Chapter 4: Carbon and Its Compounds

Introduction:

Two or more elements combine to form a compound.

There are two types of compounds: Organic Compounds and Inorganic Compounds.

Organic compounds are made up of carbon and hydrogen (generally known as hydrocarbons)

Covalent Bond

The bond formed by sharing a pair of electrons between two atoms is known as Covalent Bond.
Carbon forms covalent bonds.
Carbon exists in two forms- as a free state and as a combined state.
The free form of carbon is found in graphite, diamond and fullerene. In the combined state, carbon exists as Carbon-dioxide, Glucose, Sugar etc.

Allotropes of Carbon

Different forms of an element that have the same chemical properties but different physical properties are known as Allotropes.
There are three allotropes of carbon- diamond, graphite and fullerene.

Diamond

Diamond exists as a three-dimensional network with strong carbon-carbon covalent bonds.
Diamonds are hard in nature with high melting points.
It shines in the presence of light (due to its high refractive index and total internal reflection).
It is a bad conductor of electricity.
The most common use of diamonds is in making jewellery.
It is also used in cutting and drilling tools.

Graphite

Graphite is made from weak Van der Waal forces.
Each carbon atom is bonded with the other three carbon atoms to form hexagonal rings.
It serves as a good conductor of heat and electricity.
It is used as a dry lubricant for machine parts (generally at higher temperatures where liquid lubricants cannot be used).
It is also used in lead pencils.

Fullerene

It is a hollow cage that exists in the form of a sphere made up of several hexagonal rings.
Its structure is similar to fullerene.
Along with hexagonal rings, sometimes pentagonal or heptagonal rings are also present.

Fig.1 Structure of fullerene

Two Important Properties of Carbon

Catenation and tetravalency are the two important properties of carbon.
Catenation is a property of carbon by which carbon atoms can link one another via the covalent bonds and can form long chains, closed rings or branched chains etc.
- Carbon atoms can be linked by single, double or triple bonds.
Carbon has a valency of 4 due to which it is known to have tetravalency.
- Due to this one carbon atom can bond with the other 4 carbon atoms, with other atoms also such as Oxygen, Nitrogen etc.

Hydrocarbons

Compounds that are made up of carbon and hydrogen are known as Hydrocarbons.
There are two types of hydrocarbons found – Saturated Hydrocarbons and Unsaturated Hydrocarbons.
Saturated Hydrocarbons consist of single bonds between the carbon atoms.
- For Example, Alkanes. Alkanes are saturated hydrocarbons represented by a formula, CnH2n+2.
Unsaturated Hydrocarbons are the ones with double or triple bonds between the carbon atoms.
- For Example, Alkenes and Alkynes. Alkenes are represented as CnH2n whereas alkynes are represented as CnH2n-2.
Some saturated hydrocarbons and unsaturated hydrocarbons are represented as –

Fig.2. Saturated hydrocarbons

Fig. 3. Unsaturated hydrocarbons

The structure of hydrocarbons can be represented in the form of electron dot structure as well as open structures as shown below-

Fig.4. Electron dot structure and open structure of ethane

Fig.5. Electron dot structure and open structure of ethyne

Carbon Compounds based on the structure

Carbon Compounds can be classified as straight-chain compounds, branched-chain compounds and cyclic compounds. They are represented as –

Fig.6. Straight chain carbon compound

Fig.7. Branched-chain compounds

Fig.8. Cyclic carbon compounds

Functional Groups

One of the hydrogen atoms in hydrocarbons can be replaced by other atoms also called hetero-atoms (like halogens, oxygen, nitrogen, and sulphur) such that the valency of carbon remains satisfied.
Hetero-atoms or their groups, when present, imparts special property to the hydrocarbon.
The atoms or groups of atoms, which decide the properties of the hydrocarbon compound are known as Functional Groups.
- For Example, Cl, Br, -OH (alcohol), Aldehyde, Ketone, Carboxylic Acid, etc.

Homologous Series

It is a series of compounds, having similar chemical properties, due to the presence of the same functional group in a chain of carbon.
- For example, CH3OH, C2H5OH, C3H7OH and C4H9OH are all have the same functional group (-OH) and have very similar chemical properties.
- Such a series of compounds is called homologous series.
- Chemical properties of compounds in a homologous series remains similar.
Successive members of homologous series differ by a -CH2 unit.
- Mass of one -CH2 unit is 14 u.
- Therefore each successive member of a homologous series has 14 u more molecular mass than its previous member.
Gradation in physical properties:
- It means a regular and gradual change in the physical properties of compounds in a homologous series.
- As molecular mass increases in any homolous series, melting point and boiling point of compounds increases.
- Also, other physical properties like solubility on compound in a particular solvnet also show similar gradation.
- However, chemical properties remains similar in a homologous series.

