ORGANIC CHEMISTRY

ORGANIC CHEMISTRY

SOLVED TEST YOURSELFS

Test Yourself 11.1 (ORGANIC COMPOUNDS)

  1. No. i. Why and how carbon completes its octet?

Answer. Octet rule is Having 8 electrons in the valance shell of an atom. All the elements (atoms) complete their octet to gain stability either losing or gaining electrons or by sharing of electrons.

Carbon atom consists of 4 electrons in its valence shell. It completes its octet by sharing of its 4 electrons with other atoms. So that it gains stability

  1. No. ii. Point out the properties of carbon which are responsible for formation of long chains of carbon atom compounds.

Answer. Property of carbon atom which is responsible for formation of long chains of carbon atom compounds is catenation. It is ability of carbon atoms to link with other carbon atoms to form long chains and large rings. Carbon atom fulfils the two basic conditions to exhibit catenation.

 (a)      It has valence greater than two.

(b)      It forms stronger bonds with its own atoms than that of its bonds with other atoms especially oxygen.

For example, both silicon and carbon have similar electronic configurations but carbon shows catenation whereas silicon does not. It is mainly due to the reason that C-C bonds are much stronger (355 kJ mol-1) than Si-Si (200 kJ mol-1) bonds. On the other hand, Si-O bonds are much stronger (452 kJ mo-1) than C-O bonds (351 kJ mol-1). Hence, silicon occurs in the form of silica and silicates in nature.

  1. No. iii. Why the melting and boiling points of organic compounds are low.

Answer. Melting point of a compound depends upon the nature of the bonding of the compound. Compounds having covalent bonding have low melting points than that of compounds having ionic bonding in them. As organic compounds consists of covalent bonding their melting point are low.

Boiling point depends upon intermolecular forces between the molecules. Organic compounds are mostly non-polar or slightly polar, Intermolecular forces between the organic compounds are London forces of attraction. Because of these weak forces boiling points of organic compounds are low.

  1. No. iv. Why the organic compounds are poor conductor of electricity?

Answer. Organic compounds are covalent in nature. They do not ionize to produce ions. Thus because of covalent nature of bonding, organic compounds are poor conductor of electricity.

  1. No. v. What are the reasons for the formation of millions of organic compounds?

Answer. The reasons for the formation of millions of organic compounds are as follows:

(a)       Catenation; the ability of carbon atoms to link with other carbon atoms to form long chains and large rings.

(b)      Isomerism; a phenomenon in which compounds have same molecular formula but different structural formulas.

(c)      Strong covalent bonding of carbon atoms.

(d)      Multiple bonding ability of the carbon atom.

  1. No. vi. What is vital force theory?

Answer. In early 19th century, Swedish chemist Jacob Berzelius put forward the “Vital Force Theory”. According to this theory organic compounds could not be prepared in laboratories from inorganic substances because vital force is not available in laboratories. Organic compounds could be synthesized under the influence of a mysterious force called Vital Force, inherent only in living things.

  1. No. vii. Explain Wohler’s work.

Answer, Wohler synthesized the first organic compound urea from inorganic substance ammonium cyan ate (NH4CNO) by heating it:

NH4CNO            H2NCONH2

Rearrangement         urea

 No. viii. What is isomerism?

Answer. It is a phenomenon in which compounds have same molecular formula but different structural formulae. These different structures are called isomers. Isomers have the same molecular formula but different arrangement of atoms in molecules or different structural formulae. Isomerism also adds to the possible number of structures, e.g., molecular formula C5H12 can be represented by three different structures. Thus, C5H12 has three isomers, as shown below:

CH3

H3C-CH2-CH2-CH2– CH3     H3C-CH-CH2-CH3             H3C-C-CH3

CH3                           CH3                                       CH3

n-pentane                     isopentane                   neopentane

 

Test Yourself 11.2 (SOURCES OF ORGANIC COMPOUNDS)

  1. No. i. Name the gases which are found in coal gas.

Answer. The gases which are found in coal gas are hydrogen, methane and carbon monoxide

  1. No. ii. Is coal-tar a compound? What is importance of coal-tar?

Answer. It is a mixture of more than 200 different organic compounds, mostly

aromatic. Some of the important aromatic compounds are benzene, phenol, toluene, aniline, etc. These chemicals are used to synthesize drugs, dyes, explosives, paints, varnishes, plastics, synthetic fiber and pesticides. Besides these valuable chemicals, the black residue of the coal tar is called pitch. -It is used for surfacing of roads and roofs.

