KCSE ONLINE

Esoma Online Revision Resources

Order this CD Today to Experience the Full Multimedia State of the Art Technology!

Introduction to Organic Chemistry

To understand life as we know it, we must first understand a little bit of organic chemistry. Organic molecules contain both carbon and hydrogen. Though many organic chemicals also contain other elements, it is the carbon-hydrogen bond that defines them as organic. Organic chemistry defines life. Just as there are millions of different types of living organisms on this planet, there are millions of different organic molecules, each with different chemical and physical properties. There are organic chemicals that make up your hair, your skin, your fingernails, and so on. The diversity of organic chemicals is due to the versatility of the carbon atom. Why is carbon such a special element? Let's look at its chemistry in a little more detail.

Carbon (C) appears in the second row of the periodic table and has four valence electrons in its valence energy level. Similar to other non-metals, to be stable carbon needs eight electrons. Carbon therefore forms four bonds with other atoms (each bond consisting of one of carbon's electrons and one of the bonding atom's electrons). Every valence electron participates in bonding, thus a carbon atom's bonds will be distributed evenly over the atom's surface. These bonds form a tetrahedron (a pyramid with a spike at the top), as illustrated below:



Carbon atom

What do the modes of transport & industrial activity shown have in common?

By the end of the lesson you should be able to:
a) Define a hydrocarbon.
b) Name simple hydrocarbons.
c) Draw structures of hydrocarbons up to five carbon atoms.
d) State features of a homologous series.
e) Draw isomers of simple hydrocarbons.
f) Name the isomers of simple hydrocarbons.


In this lesson we will discuss Alkanes.

Observe the following pictures carefully. From your observation,what do the modes of transport and industrial activity shown have in common?


Note that they all use liquids obtained from crude oil as fuels.

Crude oil is a complex mixture of compounds known as Hydrocarbons.
Hydrocarbons are compounds of carbon and hydrogen only.
The hydrocarbons from the basis of a group called organic compounds.



A Hydrocarbon
Carbons form thousands of compounds having atoms linked in chains or rings and are studied in a special branch of chemistry called organic chemistry.Compounds such as carbon (II) oxide, carbon (IV) oxide, carbonates and hydrogen carbonates contains carbons not in chains or rings. These compounds are said to be inorganic.
Hydrocarbons are divided into groups or families with similar properties. These families are known as Homologous series.There are three families (homologous series) of hydrocarbons
1) Alkanes
2) Alkenes
3) Alkynes

Most hydrocarbons in crude oil belong to the alkane family of compounds.Alkanes contain Carbon atoms covalently bonded to four other carbon atoms.


The following are examples of alkanes.


Examples of alkanes

The names of the alkanes are derived by first writing a prefix and then ending with "ane". For example
(A) One carbon atom.......... [meth]ane
B) Two carbon atoms...........[Eth]ane
(C) Three carbon atoms .......[prop]ane
(D) Four carbon atoms........ [But]ane
(E) Five carbon atoms........ [pent]ane
The prefix is used to indicate the number of carbon atoms in the chain.
n Prefix
1 w Meth-
2 w Eth-
3 w Prop-
4 w But-
5 w Pent-

Click on the ENTER button showing the structural formula of methane.


Click on the ENTER button showing the structural formula of ethane.

The table below gives a summary of the first 10 alkanes. Click on each arrow to view the molecular structure of each alkane.

From the table it can be noted that alkanes contain only C-H and C-C single bonds and are therefore saturated hydrocarbons.
Homologous series have a number of things in common such as the following:
(i) They are represented by the same general formula eg alkanes:CnH2n+2
ii) Each member differs from the next by -CH
2 group and the molecular mass increase by 14.
iii) They have similar structures.
iv) They have similar chemical properties.
(v) Melting point, boiling point and density of the members increase steadily with increasing molecular mass.
(vi) Members of same series can be prepared by similar methods.

Alkanes are major component of crude oil.
Crude oil is a complex mixture of gases, liquids and solids.


