Chapter 4 Carbon and its Compounds

Carbon exists in the atmosphere in the form of Carbon dioxide in air (0.03 % only). Carbon also occurs in the form of minerals such as carbonates, fossil fuels and other organic compounds. Carbon monoxide is produced by the incomplete combustion of carbon due to limited supply of oxygen. It is pollutant that is formed when hydrocarbons such as petrol and diesel are burnt. Carbon monoxide is short-lived and is oxidized into carbon dioxide.

A molecule of ammonia (NH₃) has only single bonds. Since the valency of nitrogen is 3, one atom of nitrogen combines with three atoms of hydrogen in a molecule of ammonia. An atom of nitrogen can form three covalent bonds.

Carbon compounds are poor conductors of electricity because they form covalent bonds. In such bonds, there are no free electrons left as all are used in making the bond. Hence carbon compounds do not dissociate into ions and hence are poor conductors of electricity. Moreover, covalent bonds do not have strong forces of attraction. A covalent bond is also known as a molecular bond.

Buckminsterfullerene is an allotropic form of carbon (C60). It was discovered in 1985. Sixty carbon atoms form the shape of a ball with a carbon atom at each corner of the 20 hexagons and 12 pentagons. Each carbon atom has three bonds.

The chemical formula of butane is C₄H₁₀. Structural formulae in other options (iii) and (iv) show only 8 hydrogen atoms. Isomers are molecules that contain the same number of atoms of the same elements but differ in structural arrangement and properties. Isomers have same chemical formula.

In this reaction, alkaline KMnO₄ acts as an oxidising agent. It adds oxygen to ethanol resulting in formation of ethanoic acid. An oxidizing agent adds oxygen or takes electrons from other substances.

This reaction is an example of addition reaction (also known as hydrogenation reaction). In this reaction oil which is unsaturated hydrocarbon reacts with hydrogen in the presence of a catalyst (palladium or nickel) to form fats which are saturated hydrocarbon.

The alcohols are organic compounds. They all contain the functional group –OH. This group possess the properties of alcohols. The names of alcohols end with ‘-ol’ – e.g., ethanol, butanol.

The soap molecule has a hydrophilic head and a hydrophobic tail. When soap is mixed into the water, the soap molecules arrange themselves into tiny clusters called 'micelles'. The hydrophilic parts of the soap (water-loving) molecules point outwards, forming the outer surface of the micelle. The hydrophobic parts (oil-loving) group together on the inner side. Micelles can trap fats in the center and helps to get rid of oil and dirt.

An electron dot structure represents the valence electrons as dots placed around the chemical symbol. Electrons are placed up to two on each side of the symbol for a maximum of eight, which is the number of electrons in a filled s and p shell.

Nitrogen has 5 valence electrons. When we draw the Lewis structure for Nitrogen we draw 5 ‘dots’ or valence electrons around the element symbol (N).

The chemical formula of Ethyne is C₂H₂, the structure of which is shown in option ‘a’.

Alkanes such as propane are saturated hydrocarbons. An unsaturated compound contains carbon-carbon double bonds or triple bonds, such as those found in alkenes (propene) or alkynes (propyne), respectively.

Saturated hydrocarbons reacts with chlorine at room temperature in the presence of sunlight to form alkyl chloride by displacement reaction.

The soap molecule has a hydrophilic head and a hydrophobic tail. When soap is mixed into the water, the soap molecules arrange themselves into tiny clusters called 'micelles'. The hydrophilic parts of the soap (water-loving) molecules point outwards, forming the outer surface of the micelle. The hydrophobic parts (oil-loving) group together on the inner side. Micelles can trap fats in the center and helps to get rid of oil and dirt.

Pentane has 16 covalent bonds. The structural diagram is as follows:

The chemical formula of benzene is C₆H₆.

