TYPES OF CHEMICAL REACTIONS at a glance

  Chemical reactions are reactions in which elements or/and compounds combine chemically to form new substances.

There are different types of chemical reactions, they include 

1.   Combinations reactions 

2.   Decomposition reactions 

3.   Displacement reaction

4.   Double decomposition reaction

5.   Thermal Dissociation reaction 

6.  Catalytic reaction

1. Combination reaction: This is a type of reaction where two elements combine to form one single product.

  C_(s) + O_2(g)   CO₂

2). Decomposition reactions:- In this type of reaction a single reactant split (decomposition) to give two or more products.

CaCO_3(s)  CaO_(s)+ CO_2(g) 

When heat is applied to the reaction it is know as thermal decomposition.

3) Displacement reaction: this type of reaction occurs when one element or radical replaces another element or radical in a compound 

ALKENES at a glance

 ALKENES

Alkenes are a homologous series of unsaturated hydrocarbons with a general molecular formular C_nH_2n.

They are named by replacing the ending "ane" of the corresponding alkane with "ene" for each member of the series.

NOTE: because alkenes contain double bonds between carbon to carbon i.e C=C n=1 does not exist.

When n=	General Molecular Formulae CnH2	Name
2.	C2H2x2= C2H4	Ethene
3.	C3H2x3 = C3H6	Propene
4.	C4H2x4 = C4H8	Butene
5.	C5H2x5 = C5H10	Pentene
6.	C6H2x6 = C6H12	Hexene
7.	C7H2x7 = C7H14	Heptene
8.	C8H2x8 = C8H16	Octene
9.	C9H2x9 = C9H18	Nonem
10.	C10H2x10 = C10H20	Decene

 

NOMENCLATURE OF ALKENES

When naming the alkenes, care must be taken because unlike the alkanes which have only single bonds, the alkenes contain double bonds, and so the naming of the substituents is based on the position of the double bond. For example, the molecule CH₃CH=CHCH₃ is named but-2-ene.

Although the double bond joins carbon atoms 2 and 3, the number 2 is used because it gives the lowest number to the double bond.

other examples include 

(i)  CH₃-CH₂-CH₂-CH=CH₂           

        Pent-1-ene or Pentene

(ii) CH₃CH₂CH=CHCH

        Pent-2-ene

                     

                    CH₃
                     |
(iii)       CH₃C=CHCH₃                    

         2-methylbut-2-ene

                          CH₃

                           |

(iv)       CH₃-C=C-CH₃
                       |   

                       CH₃  

       2,3-dimethylbut-2-ene

                    CH₃         
                          |   
           CH₃CH₂C=CCH₂CH₃                                      
                               |

                              CH₃

         3,4-dimethylhex-3-ene

             CH₃            CH₃

                |                   |

(vi)   CH₃C-CH=CH-C-CH₃
                 |                  |      
                CH₃            CH₃

    2,2,5,5-tetramethylhex-3-ene

MOLECULAR STRUCTURES OF ALKENES

	ALKENES molecular formula /name	STRUCTURAL FORMULAR	Condensed  FORMULAR
2.	C2H4 Ethene	H             H            ∖         ∕              C=C          ∕         ∖       H           H	H2C=CH2
3.	C3H6 Propene	H    H     H            |      |         ∕  H — C — C=C            |       |        ∖           H     H       H	CH3CH=CH2
4.	C4H8 Butene	H H H   H         |   |   |      ∕    H-C-C-C=C         |   |   |       ∖         H H H      H	CH3CH2CH=CH2
5.	C5H10 Pentene	H H H  H        H        |   |   |    |       ∕   H-C-C-C-C=C        |   |   |   |       ∖       H H H H        H	CH3(CH2)2CH=CH2
6.	C6H12 Hexene	H H H H  H     H        |   |   |   |    |       ∕   H-C-C-C-C-C=C         |   |   |   |   |     ∖        H H H H H      H	CH3(CH2)3CH=CH2
7.	C7H14 Heptene	H H H H H H       H       |   |    |   |   |   |       ∕  H-C-C-C-C-C-C=C       |    |   |   |   |   |       ∖      H H H H H H        H	CH3(CH2)4CH=CH2
8.	C8H16 Octene	H H H H H H  H     H        |   |    |   |   |   |   |      ∕  H-C-C-C-C-C-C-C=C        |  |    |   |   |   |   |      ∖       H H H H H H H      H	CH3(CH2)5CH=CH2
9.	C9H18 Nonene	H H H H H H H  H     H       |   |   |    |   |   |    |   |      ∕                          H-C-C-C-C-C-C-C-C=C       |   |   |   |    |   |   |   |     ∖        H H H H H H H H   H	CH3(CH2)6CH=CH2
10.	C10H20 Decene	H H H H H H H H  H     H      |  |    |    |    |  |    |   |   |     ∕ H-C-C-C-C-C-C-C-C-C=C      |   |   |    |   |   |  |   |    |     ∖           H H H H H H H H H      H	CH3(CH2)7CH=CH2

LABORATORY PREPARATION OF ETHENE

Ethene is prepared in the laboratory by dehydration of alkanols such as ethanol (C₂H₅OH) by concentrated H₂SO_4.

