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Alkanes= saturated hydrocarbons Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis Simplest alkane = methane CH 4 We can build additional alkanes by adding - CH 2 – units C n H 2n+2 = saturated = no double bonds, no rings Shapes of Alkanes There is no limit on length of chain “Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation Polyethylene = an “infinite” alkane Branched alkanes At least one carbon attached to more than two others Many constitutional isomers are possible Same molecular formula but different connectivity of atoms Constitutional isomers have different physical properties (melting point, boiling point, densities etc.) 5 “hexanes” C6H 14 1 Source of alkanes = petroleum The number of constitutional isomers possible for a given molecular formula rise rapidly with the number of carbons A complex mixture of organic compounds, largely alkanes US uses 17 M barrels/day, 75% for energy, 8% chemicals Refineries not only separate the compounds, but “reform” the molecules by catalytic cracking and rearrangements smaller alkanes Crude Petroleum Refining straightchain alkanes of different sizes Cracking Isomerization Reforming branched alkanes Hyd rocarbon Fraction s from Petro le um boi li ng range of fraction ( o C) s iz e range A reaction of alkanes - Combustion nam e and use All hydrocarbons are combustible C 6 to C7 n atural gas, bottl e d gas, p etroche m icals p e trole u m "ethe r". s olve nts l igroi n, sol ven ts Heat of combustion is very high be low 20 C 1 to C 4 20 to 60 C5 to C6 60 to 100 aromatics 40 to 200 C5 to C10 s traigh t-ru n gasol in e 175 to 325 C 12 to C 18 k e ros en e and je t fu el 250 to 400 C 12 and h igh e r gas oil , fu el oil and d ie se l oi l non volati le liq ui ds C20 an d h igh er non volati le s oli ds C 20 and hig he r min e ral o il, lu bri catin g oil paraffi n wax, asph al t, tar Octane boosters (anti -knock additives) Combine with oxygen and release energy 2C8 H18 + 25O2 16CO2( g) + 18H2O( l) ∆Hcomb = -5452 kJ/mol (or -47.8 kJ/g) Examples of “reforming” to raise octane Smoother engine performance if combustion is slowed isooctane = 100 (2,2,5 -trimethylpentane) (vs heptane = 0) For decades, used Tetraethyl lead, now banned For past decade, MTBE was used, now also banned Some Octane Ra ting s of Hydrocarbons and Addit iv es Octa ne Rating hepta ne 0 1-pentene 91 2,2,4-t rimethylpentane 10 0 benzene 10 6 met ha no l 1 07 ethanol 108 met hy l t-butyl ether 116 toluene 1 18 silica-alumina CH3CH2CH2CH2CH2CH2CH3 catalyst, 500oC heptane 20 atm H2 CH3 CH2 CH 2CH2CH2CH3 hexane a cid catalyst CH3 + 4H2 CH3 CH2CH2CHCH3 CH3 + CH3CH2 CHCH2 CH3 CH3 branched a lkanes 2 lNomenclature of Unbranched Alkanes IUPAC Nomenclature t Early chemicals were given “common” or “trivial” names based on the source of the compound or a physical property t The International Union of Pure and Applied Chemistry (IUPAC) started devising a systematic approach to nomenclature in 1892 t The fundamental principle in devising the system was that each different compound should have a unique unambiguous name t The basis for all IUPAC nomenclature is the set of rules used for naming alkanes Nomenclature of Unbranched Alkyl groups Nomenclature of Unbranched Alkanes – Learn the first ten! 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 Nomenclature of Branched-Chain Alkanes (IUPAC) Nomenclature of Branched -Chain Alkanes (IUPAC) – cont. 1. Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name. 4. 7 carbons = a heptane When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain ¶ Substituents are listed alphabetically 2. Number the longest chain beginning with the end of the chain nearer the substituent 3. Designate the location of the substituent 4-ethyl -3-methylheptane 3 Nomenclature of Branched -Chain Alkanes (IUPAC) – cont. 5. 6. When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc. a) Commas are used to separate numbers from each other b) Repeat the number if two substituents on same carbom c) The prefixes are used in alphabetical prioritization 7. 