CHAPTER 4

Carbon & it’s Compounds

Brief about Carbon

1. Symbol: C
   
2. Atomic No.:
   6
   
3. Electronic configuration:
   1s², 2s²2p² (2, 4)
   
4. Valence electron:
   4, ‘p’ block elements of Periodic Table.
   
5. Nature:
   Non-metal.
   
6. Occurrence:
   Carbon occurs in nature as elemental Carbon (coal) as well as in the form of it’s compounds such as CO₂ in air, in carbonate minerals such as lime stone CaCO₃, It also occurs in animals and plants in the macromolecular form.
   
7. Bonding in Carbon: Covalent bond is formed by Carbon (C) as it is a non-metal and requires to form a bond by sharing of electrons with other atoms of an element.
   

   Example:
   Covalent bond (Double bond) formation in Carbon di-oxide:
   

   C + O₂ = CO₂
   

   
   

8. Catanation property of Carbon (C): Carbon has a peculiar characteristic to form C – C bond. This property of Carbon (C) to link with another Carbon (C) atom is called catenation property of Carbon (C). This can be shown as follows:
   

   
   

   Carbon (C) also forms stable C – H bond. Owing to this property, it forms several lakhs of important compounds which is the subject matter of Organic Chemistry. It can be shown by electron dot structure as follows:
   

   
   

Allotropic forms of Carbon (C)

2.1    Carbon exists in several allotropic forms such as Diamond, Graphite, Coal, Lamp black etc. Diamond and Graphite forms are more interesting forms of carbon due to their unique structures and properties.

2.2    Diamond:
Diamond is a precious gem. It is the hardest substance know. In diamond, Carbon (C) atoms are linked very closely to four other Carbon (C) atoms tetrahedrally. Therefore, C – C linking is in three dimensions and all the four valence electrons are involved in bond formation. The close linking of C-atoms results in it’s great hardness and involvement of it’s valence electrons in bond formation results in their non-conductivity of electricity. The C-atoms structure of diamond is shown as:

2.3    Property of Diamond: It is transparent to X-rays and hence pure diamond can be distinguished from artificial diamond which is not transparent to X-rays. When heated with K₂Cr₂O₇ and concentrated H₂SO₄ at 200C, it evolves CO₂.

2.4    Uses of Diamond: (a) Used in jewelry (b) Used as gem due to it’s hardness. (c) Used in rock-drills (d) Glass cutting instruments.

2.5    Graphite: Graphite is soft crystalline substance with a greasy feel and metallic lusture. It is conductor of electricity.

    Graphite has a layer structure in which each C-atoms are linked with three other carbon atoms in the same plane. In this way, hexagons of six-carbon atoms are formed in one layer. There are several such layers in graphite held together by weak Vanderwaals forces. Hence, graphite is soft and greasy. The C-atoms structure in graphite is shown below:

2.6    Property of graphite:
Graphite is chemically inactive. It is not acted upon by strong acids, alkalies, Cl₂, dilute HNO₃ and H₂SO₄. When heated in air it burns forming CO₂, A mixture of K₂Cr₂O₇ and conc. H₂SO₄ oxidies it to CO₂.

2.7    Uses of graphite:
(a) It makes mark on paper and hence it is used in marking lead pencils. (b) It is good conductor of electricity hence, it is used in making graphite electrodes, arcs nd battery plates. (c) It is used as lubricant in machines to prevent rusting and friction due to it’s layer structure.

Questions (Page – 61)

Q-1: What would be the electron dot structure of carbon dioxide which has the formula CO2?

Ans:
The electron dot structure of Carbon dioxide is as follows:

Q-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.)

Ans:
The electron dot structure of molecule of sulphur is as follows:

Versatile nature of Carbon (C)

3.1    Saturated carbon compounds:
Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. They are the simplest class of hydrocarbons. They are called saturated because each carbon atom is bonded to as many Hydrogen atoms as possible. In other words, the carbon atoms are saturated with Hydrogen. Saturated carbon compounds are not very reactive.

    Example: Methane: CH₄, Ethane: C₂H₆

3.2    Unsaturated carbon compounds:
An unsaturated compound is a chemical compound that contains carbon-carbon double bonds or triple bonds. These compounds are more reactive than saturated carbon compounds.

