This pack includes detailed notes on organic chemistry I, as well as some exam style questions related to the topic. Organic chemistry I is a content heavy topic consisting of many difficult-to-learn, complicated organic mechanisms that many students struggle to learn and understand. Luckily for yo...
Organic Chemistry I
Introduction to organic chemistry
A hydrocarbon is a molecule composed of only oxygen and hydrogen.
Organic molecules can be represented in different ways by different types of formula:
Empirical formula: The simplest ratio of all atoms of each element in a
molecule.
Molecular formula: The total number of all atoms of each element in a
molecule.
General formula: An algebraic formula for all members of a homologous
series.
Structural formula: A representation of a molecule which shows the
arrangement of atoms and functional groups in a compound.
Displayed formula: A representation of a molecule which shows all the bonds.
Skeletal formula: Where single lines represent single bonds and double lines
represent double bonds, with carbon atoms being represented by unlabelled
corners and ends, and hydrogen atoms not being labelled.
A homologous series is a group of compounds which all contain the same functional
group(s), the same general formula, react in similar ways, with the formula
increasing incrementally by CH2 from one member to the next.
A functional group is a group of atoms or a single halogen atom which determines
how a molecule reacts.
Reactions of organic molecules include:
Addition
Elimination
Substitution
Oxidation
Reduction
Hydrolysis
Polymerisation
Structural isomers are molecules with the same molecular formula but different
structural formulae. They can be either:
Carbon chain isomers – the length of the main carbon chain varies.
Positional isomers – the functional groups are the same but in different
locations.
Functional group isomers – the functional groups of the compounds are
different.
Stereoisomers are molecules with the same molecular formula and structural formula
but different arrangements of atoms in space. They can be either:
Geometric isomers – isomerism due to restricted rotation around the carbon-
carbon double bond (E-Z isomers).
, Optical isomers – molecules which have non-superimposable mirror images.
Alkanes
The general formula for alkanes is CnH2n+2
Alkanes and cycloalkanes are saturated hydrocarbons (hydrocarbons with the
maximum number of hydrogen atoms per carbon atoms and where all carbon-carbon
bonds are single).
Alkane fuels are obtained from fractional distillation, cracking and reforming of crude
oil.
In fractional distillation, the fractions of crude oil are separated based on their boiling
points. The fractions are collected as liquids which had been condensed from gases.
In cracking, either high temperatures or high pressures are used without a catalyst,
or low temperatures and low pressures are used with a catalyst. In modern times,
cracking takes place at around 500oC with low pressures and a zeolite catalyst.
In reforming, straight-chain alkanes are reformed into branched-chain alkanes and
cyclohexanes to increase the octane rating of petrol.
During the combustion of alkane fuels, pollutants can be formed. These include:
Carbon monoxide (from incomplete combustion of hydrocarbons)
Oxides of sulfur (from the burning of sulfur in crude oils)
Oxides of nitrogen (from the burning of nitrogen in crude oils, and the reaction
between nitrogen in the atmosphere and flames of oxygen from combustion)
Carbon particulates (from incomplete combustion of hydrocarbons)
Unburned hydrocarbons
There are problems arising from these pollutants:
Carbon monoxide is toxic as it binds with haemoglobin in the blood,
preventing the haemoglobin binding with oxygen.
Nitric oxide which is acidic and can cause damage to human lungs, buildings
and trees. It is the major constituent of acid rain.
Nitrogen dioxide has similar effects to nitric oxide
Sulfur dioxide is acidic and causes damage to buildings and the natural
environment. It can dissolve in rainwater to produce acid rain.
A catalytic converter can solve some of the issues caused by these pollutants:
2NO N2 + O2
2NO2 N2 + 2O2
2CO + O2 2CO2
HC + O2 CO2 + H2O (where HC represents any hydrocarbon)
Alternative fuels such as biodiesel and alcohols derived from renewable sources
such as plants can be used instead of non-renewable fuels such as alkanes.
, A radical is a species with an unpaired electron and is represented in mechanisms
by a single dot. It is formed from homolytic fission of a covalent bond.
Alkanes can react with oxygen (complete combustion or incomplete combustion).
Alkanes can also react with halogens by free radical substitution (initiation,
propagation, and termination steps).
Free radical substitution has limitations in synthesising organic molecules:
Further substitution reactions are required to add more halogen atoms
A mixture of products is formed, and so they need to be separated
Alkenes
The general formula for alkenes is CnH2n
Alkenes and cycloalkenes are unsaturated hydrocarbons (hydrocarbons which
contain at least one carbon-carbon double or triple bond and contain less than the
maximum number of hydrogen atoms per carbon atoms).
An electrophile is a species which attracts a pair of electrons.
Alkenes are more reactive than alkanes because they contain a double bond. The
double bond contains two sigma bonds and a pi bond. The pi bonds are an electron
rich area which are therefore attractive to electrophiles. This makes alkenes very
reactive.
Sigma bonds: This is the sharing of one pair of electrons and is formed when the
orbital (s or p) of one atom overlaps head-on with an orbital in another atom.
Pi bonds: The p orbital is perpendicular to the sigma bond (and the internuclear
axis) and forms a sideways overlap with another p orbital. Rotation of atoms around
a carbon-carbon double bond are therefore not possible, as they would cause the pi
bond to break, hence causing geometric (E-Z) isomerism.
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