  1. No. iii. What is coke? For what purpose it is used? Which is the best quality of coal?

Answer. Coke has 98% carbon. It is left behind residue of coal. When coal is subjected to destructive distillation, it loses all its volatile components and leaves behind a solid residue called coke. It is mainly used as a reducing agent in the extraction of metals especially iron. It is also used as fuel.

  1. No. iv. What is destructive distillation?

Answer. The strong heating of coal in the absence of air is called destructive distillation. The destructive distillation produces a large number of organic compounds along with a few inorganic compounds.

Test Yourself 11.3 (USES OF ORGANIC COMPOUNDS)

  1. No. i. Define petroleum

Answer. Petroleum is a dark brownish or greenish black colored viscous liquid. It is a complex mixture of several solid, liquid or gaseous hydrocarbons in water mixed with salts and earth particles.

  1. No. ii. What types of compounds are synthesized by plants?

Answer. Living plants synthesize macro-molecules, e.g., carbohydrates, proteins, oils and vitamins.

  1. No.iii. What is the basic unit of carbohydrates and how it is synthesized?

Answer. The basic unit of all types of carbohydrates is glucose which is synthesized by plants through photosynthesis. Glucose then further polymerizes to form carbohydrates like sucrose, starch, and cellulose.

  1. No. iv. CNG stands for

Answer. CNG stands for compressed natural gas.

  1. No. v. Our existence is due to organic compounds, comment.

Answer. Because organic compounds are part of everything from food we eat to the various items we use in daily life to fulfill our needs. We depend upon organic compounds that are used as food, as clothing as housing as fuel as medicines and as raw materials for various daily life atoms.

Test Yourself 11.4 (FUNCTIONAL GROUPS)

  1. No. i. What is the functional group of an ester?

Answer. Organic compounds consisting of RCOOR’ functional group are called esters. Their general formula is

                       o

R-C-OR

where R and R are alkyl groups. They may be same of different, such

as:

                              O                                              O             

                               ║                                               ║

                        H3C-C-OCH3                            H3C-C-OC2H5

Methyl acetate                        Ethy l acetate

 

  1. No: ii. What is the difference between aldehyde and ketone?

Answer. In aldehydes the carbonyl carbon is attached with at least one hydrogen atom while in ketone it is attached on both sides with alkyl groups. For example, functional group of aldehyde is:

O

II

-C-H

 

While, the functional group of ketone is:

C==== O

Examples are;

O                                      O

║                                                            ║

H-C-H                                     H3C-C-H

Form aldehyde                             Acetaldehyde

 

O                                                    O

II                                                     II

H3C-C-CH3                                                                H3C-C-CH2-CH3

Acetone (Dimethyl ketone)                   Ethyl methyl ketone

  1. No.iii. Give the functional group of alkenes and alkynes?

Answer. Hydrocarbon compounds consisting of double bonds between two carbon atoms in their molecules are called as alkenes, such as:

H2C = CH2

Ethene (Acetylene)

Hydrocarbon compounds consisting of triple bonds between two carbon atoms in their molecules are called as alkynes, such as:

HC = CH

Ethyne (Acetylene)

  1. No. iv. How an alcohol is tested?

Answer. Sodium metal test:

Take about 2-3 cm3 of the given organic liquid in a dry test tube and add a piece of sodium metal.

    Result: Hydrogen gas will evolve

  1. No. v. How ketonic group is tested?

Answer. Phenyl Hydrazine Test:

Shake a pinch of the given organic compound with about 2.0 cm3 of phenyl hydrazine solution.

Result: Orange red precipitate will be formed.

MULTIPLE CHOICE QUESTIONS

Four answers are given against each question; please (√) the correct one answer.

  1. The ability of carbon atoms to form chains is called
  2. isomerism           b. catenation
  3. resonance                            d. condensation
  1. Coal having 90% carbon contents is called
  2. peat. b. lignite.
  3. anthracite.                     d. bituminous.
  4. Main component of natural gas is
  5. methane.                     b. Propane.
  6. butane.                                      d. propene.
  1. The strong heating of coal in retorts in the absence of air is called
  2. fractional distillation. b. sublimation.
  3. roasting. d. destructive distillation.
  1. Pitch is black residue of:
  2. coke. b. coal-tar,

coal.                                 d. coal gas.