It is refined into its components in a process of fractional distillation
This is done in an oil refinery such as the one at changamwe in Mombasa.Crude oil is first heated then passed into the fractionating tower where it separates into different fractions.
The different components (fractions) separate because they have different boiling points.
The smaller molecules with lower boiling points condense up the tower.
The bigger molecules with higher boiling points condense in the lower half of the tower and are collected.

The table below shows the different alkyl
groups.


Rules applied in naming alkanes:
The following animation shows a certain alkane. Let us name the alkane by applying the rules that should be followed when naming any particular alkane.



The following an animation shows an example of an alkane. Let us find out the name of this alkane.


Let us now name the molecule shown.

Step 1:Count the longest carbon chain to get the parent name. This is a Heptane.

Step 11:Identify the branches. These are methyl-CH3 at carbon 2,Bromo -Br- at carbon 3 and Ethyl C2H5 at carbon 4. Note that Halogens branches are named as Br-Bromo, Cl-Chloro and I as Iodo.If we count from left to right we get the branches at carbon atoms 2,3 and 5. Counting from left to right places branches at 3, 5,and 6.Note that we should count from the side that will give the substituents/branches the lowest numbers. Therefore we take the number 2,3,5.The name therefore becomes 3-bromo,5-ethyl,2-methylheptane.

By the end of the lesson you should be able to
a) Describe the general methods of preparing alkanes
b) Explain the physical properties of alkanes
c) Explain the chemical properties of alkanes
d) State the uses of alkanes


In this lesson we will study the methods of preparing alkanes. We will also discuss the properties of alkanes.

Ethane is prepared by heating fused Sodium Ethanoate with soda lime and collected over water as shown below.

Soda lime is a mixture of Calcium Oxide (quick lime) and sodium hydroxide.
Soda lime is preferred to Sodium Hydroxide because
i. It is not deliquescent
ii. Does not melt readily
iii. Does not attack glass
Generally preparation of alkanes can be represented by the equation below;


Physical properties of Methane
a) It is a colourless gas
b) It is an odourless gas
c) It is less dense than air
d) It is insoluble in water but soluble in organic solvents hence is collected over water.
Generally any alkane can be prepared from the represented by the following equation.

The table below shows a summary of some physical properties of alkanes.


As the number of carbon atoms increase the molecular masses also increases hence the melting point, boiling point and density increases. This is also caused by the increase in intermolecular forces of attraction.
Alkanes are insoluble in water and highly soluble in organic compounds. eg benzene.


We will begin by looking at the combustion of alkanes in air.Alkanes burn in sufficient air to form Carbon (IV) Oxide and water.

However in insufficient air Carbon (II) oxide is formed.

Substitution reactions of alkanes.
Alkanes react with Halogens in the presence of sunlight since the energy from sunlight splits the halogen molecule into atoms.
Example
Substitution reactions of Methane and Chlorine
Methane + Chlorine gas w Chlorormethane + Hydrogen Chloride gas
Step 1:Write the chemical equation
CH 4 (g) + Cl2 (g) w CH3Cl (g) + HCl(g)
Step 11: Draw the structural formula
The Cl-Cl breaks due to the U.V light and the H combines with Cl to form HCl. One of the Cl replaces the H in methane.
Step 111: The Hydrogen in Chloromethane are replaced further by Chlorine atom.
Step IV
The Hydrogen in Dichloromethane are further replaced by the Chlorine atom.
Step V
The remaining Hydrogen in trichloromethane is finally replaced by Chlorine.


Observe the photographs that show some uses of alkanes.

By the end of the lesson you should be able to
a) Name and draw the structures of alkenes.
b) Draw and name the isomers of alkenes.


You will now learn about another family of Hydrocarbons called alkenes.



An alkene containing a double bond.



They belong to a group of Hydrocarbons called alkenes.
Alkenes are Hydrocarbons that contain at least one double bond between the carbon atoms.



Butene

Alkenes have a double bond between two Carbon atoms
The double bond is a functional group.
They have the general formula CnH2n
Consecutive members differ from each other by -CH2- group

The IUPAC rules for naming alkenes are similar in many aspects to those of naming alkanes.
Count the number of Carbon atoms present.Change the ending of the name of the alkane of the same number of Carbon atoms to end with -ene instead of -ane



The alkene shown are numbered from the right hand side because this gives the Carbon atom bearing the double bond number 2 and 3.
Note that if they are numbered from the left hand side the two Carbon atoms would be numbered 3 and 4.
Use the lower number of the carbon atoms containing the double bond in writing the name of the alkene. The name of the alkene is therefore Pent-2-ene.

