Bài giảng hoá học hữu cơ alkenes

Chapter ALKENES 2010 Outlines Nomenclature Isomerism Preparation Physical properties Chemical reactions Conjugated dienes Alkenes in Nature  Alkenes or olefins are a homologous series of unsatutared hidrocarbons with the general formula CnH2n, that contain functional groups, C=C double bond  Alkenes occur abundantly in nature, and many have important biological roles Nomenclature  Common names (trivial name) used for simple alkenes: ethylene, propylene, isobutylene,  In the IUPAC system, the continuous-chain alkenes are named after their alkane parent, but with the -ane ending changed to -ene  CH2=CH2 is ethene for 2C;  CH3-CH2=CH2 is propene for 3C  A hydrocarbon with two double bonds is called a diene, while one with three double bond is called a triene, and so on  For higher alkenes the names are given according to a series of rules  Step Find the longest carbon chain that contains the double bond (the parent hydrocarbon) and name the compound using the suffix -ene (see next slide)  Step Begin numbering at the end nearer the double bond CH3CH2 C CH3CH2 H C CH3CH2CH2 Named as pentene H C H CH3CH2CH2 NOT C H as hexene, since the double bond is not contained in the six-carbon chain  If the double bond is equidistant from the two ends, begin numbering at the end nearer the first branch point  This rule ensures that the double bond carbons receive the lowest possible numbers: CH3 CH3CH2CH2CH = CHCH3 2-Hexene CH3CHCH = CHCH2CH3 2-Methyl-3-hexene  Step Number of substituents according to their position in the chain and list them alphabetically  Indicate the position of the double bond by giving the number of the first alkene carbon CH3CH2 C H C CH3CH2CH2 2-Ethyl-1-pentene H CH3 CH2 = C-CH = CH2 2-Methyl-1,3-butadiene Ankenyl  Monovalence radicals (=Groups) containing C=C double bonds are called ankenyl  Some ankenyl groups have CH2 = trivial names methylene methylenecyclohexane CH2 = CH- vinyl CH2 = CH-Cl vinyl chloride CH2 = CH-CH2- allyl CH2 = CH-CH2-Br allyl bromide Isomersm in the alkenes   All the alkenes with more carbon atoms show structural isomerism Butene, C4H8 has structural isomers:  But-2-ene (2-butene) also exhibits in two geometric isomers  The isomer with both CH3 same side is cis-but-2-ene groups on the  When oxidation of the alkene is carried out with KMnO4 in acidic solution (or potassium dichromate), cleavage of the double bond occurs and carbonylcontaining products are obtained  If the double bond is tetrasubstituted, the two carbonyl-containing products are ketones  If a hydrogen is present on the double bond, one of the carbonyl-containing products is a carboxylic acid  If two hydrogen are present on one carbon, CO2 is formed: CH3 C + CH3 KMnO4 H3O+ O CH3 3-Methyl-1-pentene + O Cyclohexanone Isopropylidenecyclohexane CH3CH2CHCH = CH2 CH3 + KMnO4 + H3O C Acetone CH3 CH3CH2CHCOOH 2-Methylbutanoic acid CH3 + CO2 Substitution reactions (S)  At elevated temperatures (500°C), higher alkenes give substitution products with Cl2 CH3-CH=CH2 + Cl2 ClCH2-CH=CH2+HCl  Branched-chain alkenes (isobutene) give substitution reaction easily (at room temp.) Polymerization  Addition polymerization is a process by which a large number of molecules of the same species (called monomer) join together (without the elimination of simple molecules like HX, H2O, etc.,) to form a giant molecule, called a polymer  Polyethene is formed at 100 0C under 1000 atm pressure: Mechanisms of polymerization  Polymerization reactions proceed via either ionic (cationic and anionic) or freeradical mechanisms  Cationic polymerization is less efficient than free-radical polymerization because of the caustic nature  Example of cation-initiated polymerization: The reaction of ethene with sulfuric acid  The reaction continues and gives CH3 - CH2 ( CH2 - CH2 )nCH2 - CH2+  which finally reacts with HSO4− to create the polymer: CH3 - CH2 ( CH2 - CH2 )nCH2 - CH2HSO4 Free-radical polymerization  The radical polymerization can be initiated by oxygen or free-radical compounds, such as peroxides:  The reaction continues and gives  The reaction may end by bonding of two free radicals: Alkadienes  The polyenes, having two double bonds, are called "dienes"  The relative arrangement of the double bonds dictates the characteristic reactions of the systems  Double bonds can be arranged in three possible ways from most to least stable as below: Conjugated dienes  There is an electronic interaction between the two double bonds of a conjugated diene CH2=CH-CH=CH2, because the adjacent "p" orbitals can all overlap with each other across the central single bond  Adjacent π systems makes the conjugated dienes the most stable type of diene Isolated dienes  The double bond units occur separately  The π systems, CH2=CH-CH2-CH=CH2 are isolated from each other by sp3 hybridised centers  Isolated dienes have the characteristic reactivity of simple alkenes Cumulated diens  The double bond units share a common hybridised C atom  The cumulated dienes have reactivity more like simple alkynes  Allene, CH2=C=CH2 sp Preparation  Double bonds are formed by elimination reactions of unsaturated alcohols and alkyl halides  The formation of the conjugated diene is usually favored over the isolated diene… Addition of Hydrogen Halides  Conjugated dienes undergo addition reactions in a similar manner to simple alkenes, but  Two modes of addition are possible which differ based on the relative positions of H and X in the products: Addition of Halogens to Dienes  Halogens add to dienes via direct and conjugate addition pathways:  The major 1,4-addition products are usually the more stable with the E configuration of C=C [...]... consequence substituted alkenes may exist as one of two geometric isomers The E, Z notation  When an alkene has 3 or 4 different substituents, the geometric isomerism is described using the labels E and Z  Alkenes with the higher priority groups (as determined by CIP rules) on the same side of the double bond are designated Z (it comes from the German words "zusammen" meaning "together")  Alkenes with the... yourself Physical properties  The physical state depends on molecular mass, the shape of the molecule (Van der Waals forces)  The simplest alkenes: ethene, propene and butene are gases  Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids  The alkene has a boiling point which is little lower than the corresponding alkane * Ethane * Propane * n-Butane... trans-isomers Use of Alkenes  Lower alkenes are used as fuel and illuminant  For the manufacture of a wide variety of polymers, e.g., polyethene, polyvinylchloride (PVC) and teflon etc  As a raw material for the manufacture of industrial Chemicals such as alcohols, aldehydes, etc Chemical reactions 1 Addition reaction Mechanism for the addition reaction  The addition reaction of alkenes involves electrophilic... C C H X + KOH CH3CH2CHClCH3 sec-Butyl chloride alcohol KOH/alc C C + KX + H2O CH3CH = CHCH3 + CH3CH2CH = CH2 2-Butene, 80% 1-Butene, 20% Saytzeff Rule  For unsymmetrical Halides, the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate Dehydration of alcohols  Dehydration meaning elimination of a molecule of water is generally carried out neither of two ways:... carbenium ion  It is possible for YΘ to approach the carbon atom from either side, to produce either cis- or trans-addition to the double bond 1.1 Addition electrophilic (AE) 1.1.1 Addition halogen  Alkenes react with halogens to form dihaloalkanes  The order of reactivity is, chlorine > bromine > iodine