HYDROCARBONS

Chapter 12

HYDROCARBONS

SOLVED TEST YOURSELFS Test Yourself 12.1 (HYDROCARBONS)

  1. No. i. Why hydrocarbons are considered, as parent organic compounds?

Answer. Hydrocarbons are regarded as the parent organic compounds because other organic compounds are considered to be derived from then by replacement of one or more hydrogen atoms by other atoms or group of atoms. For example, by replacing hydrogen atom with hydroxyl group alcohol is produced.

CH3-H              CH3-OH

CH3-CH2-H         CH3CH2-OH

By replacing hydrogen atom with -NH2 (amino) group amine is formed.

CH3-CH2-NH2

  No. ii. What is the difference between a straight and a branched chain?

Answer. 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. Branched chain compounds are those, in which there is a branch along a straight chain, such:

H3C-CH2-CH2– CH3                     H3C-CH2-CH2      

Straight chain (n- butane)                             CH3

Branched chain (isobutane)

  1. No.iii. Give the general formula of saturated and unsaturated hydrocarbons.

Answer. The general formula of saturated hydrocarbons is CnH2n+2, while that of unsaturated hydrocarbons having one double bond is CnH2n. Where ‘n’ is the number of carbon atom in one molecule.

  1. No. iv. Define unsaturated hydrocarbons with examples.

Answer. The hydrocarbons in which two carbon atoms are linked by a double or a triple bond are called unsaturated hydrocarbons. The compounds in which two carbon atoms are linked by a double bond are called alkenes. For example, ethane and propene.

H2C=CH2                 H3C-CH=CH2

Ethene                        Propene

These compounds have general formula CnH2n and functional group >C = C<.

The hydrocarbons in which two carbon atoms are linked by a triple bond are called alkynes. For example, ethyne and propyne.

HC≡CH            H3C-O≡CH

Ethyne               Propyne

They   have   general formula   CnH2n-2   and   functional   group

Test Yourself 12.2 (ALKANES)

  1. No. i. Which is the simplest alkane?

Answer: Methane is the simplest form of alkane.

  1. No. ii. Give the structure of following compounds isopentane, and isobutene

Answer:

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

CH3                               CH3         

           Isopentane                 isobutene

  1. No. iii. Why the burning of alkanes requires sufficient supply of oxygen?

Answer: For complete combustion of alkanes into carbon dioxide sufficient supply of oxygen is required. If there is insufficient supply of oxygen, alkanes will produce carbon monoxide gas, which creates suffocation and causes death Such as;

3CH4 + 4O2 —> 2CO + C+ 6H2O

  1. No.iv. What do you mean by halogenations? Give the reaction of methane with chlorine in bright sunlight.

Answer. A reaction in which one or more hydrogen atoms of saturated compounds are replaced with halogen atoms is called halogenations. Methane reacts in the presence of direct sunlight to form carbon. It is an explosive reaction.

CH4 + 2CI2 ———> C + 4HC1

Test Yourself 12.3 (ALKENES)

  1. No. i. Why alkenes are reactive?

Answer. Alkenes are reactive compounds because the electrons of the double bond are easily available for reaction. These compounds have the tendency to react readily by adding other atoms, to become saturated compounds. As a result, the double bond is converted into a single bond-that is more stable.

  1. No. ii. How can you prepare propene from propyl alcohol?

Answer. Propene is prepared by heating a mixture of propyl alcohol and excess of concentrated sulphuric acid at 180°C. In first step, propyl hydrogen sulphate is formed which on heating .produces propene, which is collected over water.

180 °C

CH3-CH2-CH2OH + H2SO4 ———> CH3– CH2– CH2-OSO3H + H2O

heat

CH3-CH2-CH2OSO3H ———> CH3-HC= CH2 + H2SO4

  1. No.iii. Give a test used to identify unsaturation of an organic compound.

Answer. Unsaturated compounds consist of a double or triple bond. They can be identified by KMnO4 test. Alkenes decolorizes the

3CH2=CH2 + 2KMn04 + 4H2O———> 3H2C——CH2 + 2MnO2 + 2KOH

OH   OH

pink colour of acidified dilute solution of potassium permanganate

  1. No. iv. Give a few uses of ethene.

Answer. Ethene is used:

  1. For artificial ripening of fruits
  2. As a general anaesthetic

iii. For manufacture of polythene. Polythene is a plastic material used in      packaging, toys, bags.