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 4-ethyl -2,3,3 -trimethylheptane Nomenclature of Branched Alkyl Chains How many butyl groups are possible? Two alkyl groups can be derived from propane 4-isopropylheptane The neopentyl group is a common branched alkyl group Classification of Hydrogen Atoms Hydrogens take their classification from the carbon they are attached to 4 Nomenclature of Alkyl Halides What type of hydrogens in neopentane? t In IUPAC nomenclature halides are named as substituents on the parent chain l Halo and alkyl substituents are considered to be of equal ranking CH3 CH3-C-CH3 CH3 t In common nomenclature the simple haloalkanes are named as alkyl halides l Common nomenclature of simple alkyl halides is accepted by IUPAC and still used Nomenclature of Alcohols In the IUPAC naming system, there may be as many as four components to the name: IUPAC Nomenclature of Alcohols Locant indicates the position of a substituent. Prefix names the substituent group. 1. Select the longest chain containing the hydroxyl and change the suffix ending of the parent alkane from -e to -ol Pare nt is the pa rent a lka ne. 2. Number the parent to give the hydroxyl the lowest possible Suffix names a key function. number Examples 3. The other substituents take their locations accordingly OH CH3 CH2CHCH 3 CH3 CHCH2 CH 2OH CH3 parent 2-butanol su ffix locant parent 3-methyl-1 -buta no l locant prefix locantsuffix l Examples 4-methyl-1-hexanol t Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature l Common Names of simple alcohols are still often used and are approved by IUPAC 5 Nomenclature of cycloalkanes Nomenclature of Substituted Cycloalkanes t When one substituent is present it is assumed to be at position one and is not numbered t Ring compounds are very common t Named as cyclo + base name for number of C in ring t When two alkyl substituents are present the one with alphabetical priority is given position 1 l Numbering continues to give the other substituent the lowest number l Hydroxyl has higher priority than alkyl = position 1 cyclobutane cyclopentane cyclooctane Cycloalkyl group t If other parts of the molecule are dominent or if a long chain is attached to a ring with fewer carbons, the cycloalkane is considered the substituent Bicyclic Compounds – the Playground of Organic Chemists l Bicyloalkanes contain 2 fused or bridged rings l The alkane with the same number of total carbons is used as the parent and the prefix bicyclo- is used Bicyclic Compounds – the Playground of Organic Chemists Beginning at bridgehead position, number around largest ring first, then second largest ring l The number of carbons in each bridge is included in the middle of the name in square brackets 6 Nomenclature of Alkenes and Cycloalkenes t Alkenes are named by finding the longest chain containing the double bond and changing the name of the parent alkane from -ane to -ene t The compound is numbered to give one of the alkene carbons the lowest number Double bonds plus alcohol hydroxyl groups = alkenols The hydroxyl is the group with higher priority and must be given the lowest possible number t The double bond of a cylcoalkene must be in position 1 and 2 Note that positions of both the C=C and the OH must be indicated Vinyl and allyl groups The vinyl and allyl groups are common names that need to be readily recognized: Alkene nomenclature – cis and trans l If two identical groups occur on the same side of the double bond the compound is cis l If they are on opposite sides the compound is trans l Several alkenes have common names which are recognized by IUPAC Alkynes = named similarly to alkenes (4) Po sitions of substituents are determined by the usual rules . (1) The name of the pa rent alkane is modified by dropping the "ane" ending and adding "yne." (2) The parent chain is numbered to give the carbons of the alkyne lower numbers . (3) The lo cation of the triple bond is g iven by the lower positional number of the a lkynyl carbons. 5 4 3 2 CH 3CH2CH2 C 1-pen tyn e 1 CH CH 3 C CCH 3 2-butyne 4 3 C H3 C 5 4 3 2 CH3CH2CHC CH 3 2 1 CCH 2C l 1-chloro -2 -butyn e 1 CH 3-methyl-1-pentyne (5) In an alkynol, the a lcohol has priority in numbering. 