The benzene ring consists of six carbon atoms bonded in a hexagon ring. Each carbon is bonded to one hydrogen. There are three alternating double bonds between carbon atoms. This reveals that each carbon is bonded to 3 others.

When a small piece of sodium reacts with ethanol, a colourless solution of sodium ethoxide, CH₃CH₂ONa is obtained. It also gives off bubbles of hydrogen gas. Sodium ethoxide is known as an alkoxide.

This following equation shows this reaction:

2CH₃CH₂OH + 2Na 2CH₃CH₂ONa + H₂

Butanoic acid has the structural formula: C₃H₇COOH (CH₃CH₂CH₂-COOH ).

Vinegar is a solution of 5% - 8% acetic acid in water. Acetic acid is the solute and water is the solvent.

Acids ionize in a solution. Strength of an acid is determined by the ability to furnish H⁺ions in a solution. A strong acid such as mineral acids dissociate completely to furnish H⁺ions whereas weaker acids such as carboxylic acids do not dissociate completely to furnish H⁺ions. Hence, mineral acids are more acidic than carboxylic acids.

After the formation of four bonds, carbon attains the electronic configuration of neon, the nearest noble gas or inert gas.

Electronic configuration of carbon (C) = 2, 4. When it reacts with hydrogen, it forms methane (CH₄). Carbon forms four covalent bonds by sharing its four valence electrons. Now, electronic configuration of C in CH₄ = 2, 8. Therefore, after the formation of four bonds, carbon attains the electronic configuration of neon with the atomic number 10.

Oxygen (O) has 6 valence electrons and hydrogen (H) has 1. All the 8 electrons must be arranged in pairs so that oxygen completes the octet structure (8 electrons in its valence cell). Each hydrogen has now 2 electrons in its valence shell.

A straight chain hydrocarbon is one that is made from carbon atoms that are joined to not more than two other carbon atoms.

A branched chain hydrocarbon is one that is made from carbon atoms where at least one carbon atom is joined to more than two other carbon atoms.

In these options, carbon atoms show double bond and triple bonds; hence these are unsaturated hydrocarbons.

Unsaturated hydrocarbons are hydrocarbons that form double or triple covalent bonds with other carbon atoms. Those hydrocarbons that have at least one carbon-to-carbon double bond are called alkenes. Those hydrocarbons with at least one carbon-to-carbon triple bond are called alkynes.

follows the general formula C_nH_2n, whereas other options follow the general formula C_nH_2n+2.

A homologous series is a series of carbon compounds that differ in the number of carbon atoms they have but possess the same functional group. Methane, ethane, propane, butane are part of alkane homologous series. The homologous series of a class of alcohols has the hydroxyl functional group.

The functional group – CHO is used with the compounds ending with suffix ‘al’.

Propanal (also known as propionaldehyde) is an organic compound with the formula CH₃CH₂CHO. It is a saturated 3-carbon aldehyde. It is also a structural isomer of acetone. It is colourless liquid with a fruity smell.

Oxygen and chlorine are heteroatoms.
A heteroatom is any atom other than carbon or hydrogen in a molecular structure.

Soaps are made by the process of saponification. Soaps are sodium or potassium salts of long chain fatty acids. When triglycerides in fats react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called saponification or alkaline hydrolysis of esters.

Ethyne (C₂H₂) is the first member of alkyne homologous series.

Ethyne (also known as acetylene) is the simplest alkyne. Its molecular formula is C₂H₂. The structural formula for ethyne is

(a) Pentanoic acid

(b) Butyne

(c) Heptanal

(d) Pentanol

(a) Propanol (C₃H₇OH). It is commonly represented by the molecule propan-1-ol, a primary alcohol. It has –OH functional group.

(b) Carboxylic acid. It is an organic compound that contains a carboxyl group [C(=O)OH]. The general formula of a carboxylic acid is R–COOH. It has –COOH functional group.

