Unit 3: Carbon and its Compounds

               CARBON AND ITS COMPOUNDS

Introduction

Atomic Number: 6

Electronic configuration: 2, 4

Valence electrons: 4

Property: Non-metal

Abundance: Carbon is the 4th most abundant substance in universe and 15th most abundant substance in the earth’s crust.

Compounds having carbon atoms among the components are known as carbon compounds.

Bonding In Carbon:

Covalent Bond;- Bond formed by sharing of electrons is called a covalent bond. Two or more atoms share electrons to make their configuration stable. In this type of bond, all the atoms have similar rights over shared electrons. Compounds which are formed because of covalent bond are called COVALNET COMPOUNDS.

Covalent bonds are of three types: Single, double and triple covalent bond.

Single Covalent Bond: Single covalent bond is formed because of sharing of two electrons, one from each of the two atoms.

1-Formation of hydrogen molecule (H2)

Atomic Number of H = 1

Electronic configuration of H = 1

Valence electron of H = 1

Hydrogen forms a duet, to obtain stable configuration. This configuration is similar to helium (a noble gas). So, in the formation of hydrogen molecule; one electron from each of the hydrogen atoms is shared.

2-Formation of Methane (CH4)

Valence electron of carbon = 4

Valence electron of hydrogen = 1

Double covalent bond: Double bond is formed by sharing of four electrons, two from each of the two atoms.

Formation of oxygen molecule (O2):

Valence electron of oxygen = 2

In the formation of oxygen molecule, two electrons are shared by each of the two oxygen atoms to complete their stable configuration.

In oxygen, the total number of shared electrons is four, two from each of the oxygen atoms. So a double covalent bond is formed.

Triple Covalent Bond: Triple covalent bond is formed because of the sharing of six electrons, three from each of the two atoms.

Formation of Nitrogen (N2):

Atomic number of nitrogen = 7

Electronic configuration of nitrogen = 2, 5

Valence electron = 5

In the formation of nitrogen, three electrons are shared by each of the nitrogen atoms. Thus one triple bond is formed because of the sharing of total six electrons.

Properties of Covalent Bond:

•Intermolecular force is smaller.

•Covalent bonds are weaker than ionic bond. As a result, covalent compounds have low melting and boiling points.

•Covalent compounds are poor conductor of electricity as no charged particles are formed in covalent bond.

•Since, carbon compounds are formed by the formation of covalent bond, so carbon compounds generally have low melting and boiling points and are poor conductor of electricity.

ORGANIC COMPOUNDS

Initially, compounds of carbon could only be obtained from living sources and there was no way of synthesizing them. Hence, carbon compounds are also known as organic compounds.

Cause of formation of such a large number of compounds by carbon:

a.Carbon can form bonds with other carbon atoms. This property of carbon is known as CATENATION. Because of catenation, carbon can form a long chain; while making bond with other carbon atoms. Carbon can make single, double and triple bonds by catenation.

b.Carbon can form branched chain; along with straight chain; while combining with carbon atoms, i.e. because of the property of catenation.

Carbon can also form bonds with other types of monovalent atoms; apart from carbon. Carbon can make long chain combining with other atoms also. For example; carbon can form bonds with oxygen, hydrogen, nitrogen, etc.

HYDROCARBON:

(Hydrogen + Carbon = Hydrocarbon) Compounds formed because of the combination of hydrogen and carbon are known as hydrocarbons. There are two types of hydrocarbon, viz. saturated hydrocarbon and unsaturated hydrocarbon.

SATURATED HYDROCARBONS: Hydrocarbons having single bonds are known as SATURATED HYDROCARBONS. Saturated hydrocarbons are known as ALKANE. These are also known as paraffin. Example: Methane, Ethane, Propane, etc.

UNSATURATED HYDROCARBON: Unsaturated hydrocarbons are of two types Hydrocarbon with double bond and hydrocarbon with triple bond.

Hydrocarbon with double bond: Hydrocarbons having at least one double bond are known as ALKENE. Example: Ethylene, Propylene, Butylene, etc.

Hydrocarbon with triple bond: Hydrocarbons having at least one triple bond are known as ALKYNE. Example: Ethyne, Propyne, Butyne, etc.