  1. Natural gas is 85% methane. It is used to make one of following:
  2. carbon black. b. coke,
  3. coal tar.       d. coal gas.
  1. Which one of the following does not contain starch?
  2. sugar cane.           b. maize,
  3. barley. d. potatoes.
  1. Petroleum is refined by
  2. destructive distillation. b. fractional distillation.
  3. simple distillation. d. dry distillation.
  1. In laboratory urea was prepared by
  2. Wohler.                    b. Rutherford.
  3. Berzelius.              d. Dalton.
  1. General formula of alkyl radical is
  2. CnH2n+2 b. CnH2n-2
  3. CnH2n+l                                      d. CnH2n
  1. Identify which one of the following compounds is a ketone
  2. (CH3)2 CHOH b. (CH3)2 CO
  3. (CH3)2 NH                 d. (CH3)2CHC1
  1. The functional group-COOH is found in:
  2. carboxylic acid. b. Aldehydes.
  3. alcohols.        d. esters.
  1. Which one of the following statements is not true about fossil fuels?
  2. they all contain carbon. b. they are renewable
  3. they produce pollutants when burnt. d. they cause acid rain.
  1. Which one of the following is the hardest coal?
  2. peat. b. Lignite,
  3. bituminous.                      d. Anthracite.
  1. In which of the following groups, oxygen is attached to both sides of carbon atoms?
  2. ketone. b. ether.
  3. aldehyde.       d. ester.
  1. Carbonization process is the conversion of:
  2. a.coal into coal gas. b. coal into wood,
  3. wood into coal.    d. wood into coal tar.

Coal gas is a mixture of:

  1. COandCH4. b. CO, CH, CO2.
  2. CO,CH4, H2.     d. CO, H2 and GO2.

 

  1. Which one o/ the following is a synthetic fibre?
  2. cotton. b. wool,
  3. nylon.          d. silk.
  1. Which one of the following is not a fossil fuel?
  2. coal. b. natural gas.
  3. biogas.          d. petroleum.
  1. Which one of the following does not contain protein?
  2. pulses. b. potatoes,
  3. beans.         d. eggs.
  1. Conversion of dead plants into coal by the action of bacteria and heat is called:
  2. carbonization. b. catenation,
  3. hydrogenation.     d. cracking.
  1. Which one of the following compounds is an aldehyde?
  2. CH3-CH2-OH. b, CH3-COOH.
  3. CH3CHO.                   d. CH3COCH3.
  1. Formula of acetaldehyde is

                                      O

  1. CH3 — CH2OH b.  CH3 – C – OH

          O                               O

          ║                               ║                                                 

  1. CHs-C-H d.   H-C-H

 

  1. Who rejected the vital force theory first of all?
  2. Kolbe. b. Wohler.
  3. Berzelius.                    d. Newton.

 

  1. The first organic compound synthesized in the laboratory was:
  2. acetone. b. urea,
  3. methane.                   d. alcohol.
  1. Organic compounds are generally
  2. Ionic compounds. b. covalent compounds,
  3. metallic compounds. d. none of these.
  1. Organic compounds are generally
  2. soluble in water. b. soluble in inorganic solvents,
  3. insoluble in organic solvent.  d. insoluble in water.
  1. Which organic compound is used to keep insect away from clothes?
  2. naphthalene. b. benzene.
  3. acetic acid. d. ethyl alcohol.
  1. Organic compounds are mainly divided into
  2. Two classes. b. three classes,
  3. four classes. d. five classes.
  1. The percentage of carbon is highest in:
  2. Peat b. Anthracite,
  3. Lignite.             d. Bituminous.
  1. The bond energy between C-C single bond is
  2. 200 kJ mol-1 b. 351 kJ mol-1
  3. 452 kJ mol-1                   d. 355 kJ mol-1
  1. Organic compounds are
  2. good Conductor of electricity b. bad conductor of electricity.
  3. insulator. d. none of these.
  1. The percentage of carbon in lignite is
  2. 60 % b. 70%
  3. 80 % d. 85%
  1. The percentage of carbon in coke is
  2. 88% b. 96%
  3. 98% d. 80%
  1. Natural rubber is the linear polymer of:
  2. Methane. b. isoprene.
  3. butane. d. propene.
  1. Fermentation of molasses produces:
  2. alcohol. b. phenol,
  3. benzene. d. methane.
  1. Which is not synthetic fiber?
  2. nylon. b. dacron.
  3. acrylic. d. cotton.
  1. Fehling solution is used to identify
  2. ketone. b. aldehyde,
  3. amines. d. phenol.
  1. Compounds that contain at least one benzene ring in their molecule are called
  2. aromatic compound. b. heterocyclic compound.
  3. homocyclic compound d. alicyclic compound.
  1. The formula which shows sharing of electrons between various atoms in one molecule of organic compound is called
  2. molecular formula. b. structural formula.
  3. condensed formula. d. dot and cross formula.