  1. As a starting material for the manufacture of a large number of compounds such as ethylene oxide, ethyl alcohol, ethylene glycol, diethyl ether, etc.; ethylene oxide is used as a fumigant, ethylene glycol is used as an anti freeze, and diethyl ether and ethyl alcohol are used as solvents
  2. For making poisonous mustard gas which is used in chemical warfare.
  1. No. v. Which gases are present in LPG?

Answer. Propane and butane gases are present in LPG

 

Test Yourself 12.4 (ALKYNES)

  1. No. i. Why the alkynes are called acetylenes?

Answer. Alkynes are also called acetylenes because of the name of the first member of the series is acetylene.

  1. No. ii. How is tetrabromoethane prepared from acetylene?

Answer. When bromine water is added to acetylene, red-brown colour of bromine water is discharged rapidly due to formation of colorless tetrabromoethane.

Br    Br

HC≡CH +   2Br2   →  H-C — C-H

Br    Br

                         Tetrabromoethane

  1. No.iii. How can you prepare acetylene from tetrachloroethane?

Answer. When tetrachloroethane is heated with Zinc dust, the elimination of halogen atoms takes place and acetylene is formed.

   Cl  Cl                      heat

 H-C―C-H   + 2Zn (dust)           HC≡CH + 2ZnCl2

   Cl  Cl

  1. No.iv. What is the difference between glycol and glyoxalin?

Answer. The formula of glycol is:

h2c=ch2

OH  OH

It is (1,2-ethandiol): It belongs to alcohol family.

The formula of glyoxal is:

O O

║ ║

H-C-C-H

Glyoxal

It does not belong to alcohol family.

  1. No. v. Write the formula of oxalic acid

Answer. The formula of oxalic acid is:

O O

║ ║

HO-C-C-OH

oxalic acid

MULTIPLE CHOICE QUESTIONS

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

  1. Which one of these hydrocarbon molecules would have no effect on an aqueous solution of bromine?
  2. CH4 b. C10H2o
  3. C2H4 d. C2H2
  1. If an organic compound has 4 carbon atoms, all singly bonded, it will have the following characteristics except one
  2. it will be saturated hydrocarbon,
  3. it will have 8 hydrogen atoms,
  4. its name will be n-butane.
  5. it will be least reactive.
  1. The reduction of alkyl halides takes place in the presence of
  2. Zn/HCl b. Na/HCl
  3. Mg/HCl d. Cu/HCl
  1. Halogenations of methane produces following valuable chemical compounds used as solvents except:
  2. carbon tetrachloride b. chloroform
  3. carbon black d. chloromethane

 

  1. Incomplete combustion of alkanes produces
  2. carbon dioxide only.
  3. carbon monoxide only.
  4. carbon monoxide and carbon black.
  5. carbon dioxide and carbon black.
  1. Alkenes are prepared from alcohols by a process called
  2. dehydrogenation. b. dehalogenation
  3. dehydration. d. dehydrohalogenation
  1. Dehydrohalogenation takes place in the presence of
  2. NaOH aqueous. b. alcoholic KOH.
  3. aqueous KOH d. alcoholic NaOH.
  1. Oxidation of ethene with KMnO4 produces
  2. oxalic acid. b. glyoxal.
  3. ethane glycol. d. propene glycol.
  1. Which one of these is a saturated hydrocarbon?
  2. C2H4 b. C3H6
  3. C4H8 . d.C5H12
  1. A hydrocarbon has molecular formula CsHi4. Molecular formula of the next member of the same homologous series

Is?

  1. C9H18 b. C9H16

c.C9H20                          d. C9H12

 The molecular formula of the first three members of the alkane hydrocarbons are CH4, C2H6 and C3H8. What is the molecular formula for the eight alkane member, octane, which is found in petrol?

