CBSE NOTES CLASS 10 SCIENCE CHAPTER 4

CARBON AND ITS COMPOUNDS

Importance of Carbon in Life

Food, clothes, medicines, books, or many of the things are compounds of carbon.

All living structures are carbon based.

Although the amount of carbon present in the earth’s crust and in the atmosphere is quite meager (the earth’s crust has only 0.02% carbon in the form of minerals like carbonates, hydrogen carbonates, coal and petroleum and the atmosphere has 0.03% of carbon dioxide), the importance of carbon is immense.

Covalent Bond

A covalent bond is a chemical bond that is formed by sharing of electron pairs between atoms. The shared electrons ‘belong’ to the outer shells of both the atoms and lead to both atoms attaining the noble gas configuration.

Difference between ionic and covalent compounds

(i) Conductivity - Ionic compounds are good conductors of electricity in their molten form or in aqueous solutions, whereas most covalent compounds are poor conductors of electricity.

(ii) Boling and Melting Points - The boiling and melting points of ionic compounds are very high, whereas most covalent compounds have lower boliling and melting points.

(iii) Inter Molecular Forces or Bond Strength - The forces of attraction between covalent molecules are weaker, compared to between ionic molecules.

(iv) Ions - Ionic compounds are formed by complete transfer of electrons from one atom to another, hence they have ions. The bonding in these covalent compounds does not give rise to any ions.

Why does carbon not form ionic bonds ?

Carbon has atomic number 6 and electronic configuration 2, 4. Hence it has a valence of 4. It has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration. Two cases arise,

(i) It could gain four electrons forming C4– anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons.

(ii) It could lose four electrons forming C4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.

Hence carbon does not form the ionic bond, but shares electrons with other atoms to form covalent bond.

Single bond

When one pair of electrons is shared between two atoms, the bond formed is called single bond. A single bond is represented by a line between (-) the two atoms.

Double bond

When two pairs of electrons are shared between two atoms, the bond formed is called double bond. A double bond is represented by two lines (=) between the two atoms.

Triple bond

When three pairs of electrons are shared between two atoms, the bond formed is called tripple bond. A tripple bond is represented by three lines between the two atoms.

• Write the electron dot structures of CH4, C2H6, C2H4, C2H2, NH3, C2H5OH, CH3Cl, Sulphur (S8), CO2

Allotropes of carbon

Each of two or more different physical forms in which an element can exist, are called allotropes of that element.

Graphite, Charcoal, Diamond, Fullerenes are all allotropes of carbon.

Differentiate between graphite and diamond

(i) Molecular structure

• Diamond: Giant covalent structure, with each carbon covalently bonded to four other carbon atoms in a tetrahedral arrangement to form a rigid structure.

• Graphite: Giant covalent structure, with each carbon covalently bonded to three other carbon atoms in a hexagonal arrangement.

(ii) Hardness

• Diamond: Extremely hard, Due to rigid, tetrahedral arrangement of carbon atoms.

• Graphite: Soft, Layers of hexagonally arranged carbon atoms can slide over one another, as the layers are held together by van der Waals forces of attraction.

(ii) Melting and boiling points

• Diamond and graphite: Very high, A large amount of energy is required to break numerous, strong covalent bonds between carbon atgoms. However diamond has higher boiling point than graphite.

(iii) Electrical conductivity

• Diamond: Insulator, Mobile electrons are absent. All four valence electrons are used in covalent bonds.

• Graphite: Conductor, Three out of four valence electrons are used for covalent bonding with other carbon atoms. Remaining valence electrons can be delocalised across the planes of carbon atoms.

Synthetic Diamonds

Diamonds can be synthesised by subjecting pure carbon to very high pressure and temperature. These synthetic diamonds are small but are otherwise indistinguishable from natural diamonds.

Fullerenes are also carbon allotropes. The first one to be identified was C-60 which has carbon atoms arranged in the shape of a football. Since this looked like the geodesic dome designed by the US architect Buckminster Fuller, the molecule was named fullerene.

Versatile Nature of Carbon

The numbers of carbon compounds outnumbers the compounds formed by all the other elements put together. The nature of the covalent bond enables carbon to form a large number of compounds.