ANSWER KEY MCQ

1.b   2.c   3.a   4.d  5.b    6.a   7.a   8.b

9.a   10.c  11.b  12   13.b   14.d  15.b  16.c

17.c  18.c  19.c  20.b  21.a   22.c  23.c  24.b

25.b  26.b  27.d  28.a  29.a   30.b  31.d  32.b

33.b  34.c  35.b  36.a  37..d   38.d  39.a  40.d

ANSWERS TO THE SHORT QUESTIONS.

  1. No.1. What is meant by the term catenation? Give an example of a compound that displays catenation?

Answer. The ability of carbon atoms to link with other atoms to form long chains and large rings is called catenation. The compounds of carbon display catenation.

For example propane, butane etc.

I  I  I  I  I  I

―C-C-C-C-C-C―

I  I  I  I  I  I

  1. No.2. How coal is formed?

Answer. Coal was formed by the decomposition of plants buried under the Earth’s crust millions of years ago. Conversion of Wood into coal is called carbonization. It is a very slow bio-chemical process. It takes place in the absence of air under high pressure and high temperature over a long period of time (about 500 million years).

  1. No.3. What is importance of natural gas?

Answer. Natural gas is used as fuel in homes as well as in industries. It is used as fuel in auto-mobiles as compressed natural gas (CNG). Natural gas is also used to make carbon black and fertilizer.

  1. No.4. Justify that organic compounds are used as food.

Answer. The food which we eat daily such as milk, eggs, meat, vegetables, etc., contain carbohydrates, proteins, fats, vitamins, etc. are all organic so we can say organic compounds are used as food.

  1. No.5. How alkyl radicals are formed? Explain with examples.

Answer. Alkyl radicals are derivatives of alkanes. They are formed by the removal of one of the hydrogen atom of an alkane and are represented by a letter ‘R. Their name is written by replacing ‘ane’ of alkane with ‘yl’. For example, propane has a straight chain structure.

When terminal H is removed it is called n-propyl. When hydrogen from central carbon is removed it is called isopropyl, as explained below:

Removal of terminal H→ CH3-CH2-CH2–     n-propyl

H

CH3―CH2―CH3                                                                       I

Propane                                   Removal of centralH→ CH3—C―CHisopropyl

I

 

  1.     No.6. What is the difference between n-propyl and isopropyl? Explain with structure,

Answer. Propane has a straight chain structure. When terminal H of propane is removed it is called n-propyl. When hydrogen from central carbon atom of propane is removed it is called isopropyl, as shown in formula

CH3-CH2-CH2–                                      CH3-CH-CH3

n-propyl                                  isopropyl

  1. No.7. Explain different radicals of butane.

Answer.    Butane   exists   as   n-butane   and   isobutene.    Different structures of butyl radicals are explained

Removal of terminal H→ CH3-CH2-CH2–     n-butyl

H

CH3-CH2-CH2-CH2                                                                     │

Butane                           Removal of central H→ CH3—C― CH2―CH3                                    sec-butyl

                                                                                               │

                                                                                          

                                                                                                                                            CH3

                                                                                                                                             │

CH3                              Removal of terminal H→ CH3-CH2-CH2–     isopropyl

│                                                          H

CH3―CH2―CH3                                                                       │

isobutane                                 Removal of central H→ CH3—C―CH3

I

terbulyl

 

  1. No.8. Define functional group with an example.

Answer. An atom or group of atoms or presence of double or triple bond which determines the characteristic properties of an organic compound is known as the functional group. For example, -OH group is the functional group of alcohols, CH3OH is methyl alcohol.

  1. No.9. What is an ester group? Write down the formula of ethyl acetate.

Answer. Organic compounds consisting of -COOR’ functional group are called esters. Their general formula is R-CO-OR-. The Formula of ethyl acetate-is CH3 COOC2H3

  1. No.10. Write down the dot and cross formula of propane and n-butane?

Answer. The dot and cross formula of propane and n-butane is given below

Propane                     n-butane

Q.No.11. Define structural formula. Draw the structural formulae of n-butane and isobutene.

