introduction to organic chemistry

Organic chemistry is the chemistry of carbon and its compounds. These organic compounds contain carbon as the basic frame work and other elements like hydrogen, nitrogen and chlorine are attached to it.
Carbon has a unique behavior in a chemical sense because:

  1. It can form a very long chain of carbon atoms which can be up to 2000 atoms.
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These compounds consisting of chains of carbon atoms are called aliphatic compounds. These compounds can be saturated (if all the carbon atoms are joined to each other by a single covalent bond e.g. ethane,) or unsaturated (if it contains multiple covalent bonds i.e. either double or triple e.g. ethene,

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It can form a ring of carbon atom. The compounds that form rings of carbon atoms are alicyclic compounds.

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Because of these two unique behaviors, carbon can form very many and complex compounds which has made it necessary for its study under a separate branch called organic chemistry. However, for historical and conventional reasons some simpler compounds such as carbon dioxide (CO2) and sodium carbonate (Na2CO3) are usually studied under non carbon compound in inorganic chemistry.


Classification of organic compounds
Organic compounds can be classified into several groups. The simplest of the organic compounds are hydrocarbons. Other groups include: alcohols, esters, carboxylic acids, amines, ketones, alcohols and ethers.
These groups are differentiated from each other by functional groups.


Functional groups are groups of atoms that are common to a given homologous series and are responsible for chemical reactions.

Examples of functional groups include:
-OH for alcohols e.g. ethanol, CH3CH2OH; methanol, CH3OH
-COOH for carboxylic acids e.g. Ethanoic acid, CH3COOH; methanoic acid HCOOH
-NH2 for amines e.g. amino ethane, CH3CH2NH2, amino propane, CH3CH2CH2NH2


Homologous series
This is a series of organic compounds related to each other by the same functional group. Characteristics of homologous series include:
i) All members conform to a general molecular formula e.g.
CnH2n+2 for alkanes. If n=2, C2H6(ethane); if n=4, C4H10 (butane)
CnH2n for alkenes. If n=2, C2H4 (ethene); if n=3, C3H6 (propene)
ii) Members of the same homologous series have the same chemical properties (though varying in vigour/speed)

iii) The physical properties of the members change gradually with increase in molecular mass. E.g. boiling point, melting point and density increase with increase in molecular mass; there is a gradual change in state down the group (methane is a gas, pentane is a liquid and decane is a solid); solubility decreases down the group as molecular mass increases.
iv) Members in each homologous series differ from the next by –CH2 group (methylene group).
v) Members have the same general method of preparation

Hydrocarbons
These are compounds consisting of only hydrogen and carbon atoms. They have a general formula of CxHy where x and y can be any numerical whole numbers.
Hydrocarbons are classified into three main groups as alkanes, alkenes and alkynes. These three are differentiated by the following functional groups.

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Alkanes
These are saturated hydrocarbons with the general formula of CnH2n+2. Where n is the number of carbon atoms. Alkane members are referred to as the paraffin i.e. they have little affinity to react.


Sources of alkanes
The main sources of alkanes include:
i) Natural gas. This contains mainly methane with small amounts of other gases like propane and butane. Methane is formed by anaerobic decomposition of organic matter and it is found in swamps, stagnant ponds and marshes.
ii) Petroleum. This contains a wide range of alkanes ranging from molecular gases to high molecular waxy solids (C2-C40). Petroleum is formed by anaerobic decomposition of sea plants and animals. The components of petroleum are separated by fractional distillation, a process known as refining.

Nomenclature of alkanes
According to IUPAC (International Union of Pure and Applied Chemistry), all members of alkanes have their names ending with the suffix –ane.

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Straight chain alkanes have a prefix n before the normal name e.g.
CH3CH2CH2CH3 n-butane
In branched chains, the branch may be a hydrocarbon or other atoms like chlorine, and bromine.

The hydrocarbon side chains have one hydrogen less the parent alkanes and are generally referred to as alkyl groups. The alkyl groups derive their names from respective parent alkanes e.g.-CH3 (methyl); -CH2CH3 (ethyl); -CH2CH2CH3 (propyl); -CH2CH2CH2CH3 (butyl).