    Example: Ethyne – C₂H₂

3.3    Chain reaction:
In this category of reaction, C-atoms are combined in a chain in linear (straight) form. This can easily be understood by the following table:

 

3.4    Branch reaction: In this category of reaction, C-atoms are combined in a branch form.

    Example: Iso-Butane : C₄H₁₀

 

3.5    Ring structure: In this category of reaction, C-atoms are combined in a ring form.

    Example:
Benzene -C₆H₆

 

3.6    Hydro-Carbon:
The compounds formed by the combination of Hydrogen (H) & Carbon (C) are called Hydro-carbon. Saturated Hydrocarbons are called Alkanes (Compound formation formula: CnH2n+2, where n = 1, 2, 3….)

Example: Methane: CH₄, Ethane: C₂H₆. Structure of these are available at para No. 3.3

The unsaturated Hydrocarbons which contain one or more double bonds are called Alkenes (Compound formation formula: CnH2n, where n = 1, 2, 3….).

Example: Methane: CH_4. Structure of these are available at para No. 3.3

Those Hydrocarbons which contain one or more triple bond are called Alkynes (Compound formation formula: CnH2n
– 2, where n = 1, 2, 3….).

Example:
C₂H₂. The structure of C₂H₂ is given below:

3.7    Heteroatom: In Hydrocarbon series when other Hydrogen (H) is replaced by other elements such as halogens, Oxygen (O), Nitrogen (N), & Sulpher (S) are called heteroatom. Means to say that in a Hygrogen- Carbon compound, if any element replaces hydrogen then it is referred as heteroatom. These heteroatom are also present in some groups as tabulated below:

 

3.6    Homologous series: A homologous series is a family of hydrocarbons with similar chemical properties that share the same general formula (Compound formation formula). The three hydrocarbon series: Alkanes, Alkenes and the Cyclo-alkanes (Cycloalkanes have one or more rings of carbon atoms. Cycloalkanes only contain carbon-hydrogen bonds and carbon-carbon single bonds).

 

3.7    Nomenclature of carbon compound:
The names of compounds in a homologous series are based on the anme of a baci carbon chain modified by a “prefix” “phrase before” or “suffix” “phrase after” indicating the nature of the functional group. However, nomenclature of carbon compound can be done by the following method:-

    (i) Identify the number of C-atoms in the compound as elaborated in above table at para 3.6

    (ii) In case of functional group is present, it is named in the name of the coupound with either prefix or suffix viz. Methile alcohol etc.

    (iii) If the name of the functional group is to be given as a suffix, and the suffix of the functional grop begains with a vowel, a,e,i,o,u then the name of the carbon chain is modified by deleting the final ‘e’ and adding the appropriate suffix. For example, a 3-carbon chain with a ketone group would be named as: Propane – ‘e’ = Propan = one = propanone.

    (iv) If the C-atom chain is unsaturated thenn the final ‘ane’ in the name of the carbon chain is substituted by ‘ene’ or ‘yne’ as shown in para 3.6 table. For example, if 3-carbon chain with a double bond would be called ‘propene’ and if it has a triple bond, it would be called ‘propyne’.

    Consider the following table for more understanding:

 

Questions (Page – 68 to 69)

Q-1: How many structural isomers can you draw for pentane?

Ans:
The structural isomers hat can be drawn for pentane are drawn below:

Q-2:  What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Ans: Two properties of carbon which lead to the huge number of carbon compounds we see around us are given as:

• Carbon has six valence electrons which are actually a high number of valency.
  
• Covalent bonding happens easily with carbon atoms and numerous others such as oxygen, chlorine, nitrogen, Sulphur, hydrogen and etc.
  

Q-3: What will be the formula and electron dot structure of cyclopentane?
Ans: The electron dot structure of Cyclopentane is :

Q-4: Draw the structures for the following compounds. (i) Ethanoic acid (ii) Bromopentane* (iii) Butanone (iv) Hexanal.

Ans: The structure of the compounds are:

 

Q-5: How would you name the following compounds?