Answer. A formula which represents the exact arrangement of the different atoms of various elements present in a molecule or a compound of the substance is called structural formula. The structural formula of n-butane and isobutene are as follows.

 

n-Butane                                       isobutene

  1. No.12. Write classification of coal.

 Answer. Depending upon the extent of carbonization process, four types of coal are found. These types differ with respect to carbon content, volatile matter and moisture. Peat, lignite, bituminous and anthracite are the types of coal.

  1. No.13. Differentiate between homocyclic and heterocyclic compounds.

Answer. Homocyclic or carbocyclic compounds contain rings which are made up of only one kind of atoms, i.e., carbon atoms. For example,

 

Cyclic compounds that contain one or more atoms other than that of carbon atoms in their rings are called heterocyclic compounds. For example

 

                            Thiophene            Pyridine

 

  1. No.14. Define homologous series. Pyridine

Answer. The group of organic compounds consisting of same functional group, having same general formula, having similar chemical properties and gradually changing physical properties is called homologous series. Successive members of the series differ by one unit of –CH2-. For example, alkenes consist of functional group -C=C-, their general formula is CnH2n. All the members of the series have similar chemical properties. Their physical properties such as molecular masses increase gradually. Each member of the series differ from its lower or higher member by one unit –CH2― e.g. Ethene (C2H4), Propene (C3H6), Butene (C4H8) and so on.

  1. No.15. What are heterocyclic compounds? Give two examples.

Answer. Cyclic compounds that contain one or more atoms other than that of carbon atoms in their rings are called heterocyclic compounds. For example, thiophene and pyridine

                                Thiophene         Pyridine

  1. No.16. Why benzene and other homologous compounds of benzene are called aromatic compounds?

Answer. Those organic compounds which contain at least one benzene ring in their molecules are called aromatic compounds. They are called aromatic because of aroma. Aroma means smell.

  1. No.17. Which aromatic compound is used to keep insects away from clothes?

Answer. Naphthalene is an aromatic compound that is used to keep insects away from clothes. It decomposes at room temperature giving of very strong smell which keeps insects away from clothes.

ANSWER TO THE LONG QUESTIONS

  1. No 1. (a) How is coal formed? What are the different types of coal?

(b) Write down the composition and uses of different types of coal.

Answer.

(a)      Coal was formed by the decomposition of plants buried under the Earth’s crust millions of years ago. Conversion of wood into coal is called carbonization. It is a very slow bio-chemical process. It takes place in the absence of air under high pressure and high temperature over a long period of time (about 500 millions of years). Wood contains about 40% carbon, so depending upon the extent of carbonization process, four types of coal are found. These types differ with respect to carbon content, volatile matter and moisture. Peat, lignite, bituminous and anthracite are the types of coals.

 

(b)      The composition and uses of coal are represented in table given below.

 

Type of Coal

 

Carbon contents

 

            Uses

 

Peat

 

60 %

 

It is inferior quality coal used in kiln.

 

Lignite

 

70 %

 

It is soft coal used in thermal power stations.

 

Bituminous

 

80 %

 

It is common variety of coal used as household coal.

 

 

 

 

 

Type of Coal

 

Carbon contents

 

          Uses

 

Anthracite

 

90%

 

It is superior quality hard coal that is used in industry.

 

  1. No 2. (a) What is destructive distillation of coal?

(b) Name the different types of products obtained by the destructive distillation of coal.

Answer.

(a);     Coal has become a major source of organic compounds. These compounds are obtained by destructive distillation. The strong heating of coal in the absence of air is called destructive distillation. Coal contains elements like carbon, hydrogen, oxygen, nitrogen, and sulphur. So destructive distillation of coal provides a large number of organic compounds along with a few inorganic compounds.

(b)     Different types of products of coal;

(i)       Coal Gas is mixture of hydrogen, methane and carbon monoxide. It produces heat when burnt in air. Therefore, it is mainly used as a fuel in industry,It is also used to provide an inert or reducing atmosphere in various metallurgical processes.

(ii)      Ammonical Liquor is a solution of ammonia gas in water. It is used to prepare nitrogenous fertilizers. For example, when it is treated with sulphuric acid, it produces ammonium sulphate, fertilizer.

