ALKENES
Alkenes are a homologous series of unsaturated hydrocarbons with a general molecular formular CnH2n.
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 | Nonenem |
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 are 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.
(i) CH3-CH2-CH2-CH=CH2 (ii) CH3CH2CH=CHCH
Pent-1ene pent-2-ene
CH3
|
(iii) CH3C=CHCH3 (iv) CH3-C=C-CH3
| |
CH3 CH3
3-methylbut-2-ene 2,3-dimethylbut-2-ene
CH3 CH3 CH3
(v) | | |
CH3CH2C=CCH2CH3 (vi) CH3C-CH=CH-C-CH3
| | |
CH3 CH3 CH3
3,4-dimethylhex-3-ene 2,2,5,5-tetramethylhex-3-ene
MOLECULAR STRUCTURES OF ALKENES
|
ALKENES |
STRUCTURAL FORMULAR |
MOLECULAR FORMULAR |
|
|
2. |
C2H4 Ethene |
∖ ∕ C=C ∕ ∖ H H |
H2C=CH2 |
|
3. |
C3H6 Propene |
H H H | | ∕ H — C — C=C | | ∖ H H H |
CH3CH=CH2 |
|
|
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, that is, removal of water molecules from alkanols such as ethanol (C2H5OH) by concentrated H2SO4 to form ethylhydrogentetraoxosulphate VI and water as products.
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 serves 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 almost insoluble in water
4. It is less dense than air.
CHEMICAL PROPERTIES OF ETHENE
Alkenes such as ethene undergoes addition reaction (a reaction in which one molecule of a compound is simply added on to the alkenes at the position of the carbon - carbon double bond (C=C) and this is converted to carbon – carbon single bond (C-C) that is, the alkanes. Examples of addition reaction are:
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 1,2-ethan-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.
| | |
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
|
(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
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
a. C2H2Cl2
b. C2H6Cl
c. C4H10
d. C5H12
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