1. Introduction to Alkanes & Cycloalkanes

Transcription

1 Chapter Nomenclature & Conformations of Alkanes & Cycloalkanes Created by Professor William Tam & Dr. Phillis Chang Ch. - About The Authors These Powerpoint Lecture Slides were created and prepared by Professor William Tam and his wife Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of ong Kong in 990 and his Ph.D. at the University of Toronto (Canada) in 99. e was an NSERC postdoctoral fellow at the Imperial College (UK) and at arvard University (USA). e joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top % most cited Chemists worldwide. e has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 99, her M.Sc. and Ph.D. in 997 and 00 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew. Ch. -. Introduction to Alkanes & Cycloalkanes Alkanes and cycloalkanes are hydrocarbons in which all the carboncarbon (C C) bonds are single bonds ydrocarbons that contain C C: Alkenes ydrocarbons that contain C C: Alkynes Ch. - Alkanes: C n n+ e.g. hexane (C ) Cycloalkanes: C n n e.g. cyclohexane (C ) Ch. - A. Sources of Alkanes: Petroleum Petroleum is the primary source of alkanes. It is a complex mixture of mostly alkanes and aromatic hydrocarbons with small amounts of oxygen-, nitrogen-, and sulfurcontaining compounds Petroleum refining Distillation is the first step in refining petroleum. Its components are separated based on different volatility More than 00 different compounds are contained in petroleum distillates boiling below 00 o C Ch. - Ch. -

2 Petroleum refining (Cont d) The fractions taken contain a mixture of alkanes of similar boiling points Mixture of alkanes can be used as fuels, solvents, and lubricants Ch. - 7 Gasoline The demand of gasoline is much greater than that supplied by the gasoline fraction of petroleum Converting hydrocarbons from other fractions of petroleum into gasoline by catalytic cracking mixture of alkanes (C and higher) catalysts ~ 00 o C highly branched hydrocarbons (C - C 0 ) Ch. - 8 Gasoline (Cont d) C C C C C C C C,,-Trimethylpentane (isooctane) (C 8 ) Isooctane burns very smoothly (without knocking) in internal combustion engines and is used as one of the standards by which the octane rating of gasoline is established Ch. - 9 Gasoline (Cont d) "octane rating" isooctane heptane 00 0 e.g. a gasoline of a mixture: 87% isooctane and % heptane Rated as 87-octane gasoline Ch. - 0 Typical Fractions Obtained by Distillation of Petroleum Boiling Range of Fraction ( o C) # of Carbon Atoms per Molecule Use Below 0 C C Natural gas, bottled gas, petrochemicals 0 0 C C Petroleum ether, solvents 0 00 C C 7 Ligroin, solvents 0 00 C C 0 Gasoline (straightrun gasoline) 7 C C 8 Kerosene and jet fuel Ch. - Typical Fractions Obtained by Distillation of Petroleum (Cont d) Boiling Range of Fraction ( o C) # of Carbon Atoms per Molecule Use 0 00 C and higher Gas oil, fuel oil, and diesel oil Nonvolatile liquids C 0 and higher Refined mineral oil, lubricating oil, and grease Nonvolatile solids C 0 and higher Paraffin wax, asphalt, and tar Ch. -

3 . Shapes of Alkanes All carbon atoms in alkanes and cycloalkanes are sp hybridized, and they all have a tetrahedral geometry Straight-chain (unbranched) alkanes Butane Pentane C C C C C C C C C Even straight-chain alkanes are not straight. They have a zigzag geometry Ch. - Ch. - Branched-chain alkanes Isobutane Neopentane C C C C C C C C C Butane and isobutane have the same molecular formula (C 0 ) but different bond connectivities. Such compounds are called constitutional isomers Butane Isobutane Ch. - Ch. - C and higher alkanes exist as constitutional isomers. The number of constitutional isomers increases rapidly with the carbon number Molecular Formula # of Possible Molecular Const. Isomers Formula # of Possible Const. Isomers C 0 C 9 0 C C 0 7 C C 0,9 C 7 9 C 0 8,8,80,7, C Ch. - 7 Constitutional isomers usually have different physical properties exane Isomers (C ) Formula M.P. ( o C) B.P. ( o C) Density (g/ml) Refractive Index Ch. - 8