4 HC 32 1 CCH2 CH2 OH 3-butyn-1-o l 1-alkynes are also called terminal alkynes Terminal alkynes have an acidic H R C CH pKa = 25 7 Physical Properties of Alkanes Th e u nb ranch ed alk an es (CH4, CH3CH3, CH 3CH2CH3, CH3CH2CH2CH 3, etc.) form a regul ar s eri es where each mem ber di ffers from th e n ext in order by CH2-. S uch a regu lar series is called a h omo logou s s eri es . Melting points reflect size, but also packing factors Propane: -188 oC mp (oC) The b oili ng poin ts of the u nbranche d alkane s incre ase more or le ss regu larl y w i th in creasing si ze re flec tin g the i ncreasin g van de r Waals attrac tion s . pen tan e bp (oC) 36 h exan e hep tan e 69 98 Ethane: -183 oC pentane -130 hexane -95 ∆ = 35 heptane -91 ∆= 4 Methane: -182o C octane -57 ∆ = 34 o ctan e 126 Sigma Bonds and Bond Rotation Other physical properties of hydrocarbons t Ethane has relatively free rotation around the carbon-carbon bond t The staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possible l The drawing to the right is called a Newman projection Less dense than water Immiscible with water and other highly polar solvents Solvent for non -polar organic compounds t The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other How to draw a Newman projection The potential energy diagram of the conformations of ethane The staggered conformation is more stable than eclipsed by 12 kJ mol -1 Rotation rate 1011 /sec Sight down the C-C bond (sigma bond is symmetric) Energy barrier called torsional strain Groups on front carbon intersect in center Groups on back carbon end at circle 8 Bond Rotations in Propane The co nformational features are the same for the two C-C bonds. 2 equivalent C-C bonds Analysis of the Extreme Conf ormations H H H C During o ne complete rotation around the internal bond in butane, the three staggered conformations include two called "gauche" and one called "a nti. " H C H C H H CH3 H H C H Consider as ethane with 1 H replaced by a CH3 H H 120o H H C C rotation CH 3 CH3 H o H H 120 CH3 C C H CH3 rotation H H C CH3 The Newman proj ection formula s as viewed fro m the left are: CH 3 CH 3 H rotation H H HH H H H H CH 3 Barrier = 14 kJ/mol H eclipsed H C H 120 o H H H H r otation H CH 3 rotation C CH3 H C rotation CH 3 CH3 H H 12 0o H H H H rotation H CH3 ro tation CH 3 anti gauche gauche H CH 3 steric strain H CH 3 ga uche 3 minima and energy barriers between each (3 maxima) H H C CH3 H H CH 3 CH3 anti H Conformational Analysis of Butane H H 120o H C CH3 C H CH3 rotation H 120o CH3 120 o The stag gered anti is more stable than the two equivalent stagg ered gauche conforma tions. In the a nti conformation, the t wo CH 3 groups are on opposite sides of the structure. In t he ga uche co nformations, the two groups are within van der W aals repulsive intera ction distance, and 3.8 kJ/mo l of steric strain energy is introduced. Conformational Analysis of Butane Rotation around C2 -C3 of butane gives three energy minima 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 H H H H staggered Conclusion: eclipsing of methyl with H adds very little to energy barrier CH3 H H 120o H H CH 3 H CH3 gauche The Origin of Ring Strain in Cyclopropane : Relative Stabilities of Cycloalkanes: Ring Strain Angle Strain and Torsional Strain t Heats of combustion per CH2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain t Angle strain is caused by bond angles different from 109.5o t Torsional strain is caused by eclipsing C-H bonds on adjacent C’s l l C-C angle of 60 means orbital overlap is reduced Cyclopropane has both high angle and torsional strain H H C H C H C H H Normal bond length sp 3 -sp3 C-C = 1.54 Å Normal sp3 C-H =1.10 Å sp2 C-H=1.09 Å 9 Cyclobutane: Angle Strain and Torsional Strain t Cyclobutane has considerable angle strain t It bends to relieve some torsional strain Cyclopentane Th e i nternal an gles of a regu lar p en tagon are 108o, close to the idealized tetrahedral b ond an gles . Thu s, a plan ar cyclop en tan e would have very littl e an gle strain. 108o But a planar geometry would have very severe torsional strain (10 eclips ed H). Cons equently, the geometry of cyclopentane is bent. H The torsional strain is reduced in t he bent structure. Four H are st ill eclipsed, but 6 are st aggered. H H H H H H H H H bent or puckered geomet ry Cyclohexane: the perfect ring Two types of H in chair cyclohexane t Not 120o, the angle of simple hexagon t Adopts a chair conformation with no ring strain! t All bond angles are 109.5o and all C-H bonds are perfectly staggered H 6 H are axial (up and down) 6 H are equatorial (close to plane of ring) H H Rotation about single bonds flips the ring to a boat form t The boat conformation is less stable because of flagpole interactions and torsional strain along the bottom of the boat 2 1 H H H H H Newman pro jections along the C1-C6 a nd C3-C4 bonds show the different orientations of the