Two factors responsible for this are,

Catenation

Ability of carbon atom to form bonds with other atoms of carbon, giving rise to large molecules is called catenation. These compounds may have long chains, branched chains or rings of carbon atoms, to which other atoms are bonded.

In addition, carbon atoms may be linked by single, double or triple bonds.

The carbon-carbon bond is very strong and hence stable. This gives us the large number of compounds with many carbon atoms linked to each other.

Tetra-Valency

Since carbon has a valency of four, it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element. Carbon forms compounds with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elements with specific properties which depend on the elements other than carbon present in the molecule.

Also the bonds that carbon forms with most other elements are very strong making these compounds exceptionally stable.

One reason for the formation of strong bonds by carbon is its small size, which enables the nucleus to hold on to the shared pairs of electrons strongly.

The bonds formed by elements having larger atoms are much weaker.

Saturated compounds

Compounds of carbon which are linked by only single bonds between their carbon atoms are called saturated compounds. These compounds are normally not very reactive.

Unsaturated compounds

Compounds of carbon having at least one double or triple bond between their carbon atoms are called unsaturated compounds. They are more reactive than the saturated carbon compounds.

Hydrocarbons

All the carbon compounds which contain just carbon and hydrogen are called hydrocarbons.

Saturated Hydrocarbons (with only single C to C bond) are called alkanes (Methane, Ethane etc.), with general formula CnH2n+2.

Unsaturated Hydrocarbons with one C to C double bond are called alkenes (ethene, propene etc.), with general formula CnH2n.

Unsaturated Hydrocarbons with one C to C triple bond are called alkynes (ethyne, propyne etc.) General formula: CnH2n-2.

 Name Molecular Formula Molecular Structure Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12 Hexane C6H14
• List the saturated and unsaturated hydrocarbons.

• Practice the names of hydrocarbons up to 10 carbons in all the three categories.

• Draw their structures.

Straight Chain

An open-chain compound having no side chains is called a straight-chain compound. Examples propane, butane, octane etc.

Branch Chain

An open-chain compound that contains a chain of carbon atoms in which at least one carbon atom is connected to three or four others carbon atoms, thus forming a branch, are called branched chain compounds.

Cyclic Compounds

A cyclic compound (ring compound) has one or more series of atoms in the compound connected to form a ring.

Isomers

Compounds with identical molecular formula but different structures are called structural isomers.

Functional Groups

Functional groups are specific atoms, ions, or groups of atoms, which provide specific properties to the compound, regardless of the length and nature of the carbon chain.

Free valency or valencies of the group are shown by the single line.

The functional group is attached to the carbon chain through its valency by replacing one hydrogen atom or atoms.

Homologous Series

A series of compounds whith different number of carbon atoms, in which the same functional group substitutes for hydrogen in a carbon chain, is called a homologous series. Their formulae differ by –CH2 unit or their molecular masses differ by 14u.

Examples: Alkanes, Alkenes, Alkynes, Alcohols etc.

As the molecular mass increases in any homologous series, a gradation in physical properties occurs. For example, melting points and boiling points increase with increasing molecular mass.

However the chemical properties, which are determined solely by the functional group, remain similar in a homologous series.

Nomenclature of Carbon Compounds

The names of compounds in a homologous series are based on the names of the basic carbon chain modified by a “prefix (phrase before)” or “suffix (phrase after)” indicating the nature of the functional group. For example, the names of the alcohols (containing –OH group) are methanol, ethanol, propanol and butanol etc.

1. Identify the number of carbon atoms in the compound. A compound having three carbon atoms would have the name propane.

2. In case a functional group is present, it is indicated in the name of the compound with either a prefix or a suffix as given in Table

[alkyl group – methyl (-CH3), ethyl(-C2H5), propyl(-C3H7) etc.]

3. If the name of the functional group is to be given as a suffix, the name of the carbon chain is modified by deleting the final ‘e’ and adding the appropriate suffix. For example, a three-carbon chain with a ketone group would be named in the following manner, Propane – ‘e’ = propan + ‘one’ = propanone.