(iii)     Coal-Tar is a thick black liquid. It is admixture of more than 200 different organic compounds, mostly aromatic. These compounds are separated by fractional distillation. Some of the important aromatic compounds are benzene, phenol, toluene, aniline, etc. These chemicals are used to synthesize drugs, dyes, explosives, paints, varnishes, plastics, synthetic fiber and pesticides. Besides these valuable chemicals, the black residue of the coal tar is called pitch. It is used for surfacing of roads and roofs.

(iv)     Coke is 98% carbon. It is left behind residue of coal. When coal is subjected to destructive distillation, it loses all its volatile components and leaves behind a solid residue called coke. It is mainly used as a reducing agent in the extraction of metals | especially iron. It is also used as fuel.

  1. No.3. Write a detailed note on functional groups of alkenes and alkynes. How they are identified from other compounds?

Answer.   Organic   compounds   containing   double   bond   in   their molecules are called alkenes, such as:

H2C = CH2

Ethene

Organic compounds containing triple bond in their molecules are called alkynes, such as:

HC ≡ CH

Ethene (Acetylene)

 

Identification from other compounds

(i)      Bromine water test:

Dissolve a pinch of the given organic compound in 2.0cm3 of carbon tetrachloride (CCI4). Add 2cm3 of bromine water in it. Bromine will be decolorized which confirms the presence of double or triple bond.

(ii)     Baeyer’s test:

Dissolve about 0.2 g of the organic compound in water. Add 2-3 drops of alkaline KMnO4 solution in the aqueous solution of organic compound. Pink colour will disappear. This confirms the presence of double or triple bond.

  1. No.4. Give some uses of organic compounds in our daily life.

Answer. The main uses of organic compounds in daily life are

  1. Uses as Food: The food we eat daily such as milk, eggs, meat, vegetables, etc., contain carbohydrates, proteins, fats, vitamins, etc., are all organic compounds.
  1. Uses as Clothing: All types of clothing (we wear, we use as bed sheets etc.) are made up of natural fibers (cotton, silk and wool, etc.) and synthetic fibers (nylon, Dacron and acrylic, etc.) all these are organic compounds.
  1. Uses as Houses: Wood is cellulose (naturally synthesized organic compound). It is used for making houses and furniture of all kinds. All of these are organic compounds.
  2. Uses as Fuel: The fuels we use for automobiles and domestic purposes are coal, petroleum and natural gas. These are called fossil fuels. All of these are organic compounds.
  3. Uses as Medicines: A large number of organic compounds (naturally synthesized by plants) are used as medicines by us. Most of the life saving medicines and drugs such as antibiotics (inhibit or kill microorganisms which cause infectious diseases) are synthesized in laboratories.
  1. Uses as Raw Material: Organic compound are used to prepare a variety of materials, such as rubber, paper, ink, drugs, dyes, paints, varnishes, pesticides, etc.
  1. No.5. Write down the characteristics of homologous series. Answer.
  • All members of a series can be represented by a general formula for example, general formula of alkane, alkenes and alkynes are CnH2n +2, CnH2n and CnH2n -2 respectively.
  • Successive members of the series differ by one unit of -CH2– and a mass of 14 amu.
  • They have similar chemical properties because they contain the same functional group.
  • There is a regular change in their physical properties; the melting and boiling points increase gradually with the increase of molecular masses.
  • They can be prepared by similar general methods.

Hydrocarbons are regarded as parent organic compounds. All other   compounds   are   considered   to   be   derived   from   them   by substituting one or more hydrogen atoms of a hydrocarbon by one more reactive atom or group of atoms.

  1. No.6. Why organic compounds are numerous?

Answer. There are a total of 118 elements known today. The number of organic compounds (carbon compounds) is more than ten million. This number is far more than the number of compounds of all the remaining elements taken together. The existence of such a large number of organic compounds is due to the following reasons:

(i)       The main reason for the existence of a large number of organic compounds is that carbon atoms can link with one another by means of covalent bonds to form long chains or rings of carbon atoms. The chains can be straight or branched. The ability of carbon atoms to link with other carbon atoms to form long chains and large rings is called catenation.

Two basic conditions for an element to exhibit catenation are:

(a)      Element should have valence two or more than two.

(b)      Bonds made by an element with its own atoms should be stronger than the bonds made by the element with other atoms especially oxygen.Both silicon and carbon have similar electronic configurations but carbon shows catenation whereas silicon does not. It is mainly due to the reason that C-C bonds are much stronger (355 kJ mol-1) than Si-Si (200 kJ mol-1) bonds. On the other hand, Si – O bonds are much stronger (452 kJ.mol-1) than C-0 bonds (351 kJ mol-1). Hence, silicon occurs in the form of silica and silicates in nature.