(i) CH3—CH2—Br

Ans:
(i)
Bromoethane

1. Methanal or Formaldehyde
   
2. Hexane
   

Chemical properties of carbon compounds

4.1    Combustion: Carbon, in all its allotropic forms, burns in Oxygen to give Carbon dioxide along with the release of heat and light. Most carbon compounds also release a large amount of heat and light on burning.

    Example:
CH₄ + O₂ = H₂O + Heat & light

     CH₃CH₂OH + O₂ = CO₂ + H₂O + Heat & light.

    Saturated hydrocarbons generally give a clean flame while unsaturated carbon compounds give a yellow flame with lots of black smoke. However, limiting the supply of air results in incomplete combustion of even saturated hydrocarbons giving a sooty flame.

1. Substances burn with or without flame: A flame is only produced when gaseous substances burn. When wood or charcoal is ignited, the volatile substances present vapourise and burn with a flame in the beginning. A luminous flame is seen when the atoms of a gaseous substance are heated and start to glow.
   
2. Formation of Coal & Petroleum: (i) Coal: It is the remains of trees, ferns and other plants that lived millions of year ago. These were crushed into the earth, perhaps by earthquakes or volcanic eruptions. They were pressed down by layers of earth and rock. They slowly decayed into coal. (ii) Petroleum: Petroleum are the remains of millions of tiny plants and animals that lived in the sea. When they dies, their bodies sank to the sea bed and were covered by silt. Bacteria attacked the dead remains, turning them into oil and gas under the high pressures.
   

4.2    Oxidation: Carbon compounds can easily be oxidized on combustion. In addition to this complete oxidation, there are reactions in which alcohol are converted to carboxylic acids.

    Example: CH₃-CH₂OH Alkaline KMnO₄ + Heat/acidified K₂Cr₂O₇ = CH₃COOH

It is seen from the above example that some substances are capable of adding Oxygen to others. These substances are known as Oxydising agents.

4.3    Addition reaction: Unsaturated Hydrocarbons add Hydrogen in the present of catalysts such as Palladium or Nickel to give saturated Hydrocarbons. This reaction is commonly used in the hydrogenation of vegetable oils using a Nikel catalyst. Vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains.

4.4    Substitution reaction: Saturated hydrocarbons are fairly unreactive and are inert in the presence of most reagents. However, in the presence of sunlight, chlorine is added to hydrocarbons in a very fast reaction. Chlorine can replace the hydrogen atoms one by one. It is called a substitution reaction because one type of atom or a group of atoms takes place of another. A number of are usually formed with the higher homologues of alkanes.

    Example: CH₄ + Cl₂ = CH₃Cl = HCl (in the presence of sunlight)

Questions (Page – 71)

Q-1: Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Ans:
The
Conversion of ethanol to ethanoic acid involves the removal of a Hydrogen atom and addition of Oxygen. It is an oxidation reaction. In the first step, an H₂ molecule is removed from ethanol to form ethanal.  As loss of Hydrogen is oxidation therefore, the reaction is an oxidation reaction. Similarly, Oxygen atom is added to form ethanoic acid from ethanal. As gain of Oxygen is called oxidation there, the reaction is an oxidation reaction.

 

Q-2: A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Ans:
A mixture of Oxygen and ethyne is burnt for welding instead of mixture of ethyne and air because the production of heat is very important for welding metals. When Oxygen and ethyne are burnt, it burns completely and produces a higher temperature than air and ethyne. Oxygen and ethyne produce very hot blue flame but the mixture of air and ethyne gives out a sooty flame which means that there are unburnt particles resulting in lesser heat.

Some important Carbon compounds

5.1    Ethanol (Ehile alcohol):
It is liquid at room temperature. It is commonly called ‘alcohol’ and is the active ingredient of all alcoholic drinks. It is used in medicines such as tincture iodine, cough syrups, and many tonics. Ethanol is also soluble in water in all proportions. Consumption of small quantities of dilute ethanol causes drunkenness. However, intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal.

    Reactions of Ethanol: (i) Reaction with Sodium (Na): Alcohol reacts with Sodium (Na) Hydrogen (H) and Sodium ethoxide are formed.

