Alkene Flashcards
alkene
The members of this clas3 of hydrocarbons contain two hydrogen atoms less than the corresponding alkanes. The two valency bonds left free on the adjacent carbon atoms unite to form a second union between them, establishing a, double bond in tho molecule.
unaturated hydrocarbon
Since fill the valencies of the carbon atoms in alkenes are not
fully satisfued, they can take up more H-atoms and are called Un-saturated hydro carbons.
oleifine
The hydrocarbons of this cla.ss are frequently called Olefines
(Olefiant=oil forming) as the first member of the class, ethylene reacted with chlorine to form an oily product.
alkane vs alkene
Alkenes being much more reactive than alkanes seldom occur free in nature. They are, however, produced in Iarge amount during the cracking of petroleum and the decompoition of almost all
organic substances. The cracking of cotton seed oil gIves as much
as 30 per cent yield of ethylene. The lower Itlkenes occur to a minor extent in coal gas.
methods of fomation
Most of the methods of preparation of alkenes depend upon the principle that the removal of atoms or groups from adjacent carbon atoms of a chain results in the creation of a double bond.
-by dehydration of alcohols
-by dehydrohalogenation of alkyl halides
-by dehalogenation of dihaloalkanes with zinc
-by partial hydrogenation of alkynes
-by cracking
-by electrolysis of salts of dicarboxylic acids
by dehydration of alcohols
When heated with a dehydrating agent like concentrated sulphuric acid or phosphoric
acid, alcohols lose a molecule of water forming alkenes.
-The alcohol first reacts with sulphuric acitl to form the alkyl hydrogen sulphate which upon heating splits out a moleculc of sulphiric acid to form alkene.
-alkenes can also be prepared by passing the vapour of alcohol over alumina (AI2O3) at 300. Tertiary alcohols are dehydrated with greatest ease, and primary with greatest difficulty.
by dehydrohalogenation of alkyl halides
-When heated with alcoholic solution of potassium hydroxide (alcoholic potash), alkyl halides eliminate a hydrogen amI halogen atom from adjacent carbon atoms to form alkene
-This is probably the most important method fur preparingvalkenes in the laboratory. In the preparation of alkenes from alkyl
halides, as a rule, iodides are most reactive and chlorides least so.
by dehalogenation of dihaloalkanes with zinc
-When dihalogen derivatives of alkenes having the two halogen atoms on adjacent carbons arc heated with an active metal like zinc, the
latter removes the halogen atoms to form alkenes.
-When the two halogen atoms are attached to the same carbon, zinc eliminates four atoms from two molecules of the dihaloalkene
by partial hydrogenation of alkynes
The alkyne mixed with hydrogen gas is passed over finely divided platinum or palladium, or heated nickel, when it adds only two H-atoms to give alkene
by cracking
-The higher alkanes when heated to a high
temperature decompuse to give a lower alkene along with a lower alkane
-Thc first three alkenes are thus obtained by the cracking og the petroleum hydrocarbons
by electlysis of salts of dicarboxyluc acids
-Alkenes are formed by the electrolysis of aqeous solutions of potassium salts of dibasic acids of the succinic acid series
-This reaction is, in fact, an extension of Kolbe’s method for, the preparation of alkanes and is seldom used in actual practice as the product obtained is not pure.
physical charcteristics of alkenes
(1) The first three alkenes - ethene, propene, and butenes- ~ are gases ordinary temperature; the next fourteen members are
liquids and the hydrocarbons with more than 18 carbon atoms in the molecule are solids. They are all colourless and odourless except ethene which has a rather pleasent odour
‘(2) they are a little less volatile than the corresponding alkanes. there boiling and melting points are skighly higher than
those of alkane containing the same number of carbon atoms .
(3) They ate only slightly soluble in water but dissolve freely in organic solvents .
.4) Thc lower alkenes are powerful general anaesthetics.
(5) Their boiling points, melting points, and specific gravities, in general, rise with increase of molecular weight in the homologous series. The physical constants of the first few straight-chain hydro-carbons are given in the table below:
chemical charcteristics
The molecule of an alkene (other than ethylene) consists of
two parts:
(a) an alkyl group, R ; and
(b) the alkene part or group containing the doubly bonded carbon atoms.
While the alkyl group is marl{eu by the usual stability of alkanes and can give substitution reactions with difficulty, the alkene part is unusually reactive on account of the presence of the double bond in it. According to the classical Baeyer’s Strain theory, the moleoule at the double bond is under strain as the two valency bonds of each of the two carbon atoms are displaced from their
normal directions.