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iii) Name the branch (substituent group) i.e. methyl group
So, write the name of the alkane starting with the carbon position on which the branch is located (2); put a dash (-); write the name of the branch/substituent group (methyl) followed by the name of the longest straight carbon chain.
The above compound is therefore 2-methylpentane.
vi) If the branches of side chains are more than one and are similar, di, tri, etc are used.

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vii) If the side chains are different, naming follows alphabetical order

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Molecular and structural formulae
Molecular formula shows the number of each kind of atoms present in one molecule of a compound. It does not show the arrangement of atoms in the molecule.
Structural formula (graphical formula) shows the arrangement of atoms in one molecule of a compound.
Alkanes like other hydrocarbons and other organic compounds have covalent bonds between the atoms. In alkanes, the carbon atoms use all the four outer most electrons to form covalent bonds by sharing with other carbon atoms and hydrogen atoms. Because all the electrons are used up in the formation of covalent bonds, they are called saturated hydrocarbons.

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Isomerism
This is the existence of a compound with the same molecular formula but different structural formula.
Isomers are compounds with the same molecular formula but different structural formula.
All hydrocarbons with four or more carbon atoms per molecule posses isomers. E.g. butane (C4H10)

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Properties of alkanes
Alkanes are not so reactive and under go combustion and chlorination reactions only.

  1. Combustion
    Alkanes under go complete combustion in plenty of oxygen to form carbon dioxide and water vapour. For example, methane explodes in air/ oxygen on application of flame
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The combustion of alkanes produces considerable amount of heat. This explains why they are used as fuel for domestic and industrial uses.

The in complete combustion of carbon occurs in cylinders of petrol engines that results in to release of poisonous carbon monoxide and some times even carbon. It is therefore dangerous to run a car engine in a garage where there is no free air circulation.

  1. Chlorination

Alkanes under go substitution reaction with halogens. A substitution reaction is reaction in which an atom or a group of atoms in a compound is/are replaced by other atoms.
For the case of alkanes, this is only possible with halogens e.g. when sunlight shines on a mixture of methane and chlorine, the chlorine replaces hydrogen in a chain reaction i.e. substitution reaction occurs as follows:

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This reaction occurs rapidly in bright sunlight and when chlorine is in excess.


Petroleum (Crude oil)
Petroleum is formed by anaerobic decomposition of sea plants and animals. It is oil consisting of different alkanes normally ranging from C5H12 to C43H88. The oil deposits are usually found with sand and brine.


Refining fuel
The different alkanes that make up petrol can be separated by fractional distillation. This is based on the boiling points of the different components.
After the removal of impurities mainly sulphur compounds, it is heated until when most of it vaporizes. The vapour is passed into the bottom of a tall fractionating tower. The fractionating tower is divided into several compartments each cooler than the one below it.

During fractional distillation, the fraction of petroleum that is most volatile settles at the top and the non volatile heavy oil runs out and the bottom of the column

Fractionating tower

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The gas oil fraction is cracked to yield more petrol.


Cracking of gas oil
Fractional distillation of crude oil above only yields 20% of the petrol. More petrol is produced by the cracking process.
Cracking is the breaking down of large complex hydrocarbons into smaller molecules (of short carbon chain) by use of heat or catalyst. Heavy alkanes are cracked to produce useful alkenes and fuel of high quality (relatively smaller alkanes). E.g.

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i) Thermal cracking: this involves heating of large hydrocarbons at high pressures to break them into smaller molecules.

ii) Catalytic cracking: this involves the use of a catalyst to break down large and complex hydrocarbons in to simpler ones. Catalysts commonly used are silicon(IV) oxide and aluminium oxide. Catalytic cracking takes place at a relatively low temperature and pressure.


ALKENES

Alkenes are unsaturated hydrocarbons with a general formula of CnH2n. where n=2 or more. They are characterized by possession of a double bond between carbon atoms.
Nomenclature and structure
Alkenes are named as alkanes except that their names end with suffix –ene. Consider the table below.

