Alkanes, Alkenes, Alkynes Nomenclature-1 PDF

Chapter 4 Nomenclature & Conformations of Alkanes & Cycloalkanes Chapter 4 In this chapter we will consider:  How to name many simple organic molecules  The flexible, three-dimensional nature of organic molecules  An organic reaction that can c...

Chapter 4 Nomenclature & Conformations of Alkanes & Cycloalkanes Chapter 4 In this chapter we will consider:  How to name many simple organic molecules  The flexible, three-dimensional nature of organic molecules  An organic reaction that can convert alkenes and alkynes to alkanes Chapter 4 1. Introduction to Alkanes & Cycloalkanes  Alkanes and cycloalkanes are hydrocarbons in which all the carbon- carbon (C–C) bonds are single bonds  Hydrocarbons that contain C═C: Alkenes Hydrocarbons that contain C≡C: Alkynes Chapter 4  Alkanes: CnH2n+2 5 3 1 e.g. 6 4 2 hexane (C6H14)  Cycloalkanes: CnH2n e.g. cyclohexane (C6H12) Chapter 4 1A. 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 sulfur- containing compounds Chapter 4  Petroleum refining Distillation is the first step in refining petroleum. Its components are separated based on different volatility More than 500 different compounds are contained in petroleum distillates boiling below 200oC Chapter 4  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 Chapter 4  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 catalysts highly branched o (C12 and higher) ~ 500 C hydrocarbons (C5 - C10) Chapter 4  Gasoline (Cont’d) CH3 CH3 CH3 C CH2 C CH3 CH3 H 2,2,4-Trimethylpentane (isooctane) (C12H18) 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 Chapter 4  Gasoline (Cont’d) isooctane heptane "octane 100 0 rating" e.g. a gasoline of a mixture: 87% isooctane and 13% heptane  Rated as 87-octane gasoline Chapter 4  Typical Fractions Obtained by Distillation of Petroleum Boiling Range of # of Carbon Atoms Use Fraction (oC) per Molecule Below 20 C1 – C4 Natural gas, bottled gas, petrochemicals 20 – 60 C5 – C6 Petroleum ether, solvents 60 – 100 C6 – C7 Ligroin, solvents 40 – 200 C5 – C10 Gasoline (straight- run gasoline) 175 – 325 C12 – C18 Kerosene and jet fuel Chapter 4  Typical Fractions Obtained by Distillation of Petroleum (Cont’d) Boiling Range of # of Carbon Use Fraction (oC) Atoms per Molecule 250 – 400 C12 and higher Gas oil, fuel oil, and diesel oil Nonvolatile liquids C20 and higher Refined mineral oil, lubricating oil, and grease Nonvolatile solids C20 and higher Paraffin wax, asphalt, and tar Chapter 4 2. Shapes of Alkanes  All carbon atoms in alkanes and cycloalkanes are sp3 hybridized, and they all have a tetrahedral geometry  Even “straight-chain” alkanes are not straight. They have a zigzag geometry Chapter 4  “Straight-chain” (unbranched) alkanes Butane Pentane CH3CH2CH2CH3 CH3CH2CH2CH2CH3 Chapter 4  Branched-chain alkanes Isobutane Neopentane CH3 CH3 CH CH3 CH3 C CH3 CH3 CH3 Chapter 4  Butane and isobutane have the same molecular formula (C4H10) but different bond connectivities. Such compounds are called constitutional isomers Butane Isobutane Chapter 4  C4 and higher alkanes exist as constitutional isomers. The number of constitutional