  1. C8H8 b.C8H16
  2. C8H18 d. C7H20
  1. One of the hydrocarbons reacts with one mole of hydrogen to form a saturated hydrocarbon. Formula of unsaturated hydrocarbon is?
  2. C3H8 b.C6H12
  3. C4H10   d. C7H16
  1. Dehydration of alcohols can be carried out with
  2. NaOH b. KOH
  3. H2SO4 d. HC1
  1. The end product of oxidation of acetylene is
  2. oxalic acid. b. glycol.
  3. glyoxal. d. none of these.
  1. Dehalogenation of tetra halides produces acetylene. This reaction takes place in the presence of
  2. sodium metal b. zinc metal
  3. magnesium metal d. potassium metal
  1. Substitution reaction is the characteristics of
  2. alkanes. b. alkenes.
  3. alkynes. d. none of these.
  1. Halogenations of alkanes   in   the presence   of diffused sunlight takes place
  2. suddenly, only in one step.  b. slowly in one step,
  3. in a series of steps. d. fastly in two steps.
  1. Which one of the followings is a substitution reaction?
  2. Halogenation of alkynes b. Halogenation of alkenes
  3. Halogenation of alkanes  d. bromination of alkenes
  1. The order of reactivity of hydrogen halides with alkenes is?
  2. HI > HBr b. HBr > HI
  3. HCl >HBr     d. HBr < HC1
  1. Oxidation of alkenes produces
  2. glyoxal b. glycol
  3. oxalic acid d. formic acid
  1. Those compounds which are made up of only carbon and hydrogen atoms are called
  2. Hydrocarbon b. alcohol
  3. Both of above d. none of above
  1. The term paraffin is also used for
  2. Alkene b. Alkyne
  3. Alkane  d. None of above
  1. Addition of nascent hydrogen is called
  2. Oxidation b. Sulphonation
  3. Reduction    d. Hydrogenation
  1. Hydrocarbons are the compounds which contains
  2. OH group b. COOH group
  3. Carbon and hydrogen d. C-X group
  1. The saturated hydrocarbons are called
  2. Alkane b. Alkenes
  3. Alkyne d. All of above
  1. The name of saturated hydrocarbon end with suffix
  2. _yne b. _ene
  3. _ane d. _ine
  1. Methane, ethane, propane are
  2. Liquids b. Solids
  3. Semi solid d. Gases
  1. C10H22 is chemical formula of
  2. Methane b. Hexane
  3. Decane d. Octane
  1. Hydrocarbon having a double or triple bond between two carbon atoms is called
  2. Saturated b. Functional
  3. Unsaturated d. Paraffin
  1. Butane can have how many structural isomers
  2. One b. Two
  3. Few   d. Many
  1. The unsaturated hydrocarbons which have carbon-carbon double bonds are called
  2. Alkane b. Alkene
  3. Alkyne d. Both a & b
  1. The unsaturated hydrocarbons are called
  2. Paraffin’s         b. Olefin
  3. Metallic carbides d. Polymers
  1. The isomers that differ in skeleton is called
  2. Isomerism b. Chain isomerism
  3. Polymers   d. Hydrocarbon
  1. Compounds having same molecular formula but different structural formula are called
  2. alkane b. alkene
  3. alkyne d. isomers
  1. Alkane in which at least one carbon atom is linked to three or four other carbon atoms is called:
  2. Straight chain b. Branch chain
  3. Closed chain d. Normal alkane
  1. Which of the following is not an alkyne
  2. Ethyene b. Ethene
  3. Propyne d. Butyne
  1. Chloroform is used as solvent for the manufacture of
  2. Rubber b. Waxes
  3. Paints d. Both a & b
  1. Which one is used for artificial ripening of fruits?
  2. ethane b. ethene
  3. ethyne  d. methane
  1. Chloroform and carbon tetrachloride are formed when methane reacts with halogens in
  2. Dark b. Bright sunlight
  3. Diffused sunlight d. Both of dark and bright
  1. Ethylene is present in coal to the extent of
  2. 6% b. 0.6%
  3. 0.06% d. 6.6%

ANSWER KEY MCQ

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

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

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

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

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

                       

ANSWERS TO THE SHORT QUESTIONS.

  1. No.1. Differentiate between saturated and unsaturated hydrocarbons.

Answer. The compounds in which all the four valences of carbon atoms are fully satisfied (saturated) by single bonds with other carbon and hydrogen atoms are called saturated hydrocarbons. Saturated hydrocarbons are also called alkanes.