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Ethene
This is the simplest alkene with molecular formula, C2H4.
Laboratory preparation
Ethene is prepared by dehydration of ethanol using excess concentrated sulphuric acid.
Set up

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Procedure

  • To 50cm3 of ethanol, add 100cm3 of concentrated sulphuric acid slowly while shaking under a tap
  • The apparatus is set as above and the mixture heated with care to 180˚C. Ethene is evolved and is collected over water.
    NB. The wash bottle of alkali solution removes sulphur dioxide produced in small quantity as ethanol reduces sulphuric acid slightly. The alkali also removes fumes of the acid.
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Some times aluminium sulphate is added to the reaction to reduce frothing.
Ethane can also be prepared by catalytic dehydration of ethanol. Here, ethanol vapor is passed over a heated catalyst to produce ethane.


Properties of ethene
Physical properties

  • Is a colorless gas with a faint sweet smell
  • It is insoluble in water but soluble in organic solvents eg benzene and methylbenzene
  • It is slightly less dense than air
    Chemical properties
    Alkenes are generally more reactive than corresponding alkanes. They undergo the following reactions
    a) Combustion
    Ethane burns in excess oxygen with a smoky flame since it contains a relatively high percentage of carbon forming carbon dioxide and water vapor
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b) Addition reaction of ethene
Ethene and other unsaturated compounds undergo addition reactions an addition reaction is one in which a molecule adds to an unsaturated compound by breaking the double bond or triple bond
Examples;-

i) When ethene gas is bubbled through bromine water, bromine water changes from red brown to colorless i.e. bromine water is decolorized or the red brown color of bromine is discharged

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c) Hydrogenation (addition of hydrogen)
When hydrogen and ethane mixture is passed over a finely divided nickel catalyst which is heated to about 2000C ethane is formed

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d) Reaction with sulphuric acid
Ethene undergoes an addition reaction with fuming concentrated sulphuric acid to form an oily liquid called ethyl hydrogen sulphate

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e) Polymerization
Ethene under a very high pressure becomes a liquid. When this liquid is strongly heated to about 2000C in the presences of a little oxygen catalyst, a white waxy solid (polyethene) is obtained.

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Uses of ethene

  • it is used in the manufacture of ethanol
  • it is used in the ripening of fruits
  • it is used in the manufacture of plastics (synthetic polymers e.g. polythene)
  • it is also used in preparing other solvents

Polymerization
Is the combination of many molecules of the same compound with relatively small molecular masses to form one complex molecule with very large molecular mass.
The complex molecule with a large molecular mass formed by the combination of many molecules of relatively small molecular masses is called the polymer. The small molecules from which a polymer is built are called monomers


Types of polymerization
These are mainly two i.e. addition and condensation
Addition polymerization
This is a combination of many small but unsaturated molecules to form a large molecule without any other product. In this case, the polymer posses the same empirical formula as the monomer. E.g. in the formation of polyethene

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Condensation polymerization
In the condensation polymerization, two different molecules combine to form one large molecule with consequent loss of simple molecules like water, hydrogen chloride etc. so the empirical formula of the monomer and the polymer are not the same e.g. formation of starch from glucose and formation nylon 6,6


Types of polymers
Polymers can broadly be divided into two groups namely natural polymers and synthetic polymers


Natural polymers

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Classes of addition polymers
There are two main classes of addition polymers i.e. plastics and rubber.
Plastics
A plastic is a substance which when soft can be formed into different shapes
Plastics are minor products formed by cracking of crude oil eg poly ethene. Polyvinyl chloride. Melamine
All synthetic polymers are plastics in nature

Advantages of plastics

  • They can easily be shaped and molded (they are ductile)
  • They are good thermal and electrical insulator
  • They resistant to acids and alkalis and they do not rust
  • plastics can be colored when they are being manufactured and they do not need repainting
  • They are light and therefore portable
  • They are cheap
    Disadvantages
  • Produce poisonous fumes when they are burnt
  • They are non biodegradable i.e they do not decay naturally
  • Where serious fire hazards occur, molten plastics can inflict very severe burn
    Types of plastics
    Plastics can be put into two types depending on their behavior upon heating i.e thermo- softening plastics and thermo-setting plastics
    a) Thermo- softening plastics (Thermo-plastics)
    These are plastics that soften or melt when heated and can be therefore be moulded into any shape while they are still soft. The plastics only harden when they cool.
    Structure of thermo-plastics
    The long polymer chains in thermoplastics lie along side each other. They may be entwined on each other but the polymer chains are not linked (not bonded to each other). When heated, the chains slide over each other making them soft and runny.
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Examples of thermoplastics

