Alkanes Flashcards
What is the empirical formula?
Simplest whole number ratio
What is the molecular formula?
The actual number of atoms in a molecule
What is the structural formula?
Minimal detail of how the atoms are arranged in space
e.g. propane = CH3CH2CH3
What is the displayed formula?
Shows all the covalent bonds between every atom
What is the skeletal formula?
Carbon skeleton of a molecule where no hydrogen atoms are included
(Carbon atoms are represented by a dot and you don’t show hydrogen atoms)
What is the corresponding name beginning of each number?
One - meth
two - eth
three - prop
four - but
five - pent
six - hex
seven - hept
eight - oct
nine - non
ten - dec
What does the ending of an organic compound show?
The homologous series it is apart of
What are homologous series?
Families of organic compounds with the same functional group and same general formula
Do homologous series have the same chemical properties?
Yes
What does each member in a homologous series differ by?
CH2
What is the trend in physical properties of a homologous series?
There is a gradual trend - they have similar physical properties
What does it mean if something is saturated?
There are only single carbon bonds
What is a hydrocarbon?
A compound containing H and C
What is the general formula for alkanes?
CnH2n+2
How does fractional distillation work?
- Crude oil is heated and vaporised
- Fed into fractionating column
- Gases rise up column
- Condense at their boiling point
Which chains are at the bottom in fractional distillation and why?
The longer chains as they have a higher boiling point - more energy required to break London forces as there are more of them
Why are the longer chains at the bottom in fractional distillation less useful?
They are less flammable
What is cracking?
The breaking of large hydrocarbons into smaller hydrocarbons through breaking up carbon-carbon bonds
Why do we carry out cracking?
- Shorter chains are more flammable
- So more useful
- So more in demand
When does complete combustion occur?
When we have sufficient oxygen - produces CO2 and H2O
When does incomplete combustion occur?
When we have insufficient oxygen - produces CO and H2O
List the pollutants
Carbon dioxide
Carbon monoxide
Particulates of carbon(soot)
Sulfur dioxide
Oxides of nitrogen
How is carbon dioxide formed and why is it harmful?
- Formed in complete combustion
- Contributes to global warming
How is carbon monoxide formed and why is it harmful?
- Incomplete combustion
- Toxic gas
How are particulates of carbon formed and why are they harmful?
- Incomplete combustion
- Found in chimneys, coal
- Causes global dimming
- Respiratory problems
How is sulphur dioxide formed and why is it harmful?
- Sulphur containing impurities in the fuel are burnt to form SO2
- Acid rain - when it dissolves in atmospheric water vapour of liquid water, it is acidic
How are oxides of nitrogen formed and why are they harmful?
- N2 and O2 react at high temperatures in an engine to break the N-N bond
- N2 and O2 come from the air or the atmosphere
- Acid rain
Respiratory problems
Why is more energy required to break up N2 than O2?
N2 has triple covalent bonds
What does a catalytic converter do?
Converts CO, NOx and unburned fuels to N2, CO2 and water vapour
What is the catalyst in a catalytic converter made from?
Either:
- Platinum
- Rhodium
- Palladium
What shape is the catalyst in a catalytic converter in?
A honeycomb to increase surface area to increase rate of reaction (known as catalysis)
Why do we use alternative fuels?
To reduce pollution
What kind of fuel are biofuels?
- Renewable fuels
- They can be replaced over a short period of time
- The bio part means it comes from living matter
What are the 3 main biofuels and how are they made?
Biodiesel - made by refining renewable fats and oils
Bioethanol - made by fermentation
Biogas - made/released when organic waste breaks down
What are biofuels often considered as?
Carbon neutral
What does carbon neutral mean (link to biofuels)?
- As plants grow, absorb CO2 which is = amount of CO2 they release when burnt
- Not completely true as CO2 will be produced in refining and transport of plants
What do biodiesel and biofuel do?
Reduce the amount of waste going to landfill
How do biodiesel and biofuel reduce the amount of waste going to landfill?