Another reason for the abundance of organic compounds is the phenomenon of isomerism.

The compounds are said to be isomers if they have the same molecular formula but different arrangement of atoms in molecules or different structural formula.

Isomerism  also adds to the possible number of structures, e.g., molecular formula C5H12 can be represented by three different structures. Thus, C5H12 has three isomers, as shown below:

CH3

H3C -CH2-CH2-CH2-CH,     H3C-CH-CH2-CH3            H3C-C-CH3 n-pentane                                                                                                                                                                                          n-pentane                                       CH3                                       CH3

isopentane                        neopentane

Number of isomers increases with the increase in number of carbon atoms in the given molecular formula.

(iii)     Due to its very small size, carbon can form very strong covalent bonds with other carbon atoms, hydrogen, oxygen, nitrogen and halogens. This enables it to form a large number of compounds.

(iv).     In order to satisfy its tetravalancy, carbon can make multiple bonds (i.e., double and triple bonds). This further adds to the possible number of structures. . For example, two carbons in ethane are linked by a single covalent bond, by a double covalent bond in ethylene and a triple covalent bond in acetylene.

  1. No.7. What are amines? Explain the different types of amines giving an example of each type. How primary amino group is identified?

Answer. Amines contain carbon, hydrogen and nitrogen in their functional group.

Their functional group is -NH2, and their general formula is R-NH2

Primary amines: Only one hydrogen atom of an ammonia molecule has been replaced by alkyl group. For example, methyl amine

H3C-NH2

                                                             Methylamine

Secondary amine: Two hydrogen atoms of an ammonia molecule have been replaced by two alkyl groups. For example, diethyl amine

H3C-NH2

CH3

                                                               Dim ethylamine

 Tertiary amines: In a tertiary amine, all of the hydrogen atoms of an ammonia’ molecule have been replaced by three alkyl groups. For example, trimethyl amine

H3C-N-CH3

CH3

                                                              Trim ethylamine

Identification

Primary amino group is identified by Carbyl amine test. Heat about 0.2 g of the given compound and add 0.5 cm3 of chloroform and add 2-3 cm3 of alcoholic KOH, extremely unpleasant odour will be given out which confirm primary amino group.

  1. No.8. Describe the functional group of an alcohol. How alcoholic groups are identified?

Answer. The functional group of alcohols is -OH. Their general formula is ROH, where R is any alkyl group.

For example, methyl alcohol (CH3-OH), ethyl alcohol (CH3-CH2-OH) and n-propyl alcohol (CH3-CH2– CH2-OH)

Identification of alcohol

Alcohol can be identified by sodium metal test. Take about 2-3 cm3 of the given organic liquid in a dry test tube and add a piece of sodium metal. Hydrogen gas will evolve which confirms the presence of alcohol.

Ester formation test:

Heat about 1.0 cm3 of the organic compound with 1.0 cm3 of acetic acid and 1-2 drops of concentrated sulphuric acid. Fruity smell will be given out which confirm the presence of alcohol.

  1. No.9. Differentiate between aldehyde and ketone functional groups. How both are identified from each other?

            O

          ―║―C―H

Answer. Aldehyde consists of as functional group. Theirs general formula is RCHO. For example,

  O                 O       

  ║                 ║

H-C-H                H-C-C-H

Form aldehyde         Acetaldehyde

 

C=O

Ketone consists of as functional group having general formula

For example,

                    O  

                    ║

Acetaldehyde

Aldehyde is identified by Fehling’s solution test. Mix equal volumes of Fehling’s solution A and B in a test tube. Add a pinch of organic compound- and boil for five minutes. Red precipitate will be formed.

While ketone is identified by Phenyl hydrazine test. Shake a of the given organic compound with about 2.0 cm3 of phenyl  hydrazine solution. Orange red precipitate will be formed.

Q.No.10. Encircle the functional groups in the following (compounds. Also give the names of the functional groups?

Answer.

(i)     O                            (ii)         O

║                             ║

CH3-C-CH3                       CH3-C-H

 

(iii)   O                             (iv)        O

║                              ║

HO-C-CH2-CH3                H-C-CH2-CH3

 

(vi)             O

(v)       H2C = CH – CH3        CH3-O-C-CH3

 

(i) Ketone     (ii) Aldehyde          (iii) Carboxylic acid           (iv)  Aldehyde

(v) Alkane    (vi) Ester

No.11.What is general properties of organic compounds?