  1. Polythene
    Polythene is a polymer of ethene. There are two types of polythene i.e. low density polyethene and high density polythene.
    i) Low density polythene
    This is made by polymerizing ethene at a high pressure of 1000-2000 atmospheres and temperature of 200˚C. Oxygen is used as a catalyst. It has a lower softening temperature of 105˚C-120˚C. The low density is due to poor packing of the branched polymer chains.
    The low density polythene is soft, light and flexible
    Uses:
    For making polythene bags; insulation of electric cables because they can withstand bad weather conditions; making of squeeze bottles such as wash bottles; making plastic bags.
    Disadvantage:
    At boiling water temperature, they become soft so much that they become flappy and lose shape.
    ii) High density polythene
    It is made by polymerizing ethene at low pressure (5-25 atmospheres) and low temperature (20-50˚C) in the presence of a Ziegler catalyst. It has a higher softening temperature of about 140˚C. The high density is due top the close packing of the unbranched polymer chains. Very few of these polymers may be branched.
    They are much harder and stiff and do not lose shape at boiling water temperature.
    Uses
    For making crates e.g. of beer and sodas, bowls, toys, buckets, food boxes, e.t.c.
  2. Polyvinyl chloride (PVC)/Polychloroethene
    PVC is made by polymerization of vinyl chloride (chloroethene).
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PVC are more rigid than polyethene and are used for making water pipes, light switches and sockets, insulation for electric cables,, carpets, plastic rain coats e.t.c.

  1. Polypropene
    This is made by polymerizing propene at a high pressure in the presence of a Ziegler catalyst.
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b) Thermosetting plastics
These are plastics which do not soften or melt on heating and therefore cannot be remoulded into different shapes once they are set. They simply decompose upon heating. Thermosetting plastics have polymer chains which are bonded/ linked to each other. This is called cross linking.
Structure

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Examples of thermosetting plastics include: Bakelite (used for making electric plugs, sauce pan handler, switches); melamine (used for making cups and children dishes).


Natural rubber
Natural rubber is obtained from a rubber tree as a milky liquid called latex. Latex can be coagulated by addition of a little ethanoic acid to form a solid of high molecular weight.
The monomer of rubber is isoprene (2-methylbuta-1,3-diene)


Vulcanization of rubber
Rubber in its natural state is not strong or elastic enough and it is made more strong and useful by vulcanization which involves heating the rubber with sulphur. The sulphur combines with rubber forming cross linkages between natural rubber chains.

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Vulcanized rubber is stronger, more elastic and more durable.
Uses of vulcanized rubber

  • It is used in the manufacture of tyres
  • Used in the manufacture of foot wears
    Condensation polymers
    Fibres

    These are polymers which can be drawn into threads. This is because, the forces of attraction between the linear molecules are weak but those between individual atoms are strong.
  • Classification of fibres
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Advantages of synthetic/artificial polymers

  • Relatively low production cost compared to the cost of extracting natural polymers.
  • They are usually stronger and more resistant to corrosive substances like acids compared to natural polymers.
  • They can easily be modified depending on the purpose for which the polymer is required unlike natural polymers which are hard to modify. As well their quality can easily be improved in terms of appearance, strength e.t.c.
    Disadvantages of synthetic polymers
  • Many are non biodegradable causing pollution to the environment.
  • When burnt, they produce toxic gases like hydrogen cyanide (from polypropenenitrile) thus endangering lives of the people working in the factories.
    Alcohols/Alkanols
    These are organic compounds with hydroxyl (-OH) group attached to the hydro carbon. Alcohols have a general formula of CnH2n+1OH.
    Members of the series
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Alcohols are named by placing –ol in the place of –e in the corresponding alkane members.
Ethanol
Physical properties

  • It is a colourless liquid with a strong characteristic smell
  • It is a volatile liquid and boils at 78˚C
  • It is very soluble in water
    Chemical properties
  • Combustion
  • Ethanol burns completely in air with a blue non luminous flame producing carbon dioxide and water vapour
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  1. Dehydration
    When a little concentrated sulphuric acid is added to ethanol, an oily liquid called ethyl hydrogensulphate is produced and the reaction is exothermic.
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When concentrated sulphuric acid is heated with ethanol, it produces ethene.