2Na + 2CH₃CH₂OH = 2CH₃CH₂O^–Na⁺ (Sodium ethoxide) + H₂

    (ii) Reaction to give unsaturated hydrocarbon: On heating ethanol at 443 K with excess concentrated sulphuric acid, it results in the dehydration of ethanol to give ethane.

5.2    Ethanoic Acid:
Ethanoic acid is commonly called Acetic acid and belongs to a group of acids called carboxylic acids. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles. Carboxylic acids are weak acids.

    Reactions of Ethanoic acid:
(i) Esterfication reaction: Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester –

(ii) Reaction with a base: Ethanoic acid reacts with a base such as Sodium hydroxide to give salt (Sodium ethanoate or commonly called Sodium acetate) and water –

NaOH + CH₃COOH = CH₃COONa + H₂O

(iii) Reaction with carbonates and hydrogencarbonates: Ethanic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water. The salt produced is commonly called Sodium acetate –

2CH₃COOH + Na₂CO₃ = 2CH₃COONa + H₂O + CO₂

CH₃COOH + NaHCO₃ = CH₃COONa + H₂O + CO₂

Questions (Page – 74)

Q-1: How would you distinguish experimentally between an alcohol and a carboxylic acid?

Ans: On reaction with Sodium Carbonate, Carboxylic acids produces carbon dioxide gas which turns lime water milky whereas alcohols do not give this reaction. This experiment can be used to distinguish alcohol and carboxylic acid. The reaction of Carboxylic acid with sodium carbonate:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Q-2: What are oxidising agents?

Ans: Oxidising agents are those compounds which either removes Hydrogen or adds oxygen to a compound. Ex: halogens, potassium nitrate, and nitric acid.

 

6.1    Soap:
Soaps are Sodium(Na) or Potassium(K) salts of long chain fatty acids/carboxylic acids. When triglycerides in fat/oil react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called alkaline hydrolysis of esters. Since this reaction leads to the formation of soap, it is called the Saponification process.

    When soap is applied with water, soap molecules are formed and these structures are called micelles. The formula for soap is C₁₇H₃₅COONa (Sodium stearate).

6.2    Detergent:  Detergents are generally Sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc. Both have long hydrocarbon chain. They are also known as surfactants because they decrease the surface tension of water. Detergents are used to make shampoos and products for cleaning cloths.

Questions (Page – 76)

Q-1: Would you be able to check if water is hard by using a detergent?

Ans:
It is not possible to check if water is hard by using a detergent because detergents are salts of ammonium or sulphonates of long chain carboxylic acids. Unlike soaps they do not react with calcium and magnesium to distinguish nature of water.

Q-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?

Ans:
Agitation is necessary to get clean clothes as agitation aid soap micelles to trap the oil, grease or any other impurities that have to be removed. When they are being beaten or agitated, the particles are removed from the clothes’ surfaces and go into the water, thus cleaning the clothes.

Exercise (Page – 77 to 78)

    

 Q-1: Ethane, with the molecular formula C₂H₆has

(a) 6 covalent bonds.

(b) 7 covalent bonds.

(c) 8 covalent bonds.

(d) 9 covalent bonds

Ans:
Ethane, with the molecular formula C2H6 has 7 covalent bonds

Q-2: Butanone is a four-carbon compound with the functional group (a) carboxylic acid. (b) aldehyde. (c) ketone. (d) alcohol.

Ans:
Answer is (c) Ketone.

Q-3:  While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

(a) the food is not cooked completely.

(b) the fuel is not burning completely.

(c) the fuel is wet.

(d) the fuel is burning completely.

Ans:
The correct answer is (b)

While cooking, if the bottom of the vessel is getting blackened on the outside indicates that the fuel is not burning completely.

Q-4: Explain the nature of the covalent bond using the bond formation in CH₃Cl

Ans:
Carbon can neither lose 4 electrons nor do gain four electrons as these process make the system unstable due to requirement of extra energy. Therefore, CH₃Cl completes its octet configuration by sharing its 4 electrons with carbon atoms or with atoms of other elements. Hence the bonding that exists in CH₃Cl is a covalent bonding.

Here, carbon requires 4 electrons to complete its octet, while each hydrogen atom requires one electron to complete its duplet. Also, chlorine requires an electron to complete the octet. Therefore, all of these share the electrons and as a result, carbon forms 3 bonds with hydrogen and one with chlorine.

