Chemistry

LEARNING OBJECTIVES

After reading this unit, you will be able to:

•identify organic compounds and their origin.

•describe the physical and chemical properties of alkanes (methane).

•state the uses of mathane and some other alkanes.

•explain structural isomerism.

KNOWLEDGE GAIN

                  

The candle wax contains a mixture of alkanes having high molecular masses.

ACTIVITY 10.1:Distinguishing Daily Use Things as Organic or Inorganic

Make a list of things which you

•eat or drink daily

•wear daily

•use daily

Can you distinguish the objects you have listed as organic or inorganic?

Do you know whether milk is organic or not?

Make an album showing organic compounds used in our daily life.

The compounds of carbon are known as organic compounds. The oxides of carbon, carbonates, hydrogen carbonate and carbides are not organic. All living things, plants and animals are composed of organic compounds. The term ‘Organic’ is derived from organism meaning a living body. Initially, organic compounds were considered to be derived from animal and vegetable sources.

It was observed that inorganic compounds such as sodium chloride can be prepared directly from their elements in the laboratory but organic compounds such as sugar and urea could not be prepared from their elements.

This led to the common belief that the organic compounds could only be synthesised in nature by living organisms. In 1828, Friedrich Wöhler, a German chemist, prepared the first organic compound (urea) in the laboratory. This artificial synthesis of urea shook the common belief and it was discarded. Consequently, methane and acetic acid were prepared in the laboratory.

A large number of things which we use in daily life are organic compounds. For example,

•Our food materials such as grains, pulses, milk, sugar, etc.

•Our clothes materials such as cotton, silk, wool, nylon, etc.

•Fuels such as petrol, diesel, coal, wood, LPG, etc.

Other materials such as paper, cotton, plastics, leather are also carbon compounds. Carbon compounds such as carbohydrate, proteins, amino acids, vitamins, fats, oil and enzymes play important roles in life process. Without carbon compounds life is not possible.

10.1 ORGANIC CHEMISTRY

Organic chemistry is a discipline of chemistry that is concerned with the study of compounds containing carbon chemically bonded to hydrogen. Organic chemistry encompasses the synthesis, identification, modelling, and chemical reactions of such compounds.

Organic chemistry focuses on the structure, properties, and applications of various carbon-containing molecules that make up important biological molecules such as proteins, enzymes, carbohydrates, lipids, nucleic acids, and vitamins. A number of organic compounds such as cotton, wool, and natural petroleum are found in nature. There are many others that are purely man-made. Industries have been able to synthesize many organic compounds such as plastics, photographic film, synthetic fabrics, and pharmaceutical drugs using applications of organic chemistry.

10.2 DIFFERENCE BETWEEN ORGANIC AND INORGANIC CHEMISTRY

Chemistry is often divided into two subgroups known as organic and inorganic. Organic chemistry focuses on all compounds that contain the element carbon, usually in the form of carbon and hydrogen bonds. Inorganic chemistry deals with the compounds that do not have carbon, though there are some exceptions. Many of these inorganic compounds are classified as salts.

ACTIVITY 10.2: Distinction between Organic and Inorganic Compounds

Chemicals Required

Organic compounds: Urea, Sugar, Ethanol

Inorganic compounds: NaCl, MgOH, Ca(OH)₂

Materials Required

Homemade conductivity tester (as you have made in Activity 9.2)

Procedure

•Take 150 ml aqueous solution of urea in a beaker.

•Dip the open wire ends in the beaker.

•Observe the LED.

•Note your observation.

•Repeat the activity with each sample.

•Make an appropriate report of your observation.

Caution

•Do not insert the open wire end of tester into the AC plug

In Activity 10.2, you will observe that aqueous solution of organic compounds do not conduct electricity. Whereas inorganic compounds conduct electricity in aqueous solution.Table 10.1 summarizes the difference between organic and inorganic compounds.

 

10.3 OCCURRENCE OF ORGANIC COMPOUNDS

Simplest organic compounds occur naturally in crude oil and natural gas. Crude oil and natural gas are usually found together, formed by the same gradual decay of marine animals and plants. Crude oil is a mixture of different hydrocarbons; most of which are alkanes.

The alkanes are heavily exploited as fuels. Alkanes are obtained from crude oil by fractional distillation. Fractional distillation separates crude oil into different fractions, that is, the various hydrocarbons components.

The important sources of methane are:

•It occurs in natural gas (90% CH₄ + small amount of C₂H₆, C₃H₈ and C₄H₁₀) above the layer of petroleum under earth.

