Chapter 3 Structure and Stereochemistry of Alkanes Organic Chemistry, 6th Edition L. G. Wade, Jr. Chapter 3 Structure and Stereochemistry of Alkanes ã 2006, Prentice Hall

Classification Review Chapter 3

Alkane Formulas All C-C single bonds Saturated with hydrogens Ratio: CnH2n+2 Alkane homologs: CH3(CH2)nCH3 Same ratio for branched alkanes Chapter 3

Common Names Isobutane, “isomer of butane” Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. Neopentane, most highly branched Five possible isomers of hexane, 18 isomers of octane and 75 for decane! Chapter 3

Alkane Examples Chapter 3

IUPAC Names Find the longest continuous carbon chain. Number the carbons, starting closest to the first branch. Name the groups attached to the chain, using the carbon number as the locator. Alphabetize substituents. Use di-, tri-, etc., for multiples of same substituent. Chapter 3

Longest Chain The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.) If there are two possible chains with the same number of carbons, use the chain with the most substituents. Chapter 3

Number the Carbons Start at the end closest to the first attached group. If two substituents are equidistant, look for the next closest group. 1 2 3 4 5 6 7 Chapter 3

Name Alkyl Groups CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl CH3CH2CH2CH2-, n-butyl Chapter 3

Propyl Groups H H n-propyl isopropyl A primary carbon A secondary carbon Chapter 3

Butyl Groups H H n-butyl sec-butyl A primary carbon A secondary carbon Chapter 3

Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon A primary carbon Chapter 3

Alphabetize Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane Chapter 3

Complex Substituents If the branch has a branch, number the carbons from the point of attachment. Name the branch off the branch using a locator number. Parentheses are used around the complex branch name. 1 2 3 1-methyl-3-(1,2-dimethylpropyl)cyclohexane Chapter 3

Physical Properties Solubility: hydrophobic Density: less than 1 g/mL Boiling points increase with increasing carbons (little less for branched chains). Melting points increase with increasing carbons (less for odd- number of carbons). Chapter 3

Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces. Chapter 3

Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p. Chapter 3

Branched Alkanes Lower b.p. with increased branching Higher m.p. with increased branching Examples: H C 3 2 bp 60°C mp -154°C bp 58°C mp -135°C bp 50°C mp -98°C Chapter 3

Major Uses of Alkanes C1-C2: gases (natural gas) C3-C4: liquified petroleum (LPG) C5-C8: gasoline C9-C16: diesel, kerosene, jet fuel C17-up: lubricating oils, heating oil Origin: petroleum refining Chapter 3

Reactions of Alkanes Combustion Cracking and hydrocracking (industrial) Halogenation Chapter 3

Conformers of Alkanes Structures resulting from the free rotation of a C-C single bond May differ in energy. The lowest-energy conformer is most prevalent. Molecules constantly rotate through all the possible conformations. Chapter 3

Ethane Conformers Staggered conformer has lowest energy. Dihedral angle = 60 degrees model H Newman projection sawhorse Chapter 3

Ethane Conformers (2) Eclipsed conformer has highest energy Dihedral angle = 0 degrees Chapter 3

Conformational Analysis Torsional strain: resistance to rotation. For ethane, only 12.6 kJ/mol Chapter 3

Propane Conformers Note slight increase in torsional strain due to the more bulky methyl group. Chapter 3

Butane Conformers C2-C3 Highest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees totally eclipsed Chapter 3

Butane Conformers (2) Lowest energy has methyl groups anti. Dihedral angle = 180 degrees anti Chapter 3

Butane Conformers (3) Methyl groups eclipsed with hydrogens Higher energy than staggered conformer Dihedral angle = 120 degrees eclipsed Chapter 3

Butane Conformers (4) Gauche, staggered conformer Methyls closer than in anti conformer Dihedral angle = 60 degrees gauche Chapter 3

Conformational Analysis Chapter 3

Higher Alkanes Anti conformation is lowest in energy. “Straight chain” actually is zigzag. Chapter 3

Cycloalkanes Rings of carbon atoms (-CH2- groups) Formula: CnH2n Nonpolar, insoluble in water Compact shape Melting and boiling points similar to branched alkanes with same number of carbons Chapter 3

Naming Cycloalkanes Cycloalkane usually base compound Number carbons in ring if >1 substituent. First in alphabet gets lowest number. May be cycloalkyl attachment to chain. Chapter 3

Cis-Trans Isomerism Cis: like groups on same side of ring Trans: like groups on opposite sides of ring Chapter 3

Cycloalkane Stability 5- and 6-membered rings most stable Bond angle closest to 109.5 Angle (Baeyer) strain Measured by heats of combustion per -CH2 - Chapter 3

Heats of Combustion/CH2 Alkane + O2  CO2 + H2O 697.1 686.1 664.0 663.6 kJ/mol 662.4 Long-chain 658.6 kJ 658.6 Chapter 3

Cyclopropane Large ring strain due to angle compression Very reactive, weak bonds Chapter 3

Cyclopropane (2) Torsional strain because of eclipsed hydrogens Chapter 3

Cyclobutane Angle strain due to compression Torsional strain partially relieved by ring-puckering Chapter 3

Cyclopentane If planar, angles would be 108, but all hydrogens would be eclipsed. Puckered conformer reduces torsional strain. Chapter 3

Cyclohexane Combustion data shows it’s unstrained. 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

Chair Conformer Chapter 3

Boat Conformer Chapter 3

Conformational Energy Chapter 3

Axial and Equatorial Positions Chapter 3

Monosubstituted Cyclohexanes Chapter 3

1,3-Diaxial Interactions Chapter 3

Disubstituted Cyclohexanes Chapter 3

Cis-Trans Isomers Bonds that are cis, alternate axial-equatorial around the ring. One axial, one equatorial Chapter 3

Bulky Groups Groups like t-butyl cause a large energy difference between the axial and equatorial conformer. Most stable conformer puts t-butyl equatorial regardless of other substituents. Chapter 3

Bicyclic Alkanes Fused rings share two adjacent carbons. Bridged rings share two nonadjacent C’s. bicyclo[2.2.1]heptane bicyclo[3.1.0]hexane Chapter 3

Cis- and Trans-Decalin Fused cyclohexane chair conformers Bridgehead H’s cis, structure more flexible Bridgehead H’s trans, no ring flip possible. trans-decalin cis-decalin Chapter 3

Bicyclo[4.4.0]decane Chapter 3

End of Chapter 3 Chapter 3