isomers increases rapidly with the carbon number Molecular # of Possible Molecular # of Possible Formula Const. Isomers Formula Const. Isomers C4H10 2 C9H20 35 C5H12 3 C10H22 75 C6H14 5 C20H42 366,319 C7H16 9 C40H82 62,481,801,147,341 C8H18 18 Chapter 4  Constitutional isomers usually have different physical properties Hexane Isomers (C6H14) Formula M.P. B.P. Density Refractive (oC) (oC) (g/mL) Index -95 68.7 0.6594 1.3748 -153.7 60.3 0.6532 1.3714 -118 63.3 0.6643 1.3765 -128.8 58 0.6616 1.3750 -98 49.7 0.6492 1.3688 Chapter 4 3. How to Name Alkanes, Alkyl Halides, and Alcohols: The IUPAC System  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 Chapter 4  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 Chapter 4  The ending for all the names of alkanes is –ane  The names of most alkanes stem from Greek and Latin one two three four five meth- eth- prop- but- pent- Chapter 4  Unbranched alkanes Name Structure Name Structure Methane CH4 Hexane CH3(CH2)4CH3 Ethane CH3CH3 Heptane CH3(CH2)5CH3 Propane CH3CH2CH3 Octane CH3(CH2)6CH3 Butane CH3CH2CH2CH3 Nonane CH3(CH2)7CH3 Pentane CH3(CH2)3CH3 Decane CH3(CH2)8CH3 Chapter 4 3A. How to Name Unbranched Alkyl Groups  Alkyl group Removal of one hydrogen atom from an alkane Chapter 4  Alkyl group (Cont’d) For an unbranched alkane, the following is what happens when the hydrogen atom that is removed is terminal CH3 H CH3CH2 H CH3CH2CH2 H Methane Ethane Propane CH3 CH3CH2 CH3CH2CH2 Methyl Ethyl Propyl (Me) (Et) (Pr) Chapter 4 3B. How to Name Branched-Chain Alkanes  Rule 1. Use the longest continuous carbon chain as parent name 7 6 5 4 3 6 5 4 3 2 1 CH3CH2CH2CH2CHCH3 CH3CH2CH2CH2CHCH3 2CH2 CH2 NOT 1 CH3 CH3 (3-Methylheptane) (2-Ethylhexane) Chapter 4  Rule (Cont’d) 2. Use the lowest number of the substituent 3. Use the number obtained by Rule 2 to designate the location of the substituent 7 6 5 4 3 1 2 3 4 5 CH3CH2CH2CH2CHCH3 CH3CH2CH2CH2CHCH3 2CH2 6 CH2 NOT 1 CH3 7 CH3 (3-Methylheptane) (5-Methylheptane) Chapter 4  Rule (Cont’d) 4. For two or more substituents, use the lowest possible individual numbers of the parent chain. The substituents should be listed alphabetically. In deciding alphabetical order, disregard multiplying prefix, such as “di”, “tri” etc. Chapter 4  Rule (Cont’d) 2 4 6 8 1 3 5 7 (6-Ethyl-2-methyloctane) NOT NOT 7 5 3 1 2 4 6 8 8 6 4 2 1 3 5 7 (3-Ethyl-7-methyloctane) (2-Methyl-6-ethyloctane) Chapter 4  Rule (Cont’d) 5. When two substituents are present on the same carbon, use that number twice 2 4 6 8 1 3 5 7 (4-Ethyl-4-methyloctane) Chapter 4  Rule (Cont’d) 6. For identical substituents, use prefixes di-, tri-, tetra- and so on 5 3 1 6 4 2 7 5 3 1 6 4 2 (2,4-Dimethylhexane) (2,4,5-Trimethylheptane) NOT NOT 2 4 6 1 3 5 1 3 5 7 2 4 6 (3,5-Dimethylhexane) (3,4,6-Trimethylheptane) Chapter 4  Rule (Cont’d) 7. When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents 7 1 1 6 4 2 4 2 5 3 3 5 NOT 6 7 (2,3,5-Trimethyl- (only three substituents) 4-propylheptane) Chapter 4  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 6 1 NOT 5 3 1 2 4 6 4 2 3 5 (2,3,5-Trimethylhexane) (2,4,5-Trimethylhexane) Chapter 4  Example 1 