Home CHEMISTRY TOPIC 2: ORGANIC CHEMISTRY | CHEMISTRY FORM 4

TOPIC 2: ORGANIC CHEMISTRY | CHEMISTRY FORM 4

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Introduction to Organic Chemistry

Organic chemistry is the chemistry of carbon compounds. Due to the ability of carbon to form chains of atoms, and for other atoms or groups of atoms to be attached to these chains, there are a huge number of carbon compounds.

All organic compounds contain carbon together with one or more other elements such as hydrogen, oxygen, nitrogen, sulphur and the halogens.

Normally every compound of carbon is an organic compound. Even after discovering that these compounds could be synthesized in the laboratory, the definition that they are organic (of organic nature, that is, they originate from living things) has been retained. However, for conventional and historic reasons, some of the simpler compounds such as carbon dioxide (CO2) carbonates, carbon monoxide (CO3) are usually studied with other non-carbon compounds in Inorganic Chemistry.

Difference between Organic from Inorganic Chemistry

Distinguish organic from inorganic chemistry

Organic chemistry is the chemistry of carbon compounds. All organic compounds contain carbon and other elements such as H, O, N, S and the halogens.

Normally every compound of carbon is an organic compound. Examples of organic compounds/substances are plastics, milk, carbohydrates, lipids, proteins, sugar and hydrocarbons. Inorganic substances includes table salt, CO2, diamond, iron and water.

The Importance of Organic Chemistry in Life

Explain the importance of organic chemistry in life

Carbon is the most unusual atom. It has the ability to join up to itself and form very long chains of atoms. Without this ability, life on Earth would not exist because the molecules that make our bodies contain mostly long chains of carbon atoms.

All living things contain carbon compounds. Raw materials such as oil and coal, derived from living things, are also based on carbon. Our modern society is very much dependent on organic chemistry to make the fuels and materials that we use in every day of our lives. In particular, polymers, large molecules obtained from alkanes, have very widespread use. Without alkanes from crude oil our transport system would be impossible.

We also need various fractions obtained from crude oil (petrol, diesel, kerosene, oil, natural gas, etc.) to run motor vehicles and other machines to simplify our work and life.

In brief, the compounds obtained from crude oil have thousands of different uses, for example:

  • some are used as fuel or converted into fuels;
  • some
    are used for making detergents, dyes, drugs, paints, and cosmetics; and
    yet some are used for making polyethene, polyvinyl chloride (PVC) and
    other plastics.
The Origin of Organic Compounds
Explain the origin of organic compounds
Fossil
fuels were formed in the Earth’s crust from material that was once
living. Coal comes from fossils of plant material. Crude oil and natural
gas are formed from bodies of marine microorganisms. The formation of
these fuels took place over geological periods of time (many millions of
years).
Crude
oil is one of the world’s major natural resources. The oil is the
result of a process that began up to 400 million years ago. Prehistoric
marine organisms died and sunk to the sea bed and were covered by mud
and sand. The change into crude oil and natural gas was brought about by
high pressure, high temperature and bacteria acting over millions of
years. The original organic material broke down into hydrocarbons.
Compression of the mud above the hydrocarbon mixture transformed it into
shale. Then geological movements and pressure changed this shale into
harder rocks, squeezing out the oil and gas. The oil and gas moved
upwards through the porous rocks, moving from high- pressure to
low-pressure conditions. Sometimes they reached the surface, but often
they became trapped by a layer of non porous rock. Reservoirs of oil and
gas were created. These reservoirs are not lakes of oil or pockets of
gas. Instead, the oil or gas is spreadthroughout the pores in coarse
rocks such as sandstone or limestone the same as water is held in a
sponge.
The Fractional Distillation of Crude Oil
Describe the fractional distillation of crude oil
Crude
oil from an oilfield is separated from impurities such as sand and
water and is pumped through pipelines to the refinery. At the refinery,
fractional distillation is used to separate the crude oil into several
fractions, each fraction containing several hydrocarbons which boil
within a certain range of temperatures. These different boiling points
are roughly related to the number of carbon atoms in the hydrocarbon
(Table 2.1)
Separation
of the hydrocarbons takes place in a fractionating column
(fractionating tower). At the start of the refinery process, crude oil
is preheated to a temperature of 350–400°C and pumped in at
the base of the tower. As it boils, the vapour passes up the tower. It
passes through a series of bubble caps and cools as it rises further up
the column. The different fractions cool and condense at different
temperatures, and therefore at different heights in the column. The
fractions condensing at the different levels are collected on trays.
Thus,
vapour is rising and liquid falling at each level in the tower. As a
result very efficient fractionation occurs. Liquid is taken off at
several different levels, the higher the level, the lower the boiling
point of the fraction removed. Figure 2.2 shows the process of
separation of crude oil into different fractions.
After
fractional distillation, impurities are removed. The commonest impurity
is sulphur, which is removed and used to manufacture sulphuric acid. If
petrol (gasoline) containing sulphur is not purified before it is used
in an internal combustion engine, the exhaust fumes will contain oxides
of sulphur (SO2 and SO3). These are poisonous gases and will pollute the environment.
Uses of different petroleum fractions
1.
Natural gas (refinery gas). The gas fractions consist of mainly
methane, ethane, propane, and butane. The methane and ethane are usually
burnt as fuel. The propane and butane are liquefied and are distributed
in high pressure gas cylinders and tanks to be used for lighting and
heating purposes.
2.
Petrol (motor gasoline). It is mainly used as a fuel in internal
combustion engines in motor vehicles. It is also used as a solvent for
grease stains and paints.
3. Naphtha. It is used as a raw material for making many chemicals and plastics.
4.
Kerosene (paraffin). It is used in homes as a fuel in paraffin lamps
and stoves for heating, lighting and cooking food. However, in addition
to its domestic use, it is used as a fuel for jet engines in aeroplanes.
It is also used as a detergent.
5. Diesel oil. It is used as a fuel in diesel engines (e.g. in diesel train engines, tractors, lorries, diesel car engines).
6.
Lubricating oil. It is used to make petroleum jelly (e.g. Vaseline). It
is also used as oil for lubricating moving parts of cars and other
machines.
7. Fuel oil. It is used as a fuel for power stations, ships and factories.
8. Paraffin waxes. They are used to make candles, polishes and waxed papers. They are also used in water proofing and as grease.
9.
Asphalt, bitumen. They are used to make protective coatings for road
surfaces and concrete roof tops, and also as binding agents for roofing
sheets.
Carboxylic
acids form a homologous series of the general formula CnH2n+1COOH (or
CnH2n+1CO2H), where n = 1, 2, 4, etc. for successive members of the
group. All these acids have the characteristic functional (carboxyl)
group, –COOH, attached to a hydrocarbon chain.
Natural Sources of Organic Acids
Identify natural sources of organic acids
There are various natural sources of organic acids. Some of these sources are:
  • milk (lactic acid)
  • citrus fruits (citric acid);
  • tobacco (nicotinic acid); and
  • tea (tartaric acid).