H3C-CH3

Ethane

The compounds in which two carbon atoms are linked by a multiple bond i.e. double or a triple bond are called unsaturated hydrocarbons. The compounds in which two carbon atoms are linked by a double bond are called alkenes. For example, ethene and propene

H2C=CH2                                            CH3—CH=CH2

                    Ethene                                Propene

While the compounds in which two carbon atoms are linked by a triple bond are called alkynes, For example,

CH≡CH                      H3C-C ≡ CH

Ethyne                                Propyne

  1. No.2. A compound consisting of four carbon atoms has a triple bond in it. How many hydrogen atoms are present in it?

Answer. A compound consisting of triple bond is called alkyne. The general formula for alkynes is CnH2n-2. Where ‘n’ is number of carbon atoms. For a compound having 4 carbon atoms number of hydrogen atoms are calculated as:

2(n) ― 2= 2(4) ―2 = 6

Therefore, a compound consisting of four carbon atoms and a triple bond will have six hydrogen atoms.

  1. No.3. Why the alkanes are called ‘paraffin’s’?

Answer. Alkanes are the least reactive compounds so they are called paraffin’s. Paraffin is derived from Para means less, and affine means affinity or reactivity.

  1. No.4. What do you know about hydrogenation of alkenes?

Answer. Hydrogenation means addition of hydrogen in alkenes. This reaction is carried out in the presence of nickel catalyst at 250°C to 300°C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature.

h2c=ch2 + h2   Ni       H3C ― CH3

250-300°c

  1. No.5. How alkyl halides are reduced?

Answer. Nascent hydrogen changes alkyl radicals into alkanes. This reaction takes place in the presence of Zn metal and HC1.

CH3Br   + 2[H]    zn/dil.HCl        CH4 + HBr

CH3CH2Br + 2[H]    zn/dil.HCl     CH3-CH3 + HBr

  1. No.6. Why the alkanes are used as fuel?

Answer. The main constituents of fuels (coal, petroleum and natural gas) are hydrocarbons. When hydrocarbons are burnt in air CO2 and H2O along with heat are produced. This reaction is called combustion. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. It is highly exothermic reaction, i.e. it produces a lot of heat. The basic combustion reaction is

CH4 + 2O2 ——— > CO2 + 2H2O + heat

The heat energy produced is used for various processes, in homes, transportations, as well as in industries

  1. No.7. How can you prepare ethene from alcohol and ethyl bromide?

Answer. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180°C. In first step, ethyl hydrogensulphate is formed which decomposes on heating to produce ethene, which is collected over water.

180 °C

CH3-CH2OH + H2S04 ——> CH3CH2-OSO3H + H20

heat

CH3-CH2OSO3H ———> H2C=CH2 + H2SO4

On heating ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen from adjacent carbon atoms forms double bond. This method is called dehydrohalogenation, of alkyl halides.

H3C-CH2-Br + KOH (alcoholic) ———> H2OCH2 + KBr + H2O

  1. No.8. Distinguish between propane and propene with a chemical test

Answer. Propene does not react with alkaline KMnO4 solution.

CH3-CH=CH2 + KMnO4 + H2O ———> No reaction

While alkenes decolorize the pink colour of acidified dilute solution of potassium permanganate. Propene decolorizes the pink colour of KMnO4 solution and forms propene glycol that is colorless compound.

3CH3-CH=CH2+ 2KMn04 + 4H2O

3GH3-HC (OH)-CH2 (OH) + 2MnO2 + 2KOH

This reaction is also used to test the unsaturation of an organic compound.

  1. No.9. Why the alkenes are called ‘olefins’?

Answer. Olefin is derived from Latin word meaning oil forming. Lower members of alkenes form oily products when react with halogens therefore, alkenes are called olefins.

  1. No.10. Why alkane can’t be oxidized with KMnO4 solution?

Answer. Alkane has no double or triple bond. It has only sigma bond that is not broken by alkaline KMnO4 solution. So alkane cannot be oxidized by KMnO4 solution.