Find the longest chain as parent 4 2 4 6 3 1 5 7 or 5 7 3 1 6 2 Chapter 4  Example 1 (Cont’d) Use the lowest numbering for substituents 4 6 4 2 5 7 3 1 instead of 3 1 5 7 2 6 Substituents: two methyl groups  dimethyl 4 6 5 7 3 1 2 Chapter 4  Example 1 (Cont’d) Complete name 4 6 5 7 3 1 2 (3,4-Dimethylheptane) Chapter 4  Example 2 Chapter 4  Example 2 (Cont’d) Find the longest chain as parent 6-carbon chain 8-carbon chain 8-carbon chain Chapter 4  Example 2 (Cont’d) Find the longest chain as parent 9-carbon chain (correct!) ⇒ Nonane as parent Chapter 4  Example 2 (Cont’d) Use the lowest numbering for substituents 8 2 7 3 9 1 6 4 5 instead of 5 2 8 3 7 6 4 1 9 (3,4,7) (3,6,7) Chapter 4  Example 2 (Cont’d) Substituents  3,7-dimethyl  4-ethyl 8 7 9 6 5 2 3 4 1 Chapter 4  Example 2 (Cont’d) Substituents in alphabetical order  Ethyl before dimethyl (recall Rule 4 – disregard “di”) Complete name 8 7 9 6 5 2 3 4 1 (4-Ethyl-3,7-dimethylnonane) Chapter 4 3C. How to Name Branched Alkyl Groups  For alkanes with more than two carbon atoms, more than one derived alkyl group is possible  Three-carbon groups Propyl Isopropyl (or 1-methylethyl) Chapter 4  Four-carbon groups Butyl Isobutyl sec-butyl tert-butyl (1-methylpropyl) (or 1,1-dimethylethyl) Chapter 4  A neopentyl group neopentyl (2,2,-dimethylpropyl) Chapter 4  Example 1 Chapter 4  Example 1 (Cont’d) Find the longest chain as parent (a) (b) 6-carbon 7-carbon chain chain (c) (d) 8-carbon 9-carbon chain chain Chapter 4  Example 1 (Cont’d) Find the longest chain as parent (d) ⇒ Nonane as parent 1 3 4 5 6 7 8 9 9 7 6 5 4 3 2 1 2 or 8 Chapter 4  Example 1 (Cont’d) Use the lowest numbering for substituents 1 3 4 5 6 7 8 9 9 7 6 5 4 3 2 1 2 or 8 5,6 4,5 (lower numbering) ⇒ Use 4,5 Chapter 4  Example 1 (Cont’d) Substituents  Isopropyl  tert-butyl 9 8 7 6 5 4 3 2 1 ⇒ 4-isopropyl and 5-tert-butyl Chapter 4  Example 1 (Cont’d) Alphabetical order of substituents  tert-butyl before isopropyl Complete name 9 8 7 6 5 4 3 2 1 5-tert-Butyl-4-isopropylnonane Chapter 4  Example 2 Chapter 4  Example 2 (Cont’d) Find the longest chain as parent (a) (b) 8-carbon 9-carbon chain chain (c) ⇒ Decane as parent 10-carbon chain Chapter 4  Example 2 (Cont’d) 1 2 3 4 5 6 7 8 9 10 or 10 9 8 7 6 5 4 3 2 1 Chapter 4  Example 2 (Cont’d) Use the lowest numbering for substituents 5,6 1 2 3 4 5 6 7 8 9 10 or 5,6 10 9 8 7 6 5 4 3 2 1 ⇒ Determined using the next Rules Chapter 4  Example 2 (Cont’d) Substituents  sec-butyl  Neopentyl But is it 5-sec-butyl and 6-neopentyl or 5-neopentyl and 6-sec-butyl ? Chapter 4  Example 2 (Cont’d) Since sec-butyl takes precedence over neopentyl  5-sec-butyl and 6-neopentyl Complete name 10 9 8 7 6 5 4 3 2 1 5-sec-Butyl-6-neopentyldecane Chapter 4 3D. How to Classify Hydrogen Atoms 1o hydrogen atoms CH3 CH3 CH CH2 CH3 3o hydrogen atoms 2o hydrogen atoms Chapter 4 3E. How to Name Alkyl Halides  Rules Halogens are treated as substituents (as prefix) F: fluoro Br: bromo Cl: chloro I: iodo Similar rules as alkyl substituents Chapter 4  Examples 3 1 4 2 Cl Br 2-Bromo-1-chlorobutane Cl 2 6 Cl 1 3 4 5 CH3 1,4-Dichloro-3-methylhexane Chapter 4 3F. How to Name Alcohols  