ALKYNES

Week:                                                                                                  Date:                                                    

Period:                                 Duration: 1 HR 20 MIN.                                                 Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                        Class: SS TWO

Topic:                                    ALKYNES

Sub topic:  Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught alkenes

Behavioural objective: At the end of the lesson the students should be able to:

i.                     Explain alkenes

ii.                   Explain the laboratory preparation of alkenes

iii.                  State the physical properties and chemical properties of alkenes

iv.                 uses of alkenes

 

CONTENT

ALKYNES

Alkynes form a homologous series of unsaturated hydrocarbons containing at least a carbon-carbon triple bond. The general molecular formula of alkyne is C_nH_2n-2, where n is equal or greater than 2. Alkyne show a higher degree of unsaturation than the alkenes; hence, they are chemically more reactive than corresponding alkanes and alkenes. It have similar names with the suffix –yne.

Names and Structures of simple Linear Alkynes

The homologous series of linear alkynes with the triple bond attached to the first (= last) carbon atom is shown below.

Carbon atoms in chain	Name and simple formula	Simple Structure showing bond types but not accurate bond angles	Previous / Other Names *Examples not complete lists
2	ethyne (C2H2)       CHCH		Acetylene
3	propyne (C3H4)       CH3CCH		1-propyne prop-1-yne
4	butyne (C4H6)    CH3CH2CCH		1-butyne but-1-yne ethyl acetylene
5	pentyne (C5H8) CH3CH2CH2CCH		1-pentyne pent-1-yne acetylene
6	hexyne (C6H10) CH3CH2CH2CH2CCH		1-hexyne hex-1-yne butylacetylene

ETHYNE

Ethyne (acetylene) is the first member of the alkyne series. It has a molecular formula, C2H2, and a structural formula CHCH.

PREPARATION OF ETHYNE

Ethyne is prepared in the laboratory by the action of cold water on calcium carbide. The gas evolved is passed through an acidified copper(ii)tetraoxosulphate (vi) solution to remove gaseous impurities such as phosphine, PH₃, hydrogen sulphide, H₂S and ammonia, NH₃ and then collected over water. The reaction is highly exothermic; hence, it is performed on a heap of sand inside the flask to prevent the flask from cracking.

CaC_{2 s}  + 2H₂O_g     à   C₂H₂   + Ca(OH)2_aq

Note: carbide may be prepared by heating the appropriate metal or its oxide with carbon, carbon(ii)oxide or a hydrocarvbon.

PHYSICAL PROPERTIES OF ETHYNE

I.                    pure ethyne is a colourless gas with a characteristic sweet smell.

II.                  It is sparingly soluble in water.

III.                It is slightly less dense than air.

IV.                It is unstable and may explode on compression to a liquid.

CHEMICAL PROPERTIES OF ETHYNE

I.                    COMBUSTION: Ethyne burns in air or oxygen with a very smoky luminous flame owing to its high carbon content.

   Ethyne burns in air with a luminous smoky flame and hence used for lightening purpose.
          2C₂H₂ + 5O₂   4CO₂ + 2H₂O + heat

When burns in pure oxygen , there is complete combustion producing a non-luminous, very hot flame of about 2500^oC. This flame is used in the oxy-ethyne (oxyacetylene) torch which is used in welding and cutting metals.

 

II.                  ADDITION REACTION: Ethyne, being highly unsaturated, readily undergoes addition reactions, combining with a maximum of four univalent atoms or radicals per molecule. In the first step, the triple bond, is converted to double bond, while the second step convert double bond to single bond.

Addition of Hydrogen:
          C₂H₂   CH₂=CH₂   C₂H₆ (in the present of nickel catalyst at about 200oC)
   Addition of Chlorine:
          C₂H₂ +Cl₂   Cl-CH=CH-Cl   Cl2CH-CHCl2 (1,1,2,2-tetrachloroethane)
   Addition of Hydrogen Halide:
          C₂H₂ + HBr   CH2=CH-Br + HBr   CH3-CHBr₂  (1,1-di-bromo ethane)

TEST FOR UNSATURATION

Unsaturated compounds are best recognized by their addition reaction. For example, reaction with bromine water (HOBr), the reddish-brown bromine or bromine water turns colourless on reacting with unsaturated compounds. Also a purple solution of potassium tetraoxomanganate (vii) becomes colorless, when added to unsaturated compounds.

USES OF ETHYNE

I.                    As a fuel in miners lamps

II.                  It mixed with oxygen to produce a very hot and easily controllable flame, known as oxy-etyhne flame which is used for cutting and welding metals.

III.                A good starting material for the synthesis of other important organic compounds e.g  ethanoic acid.

IV.                Used in making vinylchloride which polymerize to the plastic

V.                  To produce synthetic fibres  and artificial rubber.

PRESENTATION

Step I: The teacher explain alkynes as an unsaturated hydrocarbon

Step II: The teacher leads the students to derived the molecular formula of first – ninth members of alkynes.

Step III: the teacher explain the laboratory preparation of alkynes

Step IV: The teacher states the physical and chemical properties of alkynes.

EVALUATION

The teacher evaluates the lessons by asking these questions

i.                     Explain alkynes

ii.                   Explain the laboratory preparation of alkynes

iii.                  State the physical properties and chemical properties of alkynes

iv.                 uses of alkynes

ASSIGNMENT

 Read about benzene