(c) Ketone. It is an organic compound with the structure RC(=O)R'. Here R and R' can be a carbon-containing compounds. Ketones and aldehydes are simple compounds that contain a carbon–oxygen double bond (carbonyl group). It has carbon–oxygen double bond functional group.

(d) Alkene. These are hydrocarbons that contain a carbon–carbon double bond. The number of hydrogen atoms in an alkene is just double the number of carbon atoms. For example, the number of hydrogen atoms in ethane (C₂H₄) is 4 and that of carbon is 2. In propene (C₃H₆), the number of hydrogen atoms is 6 and that of carbon is 3. It carbon-to-carbon double bond functional group.

(a) The name of carboxylic acid is ethanoic acid (CH₃COOH).

(b) The name of alcohol is ethyl alcohol (C₂H₅OH).

(c) Compound X is ester (CH₃-COOC₂H₅).

Carboxylic acid with chemical formula (C2H4O2) is Acetic acid (CH3-COOH):
Ethyl alcohol on oxidation forms acetic acid.
C₂H₅OH + KMnO₄→ CH₃-COOH
Carboxylic acids reacts with alcohols to forms ester.
CH₃-COOH + C₂H₅OH → CH₃-COOC₂H₅ (Ethyl actetate)
Therefore, X = CH₃-COOC₂H₅

Soaps work better and effectively in soft water and not in hard water. This happens because soap forms precipitate (or scum) after reacting with magnesium and calcium ions present in hard water. Detergents work good in soft water as well as hard water because they do not form precipitate (or scum) even in hard water as calcium and magnesium salts are soluble in water. Hence, detergents form more lather and clean better than soaps.

(a) CH₃COCH₂CH₂CH₂CH₃ (Ketone); Functional group: carbonyl, C=O.

(b) CH₃CH₂CH₂COOH (Carboxylic acid); Functional group: carbonyl –C=O) + the hydroxyl –OH) = carboxyl.

(c) CH₃CH₂CH₂CH₂CHO (Aldehyde); Functional group: carbonyl, −CHO. (i.e., an oxygen atom is attached to a carbon atom by a double covalent bond and a hydrogen atom is attached to the carbon atom )

(d) CH₃CH₂OH (Alcohol); Functional group: hydroxyl functional group (–OH)

When ethanol is heated with excess of concentrated sulphuric acid (at 443 K); it produces ethene. This can be shown by following equation.

CH₃CH₂OH + hot conc. H₂SO₄ (at 443 K) → H₂C = CH₂ + H₂O

Concentrated phosphoric (V) acid, H₃PO₄, can be used instead of concentrated sulphuric acid.

Hydrolysis of ethanol takes place in this reaction.

If methanol is taken in large quantities, it gets converted into methanal or formaldehyde. This substance coagulates the cells of the body and blocks the optic nerve thus causing blindness. It is toxic and cancer-causing too. It is naturally present in small quantities in fruits and in human body.

When ethanol reacts with sodium, hydrogen gas (H₂) is evolved.

This reaction can be shown by following equation.

2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂

This is an oxidation-reduction reaction. Na is oxidized, and H is reduced.

In this reaction, sulphuric acid is working as a dehydrating agent.

Sulphuric acid has a property to attract water. It removes water from other compounds without dissolving in it. Thus it is considered a good drying agent.

The chemical equation of this reaction is as follows:

CH₃CH₂OH + hot conc. H₂SO₄ (at 443 K) → H₂C = CH₂ + H₂

(a) Carbon forms carbon tetrachloride (CCl₄) with chlorine. Carbon tetrachloride is used as a solvent for oils and fats. It is also used as a refrigerant.

(b) Carbon forms carbon dioxide (CO₂) with oxygen. Solid carbon dioxide (i.e., dry ice) is used in large-scale refrigeration. It is also used in fire extinguishers.