ALKANE: Hydrocarbons having only single bonds are known as alkane. These are saturated hydrocarbons. Alkane are also known as paraffin. The general formula of alkane is CnH2n+2

Saturated Hydrocarbons (Alkane)
Name	No. of carbon atoms	Formula
Methane	1	CH4
Ethane	2	C2H6
Propane	3	C3H8
Butane	4	C4H10
Pentane	5	C5H12
Hexane	6	C6H14
Heptane	7	C7H16
Octane	8	C8H18
Nonane	9	C9H20
Decane	10	C10H22

UNSATURATED HYDROCARBONS

Alkene: Hydrocarbons having at least one double bond between two carbon atoms are known as ALKENE. General formula of alkene is CnH2n; where n is number of carbon atoms.

If C = 1 then CnH2n = C1H2x1 = CH2

Since, hydrocarbon having one carbon atom is known as Methane. Thus, Methane – ane + ene = Methene. But, alkene does not exist with one carbon atom, thus, methene does not exist.

If C = 2 then CnH2n = C2H2x2 = C2H4

Other alkenes are formed in similar way.

Alkyne

Hydrocarbons having at least one triple bond between two carbon atoms are known as alkyne. (Alkane – ane + yne = Alkyne).

General formula of alkyne is CnH2n − 2. As in case of alkene, minimum two carbon atoms are required to form alkyne.

If C = 2, then; CnH2n − 2 = C2H2x2 − 2 = C2H2

The name of this compound is ethyne. This can be shown by following structural formula.

Cyclic Hydrocarbon:

Carbon can form cyclic structure combining with carbon atoms. Such hydrocarbons are known as cyclic hydrocarbon. Structural formulae of some of the cyclic hydrocarbons are as follows:

HYDROCARBONS: NOMENCLATURE

Functional Group: in a hydrocarbon chain the element replacing hydrogen is called heteroatom. The heteroatoms confer a specific property to a compound and are called functional groups. Or a Single atom or group of atoms, that have similar chemical properties are called functional group. For example: Halogen group, Carboxyl group, Aldehyde group, etc.

Alkyl group: −R is known as alkyl group.

Examples: −CH3 (Methyl) −C2H5 (Ethyl), −C3H7 (Propyl)

Halogen group: Halogen group is also known as halo group. −Cl (Chloro),−Br(Bromo),−I(Iodo) are halogen or halo group.

Alcohol: −OH is known as alcohol group.

Aldehyde: −CHO is known as aldehyde group. Its structural formula is as follows:

Ketone Group: −CO− is known as ketone group. This is also known as carbonic group. Its structural formula is as follows:

Carboxylic Acid Group: −COOH is known as carboxylic acid group; or simply as acid group. Its structural formula is as follows:

NOMENCLATURE OF CARBON COMPOUNDS:

International Union of Pure and Applied Chemistry (IUPAC) decided some rules to name the carbon compounds. Names which are given on this basis are popularly known as IUPAC name. The rules for nomenclature are as follows:

Identify the number of carbon atoms in carbon compound. Name the carbon compounds according to the number of carbon atoms.

Example:Saturated hydrocarbon having one carbon atom is named as Methane. Saturated hydrocarbon having two carbon atoms is named as Ethane. Unsaturated hydrocarbon with double bond having two carbon atoms is named as Ethene. Unsaturated hydrocarbon with triple bond between carbon atoms is named as Ethyne.

If the structure has branched chain, identify the longest chain and then identify the number of carbon atoms. To understand this, let us observe following examples:

 In figure (a) the longest chain has eight carbon atoms, and thus the name of parent compound would be octane. In figure Identify the longest chain. Then number the carbon atoms in such a fashion that the functional group; if any; would come at the lowest number.

In case of a functional group present, write the prefix or suffix of the functional group according to the table given here. Then write the name of the parent compound.

Functional group	Prefix	Suffix
Alkyl	Alkyl	n/a
Halogen	Chloro− for chlorine, Bromo− for bromine Iodo− for iodine	n/a
Alcohol	n/a	ol
Aldehyde	n/a	al
Ketone	n/a	one
Carboxylic acid	n/a	oic acid
Double bond	n/a	ene
Triple bond	n/a	yne

HOMOLOGOUS SERIES:

Series of compounds with same general formula and functional group is known as homologous series. Compounds belonging to the same homologous series show similar properties. Compounds of homologous series differ by CH2 from their consecutive members. Each subsequent compound in a homologous series differs by 14 au. Example: Alkanes; such as, Methane, Ethane, Propane, Butane, etc. belong to same homologous series.