•It is one of the chief constituents (32–34%) of coal gas.

•It is the major constituent of gobar gas, sewage gas and biogas.

•It occurs in marshy lands where it is formed due to the microbial decomposition of vegetable and animal organic matter such as plant wastes, animal dung, poultry sweeps, etc.

10.4 HOMOLOGOUS SERIES

Hydrocarbons with related structures and properties are usually separated into “families” known as homologous series. Each member of a series differs from the next member by a single carbon atom and two hydrogen atoms. With each successive member, the properties of the compounds, such as melting and boiling points, change in a regular way.

•All members of a homologous series can be represented by same general formula.

•All the compounds of a homologous series show similar chemical properties.

•The members of a homologous series show a gradual change in their physical properties with increase in molecular mass.

•Any two adjacent homologous differ by a —CH₂ group.

10.5 GENERAL FORMULA OF ALKANES

In alkanes, each carbon atom is bonded directly to four other atoms. Since in these compounds, all the carbon atoms utilize their full combining capacity by forming single covalent bonds with other atoms, these compounds are called saturated compounds.

General formula of homologous series of alkanes is CnH₂n⁺², where n is a positive integer. Thus, alkanes may be defined as the saturated hydrocarbons having general formulaCnH₂n⁺².

10.6 NOMENCLATURE OF ALKANES

International Union of Pure and Applied Chemistry (IUPAC) Rules for Naming Straight Chain Alkanes

The name of a straight chain alkane, may be divided into two parts: Prefix and suffix.

Prefix designates the number of carbon atoms in the chain. For chains containing one to four carbon atoms, special prefixes are used and for chains of five or more carbon atoms, Greek number prefixes are used. IUPAC prefixes for a few carbon chains are given in Table 10.2.

   

In order to derive the IUPAC name, a suffix is added to the prefix. The primary suffix used for alkanes is “ane”

IUPAC names of straight chain alkanes containing carbons 1–10 are given in Table 10.3.

10.7 STRUCTURAL ISOMERISM

Isomerism is the phenomenon of two chemical compounds having the same molecular formula but a different structural formula.

Structural isomerism is a form of isomerism in which molecules with the same molecular formula have bonded together in different manner.

Alkanes can exhibit structural isomerism.

Each of the first three members of alkanes; methane, ethane and propane has only one structural formula as shown below. Structural formula shows the arrangement of atoms in a molecule of compound.

Thus, no structural isomerism is possible up to propane.

The fourth member, that is, C₄H₁₀, can have two structural formulae as shown below:

 

An alkane with unbranched chain of carbon atoms is called normal alkane or n-alkane.

Classes of Carbon Atoms

Carbon atoms in alkanes can be classified on the basis of the number of other carbon atoms to which they are attached.

A carbon atom which is attached directly to only one is classified as a primary (1°) carbon atom. A carbon atom attached directly to two other carbon atoms is called a secondary (2°) carbon atom. Similarly, a tertiary (3°) carbon atom is attached directly to three other carbon atoms.

             

Example 1

₁CH₃—₂CH₂—₃CH₂—₄CH₂—₅CH₃

Prefix used for 5 carbon atoms is Pent

Suffix is ane

IUPAC Name: n-Pentane

Example 2

₁CH₃—₂CH₂—₃CH₂—₄CH₂—₅CH₂ —₆CH₂—₇CH₃

Prefix used for 7 carbon atoms is Hept

Suffix is ane

IUPAC Name: n-Heptane.

Example 3

₁CH₃—₂CH₂—₃CH₂—₄CH₂—₅CH₂—₆CH₃

Prefix used for 6 carbon atoms is Hex

Suffix is ane

IUPAC Name: n-Hexane.

10.8 PHYSICAL PROPERTIES OF ALKANES

ACTIVITY 10.3:Illustrating Physical Properties of Alkanes

In groups, discuss the physical properties of alkanes.

1. Physical state: Alkanes contain only C—C and C—H bonds. They have very small difference of electronegativity between carbon and hydrogen atoms. Therefore, alkanes possess weak van der Waal’s forces as the inter molecular forces. Due to the weak inte rparticle forces, at room temperature, the first four members (C₁ to C₄) are gases; the next thirteen members, (C₅ to C₁₇) are liquids while the higher members are waxy solids.

2. Solubility: Hydrocarbons are non-polar in nature. In keeping with therule “like dissolves like”, they are insoluble in polar solvents like water but are soluble in non-polar solvents like ether, carbon tetrachloride, etc.The liquid hydrocarbons themselves are good solvents for other non-polar organic molecules.