(a) K, L, M – 2, 8, 7

Both carbon and silicon show catenation. Carbon shows much more catenation than silicon or any other element as it is smaller in size. This feature helps to make strong C—C bonds whereas the Si—Si bonds are comparatively weaker as it is larger in size. This is the reason why long chain compounds of silicon are not as stable as those of carbon.

To distinguish between saturated and unsaturated hydrocarbons, combustion test should be performed. A saturated hydrocarbon undergoes complete combustion and burns with blue flame. Also, it does not leave any residue behind after burning. On the contrary, a unsaturated hydrocarbon undergoes incomplete combustion and burns with yellow flame. Also, it leaves some residue behind after burning. Ethane is a saturated hydrocarbon. Thus, burns with a blue flame and does not leave residue behind. Ethene is an unsaturated hydrocarbon. Thus burns with a yellow flame and leaves some residue behind.

(a) → (iv), (b) → (i), (c) → (ii), (d) → (iii)

There are 5 isomers of hexane. They are hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. They are known as constitutional isomers because they each contain exactly the same number and type of atoms. In this case, six carbons and 14 hydrogens and no other atoms.

A catalyst is a substance or material that will change the rate of reaction without it being consumed by the reaction.

(a) Nickel is working as catalyst. Ethene molecules are adsorbed on the surface of the nickel.

(b) Sulphuric acid is working as catalyst

(c) Potassium permanganate is working as oxidising agent that removes one or more electrons from another atom.

Salt X is sodium ethanoate and the evolved gas is carbon dioxide.

Procedure: Take a test tube and a bent tube. Take ethanoic acid or acetic acid (HC2H3O2) and sodium hydrogen carbonate or sodium bicarbonate (NaHCO3) solution in the test tube. Insert the bent tube in the cork and fit the cork at the mouth of the test tube. Fill lime water in the bent tube so that lime water is in the ‘U’ portion of this tube.

After some time, it is observed that the lime water turns milky. This confirms that the evolved gas is carbon dioxide (CO2). Carbon dioxide always turns lime water milky.

This reaction can be shown by following equation.

NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2

Organic compounds that are made up of carbon and hydrogen atoms are called hydrocarbons. Some common examples of hydrocarbons are methane and ethane.

Methane is the main component of natural gas. It is also released into the atmosphere by crude oil production and other industrial activities and biological processes. Small amounts of methane are also produced in our body.

All hydrocarbons are converted to carbon dioxide and water after a series of reactions.

Based on the number of bonds between carbon atoms, hydrocarbons are of different types.

1. Hydrocarbons that have single bond between two carbon atoms are called alkanes. These are the least reactive of all the hydrocarbons. Examples: methane, ethane
2. Hydrocarbons that have a double bond between two carbon atoms are called alkenes. These are more reactive than alkanes. Examples: ethane, propene
3. Hydrocarbons that have a triple bond between two carbon atoms are called alkynes. These are the most reactive. They are commonly called as acetylenes. Examples: ethyne, propyne

In saturated hydrocarbons, all hydrogen atoms and carbon atoms are bonded together with single bonds. Methane and ethane are examples of saturated hydrocarbons.

Unsaturated hydrocarbons have double or even triple bonds between the carbon atoms.

Ethene and ethyne are examples of unsaturated hydrocarbons.

To distinguish between saturated and unsaturated hydrocarbons, combustion test can be performed. A saturated hydrocarbon undergoes complete combustion and burns with blue flame. Also, it does not leave any residue behind after burning. On the contrary, a unsaturated hydrocarbon undergoes incomplete combustion and burns with yellow flame. Also, it leaves some residue behind after burning. Ethane is a saturated hydrocarbon. Thus, burns with a blue flame and does not leave residue behind. Ethene is an unsaturated hydrocarbon. Thus burns with a yellow flame and leaves some residue behind.

An atom or group of atoms in an organic compound that defines the structure of a family of compounds and determines the properties of the family is called the functional group.

