exposed to UV light they will
Initiation stage 1
UV light has sufficient energy to break the cl - cl bond homolytically
Cl2β-> 2Cl*
Propagation stage 2
Uses a radical as a reactant then forms a radical as a product
Means reaction continues- chain reaction
Cl* + CH4ββ> CH3* +HCI
CH3* + Cl2β-> CH3Cl +Cl*
Termination stage 3
Propagation steps continue until radicals meet then reaction stops
Cl* +CH3* β-> CH3Cl
Radical reactions are hard to control
For the substitution reactions occur meaning a mixture of products will be formed
Make this an unsatisfactory method for making a high yield of a specific halogenalkane
Can be avoided if amounts of halogen use is very limited
structure of alkenes
- Unsaturated hydrocarbons
General formula CnH2n
How alkenes are formed
Formed when petroleum is cracked and although they undergo combustion they will not be generally used as fuels
Important as the starting material for organic synthesis reactions eg. Polymerisation
Ethene
- each C has structure 1s2,2s2, 2p2 ( each as 4 electrons for bonding)
- 3 βnormalβ covalent bonds formed using 3 electrons
- Bonds called Ε bonds, Involve both s electrons and one p electron in each C
P orbitals in ethene
1 p orbital electron on each C is not used to form the bonds
P orbitals overlap sideways producing pi bond- Area of high electron density above and below the plane of a molecule
Restricted movement about double bonds is due to pi bond
Meaning many alkenes exist as E-Z isomers
Properties of E-Z isomers
- In general the shape of E molecules mean that they can fit together more closely and have stronger intermolecular forces therefore higher Mt.
Electrophilic addition reactions
An electrophile is any species that can accept a lone pair of Electrons
Mechanism of reaction involves heterolytic bond vision leading overall to an addition reaction
Stability of a carboncation
The most stable a carbocation is the more likely it will form
The positive ion is more stable in reaction one than in reaction two because alkll groups tend to release electrons so that they become S+
Spreads or delocalises positive charge and stabilises the iron meaning secondary carbocations are more stable than primary
Testing for alkenes
- Reaction with bromine
Brown bromine water to clear
Liquid bromine can be very corrosive
Or decoloration of purple acidified potassium maganate
Factional distillation- why are different fractions different boiling temperatures
Boiling temperatures increase with increase in chain length
More carbon atoms result in an increase in Van der waals forces
How can fractional distillation be carried out
Petroleum is heated and evaporated
Fractions condense at different temperatures They are separated with different bOiling temperatures