  1. No.11. What are the addition reactions? Explain with an example.

Answer. Reactions in which two or more substances combine to form a big molecule are called addition reactions. Addition of hydrogen in alkenes and alkynes is called addition reaction. Alkenes and alkynes are unsaturated compound, so they have the capacity to add up atoms in them. This reaction is carried out in the presence of nickel catalyst at 250 °C to 300 °C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature,

Ni

H2C=CH2    +   H2   250-300 C     H3C-CH3

  1. No.12. Justify that alkanes give substitution reactions.

Answer. Alkanes give only substitution reactions. A reaction in which one or more hydrogen atoms of a saturated compound are replaced with some other atoms like halogen is called a substitution reaction. For example in diffused sunlight one hydrogen atom of CH4 is replaced by halogen atom to produce chloromethane.

diffused sunlight

CH4 + C12 ——————> CH3C1 + HC1

  1. No.13. Both, alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.

Answer. Although both of the compounds are unsaturated, they decolorize pink colour of acidified dilute solution of potassium permanganate. Alkenes (ethene) produce ethyl glycol (1,2-ethenediol) by addition of two hydroxyl groups at the double bond. Such as:

3CH2=CH2   +   2KMn04   +   4H2O ——>   3H2C—CH2 + 2MnO2 + 2KOH

OH OH

While alkynes (ethene) adds four hydroxyl groups at the triple bonds. Such as:

OH OH

HC=CH   +     2KMnO4   +   2H2O —————>     H-C—C-H     +   2MnO2   +   2KOH

OH OH

This intermediate product eliminates water molecules to from glyoxal, which further oxidize to form oxalic acid.

OH OH       2H2O        O O        2[O]              O O

│  │                      ║ ║        KMnO4            ║ ║

H-C—C-H                 H-C-C-H                     HO-C-C-OH

│  │                                                   oxalic acid

 No.14. Write the molecular, dot and cross and structural formula of ethyne.

Answer.

Name Molecular

formula

Condensed formula Structural formula Cross and dot formula
Acetylene

 (ethyne)

C2H2 HC ≡ CH H―C≡C―H H..C⁞⁞C..H

 No.15. Why hydrocarbons are soluble in organic solvents?

Answer. Hydrocarbons are soluble in organic solvents according to like dissolve  like”  principle. Hydrocarbons are non-polar and are soluble in organic solvents which are also non-polar.

  1. No.16. Give the physical properties of alkanes. Answer.
  2. Alkanes form a homologous series of compounds. First four members of the series are gases. The alkanes consisting of C5 to C10 are liquids while higher members of the series are solids.
  3. They are non polar, therefore, they are insoluble in water but soluble in organic solvents.
  4. The density of alkanes increases gradually with the increase of molecular size.
  5. The melting and boiling points of alkanes increase regularly with the increase of molecular sizes. This is because of increase of attractive forces between the molecules of alkanes.
  6. Viscosity of alkanes increases with the increase of molecular size.
  7. Alkanes are less flammable, i.e., large size molecules produces difficulty in burning.
  1. No.17. How can you differentiate between ethane and ethene?

Answer. Ethane and ethene can be identified by KMnO4 test. Ethene decolorizes the pink colour of acidified dilute solution of potassium permanganate .While ethane do not give this test.

3CH2=CH2    +   2KMn04 +   4H2O ——-   3H2C—CH2   +   2MnO2 + 2KOH

OH OH

  1. No.18. Why colour of bromine water discharges on addition of ethene in it?

Answer. When bromine waters (a solution of bromine in water) having red-brown colour is added to ethene in an inert solvent like car.bon tetrachloride, its colour is discharged at once. Bromine water reacts with ethene in CCU to form ethylene bromide a colorless compound.

H2C=CH2 + Br2 → Br-CH2-CH2-Br

This reaction is used to identify the unsaturation of an organic compound.

  1. No.19. State one important use of each:

(i) Ethene                                   (ii) Acetylene

(iii) Chloroform                     (iv) Carbon tetrachloride

Answer.

(i)       Ethene is used for artificial ripening of fruits.

(ii)     Acetylene is used as oxy-acetylene flame. It is highly exothermic reaction. Heat released is used for welding purpose.

(iii)    Chloroform is used as a solvent for rubber, waxes etc., for anesthesia.

(iv)    Carbon tetrachloride is used as an industrial solvent and in dry cleaning.

ANSWERS TO LONG QUESTIONS

  1. No.1. What types of reactions are given by alkanes? Explain with reference to halogenations of alkanes.

Answer. Alkanes give only substitution reactions. These reactions are a characteristic property of alkanes. Alkanes react fairly with halogens in diffused sunlight only. In dark there is no reaction. In direct sunlight reaction is explosive and carbon is deposited.