4 . IUPAC Nomenclature of Alkanes, Alkyl alides, & Alcohols One of the most commonly used nomenclature systems that we use today is based on the system and rules developed by the International Union of Pure and Applied Chemistry (IUPAC) Fundamental Principle: Each different compound shall have a unique name Although the IUPAC naming system is now widely accepted among chemists, common names (trivial names) of some compounds are still widely used by chemists and in commerce. Thus, learning some of the common names of frequently used chemicals and compounds is still important Ch. - 9 Ch. - 0 The ending for all the names of alkanes is ane The names of most alkanes stem from Greek and Latin Unbranched alkanes Name Structure Name Structure Methane C exane C (C ) C Ethane C C eptane C (C ) C one two three four five Propane C C C Octane C (C ) C meth- eth- prop- but- pent- Butane C C C C Nonane C (C ) 7 C Pentane C (C ) C Decane C (C ) 8 C Ch. - Ch. - A. Nomenclature of Unbranched Alkyl Groups Alkyl group Removal of one hydrogen atom from an alkane Alkyl group (Cont d) For an unbranched alkane, the hydrogen atom that is removed is a terminal hydrogen atom C C C C C C Methane Ethane Propane Ch. - C C C C C C Methyl Ethyl Propyl (Me) (Et) (Pr) Ch. -

5 B. Nomenclature of Branched-Chain Alkanes Rule. Use the longest continuous carbon chain as parent name 7 C C C C CC C (-Methylheptane) C C C C C CC C C (-Ethylhexane) Ch. - Rule (Cont d). Use the lowest number of the substituent. Use the number obtained by Rule to designate the location of the substituent 7 C C C C CC C (-Methylheptane) C C C C C CC 7C (-Methylheptane) C Ch. - Rule (Cont d). For two or more substituents, use the lowest possible individual numbers of the parent chain The substitutents should be listed alphabetically. In deciding alphabetical order, disregard multiplying prefix, such as di, tri etc. Ch. - 7 Rule (Cont d) 8 7 (-Ethyl--methyloctane) 7 8 (-Ethyl-7-methyloctane) 8 7 (-Methyl--ethyloctane) Ch. - 8 Rule (Cont d). When two substituents are present on the same carbon, use that number twice Rule (Cont d). For identical substituents, use prefixes di-, tri-, tetra- and so on (-Ethyl--methyloctane) (,-Dimethylhexane) (,,-Trimethylheptane) 7 Ch. - 9 (,-Dimethylhexane) (,,-Trimethylheptane) Ch. - 0

6 Rule (Cont d) 7 7. When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents (,,-Trimethyl- -propylheptane) (only three substituents) 7 Ch. - Rule (Cont d) 8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference (,,-Trimethylhexane) (,,-Trimethylhexane) Ch. - Example Find the longest chain as parent Example (Cont d) Use the lowest numbering for substituents 7 instead of 7 7 or 7 Substituents: two methyl groups dimethyl 7 Ch. - Ch. - Example (Cont d) Complete name Example 7 (,-Dimethylheptane) Ch. - Ch. -

7 Example (Cont d) Find the longest chain as parent Example (Cont d) Find the longest chain as parent -carbon chain 8-carbon chain 8-carbon chain Ch carbon chain (correct!) Nonane as parent Ch. - 8 Example (Cont d) Use the lowest numbering for substituents Example (Cont d) Substituents,7-dimethyl -ethyl instead of (,,7) (,,7) Ch. - 9 Ch. - 0 Example (Cont d) Substituents in alphabetical order Ethyl before dimethyl (recall Rule disregard di ) Complete name C. Nomenclature of Branched Alkyl Groups For alkanes with more than two carbon atoms, more than one derived alkyl group is possible Three-carbon groups (-Ethyl-,7-dimethylnonane) Ch. - Propyl Isopropyl (or -methylethyl) Ch. -

8 Four-carbon groups A neopentyl group Butyl Isobutyl neopentyl (,,-dimethylpropyl) sec-butyl (-methylpropyl) tert-butyl (or,-dimethylethyl) Ch. - Ch. - Example Example (Cont d) Find the longest chain as parent (a) (b) -carbon chain 7-carbon chain (c) (d) Ch. - 8-carbon chain 9-carbon chain Ch. - Example (Cont d) Find the longest chain as parent (d) Example (Cont d) Use the lowest numbering for substituents Nonane as parent or or 9 7 8,, (lower numbering) Use, Ch. - 7 Ch. - 8

9 Example (Cont d) Substituents Isopropyl tert-butyl Example (Cont d) Alphabetical order of substituents tert-butyl before isopropyl Complete name isopropyl and -tert-butyl -tert-butyl--isopropylnonane Ch. - 9 Ch. - 0 Example Example (Cont d) Find the longest chain as parent (a) (b) 8-carbon chain 9-carbon chain (c) Octane as parent Ch. - 0-carbon chain Ch. - Example (Cont d) or Ch. - Example (Cont d) Use the lowest numbering for substituents, Determined using the next Rules or, Ch. -