4. If the carbon chain is unsaturated, then the final ‘ane’ in the name of the carbon chain is substituted by ‘ene’ or ‘yne’. For example, a three-carbon chain with a double bond would be called propene and if it has a triple bond, it would be called propyne.
 Functional group Prefix/ Suffix Example Halogen Prefix - chloro, bromo Alcohol Suffix - ol Aldehyde Suffix - al Ketone Suffix - one Carboxylic acid Suffix - oic acid Double bond (alkenes) Suffix - ene Triple bond (alkynes) Suffix - yne

Nomenclature of branched chain alkanes

1. First of all, the longest carbon chain in the molecule is identified. For example the longest chain in the example below, has nine carbons and it is considered as the parent or root chain as shown in (I). Selection of parent chain as shown in (II) is not correct because it has only eight carbons.

2. The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms. The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers.

The numbering in the above example should be from left to right (branching at carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the carbon atoms at which branches are attached)

1. The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2-methylnonane. [The numbers are separated from the groups by hyphens and there is no break between methyl and nonane.]
1. If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered. Thus, the following compounds are named as:

1. If the two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing. Thus, the following compound is 3-ethyl-6-methyloctane and not 6-ethyl-3-methyloctane.

2. The carbon atom of the branch that attaches to the root alkane is numbered 1.

If there happen to be two chains of equal size, then that chain is to be selected which contains more number of side chains.

• After selection of the chain, numbering is to be done from the end closer to the substituent.
1. The longest chain of carbon atoms containing the functional group is numbered in such a way that the functional group is attached at the carbon atom possessing lowest possible number in the chain.

In the case of polyfunctional compounds, one of the functional groups is chosen as the principal functional group and the compound is then named on that basis (using suffix). The remaining functional groups, which are subordinate functional groups, are named as substituents using the appropriate prefixes.

For example, HOCH2(CH2)3CH2COCH3 will be named as 7-hydroxyheptan-2-one and not as 2-oxoheptan-7-ol. Similarly, BrCH2CH=CH2 is named as 3-bromoprop-1-ene and not 1-bromoprop-2-ene.

2. If more than one functional groups of the same type are present, their number is indicated by adding di, tri, etc. before the class suffix. In such cases the full name of the parent alkane is written before the class suffix. For example CH2(OH)CH2(OH) is named as ethane–1,2–diol. However, the ending – ne of the parent alkane is dropped in the case of compounds having more than one double or triple bond; for example, CH2=CH-CH=CH2 is named as buta–1,3–diene.

Chemical Properties of Carbon Compounds

Combustion of Carbon Compounds

• 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. These are the oxidation reactions.

C + O2 → CO2 + heat and light

CH4 + O2 → CO2 + H2O + heat and light

CH3CH2OH + O2 → CO2 + H2O + heat and light

• Saturated hydrocarbons give a clean flame while unsaturated carbon compounds give a yellow flame with lots of black smoke called soot.

• Limiting the supply of air, results in incomplete combustion of even saturated hydrocarbons, giving a sooty flame.

• The gas/kerosene stove used at home has inlets for air so that a sufficiently oxygen-rich mixture is burnt to give a clean blue flame.

• If the bottoms of cooking vessels get blackened, the air holes are blocked and fuel is getting wasted.

• Fuels such as coal and petroleum have some amount of nitrogen and sulphur in them. Hence they form oxides of sulphur and nitrogen on burning, which are major pollutants in the environment.

Why do substances burn with or without a flame?

• A flame is 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 the gaseous substance are heated and start to glow. The colour produced by each element is a characteristic property of that element.

• Yellow colour of flame is due to glowing of carbon particles.

Reaction of Alcohols with Oxygen

• Alcohols are converted to carboxylic acids

• Substances which add oxygen to the starting material are known as oxidising agents. Alkaline potassium permanganate and acidified potassium dichromate are oxidising agents here.

• Hydrogen is added to unsaturated hydrocarbons in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons.

• Catalyst is a substance that changes the rate of reaction, without itself being affected.

• This reaction is used in the hydrogenation of vegetable oils using a nickel catalyst.

• Vegetable oils generally have long unsaturated carbon chains while animal fats have saturated carbon chains.

• Animal fats generally contain saturated fatty acids which are said to be harmful for health.

• Oil containing unsaturated fatty acids should be chosen for cooking.

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.