Properties of Compounds of Same Homologous Series

a. Compounds of same homologous series have same general formula.

b. Compounds of same homologous series differ from their consecutive members by one carbon atom and two hydrogen atoms, homologous series differ from their consecutive members by one carbon atom and two hydrogen atoms, i.e. by CH2

c. Compounds of same homologous series have same chemical properties.

d. Compounds of same homologous series differ by physical properties with increase or decrease in molecular mass.

Chemical Properties of Carbon Compounds

1.Combustion Reaction: Carbon and carbon compounds gives carbon dioxide, vapor, heat and light on burning in air. Following are some of the examples of combustion reaction of organic compounds:

C + O2 ⇨ CO2 + Heat + Light

CH4 + 2O2 ⇨ CO2 + 2H2O + Heat + Light

CH3C2OH + O2 ⇨ CO2 + H2O + Heat + Light

2.Oxidation: In combustion reaction, carbon compounds are oxidized in the presence of oxygen. The following example is different because alkaline KMnO4 is the oxidizing agent in this reaction.

CH3CH2OH + (Alkaline KMnO4/Acidified K2Cr2O7) ⇨ CH3COOH

3. Addition Reaction: Formation of larger molecules by addition of more radicals is known as addition reaction. For example; ethene is converted into ethane when heated with the catalyst nickel.

CH2=CH2 + H2 + (Nickel catalyst) ⇨ CH3−CH3

When ethene undergoes addition reaction with chlorine, it gives dichloroethane.

4. Substitution Reaction: Replacement of a functional group or any atom by another atom or functional group is known as substitution reaction. Substitution reactions are single displacement reactions.

When methane reacts with chlorine gas in the presence of sunlight, it gives chloromethane and hydrogen chloride.

CH4 + Cl2 + Sunlight ⇨ CH3Cl + HCl

Similarly, ethane gives chloroethane when it reacts with chlorine in the presence of sunlight.

C2H6 + Cl2 + Sunlight ⇨ C2H5Cl + HCl

SOME IMPORTANT ORGANIC COMPOUNDS

                   Ethanol (C2H5OH)

Physical properties

•Ethanol is commonly known as alcohol and spirit.

•General name of ethanol is ethyl alcohol.

•Ethanol is the main constituent of all alcoholic drinks

•Ethanol is soluble in water

•Ethanol is a very good solvent

•Ethanol is used in manufacturing of medicines, such as tincture iodine, cough syrup, etc.

•Taking even small quantity of pure ethanol may prove lethal

•Taking dilute ethyl alcohol can cause drunkenness

Chemical properties

Reaction of ethanol with sodium metal:

When ethanol reacts with sodium, it gives sodium ethoxide and hydrogen gas.

2CH3CH2OH + 2Na ⇨ 2CH3CH2ONa + H2

Oxidation of ethanol: Ethanol gives ethanoic acid on oxidation.

CH3CH2OH + (Alkaline KMnO4/Acidified K2Cr2O7) ⇨ CH3COOH

Dehydration of ethanol: Ethanol gives ethene and water when it is heated with concentrated sulphuric acid.

CH3CH2OH + Conc. H2SO4 ⇨ CH2=CH2 + H2O

ETHANOIC ACID (CH3COOH)

Structural formula of ethanoic acid is as follows:

Physical properties

•General name of ethanoic acid is acetic acid.

•Melting point of ethanoic acid is 290K.

•Ethanoic acid freezes in winter and hence it is also known as glacial acetic acid.

•Ethanoic acid is a colorless liquid.

•5% to 8% solution of acetic acid in water is known as vinegar.

•Vinegar is used as preservative in pickles.

•Carboxylic acids are weak acid compared to mineral acids.

Chemical properties;- Reaction of ethanoic acid with base: Ethanoic acid gives sodium acetate when it reacts with sodium hydroxide.

CH3COOH + NaOH ⇨ CH3COONa + H2O

Esterificaiton of ethanoic acid: Ethanoic acid gives ethyl acetate when it reacts with ethanol in presence of conc. sulphuric acid. This reaction is called esterification reaction.

CH3COOH + C2H5OH ⇨ CH3COOC2H5 + H2O

The IUPAC name of Ethyl acetate is Ethyl Ethanoate. Ethyl acetate is also known as ester. Ester is a sweet smelling compound. It is used in making perfumes and as a flavouring agent. When ethyl ethanoate reacts with a base or acid, it gives back ethanol and ethanoic acid.

CH3COOC2H5 + NaOH ⇨ CH3COOH + C2H5OH

This reaction is called saponification, since it is used in making of soap.

