PREPARATION & REACTIONS OF ALKENES

Alkenes- Preparation, Reactions.

1. ALKENES
NIDHI SHARMA

2. PREPARATIONOFALKENES-
1.FromAlkynes-
Alkynescanbereducedtoalkenesusingpalladiumcharcoal
(palladisedcharcoal)catalystpartiallydeactivatedwithpoisonlike
sulphurcompoundsorquioline.Partiallydeactivatedpalladised
charcoalisknownasLindlar’scatalyst.Alkynescanalsobe
reducedtoalkeneswithsodiuminliquidammoniawhichiscalled
Birchreduction.
ForEg-
Alkenesareunsaturatedhydrocarbonscontainingcarbon-carbondouble
bond(C═C)intheirmolecules.
TheyhavethegeneralformulaCnH
2n.Thesimplestmemberofalkene
familyisethene,C
2H
4.Thealkenesarealsocalledolefins(Greekolefiant
meaningoilforming)becausethelargermemberoftheseries(suchas
ethylene,propylene,etcreactwithchlorinetoformoilyproducts.
ALKENES

3. Pd-C, H
CH3−C≡C−CH3
But-2-yne
CH3CH═CHCH 3
But-2-ene
CH3–C≡CH+ H2
Propone
CH3–CH=CH2
Propene
Pd/C
CH≡CH+H2
Ethyne
CH2=CH2
Ethene
2. From Alkyl Halides-
Alkenecanbepreparedfromalkylhalides(usuallybromidesoriodides)bytreating
withalcoholicpotash(potassiumhydroxidedissolvedinethanol).Thisreaction
removesamoleculeofHXandtherefore,thereactioniscalled
dehydrohalogenation.Inthisreaction,thehydrogenatomiseliminatedfromβ-
carbonatom(carbonatomnexttothecarbontowhichhalogenisattached).
Thereactionisalsocalledβ–eliminationreaction-
Nature of halogen atom and the alkyl group determine rate of the reaction. It is
observed that for halogens, the rate is-
Iodine>Bromine>Chlorine
While for alkyl group it is-
Tertiary> Secondary>Primary

4. 3. From Vicinal Dihalides-
Dihalogenderivativesofalkanesinwhichtwohalogenatomsareattachedto
adjacentcarbonatoms(calledvicinaldihalogenderivatives)areconvertedto
alkenesbyheatingwithzincdustinethylalcohol.ForEg:
CH3CHBr−CH2Br+ Zn
1,2-Bromo Propane
CH3CH=CH2 +ZnBr
Propene
4. From Alcohol-
Alkenes are prepared from alcohols by heating with protonic acids such as sulphuric
acid at about 443K. This reaction is called dehydration of alcohols.
H2SO4 or H3PO4
CH3CH2OH
Ethanol
CH2=CH2+H2O
Ethene
Thisreactionisalsoanexampleofβ-eliminationreactionbecause–OHgroup
takesoutonehydrogenatomfromtheβ-carbonatom
GENERAL PROPERTIES OF ALKENES -
1. Physical state –The memberscontaining two or four carbonatoms are
gases, five to seventeen, liquids,eighteen onwards,solids at room
temperature and they burn in air with a luminous smoky flame.
2.Density –Alkenes are lighter than water.
3.Solubility –Alkenes are insoluble in water and soluble in organic
solvents such as benzene etc.
4.Boiling point –The boiling points of alkenes show a gradual increase
with an increase in the molecular mass or chain length, this indicates that
the

5. intermolecular attractions become stronger with the increase in the size of the
molecule.
REACTION
1. Effect of Addition of halogens-
2.AdditionofHBrtoSymmetricalalkanes-
AdditionreactionsofHBrtosymmetricalalkenestakeplacebyelectrophilic
additionreactionswheresimilargroupsareattachedtothedoublebond.
CH2=CH2+ H–Br ⎯⎯→ CH3–CH2 –Br
Ethene ethyl bromide
CH3 –CH=CH–CH3+H-Br ⎯→CH3 –CH2 –CH-CH3
| Br
1-bromo butane
butene
3.AdditionofHBrtoasymmetricalalkenes-
WhenHBrisaddedtopropenethenityields1-bromopropane and 2-
bromopropane.

6. USESOFALKENES(Olefins)-
Thelistofusesofvariousalkenelikeethene,propeneetc.isgiven
below.
1.Manufactureofplasticslikepolytheneformakingbuckets,
bowls,bagsetc.
2.Manufactureofpolystyreneusedinmakingcarbatterycasesand
partsoftherefrigerator.
3.Makingethane-1,2-diolusedasanti-freezingformotorcar
radiators.
4.Manufactureofethanolandsyntheticfibreterylene.
5.Makingananti-knocksubstance(suchastetraethyllead)addedto
petroltoinhibitpre-ignition)forcarengines.
6.Manufactureofplastic,polypropeneformakingropesand
packagingmaterial.
7.Manufactureofpropanolusedinmakingacetone.
8.Manufactureofacrylicfibres.

7. THANK YOU