Alkanes= saturated hydrocarbons. Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis.

Transcription

1 Alkanes= saturated hydrocarbons Chapter 4 Alkanes: Nomenclature, Conformational Analysis, and an Introduction to Synthesis Simplest alkane = methane C 4 We can build additional alkanes by adding - C 2 units C n 2n+2 = saturated = no double bonds, no rings Shapes of Alkanes Straight-chain alkanes have a zig-zag orientation when they are in their most straight orientation There is no limit on length of chain 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 C

2 The number of constitutional isomers possible for a given molecular formula rise rapidly with the number of carbons Source of alkanes = petroleum 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 aromatics boi li ng range of fraction ( o C) yd rocarbon Fraction s from Petro leum s ize range nam e and use below 20 C 1 to C n atural gas, bottl ed 4 gas, p etrochem icals 20 to 60 C 5 to C 6 p etroleu m "ether". s olvents 60 to 100 C 6 to C 7 l igroin, solven ts 40 to 200 C 5 to C 10 s traigh t-ru n gasol in e 175 to 25 C 12 to C 18 k eros en e and jet fu el 250 to 400 C 12 and h igh er non volati le liq ui ds non volati le s oli ds gas oil, fu el oil and d iesel oi l C 20 an d h igher min eral o il, lu bri catin g oil C 20 and hig her paraffi n wax, asph al t, tar A reaction of alkanes - Combustion All hydrocarbons are combustible Combine with oxygen and release energy eat of combustion is very high 2C O 2 16CO 2( g) O ( l) D comb = kj/mol (or kj/g) Octane boosters (anti -knock additives) Smoother engine performance if combustion is slowed For decades, used Tetraethyl lead, now banned For past decade, MTBE was used, now also banned Examples of reforming to raise octane isooctane = 100 (2,2,5-trimethylpentane) (vs heptane = 0) Some Octane Ra ting s of ydrocarbons and Addit iv es Octane Rating heptane 0 1-pentene 91 2,2,4-t rimethylpentane 10 0 benzene 10 6 metha no l 1 07 ethanol 108 methy l t-butyl ether 116 toluene 1 18 C silica-alumina C C 2 C 2 C 2 C 2 C 2 C catalyst, 500 o C heptane 20 atm 2 a cid catalyst CC2C 2C2C2C hexane CC2C2CC + C CC2CC2C C branched a lkanes 2

3 IUPAC Nomenclature Nomenclature of Unbranched Alkanes Early chemicals were given common or trivial names 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 Nomenclature of Unbranched Alkanes Learn the first ten! 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 Nomenclature of Branched-Chain Alkanes (IUPAC) 1. Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name. 7 carbons = a heptane Nomenclature of Branched -Chain Alkanes (IUPAC) cont. 4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain ❶ Substituents are listed alphabetically 2. Number the longest chain beginning with the end of the chain nearer the substituent. Designate the location of the substituent 4-ethyl--methylheptane

4 Nomenclature of Branched -Chain Alkanes (IUPAC) cont. 6. When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents 5. 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,,-trimethylheptane Nomenclature of Branched Alkyl Chains ow 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 ydrogen Atoms ydrogenstake their classification from the carbon they are attached to 4

5 What type of hydrogens in neopentane? Nomenclature of Alkyl alides In IUPAC nomenclature halides are named as substituents on the parent chain alo and alkyl substituents are considered to be of equal ranking C C -C-C C In common nomenclature the simple haloalkanes are named as alkyl halides 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: Examples Locant indicates the position of a substituent. Prefix names the substituent group. Pare nt is the pa rent a lka ne. Suffix names a key function. IUPAC Nomenclature of Alcohols 1. Select the longest chain containing the hydroxyl and change the suffix ending of the parent alkane from -e to -ol 2. Number the parent to give the hydroxyl the lowest possible number. The other substituents take their locations accordingly O C C 2 CC 2-butanol suffix locant parent C CC 2 C 2 O C parent -methyl-1-butanol locant prefix locant suffix 4-methyl-1-hexanol Examples Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature Common Names of simple alcohols are still often used and are approved by IUPAC 5

6 Nomenclature of cycloalkanes Nomenclature of Substituted Cycloalkanes Ring compounds are very common Named as cyclo + base name for number of C in ring 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 ydroxyl has higher priority than alkyl = position 1 cyclobutane cyclopentane cyclooctane Cycloalkyl group If other parts of the molecule are dominent or if a long chain is attached to a ring with fewer carbons, the cycloalkaneis considered the substituent Bicyclic Compounds the Playground of Organic Chemists 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 Bicyclic Compounds the Playground of Organic Chemists Beginning at bridgehead position, number around largest ring first, then second largest ring The number of carbons in each bridge is included in the middle of the name in square brackets 6