IUPAC substitutive nomenclature: a name may have as many as four features Locants, prefixes, parent compound, and suffixes 6 5 4 3 2 1 OH 4-Methyl-1-hexanol Chapter 4  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 Chapter 4  Examples OH OH 2 1 3 1 4 3 2 OH 2-Propanol OH (isopropyl alcohol) 1,2,3-Butanetriol 5 1 4 3 2 OH 4-Methyl-1-pentanol (or 4-Methylpentan-1-ol) (NOT 2-Methyl-5-pentanol) Chapter 4  Example 4 OH Chapter 4  Example 4 (Cont’d) Find the longest chain as parent 8 7 6 3 1 or 1 5 7 4 2 2 4 6 5 3 OH OH Longest chain but 7-carbon chain does not contain containing the the OH group OH group ⇒ Heptane as parent Chapter 4  Example 4 (Cont’d) Use the lowest numbering for the carbon bearing the OH group 2 6 (lowest number of the carbon bearing the OH group) ⇒ Use 2 Chapter 4  Example 4 (Cont’d) Parent and suffix  2-Heptanol 1 5 7 2 4 6 3 Substituents OH  Propyl Complete name  3-Propyl-2-heptanol Chapter 4 4. How to Name Cycloalkanes 4A. How to Name Monocyclic Cycloalkanes  Cycloalkanes with only one ring Attach the prefix cyclo- H2C CH2 = H 2C CH2 = C H 2C CH2 H2 C Cyclopropane H2 Cyclopentane Chapter 4  Substituted cycloalkanes Isopropylcyclopropane tert-Butylcyclopentane Chapter 4  Example 1 4 3 2 1-Ethyl-3-methyl- 1 cyclopentane 5 NOT NOT 3 4 5 5 1 2 2 1 4 3 1-Ethyl-4-methyl- 3-Ethyl-1-methyl- cyclopentane cyclopentane Chapter 4  Example 2 Br 5 4 3 4-Bromo-2-ethyl-1-methyl 6 1 2 cyclohexane NOT Br 6 1 2 1-Bromo-3-ethyl-4-methyl 5 4 3 cyclohexane (lowest numbers of substituents are 1,2,4 not 1,3,4) Chapter 4  Example 3 4 3 2 4-Ethyl-3-methyl 5 6 1 cyclohexanol OH NOT 1 2 3 1-Ethyl-2-methyl 6 5 4 cyclohexan-4-ol OH (the carbon bearing the OH should have the lowest numbering, even though 1,2,4 is lower than 1,3,4) Chapter 4  Cycloalkylalkanes When a single ring system is attached to a single chain with a greater number of carbon atoms 1-Cyclobutylpentane When more than one ring system is attached to a single chain 1,3-Dicyclohexylpropane Chapter 4 4B. How to Name Bicyclic Cycloalkanes  Bicycloalkanes Alkanes containing two fused or bridged rings  Total # of carbons = 7 Bicycloheptane  Bridgehead Chapter 4  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[2.2.1]heptane Chapter 4  Other examples 9 2 1 3 8 7 4 6 5 7-Methylbicyclo[4.3.0]nonane 8 7 4 5 3 1 6 2 1-Isopropylbicyclo[2.2.2]octane Chapter 4 5. How to Name Alkenes & Cycloalkenes  Rule 1. 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 Chapter 4  Rule 2. 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 Chapter 4  Examples 1 2 3 4 CH2 CHCH 2CH3 1-Butene (not 3-Butene) 1 2 3 4 5 6 CH3CH CHCH 2CH2CH3 2-Hexene (not 4-Hexene) Chapter 4  Rule 3. Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached Examples 4 3 2 1 2-Methyl-2-butene (not 3-Methyl-2-butene) Chapter 4  Examples (Cont’d) 4 6 3 5 2 1 2,5-Dimethyl-2-hexene 3 1 NOT 4 2 5 6 2,5-Dimethyl-4-hexene Chapter 4  Rule 4. Number substituted cycloalkenes in the way that gives the carbon atoms of C=C the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference Chapter 4  Example 1 6 2 5 3 4 3,5-Dimethylcyclohexene 2 NOT 3 1 4 6 5 4,6-Dimethylcyclohexene