3. Boiling Points: Alkanes generally have low boiling points. The boiling points of n-alkanes increase regularly with the increase in the number of carbon atoms.

4. Melting Points: The melting points of alkanes do not show a very smooth gradation with the increase in molecular mass. For example, let us look at the melting points of propane to n-octane in Table 10.4.

The physical properties of the first ten straight chain alkanes are given in Table 10.4.It is evident that the increase in melting point is relatively more in moving from an alkane having odd number of carbon atoms to the higher alkane while it is relatively less in moving from an alkane with even number of carbon atoms to the higher alkane.

5. Density: The densities of alkanes increase with increasing molecular masses but become constant at about 0.8 g cm^–3 . This means that liquid alkanes are lighter than water.

 

EXERCISE 10.1

1. Alkanes contain only _______ and ______ bonds.

2. The first fifteen members of alkanes are gases. (True or False)

3. The density of alkanes is

(a) more than water(b) less than water (c) equal to water (d) cannot be calculated.

4. Which of the following is not the correct formula for alkanes?

(a) C2H2 (b) C2H4 (c) C3H6 (d) all of these

5. Alkanes generally have low boiling points.

                            (True or False)

10.9 CHEMICAL PROPERTIES OF ALKANES

ACTIVITY 10.4: Illustrating Chemical Properties of Alkanes

In groups, discuss the chemical properties of alkanes.

Alkanes are saturated hydrocarbons. They contain only strong C—C and C—H bonds. Hence, alkanes are quite less reactive and are known as paraffins (parum–little, affinis–reactivity or affinity). Alkanes are generally inert towards acids, bases, oxidising and reducing agents. Alkanes being saturated compounds can undergo only substitution reactions. Thus, the typical reactions of alkanes are substitution reactions.

Important reactions of alkanes are as follows:

1. Combustion: Alkanes on burning in air or oxygen get completely oxidised to carbon dioxide and water with the evolution of large amount of heat.

CH4 + 2O2→ CO2 + 2H2O + 890 kJ

The general chemical equation for combustion of alkanes may be written as:

                      

In the presence of limited supply of air, alkanes burn to give carbon monoxide, which is a highly poisonous gas.

2CH₄ + 3O₂→ 2CO+ 4H₂O

DO YOU KNOW?

Carbon black is formed during incomplete combustion of alkanes with insufficient amount of air or oxygen. It is used in the manufacture of ink, printer ink, black pigments and as filters.

2. Halogenation: Halogenation involves the replacement of one or more H atoms of alkane by corresponding number of halogen atoms.

Chlorination can be carried out by treatment of alkane with chlorine, either in the presence of diffused sunlight or ultraviolet light or at high temperature (573–773 K). For example, when methane is chlorinated, we get mixture of four different substitution products.

               

3. Thermal cracking: Cracking means the breaking down of long chain saturated hydrocarbon to form a mixture of alkane and alkenes of shorter length. When alkanes are heated to very high temperatures in the absence of air, they split into smaller molecules. Cracking of alkanes is a thermal decomposition that involves the breaking of carbon-carbon bonds to form smaller molecules of hydrocarbons.

             

Cracking of ethane gives ethene and hydrogen.

EXERCISE 10.2

1. Alkanes are also known as _______.

2. Alkanes on burning in air/oxygen get completely oxidised to ______ and ______.

3. Complete the following reactions:

(a) CH4 + Cl2?→ CH3Cl + ?         (b) C10H22600°C→ ? + ?

4. Define thermal cracking of alkanes.

5. _______ is used in the manufacture of printer ink and black pigments.

10.10 LABORATORY PREPARATION OF METHANE

Methane can be prepared in the laboratory by heating a mixture of anhydrous sodium ethanoate and soda-lime. Soda-lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO) in the ratio 3 : 1.

                 

•Take a mixture of anhydrous sodium ethanoate and soda-lime in the ratio 2 : 1 in a hard glass test-tube fitted with a delivery tube as shown in Figure 10.1.

      

•Heat the mixture slowly.

Observe what happens and record.

Methane gas is produced which is collected in a gas jar by the downward displacement of water. As methane gas is collected over water, it shows that methane is insoluble in water.

In Rwanda, methane gas is extracted in Lake Kivu. Methane is used as source of heat.

EXERCISE 10.3

1. Methane can be prepared in the laboratory by heating a mixture of _______ and ______.

2. Methane is soluble in water. (True or False)

3. In laboratory, methane gas is collected in a gas jar by _______ of water.

4. What is soda lime?

5. Methane is used as a source of heat. (True or False)

ACTIVITY 10.5: Illustrating Extraction of Methane

Carry out research and make a presentation on how methane gas is naturally formed. (A field visit may be necessary to a biogas plant).