10 Example (Cont d) Substituents sec-butyl Neopentyl Example (Cont d) Since sec-butyl takes precedence over neopentyl -sec-butyl and -neopentyl Complete name But is it -sec-butyl and -neopentyl or -neopentyl and -sec-butyl? Ch sec-butyl--neopentyldecane Ch. - D. Classification of ydrogen Atoms o hydrogen atoms C C C C C E. Nomenclature of Alkyl alides Rules alogens are treated as substituents (as prefix) F: fluoro Br: bromo Cl: chloro I: iodo o hydrogen atoms o hydrogen atoms Similar rules as alkyl substituents Ch. - 7 Ch. - 8 Examples Cl Br -Bromo--chlorobutane Cl Cl C,-Dichloro--methylhexane F. Nomenclature of Alcohols IUPAC substitutive nomenclature: a name may have as many as four features Locants, prefixes, parent compound, and suffixes O -Methyl--hexanol Ch. - 9 Ch. - 0

11 Rules Select the longest continuous carbon chain to which the hydroxyl is directly attached. Change the name of the alkane corresponding to this chain by dropping the final e and adding the suffix ol Number the longest continuous carbon chain so as to give the carbon atom bearing the hydroxyl group the lower number. Indicate the position of the hydroxyl group by using this number as a locant Ch. - Examples O -Propanol (isopropyl alcohol) O O O,,-Butanetriol O -Methyl--pentanol (or -Methylpentan--ol) ( -Methyl--pentanol) Ch. - Example O Example (Cont d) Find the longest chain as parent O 8 7 or Longest chain but does not contain the O group 7 O 7-carbon chain containing the O group Ch. - eptane as parent Ch. - Example (Cont d) Use the lowest numbering for the carbon bearing the O group 7 O or, (lower numbering) Use, 7 O, Ch. - Example (Cont d) Parent and suffix -eptanol Substituents Propyl 7 O Complete name -Propyl--heptanol Ch. -

12 . ow to Name Cycloalkanes Substituted cycloalkanes A. Monocyclic Compounds Cycloalkanes with only one ring Attach the prefix cyclo- Isopropylcyclopropane Methylcyclopropane C C C = Cyclopropane C C = C C C Cyclopentane tert-butylcyclopentane Ch. - 7 Ch. - 8 Example -Ethyl--methylcyclopentane Example Br -Bromo--ethyl--methyl cyclohexane Br -Bromo--ethyl--methyl cyclohexane -Ethyl--methylcyclopentane -Ethyl--methylcyclopentane (lowest numbers of substituents are,, not,,) Ch. - 9 Ch Example O O -Ethyl--methyl cyclohexanol -Ethyl--methyl cyclohexan--ol (the carbon bearing the O should have the lowest numbering, even though,, is lower than,,) Ch. - 7 Cycloalkylalkanes When a single ring system is attached to a single chain with a greater number of carbon atoms -Cyclobutylpentane When more than one ring system is attached to a single chain,-dicyclohexylpropane Ch. - 7

13 B. Bicyclic Compounds Bicycloalkanes Alkanes containing two fused or bridged rings Total # of carbons = 7 Bicycloheptane Bridgehead Ch. - 7 Example (Cont d) Between the two bridgeheads Two-carbon bridge on the left Two-carbon bridge on the right One-carbon bridge in the middle Complete name Bicyclo[..]heptane Ch. - 7 Other examples Methylbicyclo[..0]nonane 8 7. Nomenclature of Alkenes & Cycloalkenes Rule. Select the longest chain that contains C=C as the parent name and change the name ending of the alkane of identical length from ane to ene -Isopropylbicyclo[..]octane Ch. - 7 Ch. - 7 Rule. Number the chain so as to include both carbon atoms of C=C, and begin numbering at the end of the chain nearer C=C. Assign the location of C=C by using the number of the first atom of C=C as the prefix. The locant for the alkene suffix may precede the parent name or be placed immediately before the suffix Ch Examples C CC C -Butene (not -Butene) C C CC C C -exene (not -exene) Ch. - 78

14 Rule. Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached Examples -Methyl--butene (not -Methyl--butene) Examples (Cont d),-dimethyl--hexene,-dimethyl--hexene Ch Ch Rule. Number substituted cycloalkenes in the way that gives the carbon atoms of C=C the and positions and that also gives the substituent groups the lower numbers at the first point of difference Example,-Dimethylcyclohexene,-Dimethylcyclohexene Ch. - 8 Ch. - 8 Rule. Name compounds containing a C=C and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number Examples O -Methyl--cyclohexen--ol (or -Methylcyclohex--en--ol) Ch. - 8 Examples (Cont d) O -Methyl--penten--ol (or -Methylpent--en--ol) Ch. - 8