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Preparation of ethanol
Ethanol is manufactured/ prepared by the process of fermentation of carbohydrates such as starch and sugars.


Fermentation
This is a process in which carbohydrates like starch and sugars are converted to alcohol by enzymes. The enzymatic break down of glucose yields simple compounds like ethanol and carbon dioxide. Some heat is as well generated. Fermentation takes place in the absence of oxygen (anaerobic process).


Preparation from starch
Starch is heated with malt at a temperature of 60˚C. Malt contains an enzyme diastase which hydrolyses starch to maltose.

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Yeast is added at room temperature to the mixture and left to ferment for 2-3 days. Yeast contains two enzymes, maltase and zymase. Maltase catalyses the hydrolysis of maltose to glucose as below.

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Zymase catalyses the breakdown of glucose into ethanol, carbon dioxide, producing heat in the process.

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The crude ethanol produced can then be concentrated or purified by fractional distillation.


Preparation of ethanol from millet

  • Millet flour is mixed with little water to form paste. The mixture is then put under ground for about 8 days.
  • It is then removed, roasted and dried under the sun.
  • The dried material is then mixed with germinated millet flour (yeast).
  • Water is added and the mixture allowed to ferment for about 3 days in a warm place. This forms a local drink known as “Malwa”
    Preparation of ethanol from ripe bananas
  • Ripe bananas are squeezed to obtain the juice.
  • The juice is filtered to remove the solid particle.
  • The juice is mixed with roasted sorghum flour and the mixture allowed to ferment for 1-3 days in a warm place. A crude form of ethanol locally known as “Tonto” is obtained.

Beer is made by the fermentation of the starch in barley; wine by the fermentation of sugars in grapes. Spirits are obtained by distillation of dilute solutions produced by fermentation and there fore have an increased alcoholic content.


Uses of ethanol

  • It is used as an alcoholic beverage e.g. beers, wines and spirits
  • It is used as a solvent for paints, varnishes e.t.c
  • It is used as a fuel
  • It is used as a preservative and for sterilization
  • It is used as a thermometric liquid especially in minimum and maximum thermometers.

SOAPS AND DETERGENTS
Soap

Soap is a sodium or potassium salt of a long chain carboxylic acid known as sodium or potassium stearate.
Manufacture of soap
The process of making soap using an alkali and fat/oil (ester) is known as saponification.
Boil vegetable oil (from coconut, ground nuts, cotton e.t.c) or animal fat (from cattle or sheep) with concentrated sodium hydroxide solution until a uniform solution is obtained. Allow the solution to cool. Concentrated solution of sodium chloride (brine) is added to precipitate the soap which floats on the surface. The process of precipitating the soap is known as “salting out”. The soap is then removed and treated further to produce pure soap.
Perfumes may, dyes and disinfectants may be added to make toilet soap e.g. Geisha

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Note

  1. Potassium hydroxide can be used instead of sodium hydroxide. Potassium soaps are normally milder and there fore used mainly as toilet soaps.
  2. Oils are liquid esters at room temperature whereas fats are solids at room temperature.
    Cleaning action of soap
    Soaps and detergents act in a similar way to facilitate the cleaning process. They act by lowering the surface tension of water and thus enable the water to spread and wet more effectively i.e.to break up and disperse grease particles.
    Dirt is fixed on objects by oil films. Soap has two parts i.e. the long hydro carbon tail that is soluble in oil but insoluble in water (hydrophobic tail) and a carboxylic acid head that is soluble in water (hydrophilic head) but insoluble in oil.
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During washing, the hydrophobic tail dissolves in the oil film and the hydrophilic head remains in water, this crates tension making the grease particles to split up into tiny globules which are carried away by water. The dirt particles get suspended in water, a process known as emulsificationSoapless (synthetic) detergents
A detergent is any substance that facilitates the cleaning process. This means that soap is also a detergent although the name is used for other substitutes of soap like Omo, Nomi, Ariel, Toss e.t.c.
The synthetic detergents function in the same way as soap but they are more soluble than soap and there fore clean more effectively. Even when hard water is used, they do not form scum but soap does.
The soapless detergents are made from concentrated sulphuric acid and hydrocarbons obtained from petrol refining.