Chapter 4  Rule 5. Name compounds containing a C=C and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number OH Examples 6 1 2 5 3 4 2-Methyl-2-cyclohexen-1-ol (or 2-Methylcyclohex-2-en-1-ol) Chapter 4  Examples (Cont’d) OH 4 2 5 3 1 4-Methyl-3-penten-2-ol (or 4-Methylpent-3-en-2-ol) Chapter 4  Rule 6. Vinyl group & allyl group Vinyl group Allyl group ethenyl prop-2-en-1-yl OH 6 1 2 5 Ethenylcyclopropane 3 4 (or Vinylcyclopropane) 3-(Prop-2-en-1-yl) cyclohexan-1-ol (or 3-Allylcyclohexanol) Chapter 4  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 Cl cis-1,2-Dichloroethene trans-1,2-Dichloroethene Chapter 4  Example Chapter 4  Example (Cont’d) 6 (a) (b) 5 7 6 4 2 4 2 3 1 5 3 1 (c) (d) 6 2 4 2 4 6 7 5 3 1 1 3 5 7 Chapter 4  Example (Cont’d) Complete name 2 4 6 1 3 5 7 4-tert-Butyl-2-methyl-1-heptene Chapter 4 6. How to Name Alkynes  Alkynes are named in much the same way as alkenes, but ending name with –yne instead of –ene  Examples 6 4 3 2 1 2 3 Br 1 7 5 4 2-Heptyne 4-Bromo-1-butyne Chapter 4  Examples (Cont’d) 3 4 I Br 2 5 6 7 8 9 1 10 9-Bromo-7-iodo-6-isopropyl-8-methyl-3-decyne Chapter 4  OH group has priority over C≡C 4 3 1 2 2 3 OH NOT OH 1 4 3-Butyn-1-ol OH 5 6 7 OH 4 3 2 2 4 7 5 1 3 8 8 6 1 2-Methyl-5-octyn-2-ol NOT Chapter 4 7. Physical Properties of Alkanes & Cycloalkanes  Boiling points & melting points Chapter 4 C6H14 Isomer Boiling Point (oC) 68.7 63.3 60.3 58 49.7 Chapter 4 Physical Constants of Cycloalkanes # of C Refractive Atoms Name bp (oC) mp (oC) Density Index 3 Cyclopropane -33 -126.6 - - 4 Cyclobutane 13 -90 - 1.4260 5 Cyclopentane 49 -94 0.751 1.4064 6 Cyclohexane 81 6.5 0.779 1.4266 7 Cycloheptane 118.5 -12 0.811 1.4449 8 Cyclooctane 149 13.5 0.834 - Chapter 4 8. 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 that result from a rotation about a single bond Conformer – each possible structure of conformation Conformational analysis – analysis of energy changes that occur as a molecule undergoes rotations about single bonds Chapter 4  8A. Newman Projections & How to Draw Them Me H H Sawhorse formula Cl OH Et Look from this direction H Me H H combine Me H Cl Et OH Cl Et front carbon back carbon OH Newman Projection Chapter 4 8B. How to Do a Conformational Analysis f1 = 60o f2 = 180o Chapter 4  f = 0o Chapter 4 Chapter 4 9. Conformational Analysis of Butane Chapter 4 Chapter 4 Chapter 4  60o 0o CH3 CH3 CH3 CH3 CH3 180o CH3 anti gauche eclipsed Chapter 4 10. 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 Chapter 4 10A. Cyclopropane H H sp3 hybridized carbon q H H (normal tetrahedral bond angle is 109.5o) H H  Internal bond angle (q) ~60o (~49.5o deviated from the ideal tetrahedral angle) Chapter 4 Chapter 4 10B. Cyclobutane H H q H H H H H H  Internal bond angle (q) ~88o (~21o deviated from the normal 109.5o tetrahedral angle) Chapter 4  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 90o instead of 88o), but torsional strain would be considerably larger because all eight C–H bonds would be eclipsed Chapter 4 10C. Cyclopentane H H H HH HH H H H  If cyclopentane were planar, q ~108o, very close to the normal tetrahedral angle of 109.5o  However, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed)  Therefore cyclopentane has a slightly bent conformation Chapter 4 11. Conformations of Cyclohexane: The Chair & the Boat 6 4 2 3 3D 5 2 6 5 1 3 1 4 (chair form) (boat form) (more stable) (less stable) H H H H H 4 H H H 6 2 5 3 5 3 6 4 2 H H HH 1 HH H H 1 Chapter 4  The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to Eclipsed conformation 1,4-flagpole interactions 1 H H 4 H H H H (eclipsed) Chapter 4  (twist boat)  The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation Chapter 4  Energy diagram Chapter 4 12. Substituted Cyclohexanes: Axial & Equatorial Hydrogen Groups  The six-membered ring is the most common ring found among nature’s organic molecules  The chair conformation of a cyclohexane ring has two distinct orientations for the bonds that project from the ring: axial and equatorial Chapter 4 12A. How to Draw Chair Conformational Structures  When you draw chair conformational structures, try to make the corresponding bonds parallel in your drawings Chapter 4  Axial hydrogen atoms in chair form The axial bonds are all either up or down, in a vertical orientation H H H H H H Chapter 4  Equatorial hydrogen atoms in chair form The equatorial bonds are all angled slightly Chapter 4 12B. A Conformational Analysis of Methylcyclohexane  Substituted cyclohexane Two different chair forms H G H G Chapter 4  The chair conformation with axial G is less stable due to 1,3-diaxial interaction  The larger the G group, the more severe the 1,3-diaxial interaction and shifting of the equilibrium from the axial-G chair form to the equatorial-G chair form Chapter 4  G (equatorial) (axial) G At 25oC G % of Equatorial % of Axial F 60 40 CH3 95 5 iPr 97 3 tBu > 99.99 < 0.01 Chapter 4 12C. 1,3-Diaxial Interactions of a tert-Butyl Group  The chair conformation with axial tert- butyl group is less stable due to 1,3- diaxial interaction 1,3-diaxial interaction Chapter 4 13. Disubstituted Cycloalkanes: Cis-Trans Isomerism H H H CH3 CH3 CH3 CH3 H cis-1,2-Dimethyl trans-1,2-Dimethyl cyclopropane cyclopropane Cl Cl Cl H H H H Cl cis-1,2-Dichloro trans-1,2-Dichloro cyclobutane cyclobutane Chapter 4 13A. Cis-Trans Isomerism and Conformation Structures of Cyclohexanes  Trans-1,4-Disubstituted Cyclohexanes CH3 H ring H3C H H flip CH3 CH3 H trans-Diaxial trans-Diequatorial Chapter 4  Upper bond H CH3 trans-Dimethyl H3C cyclohexane H Lower bond  Upper-lower bonds means the groups are trans Chapter 4   Cis-1,4-Disubstituted Cyclohexanes CH3 chair-chair CH3 ring H H H3C flip CH3 H H Equatorial-axial Axial-equatorial Chapter 4   Cis-1-tert-Butyl-4-methylcyclohexane H3C CH3 CH3 H 3C CH3 ring H3C CH3 flip H3C (more stable (less stable because large because large group is group is equatorial) axial) Chapter 4  Trans-1,3-Disubstituted Cyclohexanes (ax) CH3 (eq) ring H H H3C H flip CH3 H CH3 (eq) (ax) trans-1,3-Dimethylcyclohexane Chapter 4   Trans-1-tert-Butyl-3-methylcyclohexane H3C CH3 H3C CH3 ring H3C CH3 flip H3C CH3 (more stable (less stable because large because large group is group is equatorial) axial) Chapter 4  Cis-1,3-Disubstituted