7 Nomenclature of Alkenes and Cycloalkenes Double bonds plus alcohol hydroxyl groups = alkenols Alkenes are named by finding the longest chain containing the double bond and changing the name of the parent alkane from -ane to -ene The compound is numbered to give one of the alkene carbons the lowest number The hydroxyl is the group with higher priority and must be given the lowest possible number The double bond of a cylcoalkene must be in position 1 and 2 Note that positions of both the C=C and the O 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 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 Alkynes = named similarly to alkenes (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. () The location of the triple bond is given by the lower positional number of the alkynyl carbons C C 2 C 2 C C 1-pentyne C C CC 2-butyne (4) Positions of substituents are determined by the usual rules C C CC 2 Cl C C 2 CC C C 1-chloro-2-butyne -methyl-1-pentyne (5) In an alkynol, the alcohol has priority in numbering C CC 2 C 2 O -butyn-1-ol 1-alkynes are also called terminal alkynes Terminal alkynes have an acidic R C C pk a = 25 7

8 Physical Properties of Alkanes The unbranched alkanes (C 4, C C, C C 2 C, C C 2 C 2 C, etc.) form a regul ar s eri es where each member di ffers from the next in order by - C - 2. Such a regular series is called a homo logous s eri es. The b oili ng poin ts of the u nbranched alkanes 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 der Waals attrac tion s. pentane h exane heptane o ctane bp ( o C) Melting points reflect size, but also packing factors Propane: -188 o C Ethane: -18 o C Methane: -182 o C pentane hexane heptane octane mp ( o C) D = 5 D = 4 D = 4 Other physical properties of hydrocarbons Less dense than water Sigma Bonds and Bond Rotation Ethane has relatively free rotation around the carbon-carbon bond The staggered conformation has C- bonds on adjacent carbons as far apart from each other as possible The drawing to the right is called a Newman projection Immiscible with water and other highly polar solvents Solvent for non-polar organic compounds The eclipsed conformation has all C- bonds on adjacent carbons directly on top of each other ow 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 /sec Sight down the C-C bond (sigma bond is symmetric) Groups on front carbon intersect in center Groups on back carbon end at circle Energy barrier called torsional strain 8

9 2 equivalent C-C bonds C eclipsed Bond Rotations in Propane rotation staggered The co nformational features are the s ame for the two C-C bonds. Consider as ethane with 1 replaced by a C C C C C Barrier = 14 kj/mol Conclusion: eclipsing of methyl with adds very little to energy barrier Analysis of the Extreme Conf ormations During o ne complete rotation around the internal bond in butane, the three staggered conformations include two called "gauche" and one called "a nti." C C C C 120 o C r otation C 120 o C C C C rotation C C The Newman proj ection formula s as viewed fro m the left are: C 120 o 120 o r otation C rotation C C C ster ic C strain anti gauche ga uche The stag gered anti is more s table than the two equivalen t stagg ered gauch e conforma tions. In the a nti conformation, the t wo C groups are on opp osite sides of the structure. In t he ga uche co nformations, the two group s are w ithin van der W aals repulsive in tera ction d is tance, and.8 kj/mo l of steric strain energy is introduced. Conformational Analysis of Butane Rotation around C 2 -C of butane gives three energy minima The gauche conformation is less stable than the anti conformation by.8 kj mol -1 because of repulsive van der Waals forces between the two methyls Conformational Analysis of Butane minima and energy barriers between each ( maxima) C C C C 120 o rotation C C 120 o C C C rotation C C C C 12 0 o rotation C 120 o ro tation C C C C anti gauche gauche Relative Stabilities of Cycloalkanes: Ring Strain eats of combustion per C 2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain The Origin of Ring Strain in Cyclopropane : Angle Strain and Torsional Strain Angle strain is caused by bond angles different from o C-C angle of 60 means orbital overlap is reduced Torsional strain is caused by eclipsing C- bonds on adjacent C s Cyclopropane has both high angle and torsional strain C C C Normal bond length sp -sp C-C = 1.54 Å Normal sp C- =1.10 Å sp 2 C-=1.09 Å 9