Laboratory preparation of a soapless detergent from castor oil
Procedure

  • Add 1cm3 of castor oil into a test tube, then carefully add 2cm3 of concentrated sulphuric acid while stirring with a glass rod
  • Gently warm the mixture and add about 10cm3 of 4M sodium hydroxide and stir. The mixture gets hot, viscous and dark.
  • Add 5cm3 of distilled water and stir. Then decant to separate the liquid from the solids. The solid is the soapless detergent which is then washed with distilled water.
    Advantages of soapless detergents
  • They are more soluble in water than soap and there fore clean more effectively.
  • They do not form scum with hard water there fore can be used with both hard and soft water. Soap forms scum with hard water.

Disadvantages of soapless detergents

  • Some soapless detergents are non biodegradable and there fore accumulate in the environment. Soap is biodegradable.
  • It is more expensive than soap
  • The phosphates from soapless detergents when washed in to water bodies‘ causes eutrophication. This leads to pollution of water bodies.
  • Sample questions on Organic chemistry
  1. Ethene can undergo polymerization. Explain what is meant by the term “polymerization of ethene”. Name the product of polymerization of ethene and write equation for the reaction leading to the formation of the product that you have named. State one use of the product you have named.
    On polymerization, ethene formed a compound T, molecular mass = 16,660. Determine the number of moles of ethene molecules that combined to form T. (C = 12, H = 1).(Ans.=595 moecules)State the term which is used to describe a single unit of the ethene molecule in T.
    Distinguish between the terms “Synthetic polymer” and “natural polymer”, and use silk and nylon to match with the type of polymer that you have distinguished. State one use each of silk and nylon. State (i) one characteristic property of thermosetting plastics and thermoplastics. (ii) one example each of thermosetting plastics and thermoplastics.
  2. Define the following terms as appied to organic chemistry: isomerism, polymerization, a hydrocarbon, homologous series, functional group, cracking, hydrogenation of ethene and vulcanization of rubber. Some organic compounds are said to be saturated and some unsaturated. Mention two examples of unsaturated and saturated organic compounds.
  3. Give the names of each of the following compounds: CH3OH; CH3CH2CH3; CH3CH2COOH; CH2=CH2. Which of these organic compounds are gases at room temperature? Which of them is/are saturated and give a reason? Mention the monomer in the following polymers: polyethene, polyvinyl chloride and polypropene.
  4. Discuss the reactions of a named alkane with oxygen and chlorine. Lower alkanes can be obtained by refining petroleum, describe the process of fuel refining.
  5. Describe briefly the preparation of ethene from ethanol. How is ethene converted back to ethanol? When ethene is bubbled through bromine water and acidified potassium potassium permanganate, the bromine water and permanganate solutions turn colourless. Explain with the aid of equations. With examples, differentiate between addition and condensation polymerization. Outline four uses of ethene.
  6. What are plastics? Describe in detail is meant by thermosetting and thermo softening plastics, use examples to illustrate. Explain the disadvantages of using plastics. Explain briefly why rubber has to be vulcanized and give the uses of vulcanized rubber.
  7. Explain the term fermentation and describe how ethanol is obtained from a named substance by fermentation. Use equations to illustrate. Which other product is given out during the process? Outline at least three uses of ethanol.
  8. Explain briefly what is meant by (i) soap and (ii) detergent. Describe how soap and detergents are made. Explain the cleaning action of soap. Outline the advantages and disadvantages of soap less detergents.
  9. Under what conditions does ethanol react with concentrated sulphuric acid to produce the colourless gas ethene (C2H4)? Write an equation for the reaction.
    What is the reaction of ethene upon bromine water and upon hydrogen chloride. Give the names and formula of the products formed.
  10. Select one member member of each of the following homologous series: (i) alkenes and (ii) alkanols. In each case give its name , molecular formula and structural formula, two chemical reactions and two uses in every day life.