Chapter 4 Alkanes Nomenclature Conformational Analysis and Reactions

Chapter 4 Alkanes: Nomenclature, Conformational Analysis and Reactions

t Shapes of Alkanes è“Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation H Straight chain alkanes are also called unbranched alkanes Chapter 4 2

èBranched alkanes have at least one carbon which is attached to more than two other carbons Chapter 4 3

èConstitutional isomers have different physical properties (melting point, boiling point, densities etc. ) H Constitutional isomers have the same molecular formula but different connectivity of atoms Chapter 4 4

t IUPAC Nomenclature of Alkanes, Alkyl Halides and Alcohols èBefore the end of the 19 th century compounds were named using nonsystematic nomenclature èThese “common” or “trivial” names were often based on the source of the compound or a physical property èThe International Union of Pure and Applied Chemistry (IUPAC) started devising a systematic approach to nomenclature in 1892 èThe fundamental principle in devising the system was that each different compound should have a unique unambiguous name èThe basis for all IUPAC nomenclature is the set of rules used for naming alkanes Chapter 4 5

l Nomenclature of Unbranched Alkanes Chapter 4 6

l Nomenclature of Unbranched Alkyl groups èThe unbranched alkyl groups are obtained by removing one hydrogen from the alkane and named by replacing the -ane of the corresponding alkane with -yl Chapter 4 7

l Nomenclature of Branched-Chain Alkanes (IUPAC) èLocate the longest continuous chain of carbons; this is the parent chain and determines the parent name. èNumber the longest chain beginning with the end of the chain nearer the substituent èDesignate the location of the substituent èWhen two or more substituents are present, give each substituent a number corresponding to its location on the longest chain H Substituents are listed alphabetically Chapter 4 8

èWhen two or more substituents are identical, use the prefixes di-, tri-, tetra- etc. Commas are used to separate numbers from each other H The prefixes are used in alphabetical prioritization H èWhen two chains of equal length compete to be parent, choose the chain with the greatest number of substituents èWhen branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference Chapter 4 9

l Nomenclature of Branched Alkyl Chains èTwo alkyl groups can be derived from propane èFour groups can be derived from the butane isomers Chapter 4 10

èThe neopentyl group is a common branched alkyl group èExamples Chapter 4 11

t Classification of Hydrogen Atoms èHydrogens take their classification from the carbon they are attached to Chapter 4 12

t Nomenclature of Cycloalkanes l The prefix cyclo- is added to the name of the alkane with the same number of carbons èWhen one substituent is present it is assumed to be at position one and is not numbered èWhen two alkyl substituents are present the one with alphabetical priority is given position 1 èNumbering continues to give the other substituent the lowest number èHydroxyl has higher priority than alkyl and is given position 1 èIf a long chain is attached to a ring with fewer carbons, the cycloalkane is considered the substituent Chapter 4 13

Chapter 4 14

t Bicyclic compounds èBicyloalkanes contain 2 fused or bridged rings èThe alkane with the same number of total carbons is used as the parent and the prefix bicyclo- is used èThe number of carbons in each bridge is included in the middle of the name in square brackets Chapter 4 15

t Synthesis of Alkanes and Cycloalkanes l Hydrogenation of Alkenes and Alkynes Chapter 4 16

l Reduction of Alkyl Halides Chapter 4 17

Lithum Dialkylcuprates : The Corey –Posner , Whitesides-House synthesis. the overall synthesis provides , for example coupling the alkyl groups of two alkyl halides to produce an alkane : R-X + R’-X R-R' In order to accomplish this coupling , we must transform one alkyl halide into a lithium dialkylcuprate (R 2 Cu. Li). This transformation requires two steps. 1 - R-X + 2 Li 2 - 2 RLi + Cu. I Alkyllithium RLi Li. X Alkyllithium R 2 Cu. Li + Li. I Lithiumdialkylcuprate 3 - R 2 Cu. Li + R' –X Lithiumdialkylcuprate Alkylhalide R R’ + RCu + Li. X Alkane Chapter 4 18