A functional group is responsible for the characteristic property or behavior of a class of compounds. For example, hydroxyl ( –OH) group in alcohols.

Examples of functional groups are: Hydroxyl group ( –OH), Aldehyde group ( –CHO), Ketone group ( –CO) and carboxylic group ( –COOH).

The reaction which is commonly used in the conversion of vegetable oils to fats is called hydrogenation reaction. During this reaction, hydrogen is added to unsaturated hydrocarbons. This is an addition reaction which takes place in the presence of catalyst such as palladium or nickel. Catalysts are compounds that increase the rate of a reaction without being used up during the process. When the hydrogen molecule reacts with an unsaturated hydrocarbon, double bond changes to a single bond. This can be shown by following equation.

A simple hydrogenation reaction can be represented as:

H2C=CH2+H2→CH3CH3H2C=CH2+H2→CH3CH3

alkene + hydrogen = alkane

Vegetable oils are commonly called polyunsaturated because there are several double bonds in it. Vegetable oils may be converted from liquids to solids by the hydrogenation reaction. Hydrogenated fats and oils are common ingredients in many foods. They are used to extend the longevity of a food item and retain the food flavor for a longer time.

Carbon tetrachloride: CCl₄.

Carbon forms carbon tetrachloride (CCl₄) with chlorine. Carbon tetrachloride is used as a solvent for oils and fats. It is also used as a refrigerant.

Soaps are made by the process of saponification. Soaps are sodium or potassium salts of long chain fatty acids. When triglycerides in fats react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called saponification or alkaline hydrolysis of esters.

CH₂COOC₂H₅ + NaOH → CH₃ COONa + C₂H₅OH

The soap molecule has two parts: a polar group (-COO-Na+) and a non-polar group (R-hydrocarbon part). The polar group is called the head and the non-polar group is called the tail. Therefore, the soap molecule has a polar head and a non-polar hydrocarbon tail. The polar head is water loving in nature (hydrophilic) and the non-polar tail is water repelling (hydrophobic) in nature.

The saponification reaction is exothermic in nature.

Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid.

Activity:

1. Take a test tube and add ethanol (1 ml) and glacial acetic acid (1 ml) in it. Put a few drops of concentrated sulphuric acid in the test tube as well.

2. Now, heat the test tube in a water bath at about 60°C for about 15 minutes.

3. After heating, transfer the contents of the test tube into a beaker which is filled with about 50 ml water.

4. The product, that is obtained, smells sweet.

This is how esters are made.

Formation of esters can be shown by following equation:

CH₃COOH + CH₃CH₂OH + Conc. H₂SO₄→ CH₃COOCH₂CH₃ + H₂O

Diagram:

C is ethanoic acid or acetic acid (CH₃COOH).

R is sodium acetate or sodium ethanoate (CH₃COONa) and the gas evolved is hydrogen.

A can be methanol (CH₃OH) or ethanol (C₂H₅OH).

S is ester or methyl acetate (CH₃COOCH₃), an ester.

The reactions involved in this process are as follows:

1. 2CH₃COOH + 2Na → 2CH₃COONa + H₂

2. CH₃COOH + CH₃OH + Conc. H₂SO₄→ CH₃COOCH₃ + H₂O

3. CH₃COOH + NaOH → CH₃COONa + H₂O

4. CH₃COOH + NaOH → CH₃COONa+ CH₃OH

(a) Calcium hydroxide solution will turn milky.

(b) 2CH₃COOH + Na₂CO₃→ 2CH₃COONa + H₂O + CO₂ (Test tube A)

Ca(OH)₂ + CO₂→ CaCO₃ + H₂O (Test tube B)

The white colour of the liquid disappears with excess CO₂.

CaCO₃ + H₂O + CO₂→ Ca(HCO₂)₂

(c) Ethanol (C₂H₅OH) does not react with sodium carbonate (Na₂CO₃). Hence no similar reaction takes place.