CH4 + 2C12———> C + 4HC1

In diffused sunlight a series of reactions take place and in each step one hydrogen atom is substituted by halogen atom in four steps, all the hydrogen atoms are substituted one by one by halogen atoms

diffused

CH4      +     C12                         CH3C1   +   HC1

Sunlight                Chloromethane

 

CH3C1 +    C12                        CH2C12 +    HC1

Dichloromethane

CH2C12 +   C12                        CHCl3   +    HCl

Trichloromethane

(Chloroform)

CHC13 +   C12                         CCl4    +    HC1

Tetrachloromethane

(Carbon tetrachloride)

  1. No.2. Alkanes are a source of heat. Explain it.

Answer. The main constituents of fuels (coal, petroleum and natural gas) are hydrocarbons. When hydrocarbons-are burnt in air, CO2 and H2O along with heat are produced. This reaction is called combustion. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. It is highly exothermic reaction, i.e. it produces a lot of heat. The basic combustion reaction is

CH4 + 2O2 ———> CO2 + 2H20 + heat

The heat energy produced is used for various processes, in homes, transportations, as well as in industries.

  1. No.3. Prepare the following as directed.

(a) ethane from ethene

When ethene reacts with hydrogen in the presence of Ni at 250 to 300 °C ethane is formed.

Ni

H2C=CH2   +   H2             H3C-CH3

250-300°C

(b) Acetylene from tetra-halide

When an alkyl tetra-halide is heated with zinc dust, the elimination of halogen atoms takes place and acetylene is formed.

Cl  Cl

H-C―C-H        +2Zn(dust)     heat           HC≡CH   +  2ZnCI2 Cl

Cl   Cl                                    (Ethyne)

 

(c) Carbon tetra-chloride from methane

Alkanes react fairly with halogens in diffused sunlight only. CH4 reacts with Cl2 in diffused sun light to form chloromethane, dichloromethane tri chloromethane and finally Tetrachloromethane

diffused

CH4      +     C12                         CH3C1   +   HC1

Sunlight                Chloromethane

 

CH3C1 +    C12                        CH2C12 +    HC1

Dichloromethane

CH2C12 +   C12                        CHCl3   +    HCl

Trichloromethane

(Chloroform)

CHC13 +   C12                         CCl4    +    HC1

Tetrachloromethane

(Carbon tetrachloride)

  (d) Ethylene glycol from ethene

When ethene is treated with aq. alkaline KMn04 solution, ethylene glycol is formed in cold state.

3CH2 = CH2   + 2KMnO4 +   4H2O         3CH2 ― CH2   +   2KMnO4   + 2KOH

(e)  1,2-dibromoethane from ethene

When ethene is treated with bromine 1, 2-dibromoethane is formed.

H2C=CH2 + Br2 + Br-CH2-CH2-Br

(f) glyoxal from acetylene

When acetylene is oxidized by alkaline KMn04 hydroxyl groups add to triple bond. This intermediate product eliminates water molecules to form glyoxal.

HC≡CH   +   2KMnO   +   2H20   OH OH   +   2MnO2   +   2KOH

H-C—C-H

OH OH

O O

OH OH                           -2H2O         ║ ║

H-C—C-H                             H-C-C-H

OH OH                                  Glyoxal

  1. No.4. Explain the oxidation of acetylene.

Answer. Acetylene is oxidized by alkaline KMn04. Hydroxyl groups add to the triple bond to form an intermediate product.

HC≡CH   +   2KMnO4   +   2H20   OH OH   +   2MnO2   +   2KOH

H-C—C-H

OH OH

O O

OH OH                        -2H2O        ║ ║

H-C—C-H                        H-C-C-H

OH OH                             Glyoxal

This intermediate product eliminates water molecules to from glyoxal, which on oxidation forms oxalic acid

O O        2[O]                 O O

OH OH                           -2H2O         ║ ║       KMnO4              ║ ║

H-C—C-H                             H-C-C-H                    HO-C-C-OH

OH OH                                  Glyoxal                     Oxalic acid

This reaction is also used to test the unsaturation in an organic compound.