t The Reactions of Alkanes with Halogens èAlkanes undergo substitution reactions with halogens such as fluorine, bromine and chlorine in the presence of heat or light l Multiple Substitution Reactions versus Selectivity èRadical halogenation can yield a mixture of halogenated compounds because all hydrogen atoms in an alkane are capable of substitution H In the reaction above all degrees of methane halogenation will be seen èMonosubstitution can be achieved by using a large excess of the alkane H A large excess of methane will lead to predominantly monohalogenated product and excess unreacted methane Chapter 10 19

t Chlorination of Methane: Mechanism of Reaction èThe reaction mechanism has three distinct aspects: Chain initiation, chain propagation and chain termination èChain initiation Chlorine radicals form when the reaction is subjected to heat or light H Chlorine radicals are used in the chain propagation steps below H èChain propagation A chlorine radical reacts with a molecule of methane to generate a methyl radical H A methyl radical reacts with a molecule of chlorine to yield chloromethane and regenerate chlorine radical H A chlorine radical reacts with another methane molecule, continuing the chain reaction H A single chlorine radical can lead to thousands of chain propagation cycles H Chapter 10 20

èThe entire mechanism is shown below Chapter 10 21

èChain reaction: a stepwise mechanism in which each step generates the reactive intermediate that causes the next cycle of the reaction to occur èChain termination Occasionally the reactive radical intermediates are quenched by reaction pathways that do not generate new radicals H The reaction of chlorine with methane requires constant irradiation to replace radicals quenched in chain-terminating steps H Chapter 10 22

t Halogenation of Higher Alkanes èMonochlorination of alkanes proceeds to give some selectivity Teritiary hydrogens are somewhat more reactive than secondary hydrogens which are more reactive than primary hydrogens H Eact for abstraction of a tertiary hydrogen is lower because of increased stability of the intermediate tertiary radical H The differences in rate of abstraction are not large and chlorination occurs so rapidly it cannot distinguish well between classes of hydrogen and so is not very selective H Chapter 10 23

l Selectivity of Bromine èBromine is much less reactive but more selective than chlorine in radical halogenation H Fluorine shows almost no discrimination in replacement of hydrogens because it is so reactive Chapter 10 24

Oxidation : Combuston to carbon dioxide and water is characteristic of organic compound under special condition : CH 4 + 2 O 2 Prylosis : CH 3 -CH 2 -CH 3 550 CO 2 +2 H 2 O + Energy • • • CH 2 CH CH 3 +2 H • • CH 3+CH 3 CH 2 Chapter 4 -H • CH 2 =CH CH 3 +H 2 CH 2 = CH 2 +CH 4 25

t Nomenclature of Alkenes and Cycloalkenes èAlkenes are named by finding the longest chain containing the double bond and changing the name of the corresponding parent alkane from -ane to -ene èThe compound is numbered to give one of the alkene carbons the lowest number èThe double bond of a cylcoalkene must be in position 1 and 2 Chapter 4 26

èCompounds with double bonds and alcohol hydroxyl groups are called alkenols H The hydroxyl is the group with higher priority and must be given the lowest possible number èTwo groups which contain double bonds are the vinyl and the allyl groups Chapter 4 27

èIf two identical groups occur on the same side of the double bond the compound is cis èIf they are on opposite sides the compound is trans èSeveral alkenes have common names which are recognized by IUPAC Chapter 4 28

t Sigma Bonds and Bond Rotation èEthane has relatively free rotation around the carbon-carbon bond èThe staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possible H The drawing to the right is called a Newman projection èThe eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other Chapter 4 29

èThe potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 k. J mol-1 Chapter 4 30

t Conformational Analysis of Butane èRotation around C 2 -C 3 of butane gives six important conformations H The gauche conformation is less stable than the anti conformation by 3. 8 k. J mol -1 because of repulsive van der Waals forces between the two methyls Chapter 4 31