The following steps are taken in naming branched chain alkenes.
The longest continous Carbon chain containing the double bond is identified and an appropriate name prefix given.The longest Carbon chain contains 5 Carbon atoms
The lowest number possible for the Carbon atom with double bond is then ascribed.Indicate the location of substituent group by the number of the Carbon atoms to which they are attached.When naming the alkene the double bond should have the lowest number from the terminal of the longest chain.

A and B have the molecular formula but different structural formula. They are Isomers of Butane.
Butane has two isomers that contain the double bond in different positions. Such Isomers are known as positional Isomers.



But-1-ene


Alkenes also exhibit chain Isomerism. Chain isomers contain the double bond in the same position while the branches are in different positions.

By the end of the lesson you should be able to:
a. Describe the general method of preparing alkenes
b. Explain the physical and chemical properties of alkenes
c. State some uses of alkenes.

In this lesson we will discuss how alkenes are prepared in the laboratory.

Examples of alkenes



Ethene can be prepared in the laboratory using the following procedure.

Click to play the following video to observe how ethene gas is prepared.

Note that the melting points, boiling points and density increase with increase in molecular mass of the alkenes.

All alkenes are in soluble in water.

All Alkenes are soluble in Organic Solvents.

Ethene gas is colourless and has no smell. It is insoluble in water.The table below gives a summary of some physical properties of Alkenes.


The video clip shows different tests carried out with ethene gas.

Click to play the video and observe carefully.

.


The following observations were made.

When bromine gas is added to ethene gas the following reaction occurs.
Ethene has a double bond between its two carbon atoms. It is said to be unsaturated. Generally Hydrocarbons with double bonds or triple bonds between Carbon atom are called unsaturated Hydrocarbons.
In the reaction between ethene and bromine water OH- and Br- groups add across the double bonds.
The product formed is colourless. This is why bromine water is decolourised. The type of reaction in which various groups add across a double bond is called an addition reaction.
Alkenes undergo addition reactions with hydrogens, Chlorine gas, bromine and Hydrogen halides.

Ethene reacts with Hydrogen gas in the presence of nickel catalyst to form ethane gas.This reaction is used in the manufacture of Margarine.

Alkenes are extremely important in the manufacture of plastics. All plastics are in some way related to alkenes. The names of some plastics (Polythene or Poly Ethene, Polypropene), relate to their alkene partners. Plastics are used for all kinds of tasks, from packaging and wrapping, to clothing and outdoor apparel.

Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol.

For the manufacture of a wide variety of polymers, e.g., polyethene, polyvinylchloride (PVC) and teflon etc.

As raw materials for the manufacture of industrial Chemicals such as alcohols.

Besides, alkenes are also used for artificial ripening of fruits, as a general anesthetic, for making poisonous mustard gas (War gas) and ethylene-oxygen flame.


The following video clip shows the manufacture of Margarine.


By the end of the lesson you should be able to
i. Name and draw the structures of alkynes
ii. D raw and name isomers of alkynes


In this lesson we will be looking at another group of the Hydrocarbons known as alkynes.


Remember that each carbon forms four covalent bonds with other atoms. In model c the two carbon atoms are joined to each other by three covalent bonds. This leaves the fourth bond in each atom to join other atoms. The three covalent bonds formed between the two carbon atoms are referred to as triple bonds.
Hydro carbons which contain a triple bond between any two adjacent Carbons are reffered to as Alkynes.
Alkynes have a general formula Cn H2n-2 where n= 2,3,4...........n is the number of carbon atoms.

When naming alkynes the suffix -ane of the corresponding alkanes is replaced with -yne. For example ethane becomes Ethyne while propane becomes propyne.The table below shows names and structures of the first six members of the alkynes series.

Naming of alkynes is similar to that of alkenes. Preference is given to the position of the triple bond in the alkyne molecule. In branched alkynes , first identify the longest continuous carbon chain in order to give the parent name . From the end that gives the triple bond the lowest possible number ,assign the branches the number representing the position.