(d) For preparing lime water in the laboratory, calcium oxide (quick lime) is dissolved in water and the mixture is allowed to settle. The supernatant liquid is then decanted lime water is obatined.

(a) Ethanol is dehydrated in the presence of concentrated sulphuric acid to obtain ethene.

This can be shown by the following equation:

CH₃CH₂OH + conc. H₂SO₄→ CH₂=CH₂ + H₂O

(b) Propanol is oxidised by an oxidizing agent potassium permanganate to obtain propanoic acid.

This can be shown by the following equation:

CH₃CH₂CH₂OH + alkaline KMnO₄→ CH₃CH₂OOH

Isomers are the molecules with same molecular formula but different structural formula.

Electron dot structure of propanone

Electron dot structure of propanol

(a) Hydrogenation reaction: The reaction which is commonly used in the conversion of vegetable oils to fats is called hydrogenation reaction. During this reaction, hydrogen is added to unsaturated hydrocarbons. This is an addition reaction which takes place in the presence of catalyst such as palladium or nickel. Catalysts are compounds used to speed up the rate of a reaction without being eaten up during the process. When the hydrogen molecule reacts with an unsaturated hydrocarbon, double bond changes to a single bond. This can be shown by following equation.

A simple hydrogenation reaction can be represented as:

H2C=CH2+H2→CH3CH3H₂C=CH₂+H₂→CH₃CH₃

alkene + hydrogen = alkane

Vegetable oils are commonly called polyunsaturated because there are several double bonds in it. Vegetable oils may be converted from liquids to solids by the hydrogenation reaction. Hydrogenated fats and oils are common ingredients in many foods. They are used to extend the longevity of a food item and retain the food flavor for a longer time.

(b) Oxidation reaction: Oxidation is the loss of electrons during a reaction by a molecule, atom or ion. The opposite process is called reduction, which occurs when there is a gain of electrons during a reaction by a molecule, atom or ion. Metals such as sodium, magnesium, and iron are easily oxidized.

After oxidation, properties of an atom or compound change. For example, when an iron undergoes oxidation, it losses electrons. Unoxidized iron is a strong, while oxidized iron is a weak and brittle.

The following equation shows an oxidation reaction.CH₃CH₂CH₂OH + alkaline KMnO₄→ CH₃CH₂OOH

(c) Substitution reaction: A substitution reaction is a reaction in which an atom or a group of atoms replaces other atom or atoms in reactants.

There are two types of substitution reactions: nucleophilic and electrophilic. In nucleophilic reactions, the new atom is electron-rich, while in electrophilic reactions, the new atom is electron-deficient.

CH₃Cl react with a hydroxy ion (OH-) to produce CH₃OH and chlorine. This substitution reaction replaces the chlorine atom on the original molecule with the hydroxyl ion.

The following equation is an example of substitution reaction.CH₄ + Cl₂ + sunlight → CH₃Cl + HCl

(d) Saponification reaction: Soaps are made by the process of saponification. Soaps are sodium or potassium salts of long chain fatty acids. When triglycerides in fats react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called saponification or alkaline hydrolysis of esters.

CH₂COOC₂H₅ + NaOH → CH₃ COONa + C₂H₅OH

(e) Combustion reaction: Combustion reaction occurs when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. It is usually accompanied by the production of heat and light. Combustion involves a reaction between any combustible substance and an oxidizing agent to form an oxidized product.

The following equation is an example of combustion.CH₄ + 2O₂→ CO₂ + 2H₂O + Heat and light

Compound A is ethanol (C₂H₅OH).

Compound B is ethane (C₂H₄).

Compound C is ethane (C₂H₆).

The following equations show the reactions involved in this process.

C₂H₅OH + conc. H₂SO₄→ C₂H₄ + H₂O

C₂H₄ + H₂ + Ni → C₂H₆

2C₂H₆ + 7O₂→ 4CO₂ + 6H₂O + Heat and light