The waste can be used to produce them
How could biofuel production provide money for less developed countries?
They have space to grow the crops required
What is a disadvantage of biofuels in terms of cost?
The cost of converting engines and machinery to run on biofuels instead of petrol/diesel
What is a disadvantage of biofuels in conjunction with developed countries?
Many developed countries don’t have space to be able to produce enough plants to make the biofuels as land is needed for food production
What is reforming?
It is the process of turning straight chain hydrocarbons into branched-chain alkanes and cyclic hydrocarbons
Why are cyclic hydrocarbons more useful?
They are used for more efficient combustion
What is free radical substitution?
Replacing H atoms with halogen atoms
How do we carry out free radical substitution?
We react halogens with UV light
What do we form in free radical substitution?
Halogenoalkanes
What are free radicals and what is their reactivity like?
Free radicals have an odd number of electrons
They are very reactive
How do we represent free radicals?
It is drawn as a . on the atom with the odd number of electrons
What are the 3 steps to free radical substitution?
Initiation
Propagation
Termination
What is initiation?
It involves the splitting of the halogen bond using UV light (e.g. splitting of Cl-Cl)
What is propagation?
- The halogen free radicals produced are extremely reactive
- In the propagation step a free radical reacts with a reactant and produces a molecule and another free radical
What is termination?
When any 2 free radicals meet they react to form a molecule
This removes free radicals and stops the chain reaction
What is formed in free radical substitution?
Alkane + halogen = halogenoalkane + H-X
What is the definition of a free radical substitution?
A reactive species that has an unpaired electron
What is 1 limitation of free radial substitution?
Impurities - multiple termination steps
What is another limitation of free radical substitution?
Further substitutions - more than 1 halogen is substituted onto a C
What is another limitation of free radical substitutuion?
Substitution of different C atoms
- Halogen substituted onto different C atoms
How is thermal cracking done?
Takes place at high temperatures and high pressure
How is catalytic cracking done?
Uses a zeolite catalyst at a slight pressure and high temperature
What is knocking?
It is where alkanes explode of their own accord when the fuel/air mixture in the engine is compressed
What is homolytic fission?
When a covalent bond breaks into two and each atom receives 1 electron from the bond
What is a structural isomer?
These are compounds that have the same molecular formula but different structural formulae
What are the 2 types of structural isomers?
Chain and Positional
What is chain isomerism?
When compounds have the same molecular formula, but their longest hydrocarbon chain is not the same
What is chain isomerism caused by?
Branching
What does positional isomerism arise from?
The differences in the position of a functional group in each isomer
Where can the functional groups be located due to positional isomerism?
Different carbons
What is stereoisomerism?
Compounds that have the same atoms connected to each other, but the atoms are arranged differently in space
Where is E/Z isomerism seen?
In unsaturated or ring compounds that have the same molecular formula and order of atoms but different shapes
Where are the groups on a Z isomer?
The same side of the double bond/carbon ring
Where are the groups on an E isomer?
Opposite sides of the double bond/carbon ring
What is E/Z isomerism sometimes referred to as?
Cis/trans isomerism
How does cis/trans isomerism correlate with E/Z isomerism?
Cis = Z
Trans = E
What does Cis/trans isomerism cause compounds to have?
Different chemical and physical properties
What are the limitations of cis/trans naming?
It cannot be used with 3 or 4/groups of atoms when they are all different
When molecules have 4 different groups, what are they known as?
Chiral/asymmetric
What rules do we use when we use the E/Z naming system?
Cahn, Ingold, Prelog - CIP priority rules
How do we use the CIP rules?
We look at the atomic number of the 1st atom attached to the carbon in question - the higher the atomic number, the higher the priority
What are the steps for deciding whether a compound with 4 different atoms or groups of atoms attached to the carbon bond is E or Z?
Step 1 - Apply CIP rules
Step 2 - Deduce E or Z
What are optical isomers?
2 compounds which contain same number, kind of atoms and bonds and different spatial arrangements but have non-super imposable mirror images
What is the structure of optical isomers called?
Enantiomer