15 Rule. Vinyl group & allyl group Vinyl group ethenyl Ethenylcyclopropane (or Vinylcyclopropane) Allyl group prop--en--yl O -(Prop--en--yl) cyclohexan--ol (or -Allylcyclohexanol) Ch. - 8 Rule 7. Cis vs. Trans Cis: two identical or substantial groups on the same side of C=C Trans: two identical or substantial groups on the opposite side of C=C Cl Cl Cl cis-,-dichloroethene trans-,-dichloroethene Cl Ch. - 8 Example Example (Cont d) (a) 7 (b) (c) (d) 7 7 Ch Ch Example (Cont d) Complete name 7 -tert-butyl--methyl--heptene. Nomenclature of Alkynes Alkynes are named in much the same way as alkenes, but ending name with yne instead of ene Examples 7 Br Ch eptyne -Bromo--butyne Ch. - 90

16 Examples (Cont d) O group has priority over C C I Br O -Butyn--ol O 9-Bromo-7-iodo--isopropyl-8-methyl--decyne O 7 8 -Methyl--octyn--ol O 7 8 Ch. - 9 Ch Physical Properties of Alkanes & Cycloalkanes Boiling points & melting points C Isomer Boiling Point ( o C) Ch Ch. - 9 Physical Constants of Cycloalkanes # of C Refractive Atoms Name bp ( o C) mp ( o C) Density Index Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Ch Sigma Bonds & Bond Rotation Two groups bonded by a single bond can undergo rotation about that bond with respect to each other Conformations temporary molecular shapes result from a rotation about a single bond Conformer each possible structure of conformation Conformational analysis analysis of energy changes occur as a molecule undergoes rotations about single bonds Ch. - 9

17 8A. Newman Projections 8B. ow to Do a Conformational Analysis Me Sawhorse formula Cl O Et Look from this direction Me Cl Et O front carbon back carbon combine Me Cl Et O Newman Projection Ch Look from this direction φ = 80 o a c b φ = 0 o staggered confirmation of ethane Ch o 0 o C C C C C 80 o C anti gauche eclipsed Look from this direction φ = 0 o eclipsed confirmation of ethane Ch Ch Conformational Analysis of Butane Me Me Sawhorse formula Me Me New Projection formula Ch. - 0 Ch. - 0

18 C C C C C anti conformer (I) (lowest energy) C eclipsed conformer (II) = C on front carbon rotates 0 o clockwise gauche conformer (III) C C C C C C eclipsed conformer (VI) gauche conformer (V) eclipsed conformer (IV) (highest energy) Ch. - 0 Ch The Relative Stabilities of Cycloalkanes: Ring Strain Cycloalkanes do not have the same relative stability due to ring strain Ring strain comprises: Angle strain result of deviation from ideal bond angles caused by inherent structural constraints Torsional strain result of dispersion forces that cannot be relieved due to restricted conformational mobility Ch A. Cyclopropane θ sp hybridized carbon (normal tetrahedral bond angle is 09. o ) Internal bond angle (θ) ~0 o (~9. o deviated from the ideal tetrahedral angle) Ch B. Cyclobutane θ Internal bond angle (θ) ~88 o (~ o deviated from the normal 09. o tetrahedral angle) Ch Ch. - 08

19 Cyclobutane ring is not planar but is slightly folded. If cyclobutane ring were planar, the angle strain would be somewhat less (the internal angles would be 90 o instead of 88 o ), but torsional strain would be considerably larger because all eight C bonds would be eclipsed Ch C. Cyclopentane If cyclopentane were planar, θ ~08 o, very close to the normal tetrahedral angle of 09. o owever, planarity would introduce considerable torsional strain (i.e. 0 C bonds eclipsed) Therefore cyclopentane has a slightly bent conformation Ch Conformations of Cyclohexane: The Chair & the Boat D (chair form) (more stable) (boat form) (less stable) The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to Eclipsed conformation,-flagpole interactions (eclipsed) Ch. - Ch. - Energy diagram (twist boat) The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation Ch. - Ch. -