Q-5: Draw the electron dot structures for

(a) ethanoic acid.

(b) H₂S.

(c) propanone.

(d) F₂

Ans: The electron dot structures are:

Q-6: What is a homologous series? Explain with an example.

Ans: A homologous series is a series of compounds, which has the same functional group. This also contains similar general formula and chemical properties. Since there is a change in the physical properties, we can say that there would be an increase in the molecular size and mass.

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.

Q-7: How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Ans:The difference between Ethanol & Ethanoic acid are as follows:

Q-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?

Ans: Micelle formation takes place because of the dirt particles in water and clean water. There are two mediums that are involved: one is pure water and the other being dirt (also called as impurities). The soap also has two mediums: (i) organic tail and (ii) ionic head. So the organic tail mixes and dissolves with the dirt whereas the oil or grease and ionic head dissolves and mixes with the water. Therefore, when the material to be cleaned is removed from the water, the dirt is taken off by the soap molecules in the water. Hence, the soap cleans by forming closed structures by the mutual repulsion of the micelles (positively charged heads).

Q-9: Why are carbon and its compounds used as fuels for most applications?

Ans: Carbon and its compounds used as fuels for most applications for they have high calorific values and give out a lot of energy. Most of the carbon compounds give a lot of heat and light when burnt in air.

 Q-10:  Explain the formation of scum when hard water is treated with soap?

Ans: Scrum is produced from reaction of hard water with soap. Calcium and magnesium present in the hard water form an insoluble precipitate that stick as a white which is also called as scrum.

Q-11: What change will you observe if you test soap with litmus paper (red and blue)?

Ans:
When we test soap with litmus paper color of litmus paper turns blue from red. As soap is a base it turns red litmus paper into blue.

Q-12: What is hydrogenation? What is its industrial application?

Ans:
Hydrogenation is a process or a chemical reaction between hydrogen and other compounds. It is usually done in the presence of catalysts: for example- Nickel, Palladium or Platinum. Hydrogenation is used mainly to saturate organic compounds.

Q-13: Which of the following hydrocarbons undergo addition reactions: C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄.

Ans: Unsaturated hydrocarbons undergo addition reactions. C₃H₆ and C₂H₂ are unsaturated hydrocarbons which undergo addition reactions.

Q-14: Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Ans: Bromine water test – is used to differentiate between the unsaturated compounds (like alkenes and alkynes) and the saturated compounds. For this purpose, bromine is used in the form of bromine water. A solution of bromine in water is called bromine water. Bromine water has a red-brown colour due to the presence of bromine in it. When bromine water is added to an unsaturated compound, then bromine gets added to the unsaturated compound and the red-brown colour of bromine water is discharged. So, if an organic compound decolorizes bromine water, then it will be an unsaturated hydrocarbon (containing a double bond or a triple bond), but saturated hydrocarbon (alkanes) do not decolorize bromine water.

Bromine water test is performed to differentiate between the unsaturated compounds (like alkenes and alkynes) and the saturated compounds. Bromine water is added to an un-saturated hydrocarbon red brown colour of bromine solution is discharged. Si if there is dis-coloration then the compound will be an unsaturated Hydrocarbon.

Q-15: Explain the mechanism of the cleaning action of soaps.

Ans: There are so many impurities and dirt mixed in water, and most of all the dirt do not dissolve in the water. Soap molecules are a combination of salts such as sodium or potassium. The molecules are of a long chain of carboxylic acids. So, when the carbon chain is dissolved in oil and the ionic end is dissolved in the water, the soap starts cleansing and trapping the dirt. When this happens, the soap molecules form structures that are called micelles are used for capturing the oil droplets and then the other end being the ionic faces. This will then form an emulsion in water and help in dissolving the dirt or impurities when the clothes are washed.

The soap molecules have different properties at different ends. The first end being the hydrophilic end which dissolves in the water and is attracted towards the water and the second one being the hydrophobic end is dissolved in the hydrocarbons and is repulsive to water. The hydrophobic tail aligns itself along the surface of the water because it is not soluble in the water.

 

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