Carbon and Its Compounds 

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

  1. 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

  1. 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.

 

  1. 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.
  1. 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.

  1. 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.
  1. 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.

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