The Oxidation of Ethanol to Ethanoic Acid
Explain the oxidation of ethanol to ethanoic acid
When exposed to open air, ethanol is oxidized (by oxygen of the air) to ethanoic acid. The reaction for the process occurs thus:
The Structures of the Homologues of Carboxylic Acids up to Five Carbon Atoms
Write the structures of the homologues of carboxylic acids up to five carbon atoms
Carboxylic
acids are named as if they are derived from alkanes by the replacement
of one hydrogen atom by the –COOH group. The two lowest members,
containing one atom and two carbon atoms respectively, are:
The other members of the homologous series are as shown below:
The
successive members of the series have molecular formulae which differ
by –CH2 It is important to remember that every carboxylic acid molecule
contains the functional group –COOH which is called the carboxyl group.
The Isomers of Carboxylic Acids up to Five Carbon Atoms
Name the isomers of carboxylic acids up to five carbon atoms
Like
other organic compounds, carboxylic acids also exhibit isomerism.
Isomers of carboxylic acid are a result of branching of the hydrocarbon
end (R) rather than the position of the carboxyl group in a molecule of
the carboxylic acid. More isomers of the carboxylic acids can be created
by branching the hydrocarbon end in as many different ways as possible.
Rules
The carbon of the carboxyl group (–COOH) is considered as carbon atom number 1.
Identify the positions of the alkyl group(s) attached to the (longest) acid chain. For example, in a molecule,
the alkyl group is methyl (–CH3) and it is attached to carbon number 2.
Name
the branched alkyl group, followed by the name of the acid to which the
alkyl group is attached. For example, in the case above (rule no.2):
  • the alkyl group is methyl;
  • it is attached to carbon number and
  • the acid to which it is attached is butanoic acid,CH3CH2CH2COOH
Therefore, the name of the compound is 2-methylbutanoic acid.
In
case there occurs more than one alkyl groups in the compound the
prefixes di(2), tri(3), tetra(4) etc (as it was the case in alkanes) may
be used. For, example in the compound
  • there are two methyl groups, one attached to carbon number 2 and the other to carbon number 3; and
  • they are both attached to butanoic acid chain.
Therefore, the name of the compound is 2,3-dimethylbutanoic acid.
Isomerism and nomenclature
Branching
isomerism is found in this homologous series. Isomerism in carboxylic
acids begins from butanoic acid, C3H7COOH. The first three members of
the series do not show isomerism because their hydrocarbon ends do not
form branches. The following are the structures and names of the isomers
of carboxylic acids up to five carbon atoms:
  • Butanoic acid, C3H7COOH or C3H7CO2H or CH3CH2CH2COOH
Isomers:
  • Pentanoic acid, C4H9COOH
Isomers
The Properties of Carboxylic Acids
Explain the properties of carboxylic acids
Carboxylic acids are weak acids. They are slightly ionized in dilute solutions.
Like
inorganic acids, their solutions contain H+ ions. The presence of H+
ions give the solutions acidic behaviour, that is, their solutions
affect indicators, just like the inorganic acids do.
Neutralization
Like
inorganic acids, carboxylic acids react with metals, alkalis,
carbonates, and hydrogen carbonates to form salts. For example:
Esterification
The
reaction between carboxylic acids and alcohols is called
esterification. The acids will react reversibly with alcohols to form
sweet–smelling esters. Concentrated sulphuric acid is a catalyst for the
reaction.
The
reaction can be reversed to recover an acid and alcohol again by
boiling the products (an ester + water) with a mineral acid (HCl or
H2SO4) or with an aqueous alkali (KOH or NaOH) as a catalyst.
Esters
are manufactured for use as solvents, food flavourings, and fragrance
for perfumes and beauty products. Ethyl ethanoate is just one example of
many esters. The esters usually have strong and pleasant smells. Many
of these compounds occur naturally. They are responsible for the
flavours in fruits and for the scents of flowers. Fats and oils are
naturally occurring esters used for energy storage in plants and
animals. Some of the naturally occurring esters include:
  1. vegetable oils e.g. palm oil, groundnut oil, cashewnut oil, olive oil, sunflower oil, etc; and
  2. animal fats.
All esters contain the functional group,
, where R is any alkyl group.
Preparation of Soap from Animal Fats or Vegetable Oil
Prepare soap from animal fats or vegetable oil
Vegetable
oils are formed from fatty acids and an alcohol called glycerol (also
called glycerine). Fatty acids are carboxylic acids with long chains of
carbon atoms. They are called “fatty” because the long chains repel
water, making them immiscible with water. Glycerol or glycerine (or
propane–1,2,3-triol) has three –OH groups. This is how fatty acids and
glycerol react:
Preparation of soap from oils
Soap
is made by heating animal fats or vegetable oils with sodium hydroxide
solution. The oils react with the solution of sodium hydroxide and break
down to form glycerol and the sodium salts of their fatty acids. These
salts are used as soap. The reaction equation is:
This
process is known as saponification. The soap you buy is made from a
blend of different oils. When soap dissolves in water it ionizes thus:
The cleansing agent in soap is the ion, RCOO–

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