Organic Chemistry, 5th Edition Alkane Formulas L. G. Wade, Jr. • All C-C single bonds • Saturated with hydrogens Chapter 3 • Ratio: CnH2n+2 Structure and Stereochemistry • Alkane homologs: CH3(CH2)nCH3 of Alkanes • Same ratio for branched alkanes H H H H H H CH H C C C C H H H H C CCH Jo Blackburn H H H H H H Richland College, Dallas, TX H => Butane, C4H10 Dallas County Community College District Chapter 3Isobutane, C H 2 © 2003, Prentice Hall 4 10 Common Names Pentanes H H H H H H H CH • Isobutane, “isomer of butane” H C C C C C H H H • Isopentane, isohexane, etc., methyl H H H H H H H C CCC H branch on next-to-last carbon in chain. H H H H • Neopentane, most highly branched n-pentane, C5H12 isopentane, C5H12 • Five possible isomers of hexane, CH3 18 isomers of octane and 75 for H3C C CH3 decane! CH3 => => neopentane, C5H12 Chapter 3 3 Chapter 3 4 Longest Chain IUPAC Names • The number of carbons in the longest • Find the longest continuous carbon chain determines the base name: chain. ethane, hexane. (Listed in Table 3.2, • Number the carbons, starting closest to page 81.) the first branch. • If there are two possible chains with the • Name the groups attached to the chain, same number of carbons, use the chain using the carbon number as the locator. with the most substituents. • Alphabetize substituents. H3C CH CH CH • Use di-, tri-, etc., for multiples of same 2 3 CH3 substituent. => H3C CH2 C CH CH2 CH2 CH3 CH3 Chapter 3 5 Chapter 3=> 6 1 Number the Carbons Name Alkyl Groups • Start at the end closest to the first •CH-, methyl attached group. 3 CH3 •CH3CH2-, ethyl • If two substituents are equidistant, look CH CH CH •CHCH CH -, n-propyl 3 2 for the next closest group. 3 2 2 isobutyl •CHCH CH CH -, n-butyl 3 2 2 2 CH CH CH 3 1 3 3 34 5 CH3 CH CH3 CH3 CH CH2 CH3 H3C C CH3 H3C CH CH CH2 CH2 CH CH3 tert-butyl 2 isopropyl sec-butyl CH CH 67 2 3 => => Chapter 3 7 Chapter 3 8 Propyl Groups Butyl Groups H H H H H H H H H H H H H H H C C C H H C C C H H C C C C H H C C C C H H H H H H H H H H H H H H H n-propyl isopropyl n-butyl sec-butyl A primary carbon A secondary carbon A primary carbon A secondary carbon => => Chapter 3 9 Chapter 3 10 Isobutyl Groups Alphabetize H H H H • Alphabetize substituents by name. H C H C • Ignore di-, tri-, etc. for alphabetizing. H H H H H C C C H H C C C H CH3 CH3 H HH H H H H3C CH CH CH2 CH2 CH CH3 isobutyl tert-butyl CH2CH3 A primary carbon A tertiary carbon 3-ethyl-2,6-dimethylheptane => => Chapter 3 11 Chapter 3 12 2 Complex Substituents Physical Properties • If the branch has a branch, number the carbons from the point of attachment. • Solubility: hydrophobic • Name the branch off the branch using a • Density: less than 1 g/mL locator number. • Boiling points increase with • Parentheses are used around the increasing carbons (little less for complex branch name. branched chains). 1 2 • Melting points increase with increasing carbons (less for odd- 3 1-methyl-3-(1,2-dimethylpropyl)cyclohexane => number of carbons). Chapter 3 13 Chapter 3 14 Boiling Points of Alkanes Melting Points of Alkanes Branched alkanes have less surface area contact, Branched alkanes pack more efficiently into so weaker intermolecular forces. a crystalline structure, so have higher m.p. => => Chapter 3 15 Chapter 3 16 Branched Alkanes Major Uses of Alkanes • Lower b.p. with increased branching •C1-C2: gases (natural gas) • Higher m.p. with increased branching •C3-C4: liquified petroleum (LPG) • Examples: •C5-C8: gasoline CH3 •C9-C16: diesel, kerosene, jet fuel CH3 C CH2 CH3 CH3 CH3 CH3 •C17-up: lubricating oils, heating oil CH CH2 CH2 CH3 CH CH CH3 CH CH3 CH3 3 bp 60°C bp 58°C bp 50°C • Origin: petroleum refining mp -154°C mp -135°C mp -98°C => Chapter 3 17 => Chapter 3 18 3 Reactions of Alkanes Conformers of Alkanes • Combustion • Structures resulting from the free heat rotation of a C-C single bond 2 CH3CH2CH2CH3 + 13 O2 810CO2 + H2O • May differ in energy. The lowest-energy • Cracking and hydrocracking (industrial) catalyst