Alkena disebut juga olefin Unsaturated hidrocarbon Double bond Pembentuk minyak Rumus: C n H 2n
Sifat BM alkena < BM alkana Titik didih alkena < alkana Berat jenis alkena < air Tidak larut dalam air & larut dalam pelarut organik Alkena lebih reaktif daripada alkana Produksi alkena menghasilkan alkana & alkohol
Nomenclature (1) 1. Select the longest carbon-carbon chain that cantains the double bond. 2. Name the parent compound as you would an alkene but change the –ane ending to –ene; for example, propane is change to propene. CH 3 CH 2 CH 3 CH 3 CH=CH 2 Propane Propene
Nomenclature (2) 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name; for example, 2-butene means that the carbon-carbon double bond is between carbon numbers 2 and 3.
Nomenclature (3) 4. Side chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded. College Chemistry_ch.22.2_p.551
Practice 1 : Write structural formulas of : 1. 2-pentene 2. 7-methyl-2-octene 3. 3-hexene 4. 4-ethyl-2-heptene 5. 3,4-dimethyl-2-pentene
Geometric Isomerism in Alkenes (1) Geometric isomer = isomer that differ from each other only in the geometry of the molecules and not in the order of their atoms.
Geometric Isomerism in Alkenes (2) Geometric isomers are also called cis-trans isomers. If the two chlorine atoms are locked on opposite sides of the double bond, it is known as the trans isomer. (trans : from latin meaning "across" - as in transatlantic). If the two chlorine atoms are locked on the same side of the double bond, it is known as the cis isomer. (cis : from latin meaning "on this side")
Practise 2 Draw structural formulas and names for all the isomers of pentene, C 5 H 10. Identify all geometric isomers. Identify which one that have cis-trans isomers, and draw the cis-trans isomers.
Cycloalkenes A cycloalkene or cycloolefin is a type of alkene hydrocarbon which contains a closed ring of carbon atoms, but has no aromatic character. alkenehydrocarboncarbonatoms aromatic cyclopropene cyclobutene cyclopentene cycloheptene
Preparation of Alkenes Cracking/Pyrolisis Adalah pemanasan hidrokarbon jenuh pada suhu yang sangat tinggi dan dengan bantuan katalis silika-alumina Contoh: C 16 H 34 C 8 H 18 + C 8 H 16 alkane alkane alkene
Dehydration of Alcohols Alkohol dipanaskan pada asam sulfat terkonsentrasi Reaksi: Alkohol con.H2SO4 alkena + H 2 O Contoh:
C – C – C – C +H2SO4 OH C – C = C – C +H2O C – C – C = C +H2O
Alkenes Physical Properties The first three alkenes are gases, the intermediate alkenes are liquids and higher members of the olefin series are wax like solids at room temperature. The alkenes are insoluble in water, but are soluble in organic solvents. The liquids and solids have a density less than water. Compound Formula MP BP Density o C o C (g/ml) Ethylene C 2 H 4 -170 -102 0.6128 Propene C 3 H 6 -185 -47 0.6142 Butene C 4 H 8 -130 -6.5 0.6356
Chemical Properties of Alkenes (1) Combustion of Alkenes The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. C 2 H 4 + 3 O 2 2 CO 2 + 2 H 2 O
Chemical Properties of Alkenes (2) Addition: Addition of Hydrogen (hidrogenation) Addition of Halogen (halogenation) Addition of Hydrogen Halide Addition of Water
Chemical Properties of Alkenes (3) Addition Reactions across the Double Bond (oxidation) 3 H2C=CH 2 + 2 KMnO 4 + 4 H 2 O 2MnO 2 + 2KOH + CH 2 OHCH 2 OH Ethylene Glycol During the oxidation of alkenes, the purple colour of the permanganate solution disappears and the reaction constitutes a test, known as Baeyer's Test, to detect unsaturation in any compound.
Chemical Properties of Alkenes (4) Reaction with Sulphuric Acid Similarly, fuming sulphuric acid absorbs ethylene at room temperature to form ethyl hydrogen sulphate, with much evolution of heat. C 2 H 4 + H 2 SO 4 C 2 H 5.HSO 4 If this is treated with water and warmed, ethanol is formed. heat C 2 H 5.HSO 4 + H 2 O C 2 H 5 OH + H 2 SO 4
Chemical Properties of Alkenes (5) Polymerisation Reactions due to the Double Bond When ethylene is heated under great pressure in the presence of a catalyst a large number of the molecules combine to form polythene, (C2H4)n, (i.e. Polyethylene). This particular kind of reaction is called an addition polymerisation and the mechanism by which it takes place is a reaction is a free radical chain reaction. The overall reaction is n(C 2 H 4 ) (C 2 H 4 )n Ethene Polythene
Sifat: Sifat hampir sama dengan alkena Berupa gas tidak berwarna Tidak larut dalam air & gas pada tekanan & suhu normal Sangat mudah terdekomposisi Sebagai asetilen cair, bersifat sangat sensitif dan dapat meledak.
Nomenclature The procedure for naming alkynes is the same as that for alkenes, but the ending used is –yne to indicate the presence of a triple bond.
The name of some common alkynes : _________________________________________________ MolecularStructuralCommon IUPAC Formulaformulaname name _________________________________________________ C 2 H 2 H-C≡C-HAcetylene Ethyne C 3 H 4 CH 3 -C≡C-HMethylacetylene Propyne C 4 H 6 CH 3 CH 2 -C≡C-HEthylacetylene 1-Butyne C 4 H 6 CH 3 -C≡C-CH 3 Dimethylacetylene2-Butyne
Prepraration From calcium carbide and water CaC 2 + 2 H 2 O HC ≡ CH + Ca(OH) 2 From cracking mathane 2 CH 4 HC ≡ CH + 3 H 2
Chemical Properties Hidrogenation Halogenation Hidrogen halida