Fig.9. Homologous series

Nomenclature of Carbon Compounds

First of all, identify the number of carbon atoms in compounds. And in it identify the longest chain.
Then the functional group can be indicated by suffix or prefix.
Cyclic hydrocarbon is designated by the prefix cyclo.
If there are two or more different substituents they are listed in alphabetical order.
If the same substituent occurs more than once, the location of each point on which the substituent occurs is given

Fig.10. Different functional groups

Chemical Properties of Carbon Compounds

Combustion

Carbon (coal, coke, etc) along with its compound is used as a fuel as it burns in presence of oxygen to release energy and carbon dioxide
Even diamond and graphite burns in oxygen to give carbon dioxide and energy.
Saturated hydrocarbons produce blue and non-sooty flame whereas unsaturated hydrocarbons produce a yellow sooty flame.
Fuels like coal and petroleum have some amount of nitrogen and sulphur impurity in them and their combustion results in formation of oxides of nitrogen and oxides of sulphur.
Nitrogen oxides and sulphur oxides are major pollutants in the environment.

CH4+2O2-> CO2+2H2O

Oxidation

Carbon compounds oxidise easily on combustion to form oxides and water.
With controlled oxidation we can get several industrially important products by using oxidising agents.
- For example, Alcohol can be oxidised to aldehydes whereas aldehydes, in turn, can be oxidised to carboxylic acid.
- Oxidising agents such as potassium permanganate and potassium dichromate can be used for oxidation.

Addition Reaction

It is the property of unsaturated hydrocarbon.
Hydrogenation of vegetable oil is an example of an additional reaction.
Addition of hydrogen to unsaturated hydrocarbons in presence of catalysts such as nickel or palladium is an example of an addition reaction.
- This converts the oil into vanaspati ghee.
Oils or fats having unsaturated carbon chains are healthy and should be chosen for cooking.
Vegetable oils are unsaturated while animal fat and vanaspati ghee are saturated.
Addition of bromine to ethylene is another example of an addition reaction.

Substitution Reaction

When one atom in hydrocarbon is replaced by other atoms like chlorine, bromine, etc. this is known as a Substitution Reaction.
Saturated hydrocarbons are the least reactive and they undergo substitution reactions.
Chlorine atom substitutes hydrogen atom one by one in saturated hydrocarbon in presence of sunlight.
It is called substitution reaction because one type of atom or a group of atoms takes the place of another.

Important Carbon Compounds: Ethanol and Ethanoic Acid

Ethanol and ethanoic acids are two commercially important carbon compounds.

Properties of Ethanol

Ethanol is a volatile liquid at room temperature.
It has low melting and boiling point.
It is active ingredient of all alcoholic drinks like wine, whiskey, rum, etc.
It is also a very good solvent and used in medicines such as tincture iodine, cough syrups and tonics.
It is soluble in water.
Consumption of dilute ethanol (wine, rum, whiskey, etc.) is a socially widespread practice, though it causes drunkenness, and may leads to many health problems.
Intake of even small quantity of pure ethanol (absolute alcohol, i.e. no water added) can be leathal.
It reacts with sodium to form sodium ethoxide.

This reaction is used to test the presence of ethanol by the evolution of hydrogen gas.

Dehydration of ethanol in presence of hot and concentrated sulphuric acid forms ethene.

Properties of Ethanoic Acid

Commonly called as acetic acid.
Ethanoic acid is a colourless liquid.
It belongs to a group of acids called carboxylic acids.
5% to 8% solution of acetic acid in water is called vineger.
Pure ethanoic acid freezes like ice at around 16.6 degree celcius during cold winter like ice, hence it is also known by the name Glacial Acetic Acid.
Carboxylic acids including acetic acid are weak acids.
Ethanoic Acid/Acetic acid when reacts with ethanol forms an ester. Ester can be identified by its sweet and fruity smell.

Ester is used in making perfumes and flavouring essence like vanilla essence, etc.
The reaction of esters with a strong base is used to form soap. This is known as Saponification Reaction.
Acetic acid also reacts with a strong base like sodium hydroxide to form sodium acetate and water.

NaOH + CH3COOH + CH3COONa + H2O

Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt (acetate), carbon dioxide and water.

Soaps and Detergents

Sodium or potassium salt of a long-chain carboxylic acid is known as Soap.
- They work most effectively in soft water.
Detergents are sulphonate or ammonium salt of a long-chain of carboxylic acid.
- They can work effectively on soft as well as hard water.

Cleansing Action of Soaps and Detergents

When soap is dissolved in water, it ionises, one end of soap molecule is anion and other end consists of long hydrocarbon chain.
The ionic end of soap is hydrophilic , while the long hydrocarbon chain is hydrophobic in nature.
The hydrophobic part of soaps and detergents are soluble in grease and the hydrophilic part is soluble in water.
Thus, the cleansing action of soaps and detergents is due to the ability to emulsify oil or grease and hold them in a suspension of water.

Micelle Formation

When dirt and grease are mixed with soap water, soap molecules arrange them in tiny clusters known as Micelle.
The hydrophilic part sticks to the water and forms the outer surface of the micelle and the hydrophobic part binds to oil and grease.
- This cluster formation is called miscelle.
Oily and dirt is collected in the centre of the miscelle, remained suspended in water and is also rinsed away easily.