Alkanes Nomenclature ch04.ppt

Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis  Shapes of Alkanes  “Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation  Straight chain alkanes are also called unbra...

Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis  Shapes of Alkanes  “Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation  Straight chain alkanes are also called unbranched alkanes Chapter 4 2  Branched alkanes have at least one carbon which is attached to more than two other carbons Chapter 4 3  Constitutional isomers have different physical properties (melting point, boiling point, densities etc.)  Constitutional isomers have the same molecular formula but different connectivity of atoms Chapter 4 4  The number of constitutional isomers possible for a given molecular formula increases rapidly with the number of carbons Chapter 4 5  IUPAC Nomenclature of Alkanes, Alkyl Halides and Alcohols  Before the end of the 19th century compounds were named using nonsystematic nomenclature  These “common” or “trivial” names were often based on the source of the compound or a physical property  The International Union of Pure and Applied Chemistry (IUPAC) started devising a systematic approach to nomenclature in 1892  The fundamental principle in devising the system was that each different compound should have a unique unambiguous name  The basis for all IUPAC nomenclature is the set of rules used for naming alkanes Chapter 4 6  Nomenclature of Unbranched Alkanes Chapter 4 7  Nomenclature of Unbranched Alkyl groups  The 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 8  Nomenclature of Branched-Chain Alkanes (IUPAC)  Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name.  Number the longest chain beginning with the end of the chain nearer the substituent  Designate the location of the substituent  When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain  Substituents are listed alphabetically Chapter 4 9  When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc.  Commas are used to separate numbers from each other  The prefixes are used in alphabetical prioritization  When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents  When 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 10  Nomenclature of Branched Alkyl Chains  Two alkyl groups can be derived from propane  Four groups can be derived from the butane isomers Chapter 4 11  The neopentyl group is a common branched alkyl group  Examples Chapter 4 12  Classification of Hydrogen Atoms  Hydrogens take their classification from the carbon they are attached to Chapter 4 13  Nomenclature of Alkyl Halides  In IUPAC nomenclature halides are named as substituents on the parent chain  Halo and alkyl substituents are considered to be of equal ranking  In common nomenclature the simple haloalkanes are named as alkyl halides  Common nomenclature of simple alkyl halides is accepted by IUPAC and still used Chapter 4 14  IUPAC Substitutive Nomenclature  An IUPAC name may have up to 4 features: locants, prefixes, parent compound and suffixes  Numbering generally starts from the end of the chain which is closest to the group named in the suffix  IUPAC Nomenclature of Alcohols  Select the longest chain containing the hydroxyl and change the suffix name of the corresponding parent alkane from -ane to -ol  Number the parent to give the hydroxyl the lowest possible number  The other substituents take their locations accordingly Chapter 4 15  Examples  Common Names of simple alcohols are still often used and are approved by IUPAC Chapter 4 16  Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature Chapter 4 17  Nomenclature of Cycloalkanes  The prefix cyclo- is added to the name of the alkane with the same number of carbons  When one substituent is present it is assumed to be at position one and is not numbered  When two alkyl substituents are present the one with alphabetical priority is given position 1  Numbering continues to give the other substituent the lowest number  Hydroxyl has higher priority than alkyl and is given position 1  If a long chain is attached to a ring with fewer carbons, the cycloalkane is considered the substituent Chapter 4 18 Chapter 4 19  Bicyclic compounds  Bicyloalkanes contain 2 fused or bridged rings  The alkane with the same number of total carbons is used as the parent and the prefix bicyclo- is used  The number of carbons in each bridge is included in the middle of the name in square brackets Chapter 4 20  Nomenclature of Alkenes and Cycloalkenes  Alkenes are named by finding the longest chain containing the double bond and changing the name of the corresponding parent alkane from -ane to -ene  The compound is numbered to give one of the alkene carbons the lowest number  The double bond of a cylcoalkene must be in position 1 and 2 Chapter 4 21  Compounds with double bonds and alcohol hydroxyl groups are called alkenols  The hydroxyl is the group with higher priority and must be given the lowest possible number  Two groups which contain double bonds are the vinyl and the allyl groups Chapter 4 22  If two identical groups occur on the same side of the double bond the compound is cis  If they are on opposite sides the compound is trans  Several alkenes have common names which are recognized by IUPAC Chapter 4 23  Physical Properties of Alkanes and Cycloalkanes  Boiling points of unbranched alkanes increase smoothly with number of carbons  Melting points increase in an alternating pattern according to whether the number of carbon atoms in the chain is even or odd Chapter 4 24  Sigma Bonds and Bond Rotation  Ethane has relatively free rotation around the carbon-carbon bond  The staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possible  The drawing to the right is called a Newman projection  The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other Chapter 4 25  The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kJ mol-1 Chapter 4 26  Conformational Analysis of Butane  Rotation around C2-C3 of butane gives six important conformations  The gauche conformation is less stable than the anti conformation by 3.8 kJ mol -1 because of repulsive van der Waals forces between the two methyls Chapter 4 27  The Relative Stabilities of Cycloalkanes: Ring Strain  Heats of combustion per CH2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain Chapter 4 28  The Origin of Ring Strain in Cyclopropane and Cyclobutane : Angle Strain and Tortional Strain  Angle strain is caused by bond angles different from 109.5 o  Tortional strain is caused by eclipsing C-H bonds on adjacent carbons  Cyclopropane has both high angle and tortional strain  Cyclobutane has considerable angle strain  It bends to relieve some tortional strain  Cyclopentane has little angle strain in the planar form but bends to relieve some tortional strain Chapter 4 29  Conformations of Cyclohexane  The chair conformation has no ring strain  All bond angles are 109.5 o and all C-H bonds are perfectly staggered Chapter 4 30  The boat conformation is less stable because of flagpole interactions and tortional strain along the bottom of the boat  The twist conformation is intermediate in stability between the boat and the chair conformation Chapter 4 31  Substituted Cyclohexanes: Axial and Equatorial Hydrogen Atoms  Axial hydrogens are perpendicular to the average plane of the ring  Equatorial hydrogens lie around the perimeter of the ring  The C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines  The axial hydrogens are drawn straight up and down Chapter 4 32  Methyl cyclohexane is more stable with the methyl equatorial  An axial methyl has an unfavorable 1,3-diaxial interaction with axial C-H bonds 2 carbons away  A 1,3-diaxial interaction is the equivalent of 2 gauche butane interactions Chapter 4 33  Disubstitued Cycloalkanes  Can exist as pairs of cis-trans stereoisomers  Cis: groups on same side of ring  Trans: groups on opposite side of ring Chapter 4 34  Trans-1,4-dimethylcylohexane prefers a trans- diequatorial conformation Chapter 4 35  Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation  A very large tert-butyl group is required to be in the more stable equatorial position Chapter 4 36  Bicyclic and Polycyclic Alkanes  The bicyclic decalin system exists in non-interconvertible cis and trans forms Chapter 4 37  Synthesis of Alkanes and Cycloalkanes  Hydrogenation of Alkenes and Alkynes Chapter 4 38  Reduction of Alkyl Halides Chapter 4 39  Alkylation of Terminal Alkynes  Alkynes can be subsequently hydrogenated to alkanes Chapter 4 40  Retrosynthetic Analysis-Planning Organic Synthesis  The synthetic scheme is formulated working backward from the target molecule to a simple starting material  Often several schemes are possible Chapter 4 41