Hydrolysis of ester (Ethyl ethanoate): Ethyl ethanoate gives parent alcohol and sodium ethanoate when heated with sodium hydroxide solution.

CH3COOC2H5 + NaOH ⇨ CH3COONa + C2H5OH

Saponification: Ester of higher fatty acids gives sodium salt of higher fatty acid; when heated with glycerol and sodium hydroxide. Sodium salts of higher fatty acid are known as soaps. This reaction is called saponification (soap making).

Reaction of ethanoic acid with sodium carbonate and sodium bicarbonate:

Ethanoic acid gives sodium acetate, water and carbon dioxide when reacts with sodium carbonate or sodium bicarbonate (sodium hydrogen carbonate).

2CH3COOH + Na2CO3 ⇨ 2CH3COONa + CO2 + H2O

CH3COOH + NaHCO3 ⇨ CH3COONa + CO2 + H2O

SOAPS AND DETERGENTS:

Soap: Ester of higher fatty acids is called soap. It is manufactured by the reaction of easter of higher fatty acid with sodium hydroxide. The sodium salt so formed has cleansing property.

Detergent: Soap cannot form lather in hard water. To overcome this problem, detergents were introduced. Detergent is also known as soapless soap. Detergent is sodium salt of benzene sulphonic acid or sodium salt of long chain alkyl hydrogen sulphate.

Cleansing action of soap:

Soap molecule has two ends. One end is hydrophilic and another end is hydrophobic. In other words, one end is lipophobic (hydrophilic) and another end is lipophilic (hydrophobic). When soap is dissolved in water and clothes are put in the soapy solution, soap molecules converge in a typical fashion to make a structure; called micelle. The hydrophobic ends of different molecules surround a particle of grease and make the micelle; which is a spherical structure. In this, the hydrophilic end is outside the sphere and hydrophobic end is towards the centre of the sphere. That is how, soap molecules wash away dirt and grease by making micelles around them.

Soap and Hard Water: Hard water often contains salts of calcium and magnesium. Soap molecules react with the salts of calcium and magnesium and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum. Detergents are used; instead of soaps; in hard water to overcome the problem. Detergents are usually ammonium or sulphonate salts of carboxylic acids. The charged ends of these compounds do not form precipitate with calcium or magnesium salts in hard water. Hence, detergents retain their cleansing property in hard water.

                             TEXTUAL QUESTIONS

1. What would be the electron dot structure of carbon dioxide which has the formula CO2?
Answer

2. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint - the eight atoms of sulphur are joined together in the form of a ring.)
Answer

1. How many structural isomers can you draw for pentane?

Answer

Three structural isomers are possible for pentane.

2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer-The two features of carbon that give rise to a large number of compounds are as follows:
→ Catenation - It is the ability to form bonds with other atoms of carbon.
→ Tetravalency - With the valency of four, carbon is capable of bonding with four other atoms.
3. What will be the formula and electron dot structure of cyclopentane?
Answer-The formula for cyclopentane is C5H10. Its electron dot structure is given below.

4. Draw the structures for the following compounds.
(i) Ethanoic acid (ii) Bromopentane*
(iii) Butanone (iv) Hexanal
*Are structural isomers possible for bromopentane?

Answer

(ii) There are many structural isomers possible for bromopentane. Among them, the structures of three isomers are given.

5. How would you name the following compounds?

Answer-(i) Bromoethane
(ii) Methanal (formaldehyde)
(iii) Hexyne

1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Answer-CH3CH2OH + (Alkaline KMnO4) → CH3COOH
Since, in this reaction one oxygen is added to ethanol, hence it is an oxidation reaction.
2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer-2HC ≡ CH + 5O2 → 4CO2 + 2H2O + Heat
When ethyne is burnt in air, it gives a sooty flame. This is due to incomplete combustion caused by limited supply of air. However, if ethyne is burnt with oxygen, it gives a clean flame with temperature 3000°C because of complete combustion. This oxy-acetylene flame is used for welding. It is not possible to attain such a high temperature without mixing oxygen. This is the reason why a mixture of ethyne and air is not used.
1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer-We can distinguish between an alcohol and a carboxylic acid on the basis of their reaction with carbonates and hydrogen carbonates. Acid reacts with carbonate and hydrogen carbonate to evolve CO2 gas that turns lime water milky. Alcohols, on the other hand, do not react with carbonates and hydrogen carbonates.