axial and equatorial H. H 6 H 1 H 5 H 3 2 CH2 CH2 5 4 H H H H H H 3 H eq axial H Ring flipping leads to a second chair Proceeds via boat (or slightly twisted boat) Every axial position becomes equatorial, and vice versa chair 1 boat chair 2 Ring flipping occurs rapidly at room temperature 10 Conformational Energy Diagram for cyclohexane Another view of energy barrier half-chair half-chair 42 kJ/mol Flips about 100 x per sec boat 99.9% of molecules in chair form en ergetically preferred conformation is the tw is t boat 0 chair chair Note: kcal/mol x 4.2 = kJ/mol Energy and Equilibrium Drawing Cyclohexanes t The C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines èThe axial hydrogens are drawn straight up and down Note that 5 kJ/mol produces nearly a 90:10 preference To draw chair cyclohexanes, follow these steps: 1.) Draw the carbon chair. Cyclohexane structure can be elaborated 2.) Add the axial hydrogens. Adamantane C10 H16 3.) Draw the C1 and C4 equatorial hydrogens. 4.) Draw the remaining equatorial hydrogens. Diadamantane C14 H18 11 Substituted cyclohexanes Diamond = “infinite” cyclohexane rings Methyl cyclohexane is more stable with the methyl equatorial l An axial methyl has an unfavorable 1,3-diaxial interaction with axial C-H bonds 2 carbons away More stable by 7.6 kJ/mol 95% eq at R.T. Destabilization of axial groups due to steric interactions Equatorial groups are free of torsional strain In axial methylcyclohexane there are two gauche interactions H H H 4 H 5 H CH 3 H H 6 H 3 H 2 H view a long t he C 1-C 2 bo nd gauche CH3 2 1 CH 2 6 1 H H CH2 H H 3 H 5 H 4 view along the C1-C2 bond H 3 H H H H 6 1H H 2 CH 3 H eq uatorial-meth ylcyclohexane 3 CH2 H 2 6 CH2 1 CH3 H anti H H axial-methylcyclohe xane Each gauche interaction introduces 3.75 kJ/mol of steric strain view along the C 3 -C 2 bond CH g auche 3 H 2 3 H CH3 4 CH2 anti H CH 2 H H 2 1 CH2 3 4 CH2 H 1 Disubstituted cycloalkanes The larger the group, the greater the equatorial preference t-butylcyclohexane is >99.9% equatorial view al on g the C 3-C 2 b on d Equivalent to a staggered butane With two substituents on the ring in different positions, configurational isomers arise Cis: groups on same side of ring Trans: groups on opposite side of ring e.g.: 1,2 -dichlorocyclobutane trans isomer cis isomer Cl H Cl H Cl Cl Cl H H Cl Cl Cl 12 For 3, 4, or 5 membered rings, can treat as if planar Three options for pairs of cis-trans stereoisomers 1,3-dimethylcyclopentanes H H 2 1 Configurational Isomers in Disubstituted Cyclohexanes H C H3 3 C H3 CH 3 H cis CH3 tr ans Preferred comformations Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation Trans-1,4-dimethylcylohexane prefers trans-diequatorial conformation Note that both are on the same side of ring upper side in this projection Note methyls on opposite sides of ring Confirming whether cis or trans Is diequatorial always trans? Test one: identify each substituent as on the upper or lower side of ring Di-equatorial isomer CH3 H u pp er b on d CH3 low er b on d up p er b on d Since the two bo nds to the CH3 g ro ups are "upper", meaning sa me side, they a re "cis." low er b on d H H Test Two: Flatten the Ring H 1,3 CH3 H CH3 trans 1,4 CH3 CH3 H H CH3 CH3 H 1,2 H cis Since the CH3 groups are on the sa me side, they a re "cis." CH3 CH3 trans H SQUEEZE IT 13 Trans 1,2 dimethylcyclohexane: an energy analysis CH3 H 2 The bicyclic decalin system exists in non-interconvertible cis and trans forms H 1 H 1 CH3 CH3 1,2-diaxial Each axial m ethyl group generates 2 x 3.75 = 7.5 kJ/m ol of strain energy, for a total of 15 kJ/mol. Bicyclic and Polycyclic Alkanes 2 CH 3 H 1,2-diequatorial There is one gauche interaction from the interacting CH 3 groups: H But the energy difference between the 1,2 -axial and the 1,2-diequa torial co nformat ions is only 3 ga uche intera ctions or 3 x 3.7 5 = 1 1.25 kJ/m ol. CH2 1 CH2 2 CH 3 gauche CH3 H Synthesis of Alkanes and Cycloalkanes Steroid backbone 1. Hydrogenation of Alkenes and Alkynes cholesterol Synthesis of Alkanes and Cycloalkanes 2. Reduction of Alkyl Halides Synthesis of Alkanes and Cycloalkanes 3. Alkylation of Terminal Alkynes l Alkynes can be subsequently hydrogenated to alkanes 14 Retrosynthetic Analysis-Planning Organic Synthesis l The synthetic scheme is formulated working backward from the target molecule to a simple starting material l Often several schemes are possible 15