A reaction, in which one type of atom or a group of atoms takes the place of another, is called a substitution reaction.

CH4 + Cl2 $\stackrel{\mathrm{s}\mathrm{u}\mathrm{n}\mathrm{l}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\to }$ CH3Cl + HCl

CH3Cl + Cl2 $\stackrel{\mathrm{s}\mathrm{u}\mathrm{n}\mathrm{l}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\to }$ CH2Cl2 + HCl

SOME IMPORTANT CARBON COMPOUNDS

ETHANOL AND ETHANOIC ACID

Physical Properties of Ethanol (CH3CH2OH)

• Ethanol is a liquid at room temperature.

• Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks.

• Since it is a good solvent, it is used in medicines such as tincture of iodine, cough syrups, and many tonics.

• Ethanol is soluble in water in all proportions.

• Consumption of small quantities of dilute ethanol causes drunkenness.

• Intake of even a small quantity of pure ethanol (absolute alcohol) can be lethal.

• Long-term consumption of alcohol leads to many health problems.

How does alcohol affect living beings?

When large quantities of ethanol are consumed, it tends to slow metabolic processes and to depress the central nervous system. This results in lack of coordination, mental confusion, drowsiness, lowering of the normal inhibitions, and finally stupour. The individual may feel relaxed but does not realise that his sense of judgement, sense of timing, and muscular coordination have been seriously impaired.

Why Methanol is dangerous to drink?

• Intake of methanol even in very small quantities can cause death.

• Methanol is oxidised to methanal in the liver.

• Methanal reacts rapidly with the components of cells. It causes the protoplasm to get coagulated, in much the same way an egg is coagulated by cooking.

• Methanol also affects the optic nerve, causing blindness.

What is Denatured Alcohol?

To prevent the misuse of ethanol produced for industrial use, it is made unfit for drinking by adding poisonous substances like methanol to it. Dyes are also added to colour the alcohol blue so that it can be identified easily. This is called denatured alcohol.

Alcohol as a fuel

Sugarcane juice is used to prepare molasses which is fermented to give alcohol (ethanol). Alcohol (ethanol) is used as an additive in petrol since it is a cleaner fuel which gives rise to only carbon dioxide and water on burning in sufficient air (oxygen).

Reactions of Ethanol

1. Reaction of ethanol with sodium –

1. Reaction of ethanol with concentrated sulphuric acid to give unsaturated hydrocarbon:

Heating ethanol at 443 K with excess concentrated sulphuric acid results in the dehydration of ethanol to give ethene,

• The concentrated sulphuric acid can be regarded as a dehydrating agent, which removes water from ethanol.

Physical Properties of Ethanoic Acid (CH3COOH)

• It is called acetic acid

• Compounds with functional group –COOH, are called carboxylic acids.

• 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles.

• The melting point of pure ethanoic acid is 290 K and hence it freezes during winter in cold climates. That is why it is called glacial acetic acid.

• It is a weak acid, does not get ionized completely.

Reactions of Ethanoic Acid

Esterification reaction

Esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents.

Saponification

Esters react in the presence of an acid or a base to give back the alcohol and carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.

Reaction of ethanoic acid with a base

Gives a salt (sodium ethanoate or sodium acetate) and water:

NaOH + CH3COOH → CH3COONa + H2O

Reaction of ethanoic acid with carbonates and hydrogencarbonates

2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2

CH3COOH + NaHCO3 → CH3COONa + H2O + CO2

Soaps and Detergents

The molecules of soap are sodium or potassium salts of long-chain carboxylic acids.

Formation of Miscelle

A soap molecule has two parts namely hydrophilic (water loving) hydrophobic (water hating).

Soap molecules form clusters called micelles around grease or dirt particle.

The hydrophobic part points towards the dirt and hydrophilic end towards water (or outside).

This forms an emulsion in water. The soap micelle thus helps in dissolving the dirt in water and we can wash our clothes clean.

Hard Water

Water that has high content of minerals like calcium and magnesium carbonates.

While bathing foam is formed with difficulty and an insoluble substance (scum) remains after washing with hard water. This is caused by the reaction of soap with the calcium and magnesium salts.

Detergents

Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water.

Detergents are usually used to make shampoos and products for cleaning clothes.