2. What are oxidising agents?
Answer-Oxidising agents are the substances that gain electrons in redox reaction and whose oxidation number is reduced.
1. Would you be able to check if water is hard by using a detergent?
Answer-Detergent gives lather with hard and soft water both, while a soap gives lather with soft water only. Thus, it is not possible to check if water is hard; by using a detergent.
2. People use a variety of methods to wash clothes. Usually after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Answer-A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid. To remove these micelles, it is necessary to agitate clothes.
EXCERCISE
4. Explain the nature of the covalent bond using the bond formation in CH3Cl.
Answer-The structure of CH3Cl is given below:

Carbon has four valence electrons. It shares 1 electron each with 3 hydrogen atoms and 1 electron with chlorine. The bond between C and Cl atoms is covalent but due to higher value of electro-negativity of Cl, the C–Cl bond is polar in nature.
5. Draw the electron dot structures for
(a) ethanoic acid.
(b) H2S.
(c) propanone.
(d) F2.
Answer

6. What is a homologous series? Explain with an example.
Answer-A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group.

For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is CnH2n+2.
-Methane CH4    -Ethane CH3CH3  -Propane CH3CH2CH3  -Butane CH3CH2CH2CH3
It can be noticed that there is a difference of -CH2 unit between each successive compound.
7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Answer-Ethanol and Ethanoic acid be differentiated on the basis of their following properties:
→ Ethanol is a liquid at room temperature with a pleasant smell. Ethanoic acid has a melting point of 17°C. Since it is below the room temperature so, it freezes during winter. Moreover, ethanoic acid has a smell like vinegar.
→ Ethanol does not react with metal carbonates while, ethanoic acid reacts with metal carbonates to form salt, water and carbon dioxide. For example,
2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O
→ Ethanol does not react with NaOH while ethanoic acid reacts with NaOH to form sodium ethanoate and water. For example,
CH3COOH + NaOH → CH3COONa + H2O→ Ethanol is oxidized to give ethanoic acid in presence of acidified KMnO4 while, no reaction takes place with ethanoic acid in presence of acidified KMnO4.
8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Answer-Soap molecule has two ends. One end is hydrophilic and another end is hydrophobic. When soap is dissolved in water and clothes are put in the soapy solution, soap molecules converge in a typical fashion to make a structure; called micelle. The hydrophobic ends of different molecules surround a particle of grease and make the micelle; which is a spherical structure. In this, the hydrophilic end is outside the sphere and hydrophobic end is towards the centre of the sphere. This is why micelle formation takes place when soap is added to water.

Since ethanol is not as polar as soap, so micelles will not be formed in other solvents such as ethanol.

9. Why are carbon and its compounds used as fuels for most applications?
Answer-Carbon and its compounds give large amount of heat on combustion due to high percentage of carbon and hydrogen. Carbon compounds used as fuel have optimum ignition temperature with high calorific values and are easy to handle. Their combustion can e controlled. Therefore, carbon and its compounds are used as fuels.
10. Explain the formation of scum when hard water is treated with soap.
Answer-Hard water often contains salts of calcium and magnesium. Soap molecules react with the salts of calcium and magnesium and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.
11. What change will you observe if you test soap with litmus paper (red and blue)?
Answer-Since soap is basic in nature, it will turn red litmus blue. However, the colour of blue litmus will remain blue.
12. What is hydrogenation? What is its industrial application?
Answer-Hydrogenation is the chemical reaction between hydrogen and other compounds in the presence of catalyst. Hydrogenation is used mainly to reduce saturated hydrocarbons. Hydrogenation is an addition reaction.For Example: When ethene is heated with the catalyst nickel it is reduced to ethane.

Industrial application:

→ >Hydrogenation is used in many industrial applications. For example; in Petrochemical Industry, hydrogenation is used to convert alkenes into alkanes (paraffins) and cycloalkanes.
→ It is also used to prepare vegetable ghee from vegetable oils.
13. Which of the following hydrocarbons undergo addition reactions:
C2, H6, C3H8, C3H6, C2H2 and CH4.
Answer-Unsaturated hydrocarbons undergo addition reactions. Being unsaturated hydrocarbons, C3H6 and C2H2 undergo addition reactions.
14. Give a test that can be used to differentiate chemically between butter and cooking oil.
Answer-Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other hand, oil has unsaturated fats. That is why it can be hydrogenated to saturated fats (solids).
15. Explain the mechanism of the cleaning action of soaps.

Answer-The dirt present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. Hence, the dust particles are easily rinsed away by water.