10.11 USES OF ALKANES

Important uses of alkanes are as follows:

1. Petroleum is largely a mixture of different alkanes. On refining, petroleum gives Liquefied Petroleum Gas (LPG), gasoline, kerosene, diesel, furnace oil and wax which are used as fuels.Natural gas is mainly methane. Compressed Natural Gas (CNG) is used as fuel to run automobiles

2. Some higher alkanes are used as lubricating oils and as vaseline. Vaseline is used as a basic material in many cosmetics.

3. Higher alkanes on sulphonation yield corresponding sulphonic acids, which are used for preparing detergents.

4. Alkanes are used as starting materials for the preparation of many other useful organic compounds.

For example, methane on chlorination gives chloromethane, dichloromethane, trichloromethane (chloroform) and tetrachloromethane which are used as solvents.

5. Carbon formed during decomposition of methane is in the form of finely divided particles and is known as carbon black. Carbon black is used for making printer’s ink and paints. It is also used in rubber industry.

6. Natural gas is a rich source of hydrogen gas which is needed in the manufacture of fertilizers.

EXERCISE 10.4

1. _______ is largely a mixture of different alkanes.

2. Name two products obtained on refining of petroleum.

3. Write full form of

           (a) LPG,                (b) CNG.

4. What do you mean by carbon black?

5. Alkanes are used as lubricating oils and vaseline.

                                                            (True or False)

10.12 SUMMARY

•Organic chemistry is the branch of chemistry that deals with the carbon compounds.

•In organic chemistry, we study the structure, properties, composition, reaction and preparation of carbon containing compounds. The most organic (carbon) compounds comprise at least one carbon-hydrogen bond.

•Alkanes occur naturally in crude oil and natural gas. Crude oil and natural gas are formed by the gradual decay of marine animals and plants.

•Crude oil is a mixture of different hydrocarbons; most of which are alkanes.

•The compounds in a homologous series:

               ¢     are represented by a general formula.

               ¢     show gradation in physical properties.

               ¢     have similar chemical properties because they have the same type of bonding.

               ¢     have successive members differ by a CH₂ group.

•General formula of homologous series of alkanes is CnH_2n+2, where n is a positive integer.

•In order to derive the IUPAC name, a suffix is added to the prefix. The primary suffix used for alkanes is “ane”.

•Isomerism is the phenomenon of two chemical compounds having the same molecular formula but a different structural formula.

•Structural isomerism is a form of isomerism in which molecules with the same molecular formula have bonded together in different manner.

•No structural isomerism is possible from methane to propane.

•Alkanes are colorless. The first four members of alkanes (C₁ to C₄) are gases; the next thirteen members (C₅ to C₁₇) are liquids while the higher members are waxy solids.

•The boiling points and melting points of alkanes directly correspond to the size of the molecule. The melting and boiling points of the shorter chain alkanes are low, but the melting and boiling points of alkanes increase as the number of carbon atoms in the carbon chain increases.

•Alkanes are insoluble in water but soluble in ether. The densities of alkanes increase with increasing molecular masses but become constant at about 0.8 g cm^–3. Liquid alkanes are less dense than water (alkanes float on top of water).

•Alkanes react rapidly with oxygen releasing energy, which makes alkanes useful as fuels.Alkanes react with halogens such as chlorine gas and bromine water in the presence of ultraviolet light.

•Cracking of alkanes is a thermal decomposition that involves the breaking of carbon-carbon bonds to form smaller molecules of hydrocarbons.

•Common name of trichloromethane is chloroform.

•Methane can be prepared in laboratory by heating a mixture of anhydrous sodium ethanoate and soda-lime.

•Soda-lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO) in the ratio 3:1.

•Alkanes are used as fuels.

•Alkanes are used for preparing detergents, lubricating oils and vaseline.

10.13 GLOSSARY

•Cosmetics: substances used to enhance the appearance or odor of the human body.

•Hydrocarbon:a compound of hydrogen and carbon.

•Inorganic compound:any compound that lacks a carbon atom, for example, CO₂, CO, carbonates, cyanides and carbides.

•Lipids: a group of naturally occurring molecules that include fats, waxes, fat-soluble vitamins such as vitamins A, D, E, and K.

•Nucleic acid:a complex organic substance present in living cells, especially DNA or RNA.

•Organic compounds:compounds containing carbon.