Determine the longest continuous carbon chain containing the triple bond.Assign the numbers to the carbon atom beginning from the end that gives the triple bond the lowest possible number.

Specify the carbon atom in the longest carbon chain to which the two methyl groups are attached. The methyl groups are attached to carbon atom 4of the hex-2-yne molecule. The name of the alkyne molecule is 4, 4-dimethylhex-2-yne.

Isomers are compounds which have the same molecular formula but different in structural formula. Alkynes exhibit branching isomerism and positional isomerism. The following is an examples of isomers.

From the activity, we conclude that the two compounds have the same molecular formula but differ in the position of the triple bond . The two molecules are examples of isomers.The isomerism in which the position of the triple bond changes within the the molecule is referred to as positional isomerism.

In this lesson we will discuss preparation,properties of alkynes.

Ethyne gas is prepared by the reaction of Calcium Carbide and water. The following procedure is used to prepare Ethyne gas.Click to play the video to observe how ethyne gas is produced and carefully observe its properties from various tests carried out.


Click to play the video to observe its properties from various tests carried out.



By the end of the lesson the learners should be able to
i. Describe the methods of preparing ethyne
ii. Explain the physical and chemical properties of alkynes
iii. State the uses of alkynes


In this lesson we will learn about preparation and properties of alkynes.

The following conclusions can be made.
i. Ethyne is a colourless gas prepared by reacting Calcium Carbide and water
ii. Ethyne gas is insoluble inn water and that is why it is collected over water.
iii. Ethyne gas decolorizes brown bromine water
iv. Ethyne gas decolorizes purple acidified potassium Mangnate (VII) solution
v. Ethyne gas changes acidified orange potassium Chromate (VI) solution to green.

The observations made by reacting brown bromine water, purple potassium manganate(VII) solution and orange potassium Chromate (VI) solutions indicated that Ethyne is an unsaturated hydrocarbon. These reactions can be used to test for alkynes specifically potassium manganate(VII), bromine water and potassium Chromate (VI).Ethyne is collected over water.

The following are some physical properties.

The melting points of alkynes are low because the molecules are held together by weak vander waals forces of attraction. As the size of the molecule increases (increase in molecular mass )the strength of the strength of the vander waal forces also increase. This makes the molecules to change from gas to liquid and solid.

Alkynes generally burn with a yellow luminous flame. Ethyne burns in excess Oxygen to form Carbon (IV) Oxide and water as shown in the following equation.

Ethyne burns in limited Oxygen to form Carbon (II) Oxide and water. This is illustrated in the following equation.



Alkynes undergo addition reactions to produce alkenes. Alkenes on the other hand undergo addition reactions to form alkanes. Examples of addition reactions include:i. Ethyne reacts with Hydrogen in the presence of Nickel catalyst at 1500c to form ethene and the ethane.

Alkynes react explosively with halogens at room temperature.
Reaction with Chlorine.

Addition of Hydrogen Halide.
Ethyne reacts with Hydrogen bromide in two steps.

The following pictures give a summary of some uses of alkynes.The video clip below shows a jua kali artisan using Oxy-acetylene flame.


Order this CD Today to Experience the Full Multimedia State of the Art Technology!

For Best results INSTALL Adobe Flash Player Version 16 to play the interactive content in your computer. Test the Sample e-Content link below to find out if you have Adobe Flash in your computer.

Sample Coursework e-Content CD

Other Goodies for KCSE ONLINE Members!

Coursework e-Content CD covers all the topics for a particular class per year and costs 1200/- ( Per Subject per Class ).

Purchase Online and have the CD sent to your nearest Parcel Service. Pay the amount to Patrick 0721806317 by M-PESA then provide your address for delivery of the Parcel. Alternatively, you can use BUY GOODS TILL NUMBER 827208 Ask for clarification if you get stuck.

Install ADOBE Flash Player for Best Results

For Best results INSTALL Adobe Flash Player Version 16 to play the interactive content in your computer. Test the link below to find out if you have Adobe Flash in your computer.

Search

Subject Menu