Cyclohexanes H H ring CH3 flip H H CH3 CH3 CH3 (more stable) (less stable) Chapter 4  Trans-1,2-Disubstituted Cyclohexanes (ax) (eq) CH3 CH3 ring CH3 flip (ax) (eq) CH3 diequatorial diaxial (much more stable) (much less stable) trans-1,2-Dimethylcyclohexane Chapter 4  Cis-1,2-Disubstituted Cyclohexane (ax) (ax) CH3 CH3 ring CH3 flip CH3 (eq) (eq) (equatorial-axial) (axial-equatorial) cis-1,2-Dimethylcyclohexane (equal energy and equally populated conformations) Chapter 4 14. Bicyclic & Polycyclic Alkanes Decalin (Bicyclo[4.4.0]decane) H H H H cis-Decalin trans-Decalin H H H H Chapter 4 Adamantane Cubane Prismane C60 (Buckminsterfullerene) Chapter 4 15. Chemical Reactions of Alkanes  Alkanes, as a class, are characterized by a general inertness to many chemical reagents  Carbon–carbon and carbon–hydrogen bonds are quite strong; they do not break unless alkanes are heated to very high temperatures Chapter 4  Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized  This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins (parum affinis, Latin: little affinity) Chapter 4 16. Synthesis of Alkanes and Cycloalkanes 16A. Hydrogenation of Alkenes & Alkynes H2 H H Pt, Pd or Ni C C solvent heat and pressure 2H2 H H Pt, Pd or Ni C C solvent heat and pressure H H Chapter 4  Examples Ni + H2 EtOH H H o 25 C, 50 atm. Pd H + H2 EtOH H o 25 C, 1 atm. Pd H H EtOAc H H + 2 H2 o 65 C, 1 atm. Chapter 4 17. How to Gain Structural Inform- ation from Molecular Formulas & Index of Hydrogen Deficiency  Index of hydrogen deficiency (IHD) 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) Chapter 4  Index of hydrogen deficiency (Cont’d) Saturated acyclic alkanes: CnH2n+2 Each double bond or ring: 2 hydrogens less Each double bond or ring provides one unit of hydrogen deficiency Chapter 4  e.g. Hexane: C6H14 and C6H12 1-Hexene Cycloheane C6H14 Index of hydrogen – C6H12 = deficiency (IHD) H2 = one pair of H2 =1 Chapter 4  Examples IHD = 2 IHD = 3 IHD = 2 IHD = 4 Chapter 4 17A. Compounds Containing Halogen, Oxygen, or Nitrogen  For compounds containing Halogen – count halogen atoms as though they were hydrogen atoms Oxygen – ignore oxygen atoms and calculate IHD from the remainder of the formula Nitrogen – subtract one hydrogen for each nitrogen atom and ignore nitrogen atoms Chapter 4  Example 1: IHD of C4H6Cl2 Count Cl as H C4H10  C4H6Cl2 ⇒ C4H8 – C4H8 A C4 acyclic alkane: C4H2(4)+2 = C4H10 H2 IHD of C4H6Cl2 = one pair of H2 = 1 Possible structures Cl Cl or or Cl Cl... etc. Cl Cl Chapter 4  Example 2: IHD of C5H8O Ignore oxygen C5H12  C5H8O ⇒ C5H8 – C5H8 A C5 acyclic alkane: C5H2(5)+2 = C5H12 H4 IHD of C4H6Cl2 = two pairs of H2 = 2 Possible structures O or OH or OH... etc. Chapter 4  Example 3: IHD of C5H7N Subtract 1 H for each N C5H12  C5H7N ⇒ C5H6 – C5H6 A C5 acyclic alkane: C5H2(5)+2 = C5H12 H6 IHD of C4H6Cl2 = three pair of H2 = 3 Possible structures N or C N... etc. CH3 Chapter 4 18. Applications of Basic Principles  Nature prefers states of lower potential energy  Steric factors (spatial factors) can affect the stability and reactivity of molecules  Like charges repel Chapter 4  END OF CHAPTER 4  Chapter 4