DIAG.

equation for the reaction

step i:      C₂H₅OH + H₂SO₄ → C₂H₅HSO₄ + H₂O.

  step ii.   C₂H₅HSO₄ →C₂H₄ + H₂SO₄

When ethylhydrogentetraoxosulphate (VI) is heated, it releases ethene which is collected over water.

 

NOTE: The wash bottle containing sodium hydroxide is to remove Sulphur (iv)oxide.

PHYSICAL PROPERTIES OF ETHENE (ALKENES)

1. It is colourless and odourless gas

2. It is neutral litmus paper

3. It is almost insoluble in water

4. It is less dense than air.

Chemical Properties Of Ethene

Alkenes such as ethene undergoes addition reaction 

1. Reaction of ethene with hydrogen in the presence of nickel as a catalyst

                                  Ni    
          H₂C=CH₂ + H₂ →   CH₃CH₃         
              ethene                   ethane

This reaction is important in the conversion of oil into margarine by the process known as HYDROGENATION.

2. Reaction of ethene with chlorine to produce 1,2-dichloroethane.

 CH₂=CH₂ + Cl₂ → ClCH₂-CH₂Cl

3. Reaction of ethene with bromine to produce 1,2-dibromoethane

 CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br

4. Reaction of ethene with oxygen or combustion reaction of ethene (alkenes) to produce carbon(iv)oxide and water

  CH₂=CH₂ + 3O₂ →2CO₂ + 2H₂O

5. Reaction of ethene with hydrogen halide (Hydrogen chloride) (HCl) to produce ethylchloride.

 CH₂=CH₂ + HCl → CH₃CH₂Cl  

6. Reaction of ethene with water in the presence of dilute H₂SO₄ to produce ethanol

   CH₂=CH₂ + H₂O →CH₃CH₂OH

7. Reaction of ethene with neutral KMnO₄ to produce 1,2-ethan-diol (glycol)

 CH₂=CH₂ + KMnO₄ → CH₂—CH₂
                                            |          |
                                           OH     OH    

                                     ethan-1,2-diol

 KMnO4 is decolorized in the above reaction, and this reaction distinguishes alkenes from alkanes which do not decolorize KMnO₄

Uses Of Ethene

1. In the production of polythene which is used for making nylon or polythene bags and wrappers

2. In the manufacturing of margarine by the process of hydrogenation.

 Other examples of alkenes are as follows

(i)      CH₃CH₂C = CCH₃                                           (ii)     CH₃C = CHCH₃
                                 |       |                                                                   |
                           CH₃ CH₃                                                            CH₃
            

             2,3-dimethylpent-2-ene                                  3-methylbut-2-ene

                       CH₃                                                                                                CH₃ 
                        |                                                                                                      |    
(iii)       CH₂= C — CH—CH₂CH₂CH=CH₂        (iv)        CH₃CH₂CHCH=CHCH=C=CH₂ 
                                |                                                                            |
                                CH₂CH₃                                                              CH₂CH₃  
         

              2-methyl, 3-ethylhept-1,6-diene                                6-ethyl, 3-methyloct-1,2,4-triene

                                                                                    Cl
                                                                                     |
(v)       CH₃CH=CHCH₃                        (vi)      CH₃-C-CH=CH₂ 
                            |                                                         |
                           Cl                                                      Cl
            3-chlorobut-2-ene                               3,3-dichlorobut-1-ene

(vii)     CH₃CH₂C=CH=CHCH₃              (viii)     CH₃C=CHC=CH₃  
                           |            |                                              |           |
                          Cl         Br                                           Cl        Cl
            2-bromo, 4-chlorohex-2,3-diene        2,4-dichloropent-1,3-diene

                           

                                 Cl                                                                   Cl
                                  |                                                                       |
(ix)      CH₂=C=CH-C=C-CH₃                         (x)     CH₃CHCH=C-CH₃  
                                       |                                                    |             |
                                      Br                                                 Cl         Cl

                5-bromo,3-chlorohex-1,2,4-triene                 4,2,2-trichloropent-2-ene

              

          H H H            H                                                        CH₂CH₃
               |    |  |            ∕                                                          ∕
(xi)  H-C-C-C-C=C                              (xii)   CH₃CH = C
             |    |   |   |      ∖                                                        ∖                                     
            H H   | H        H                                                       CH₂CH₂Cl
                      |                                                                     5-chloro -3-ethylpent-2-ene
                 H-C-H                                              
                      |
                     H                                                                       
       3-methylpent-1-ene

                       

                                    CH₃
(xiii)                          ∕
            H₂C = C = C
                                 ∖
                                   H
            But-1,2-diene 

OBJECTIVE QUESTIONS

1. Hydrogenation of butene yields 

a. Butyne 

b. butane

c. pentene

d. butanol

2. Geometric (cis- trans) isomerism is exhibited by

a. C₂H₂Cl₂

b. C₂H₆Cl

c. C₄H₁₀

d. C₅H₁₂

3. which of the following is the general formula for the alkenes

a. CnH2n-2

b. CnH2n

c. CnH2n+2

d. CnH2n+1

4. Alkenes underg the reactions

THEORY QUESTIONS

1. Use the reaction scheme below to answer the following questions 

(i).