•Pharmaceutical drugs:a drug used to diagnose, cure, treat, or prevent disease.

10.14 UNIT ASSESSMENT

I. Multiple Choice Questions

1. Organic chemistry deals with the compounds containing

(a) carbon (b) phosphorus (c) silicon (d) chlorine

2. Alkanes occur naturally in

(a) crude oil (b) natural gas (c) both (a) and (b) (d) none of these

3. The compounds in a homologous series have

(a) Same general formula (b) Similar chemical properties (c) Differ by a CH₂ group. (d) All of these

4. The general formula of alkane is

(a) CnH₂n (b) Cn H_2n–2 (c) CnH_2n+2 (d) none of these

5. IUPAC name of alkane containing four carbon atoms is

(a) Butane (b) Propane (c) Pentane (d) Hexane

6. IUPAC name of CH₃—CH₂—CH₂—CH₂—CH₂—CH₂—CH₃ is

(a) Octane (b) Heptane (c) Heptene (d) Hexane

7. The fourth member of alkane has ................ structural isomers.

(a) one (b) two (c) three (d) four

8. Alkanes are soluble in

(a) ether (b) CCl₄ (c) both (a) and (b) (d) water

9. Alkane reacts with excess oxygen to give

(a) CO₂ + H₂O                        (b) CO + H₂O

c) CO₂ + H₂O + Heat              (d) CO + H₂O + Heat

10. Methane is prepared in laboratory by heating a mixture of

(a) CH₃COONa + NaOH                 (b) CH₃COONa + Ca(OH)₂

(c) CH₃COOH + NaOH                   (d) CH₃COOH + Ca(OH)₂

II. Open Ended Questions

1. Distinguish between organic and inorganic chemistry.

2. Compare and contrast the physical properties of organic and inorganic compounds.

3. Define homologous series.

4. Write IUPAC names of first ten alkanes.

5. Discuss the physical and chemical properties of alkane.

6. How is methane gas prepared in the laboratory?

7. State the uses of alkanes in daily life.

III. Practical-based Questions

General Instruction: In the following figures, black balls represent carbon and white balls represent hydrogen.

1. What is the name of the organic compound shown in the figure?

              

           (a) Methane               (b) Ethane            (c) Propane               (d) Butane

2. Name the compound shown in the following figure.

                              

          (a) Propane                (b) iso-butane            (c) Butane              (d) None of these

3. Which compound has higher melting point?

        

(a) Compound A

(b) Compound B

(c) Both compounds have same melting point

(d) Compounds do not exist

4. Choose the correct statement for the following compound.

                   

(a) This is the simplest alkane

(b) It can be prepared in the laboratory

(c) This can be used as a source of heat

(d) All of the above

5. If the figure shows the laboratory preparation of methane, which of the given statements is correct?

         

(a) A = Sodium Acetate + Soda Lime

(b) A = Sodium Chloride + Soda Lime

(c) A = Sodium Carbonate + Slaked Lime

(d) A = Sodium Hydrogen-carbonate + Lime

                       PROJECT

Make a 3D model of heptane using plastic balls and sticks.

Bibliography

Books

1. Childs, A. (2000). Macmillan Secondary Chemistry, Nairobi: MacMillan Publishers.

2. Mbasu, E. (2004). Certificate Chemistry. Nairobi: East African Educational Publishers Ltd.

3. Otim, E, Lwanga, & M and Jimmy, A. (2013). Junior Secondary Chemistry, Kigali: MK Publishers.

4. Mbaka Njeru and Mwangi Wamae. (2004). Comprehensive Secondary Chemistry, Nairobi: Oxford University Press.

5. Laidler, K. J. (1993). The World of Physical Chemistry. Oxford University Press.

6. Atkins, Peter; Loretta Jones (1997). Chemistry: Molecules, Matter and Change. New York: W. H. Freeman & Co.

7. Russo, Steve, and Mike Silver. (2007). Introductory Chemistry. San Francisco: Pearson.

8. Atkins, Peter W.; Julio de Paula (2006). Physical Chemistry. (4th ed.). Weinheim: Wiley-VCH.

9. Clayden, J.; Greeves, N. and Warren, S. (2012). Organic Chemistry. Oxford University Press.

Websites

1. http://www.waterwise.co.za/site/water/environment/causes-of-water-pollution.html

2. http://www.waterwise.co.za/site/water/diseases/

3. http://chem.libretexts.org/Core/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends

4. http://www.bbc.co.uk/education/guides/zptrd2p/revision

5. http://chemistry.tutorvista.com/inorganic-chemistry/list-of-electrolytes.html