Answer.     Organic     compounds     have     the     following     general characteristics:

(i)       Origin:   Naturally   occurring   substances   are   obtained   from plants and animals. On the other hand, inorganic compounds    are obtained from minerals and rocks.

(ii)      Composition: Carbon is an essential constituent of all organic compounds. They are made up of few elements such as carbon, hydrogen, nitrogen, oxygen, halogen, sulphur etc. On the other hand inorganic compounds are made up of almost all the elements of the Periodic Table known so far.

(iii)     Covalent linkage: Organic compounds contain covalent bonds that may be polar or non-polar, while the inorganic compounds mostly contain ionic bonds.

(iv)    Solubility: Organic compounds haying non-polar linkages are generally soluble in organic solvents like alcohol, ether, benzene, carbon disulphide etc. On the other hand, the inorganic compounds with ionic bonds are soluble in polar solvents like water.

(v)      Electrical conductivity: Due to the presence of covalent bonds, organic compounds are poor conductor of electricity, whereas inorganic compounds being ionic in nature are good conductors of electricity.

(vi).    Melting and boiling points: Generally organic compounds have low melting and boiling points and are volatile in nature. Inorganic compounds, on the other hand, have comparatively high melting and boiling points.

(vii)    Stability: Since organic compounds have low melting and boiling points they are less stable than inorganic compounds.

(viii)   Combustibility: Organic compounds with high percentage of carbon are generally combustible. On the other hand, inorganic compounds are mostly non-combustible.

  1. No.12. Write a detailed note on classification of organic compounds.

Answer. All known organic compounds have been broadly divided into two categories depending upon their carbon skeleton. These are:

  • Open chain or acyclic compounds.
  • Closed chain or cyclic compounds.

 (i)     Open chain or acyclic compounds.

Acyclic compounds contain open chains of carbon atoms in their molecules. These chains may be either straight or branched. For example,

(a)      Straight chain compounds are those in which carbon atom links with other carbon atoms or other atoms through a single, double or triple bonds forming a straight chain, such as

I  I  I  I  I  I

―C-C-C-C-C-C―

I  I  I  I  I  I                               H3C-CH2-CH2-CH3  

Straight chain                                    Straight chain (n-Butane)

(b)      Branched chain compounds are those in which there is a branch along a straight chain, such as:

H3O-CH-CH3

CH3

Branched chain                          Branched chain (isobutane)

 

Open chain compounds are also called aliphatic compounds.

(ii) Closed chain or Cyclic compounds

Closed chain or cyclic compounds contain one or more closed chains, i.e., the carbon atoms at the end of the chain are not free. They are further divided into two classes:

(a)      Homocyclic or carbocyclic compounds.

(b)      Heterocyclic compounds.

 (a) Homocyclic or Carbocyclic compounds.

Homocyclic or carbocyclic compounds contain rings which are made up of only one kind of atoms, i.e., carbon atoms. These are further divided into two classes:

  • Aromatic compounds
  • Alicyclic compounds

Aromatic compounds: These organic compounds contain at least one benzene ring in their molecule. A benzene ring is made up of six carbon atoms with three alternating double bonds. They are called aromatic because of aroma. Aroma means smell. Examples,

              Benzene                   Napthehatene

They are also called benzenoid compounds.

Alicyclic or non-benzenoid compounds: Carbocyclic compounds which do not have benzene ring in their molecules are called alicyclic or non-benzenoid compounds. For example

                     Cydobutan                     Cyclohexane

 Heterocyclic compounds

Cyclic compounds that contain one or more atoms other than that of carbon atoms in their rings are called heterocyclic compounds.

                               Thiophene        Pyridine

The above classification may be summarized as follows:

 

  1. No.13.What do you know about the sources of organic compounds?

Answer. Organic compounds are prepared naturally by animals and plants. Animals synthesize two main groups of organic compounds: proteins and fats. Proteins are meat, mutton, chicken and eggs, etc. Fats are present in milk, butter, etc. Plants synthesize; carbohydrates, proteins, fats, vitamins, etc.

Moreover, dead plants buried under Earth’s crust are converted through biochemical processes to coal, petroleum and gas. These materials are the main sources of organic compounds. We can get thousands of organic compounds by the destructive distillation of coal and fractional distillation of petroleum. Detail for each source is given in figure b

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