10 Cyclobutane: Angle Strain and Torsional Strain Cyclobutane has considerable angle strain It bends to relieve some torsional strain Cyclopentane The internal angles of a regular pentagon are 108 o, close to the idealized tetrahedral bond angles. Thus, a planar cyclopentane would have very little angle strain. 108 o But a planar geometry would have very severe torsional strain (10 eclipsed ). Consequently, the geometry of cyclopentane is bent. The torsional strain is reduced in t he bent structure. Four are st ill eclipsed, but 6 are st aggered. bent or puckered geomet ry Cyclohexane: the perfect ring Not 120 o, the angle of simple hexagon Adopts a chair conformation with no ring strain! All bond angles are o and all C- bonds are perfectly staggered Two types of in chair cyclohexane 6 are axial (up and down) 6 are equatorial (close to plane of ring) Newman projections along 2 the C 1 -C 6 and C -C 4 bonds C 2 1 show the different orientations C 2 of the axial and equatorial. 5 eq axial Rotation about single bonds flips the ring to a boat form The boat conformation is less stable because of flagpole interactions and torsional strain along the bottom of the boat 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

11 Conformational Energy Diagram for cyclohexane Another view of energy barrier 42 kj/mol half-chair half-chair Flips about 100 x per sec boat en ergetically preferred conformation is the tw is t boat 99.9% of molecules in chair form 0 chair chair Note: kcal/mol x 4.2 = kj/mol Energy and Equilibrium Drawing Cyclohexanes The C-C bonds and equatorial C- bonds are all drawn in sets of parallel lines The 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. 2.) Add the axial hydrogens. Cyclohexane structure can be elaborated Adamantane C ) Draw the C1 and C4 equatorial hydrogens. 4.) Draw the remaining equatorial hydrogens. Diadamantane C

12 Diamond = infinite cyclohexane rings Substituted cyclohexanes Methyl cyclohexane is more stable with the methyl equatorial An axial methyl has an unfavorable 1,-diaxial interaction with axial C- 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 view a long t he C1-C2 bo nd C 4 gauche C 2 C C 2 axial-methylcyclohexane Each gauche interaction introduces.75 kj/mol of steric strain view along the C-C2 bond C g auche 4 2 C 2 C C equatorial-methylcyclohexane Equivalent to a staggered butane view along the C 1 -C 2 bond C 2 6 C 2 C 2 anti 2 C 1 anti view al ong the C -C 2 bond 1 C 2 4 C 2 The larger the group, the greater the equatorial preference t-butylcyclohexaneis >99.9% equatorial Disubstituted cycloalkanes 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 cis isomer trans isomer Cl Cl Cl Cl Cl Cl Cl Cl 12

13 For, 4, or 5 membered rings, can treat as if planar Configurational Isomers in Disubstituted Cyclohexanes 1,-dimethylcyclopentanes Three options for pairs of cis-trans stereoisomers 2 C 1 C C cis trans C Preferred comformations Trans-1,4-dimethylcylohexane prefers trans-diequatorial conformation Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation Note that both are on the same side of ring upper side in this projection Note methyls on opposite sides of ring u pp er b on d C lower b ond Confirming whether cis or trans Test one: identify each substituent as on the upper or lower side of ring up p er b ond C low er b on d Since the two bo nds to the C g ro ups are "upper", meaning sa me side, they a re "cis." Di-equatorial isomer 1,4 Is diequatorial always trans? trans Test Two: Flatten the Ring C C SQUEEZE IT C C Since the C groups are on the sa me side, they a re "cis." 1, 1,2 C C C C cis trans 1

14 Trans 1,2 dimethylcyclohexane: an energy analysis C C C C Bicyclicand Polycyclic Alkanes The bicyclic decalin system exists in non-interconvertible cis and transforms 1,2-diaxial 1,2-diequatorial Each axial m ethyl group generates 2 x.75 = 7.5 kj/m ol of strain energy, for a total of 15 kj/mol. But the energy difference between the 1,2 -axial and the 1,2-diequa torial co nformat ions is only ga uche intera ctions or x.7 5 = kj/m ol. There is one gauche interaction from the interacting C groups: C C gauche C 2 C Steroid backbone Synthesis of Alkanes and Cycloalkanes 1. ydrogenation of Alkenes and Alkynes cholesterol Synthesis of Alkanes and Cycloalkanes 2. Reduction of Alkyl alides Synthesis of Alkanes and Cycloalkanes. Alkylation of Terminal Alkynes Alkynes can be subsequently hydrogenated to alkanes 14

15 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 15