20 . Substituted Cyclohexanes: Axial & Equatorial ydrogen Atoms Equatorial hydrogen atoms in chair form Substituted cyclohexane Two different chair forms G G Axial hydrogen atoms in chair form G Ch. - (equatorial G) (more stable) (axial G) (less stable) G Ch. - The chair conformation with axial G is less stable due to,-diaxial interaction,-diaxial interaction (equatorial) G (axial) G The larger the G group, the more severe the,-diaxial interaction and shifting the equilibrium from the axial- G chair form to the equatorial-g chair form G Ch. - 7 At o C G % of Equatorial % of Axial F 0 0 C 9 i Pr 97 t Bu > < 0.0 Ch Disubstituted Cycloalkanes Cis-Trans Isomerism C A. A.Cis-Trans Isomerism & Conformation Structures of Cyclohexanes Trans-,-Disubstituted Cyclohexanes C C cis-,-dimethyl cyclopropane Cl Cl cis-,-dichloro cyclobutane C trans-,-dimethyl cyclopropane Cl Cl trans-,-dichloro cyclobutane Ch. - 9 C C trans-diaxial ring flip C C trans-diequatorial Ch. - 0

21 Upper bond C C Lower bond trans-dimethyl cyclohexane Upper-lower bonds means the groups are trans Cis-,-Disubstituted Cyclohexanes C chair-chair C C ring flip C Equatorial-axial Axial-equatorial Ch. - Ch. - Cis--tert-Butyl--methylcyclohexane Trans-,-Disubstituted Cyclohexanes C C C (more stable because large group is equatorial) C ring flip C C C (less stable because large group is axial) C Ch. - (eq) C (ax) C ring flip C C (eq) (ax) trans-,-dimethylcyclohexane Ch. - Trans--tert-Butyl--methylcyclohexane Cis-,-Disubstituted Cyclohexanes C C C (more stable because large group is equatorial) C ring flip C C C (less stable because large group is axial) C Ch. - C ring C flip C C (more stable) (less stable) Ch. -

22 Trans-,-Disubstituted Cyclohexanes (ax) (eq) C ring C C flip (ax) (eq) C diequatorial diaxial (much more stable) (much less stable) trans-,-dimethylcyclohexane Ch. - 7 Cis-,-Disubstituted Cyclohexane C (ax) C (eq) (equatorial-axial) ring flip cis-,-dimethylcyclohexane (equal energy and equally populated conformations) (ax) C C (eq) (axial-equatorial) Ch Bicyclic & Polycyclic Alkanes Decalin (Bicyclo[..0]decane) Adamantane Cubane Prismane cis-decalin trans-decalin Ch. - 9 C 0 (Buckminsterfullerene) Ch Synthesis of Alkanes and Cycloalkanes A. A.ydrogenation of Alkenes & Alkynes Pt, Pd or Ni solvent heat and pressure Pt, Pd or Ni solvent heat and pressure C C C C Ch. - Examples Ni + EtO o C, 0 atm. + Pd EtO o C, atm. Pd EtOAc + o C, atm. Ch. -

23 7. ow to Gain Structural Information from Molecular Formulas & Index of ydrogen Deficiency Index of hydrogen deficiency (ID) The difference in the number of pairs of hydrogen atoms between the compound under study and an acyclic alkane having the same number of carbons Also known as degree of unsaturation or double-bond equivalence (DBE) Ch. - Index of hydrogen deficiency (Cont d) Saturated acyclic alkanes: C n n+ Each double bond on ring: hydrogens less Each double bond on ring provides one unit of hydrogen deficiency Ch. - e.g. exane: C Examples -exene and Index of hydrogen deficiency (ID) Cycloheane = C C C = one pair of = Ch. - ID = ID = ID = ID = Ch. - A. A.Compounds Containing alogen, Oxygen, or Nitrogen For compounds containing alogen count halogen atoms as though they were hydrogen atoms Oxygen ignore oxygen atoms and calculate ID from the remainder of the formula Nitrogen subtract one hydrogen for each nitrogen atom and ignore nitrogen atoms Ch. - 7 Example : ID of C Cl Count Cl as C 0 C Cl C 8 C A C acyclic alkane: 8 C ()+ = C 0 ID of C Cl = one pair of = Cl Possible structures Cl or Cl Cl... etc. or Cl Cl Ch. - 8

24 Example : ID of C 8 O Ignore oxygen C C 8 O C 8 C A C acyclic alkane: 8 C ()+ = C ID of C Cl = two pair of = Possible structures O or O or... etc. O Ch. - 9 Example : ID of C 7 N Subtract for each N C C 7 N C C A C acyclic alkane: C ()+ = C ID of C Cl = three pair of = Possible structures N C or C N... etc. Ch. - 0 END OF CAPTER Ch. -