conformer is most prevalent. long-chain alkanes shorter-chain alkanes • Molecules constantly rotate through all • Halogenation the possible conformations. heat or light CH + Cl 4 2 CH3Cl + CH2Cl2 + CHCl3 + CCl4 => => Chapter 3 19 Chapter 3 20 Ethane Conformers (2) Ethane Conformers • Eclipsed conformer has highest energy • Dihedral angle = 0 degrees • Staggered conformer has lowest energy. • Dihedral angle = 60 degrees H H H HH => H => Newman sawhorse model projection Chapter 3 21 Chapter 3 22 Conformational Analysis Propane Conformers • Torsional strain: resistance to rotation. Note slight increase in torsional strain • For ethane, only 3.0 kcal/mol due to the more bulky methyl group. => => Chapter 3 23 Chapter 3 24 4 Butane Conformers C2-C3 Butane Conformers (2) • Highest energy has methyl groups eclipsed. • Lowest energy has methyl groups anti. • Steric hindrance • Dihedral angle = 180 degrees • Dihedral angle = 0 degrees anti totally eclipsed => => Chapter 3 25 Chapter 3 26 Butane Conformers (3) Butane Conformers (4) • Methyl groups eclipsed with hydrogens • Gauche, staggered conformer • Higher energy than staggered • Methyls closer than in anti conformer conformer • Dihedral angle = 60 degrees • Dihedral angle = 120 degrees => eclipsed gauche => Chapter 3 27 Chapter 3 28 Conformational Analysis Higher Alkanes • Anti conformation is lowest in energy. • “Straight chain” actually is zigzag. CH3CH2CH2CH2CH3 H HH H H H C C C H C C H H H HH => => Chapter 3 29 Chapter 3 30 5 Cycloalkanes Naming Cycloalkanes • Rings of carbon atoms (CH2 groups) • Cycloalkane usually base compound • Formula: CnH2n • Number carbons in ring if >1 substituent. • Nonpolar, insoluble in water • First in alphabet gets lowest number. • Compact shape • May be cycloalkyl attachment to chain. • Melting and boiling points similar to branched alkanes with same number of CH2CH3 carbons => CH2CH3 CH3 => Chapter 3 31 Chapter 3 32 Cis-Trans Isomerism Cycloalkane Stability • 5- and 6-membered rings most stable • Bond angle closest to 109.5° • Angle (Baeyer) strain • Measured by heats of combustion • Cis: like groups on same side of ring per -CH2 - • Trans: like groups on opposite sides of ring => => Chapter 3 33 Chapter 3 34 Heats of Combustion Alkane + O2 → CO2 + H2O Cyclopropane 166.6 164.0 • Large ring strain due to angle compression 158.7 158.3 158.6 157.4 157.4 • Very reactive, weak bonds Long-chain => => Chapter 3 35 Chapter 3 36 6 Cyclopropane (2) Cyclobutane • Angle strain due to compression Torsional strain because of eclipsed • Torsional strain partially relieved by hydrogens ring-puckering => => Chapter 3 37 Chapter 3 38 Cyclopentane Cyclohexane • If planar, angles would be 108°, but all hydrogens would be eclipsed. • Combustion data shows it’s unstrained. • Puckered conformer reduces torsional strain. • Angles would be 120°, if planar. • The chair conformer has 109.5° bond angles and all hydrogens are staggered. • No angle strain and no torsional strain. => => Chapter 3 39 Chapter 3 40 Chair Conformer Boat Conformer => => Chapter 3 41 Chapter 3 42 7 Axial and Equatorial Conformational Energy Positions => Chapter 3 43 Chapter 3 44 => Monosubstituted 1,3-Diaxial Interactions Cyclohexanes => => Chapter 3 45 Chapter 3 46 Disubstituted Cis-Trans Isomers Cyclohexanes Bonds that are cis, alternate axial- equatorial around the ring. CH3 CH3 => => Chapter 3 47 Chapter 3 48 8 Bulky Groups Bicyclic Alkanes • Groups like t-butyl cause a large energy difference between the axial and equatorial • Fused rings share two adjacent carbons. conformer. • Bridged rings share two nonadjacent C’s. • Most stable conformer puts t-butyl equatorial regardless of other substituents. bicyclo[3.1.0]hexane bicyclo[2.2.1]heptane => Chapter 3 49 => Chapter 3 50 Cis- and Trans-Decalin • Fused cyclohexane chair conformers • Bridgehead H’s cis, structure more flexible • Bridgehead H’s trans, no ring flip possible. End of Chapter 3 H H H => H cis-decalin trans-decalin Chapter 3 51 Chapter 3 52 9.