  1. No.5. Write balanced chemical equations for the following reactions. Also name the products that formed.

(i) A mixture of ethyne and hydrogen is passed over heated nickel

Answer. When mixture of ethyne and hydrogen is passed over heated nickel at 250-300 °C ethane is formed.

                      

                          Ni

      HC≡CH    +   H2   250-300 C     H2C=CH2

                               Ni

      H2C=CH2    +   H2   250-300 C     H3C-CH3

 (ii) Ethene is treated with chlorine

Answer. When ethene is treated with chlorine tetrachloroethane is formed.

      2CH≡CH + 2C12             Cl  Cl

                                 H-C―C-H

                                   Cl  Cl

(iii) Ethene is burnt in air

Answer. When ethene is burnt in the presence of excess of air or oxygen carbon dioxide, water and a lot of heat are produced.

2CH=CH+ 5O2 ———> 4CO2+ 2H20 + heat

 (iv). Ethene is passed through bromine water

Answer.     When    ethene    is    passed    through    bromine    water tetrabromoethane is formed.

                           Br    Br

HC≡CH +   2Br2   →   H-C — C-H

Br    Br

                         Tetrabromoethane

  1. No.6. Explain briefly.

(i) Why butane undergoes substitution reactions?

Answer.   Alkanes   are   least   reactive   compound   being   saturated hydrocarbons they undergo substitution reactions.

diffused sunlight

CH3-CH2-CH2-CH3 + C12                   CH3-CH2-CH2-CH2-C1 + HC1

 

(ii) There are millions of organic compounds.

Answer. The ability of carbon atom to link with other carbon atom to link with other carbon atom to form long chain, large rings in an organic compound. The compounds of carbon display catenation. So they form large number of organic compounds. They are present in basic skeleton of all living things.

(iii) Acetylene undergoes addition reactions in two stages.

Answer. In addition reaction first triple bond is changed to double bond and again double bond changes to single bond. That is the reason acetylene undergoes addition reactions in two stages. For example, hydrogenation of acetylene

                           Ni

      HC≡CH    +   H2   250-300 C     H2C=CH2

                               Ni

      H2C=CH2    +   H2   250-300 C     H3C-CH3

 (iv). Alkynes are more reactive than alkanes

Answer. Alkynes are reactive compounds because of presence of a triple bond. A triple bond consists of two weak bonds and a strong bond. When alkynes react with other substances, two weak bonds are readily broken one by one and addition takes place easily. In case of alkanes only sigma bonds are present which are strong and are not

easily broken. Sigma bond is not broken in simple addition reactions.

  1. No.7. Explain the different uses of hydrocarbons in industry.

Answer. Hydrocarbons are not only used as fuel in auto-mobiles or industries; they are also used as raw materials in industries. It is explained as

(i)       Petrochemical Industry: The organic compounds prepared from hydrocarbons (petroleum and natural gas) are called petrochemicals. Some of the important petrochemicals are methyl alcohol, ethyl alcohol, methanoic acid, chloroform, carbon tetrachloride, ethylene, butadiene, benzene, toluene etc.

(ii)     Plastic Industry: Hydrocarbons are used as raw materials for the preparation of a large variety of items used in daily life. Such as, plastics like polythene, polyester. Plastics are synthetic materials which can be given any shape when soft, and on hardening make a useful durable article. For example, crockery items (cups, glass, jug, plates, spoon) furniture items (chair, table, stool) auto-mobile parts, electric and sewages items and a lot of other house hold items.

(iii)    Rubber Industry: Hydrocarbons are used to prepare synthetic rubber. For example, acetylene is used to prepare butadiene rubber used for making foot wear, tyres and toys. Similarly a good quality rubber neoprene is prepared from chloroprene.

(iv)   Synthetic fiber Industry: Hydrocarbons are used to prepare synthetic fibers like nylon, rayon, polyesters. These fibers have better qualities like greater strength, good elasticity, and resistance to wear and tear. So clothes made of synthetic fibers are long lasting than that of natural fibers,

(v)      Synthetic detergents: Long chain hydrocarbons obtained from petroleum are used to make synthetic detergents and washing powders. These detergents have long chain of alkyl hydrogen sulphate. These detergents have better and stronger cleaning properties than that of soaps. They can be used even in hard water.

 

 

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