ALKANE FORMULAS. Chapter 3 Structure and Stereochemistry of Alkanes. Lower Membered Alkanes trivial (common) names CH 3 CH 3 CH 3 CH 2 CH 3 ETHANE

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1 Organic hemistry, 5 th Edition L. G. Wade, Jr. hapter 3 Structure and Stereochemistry of Alkanes ALKANE FORMULAS Four carbon/or hydogen atoms bonded to each carbon atom All - single bonds Ratio: n 2n+2 NOTE: always an even number of hydrogen atoms in a hydrocarbon Alkane homologs : each member in a alkane series different from the next member by a 2 group hapter Lower Membered Alkanes trivial (common) names METANE ETANE PROPANE ondensed Line-angle hapter 3 3 hapter 3 4 1

2 BUTANES Pentanes n-butane iso-butane n-pentane, 5 12 isopentane, Note: 1 here hapter neopentane, 5 12 hapter 3 6 Writing Isomers - eptanes 3. Shorten chain by one more carbon. This gives a 5-carbon chain (pentane). Two carbons are to be added as two methyl groups or a single two-carbon group (ethyl group). 1. Start with longest straight chain possible 2. Shorten chain by one carbon and add the "extra" at all possible posi Obviously you do not add the extra carbon to end carbon!!! 2-Methylhexane 3-Methylhexane TIS is 3-methylhexane!! 2-Methylhexane hapter 3 3-Methylhexane 7 hapter 3 8 2

3 Shorten chain by yet one more carbon. This gives a 4-carbon chain (butane). Three carbons are to be added as three methyl group. NOTE: an t derive another butane chain by adding an ethyl group. 2,2,3-trimethylbutane 3,3-dimethylpentane ommon 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! hapter 3 9 hapter 3 10 IUPA 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. hapter 3 11 Longest hain The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.) If there are two possible chains with the same number of carbons, use the chain with the most substituents hapter

4 Number the arbons Start at the end closest to the first attached group. If two substituents are equidistant, look for the next closest group hapter 3 13 Name Alkyl Groups 3 -, methyl 3 2 -, ethyl , n-propyl , n-butyl 3 3 isopropyl sec-butyl isobutyl 3 3 tert-butyl hapter 3 14 Propyl Groups Butyl Groups n-propyl isopropyl n-butyl sec-butyl A primary carbon A secondary carbon A primary carbon A secondary carbon hapter 3 15 hapter

5 Isobutyl Groups Alphabetize Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing isobutyl A primary carbon tert-butyl A tertiary carbon hapter ethyl-2,6-dimethylheptane hapter 3 18 omplex 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. 3 1-methyl-3-(1,2-dimethylpropyl)cyclohexane hapter 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 oddnumber of carbons). hapter

6 Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces. Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p. hapter 3 21 hapter 3 22 Branched Alkanes Major Uses of Alkanes Lower b.p. with increased branching igher m.p. with increased branching Examples: bp 60 mp bp 58 mp bp 50 mp -98 hapter : gases (natural gas) 3-4 : liquified petroleum (LPG) 5-8 : gasoline 9-16 : diesel, kerosene, jet fuel 17 -up: lubricating oils, heating oil Origin: petroleum refining hapter

7 Reactions of Alkanes onformers of Alkanes ombustion heat O 2 8 O O racking and hydrocracking (industrial) long-chain alkanes alogenation catalyst shorter-chain alkanes heat or light 4 + l 2 3 l + 2 l 2 + l 3 + l 4 hapter 3 25 Structures resulting from the free rotation of a - single bond May differ in energy. The lowest-energy conformer is most prevalent. Molecules constantly rotate through all the possible conformations. hapter 3 26 Ethane onformers Staggered conformer has lowest energy. Dihedral angle = 60 degrees Ethane onformers (2) Eclipsed conformer has highest energy Dihedral angle = 0 degrees model Newman projection sawhorse hapter 3 27 hapter

8 onformational Analysis Torsional strain: resistance to rotation. For ethane, only 3.0 kcal/mol Propane onformers Note slight increase in torsional strain due to the more bulky methyl group. hapter 3 29 hapter 3 30 Butane onformers 2-3 ighest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees Butane onformers (2) Lowest energy has methyl groups anti. Dihedral angle = 180 degrees totally eclipsed hapter 3 31 anti hapter

9 Butane onformers (3) Methyl groups eclipsed with hydrogens igher energy than staggered conformer Dihedral angle = 120 degrees Butane onformers (4) Gauche, staggered conformer Methyls closer than in anti conformer Dihedral angle = 60 degrees eclipsed hapter 3 33 gauche hapter 3 34 onformational Analysis igher Alkanes Anti conformation is lowest in energy. Straight chain actually is zigzag hapter 3 35 hapter

10 ycloalkanes Rings of carbon atoms ( 2 groups) Formula: n 2n Nonpolar, insoluble in water ompact shape Melting and boiling points similar to branched alkanes with same number of carbons Naming ycloalkanes ycloalkane usually base compound Number carbons in ring if >1 substituent. First in alphabet gets lowest number. May be cycloalkyl attachment to chain hapter 3 37 hapter 3 38 is-trans Isomerism is: like groups on same side of ring Trans: like groups on opposite sides of ring ycloalkane Stability 5- and 6-membered rings most stable Bond angle closest to Angle (Baeyer) strain Measured by heats of combustion per hapter 3 39 hapter

11 eats of ombustion Alkane + O 2 O O yclopropane Large ring strain due to angle compression Very reactive, weak bonds Long-chain hapter 3 41 hapter 3 42 yclopropane (2) Torsional strain because of eclipsed hydrogens yclobutane Angle strain due to compression Torsional strain partially relieved by ring-puckering hapter 3 43 hapter

12 yclopentane If planar, angles would be 108, but all hydrogens would be eclipsed. Puckered conformer reduces torsional strain. yclohexane ombustion data shows it s unstrained. Angles would be 120, if planar. The chair conformer has bond angles and all hydrogens are staggered. No angle strain and no torsional strain. hapter 3 45 hapter 3 46 hair onformer Boat onformer hapter 3 47 hapter

13 onformational Energy Axial and Equatorial Positions hapter 3 49 hapter 3 50 Monosubstituted yclohexanes 1,3-Diaxial Interactions hapter 3 51 hapter

14 Disubstituted yclohexanes is-trans Isomers Bonds that are cis, alternate axialequatorial around the ring. 3 3 hapter 3 53 hapter 3 54 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. Bicyclic Alkanes Fused rings share two adjacent carbons. Bridged rings share two nonadjacent s. bicyclo[3.1.0]hexane bicyclo[2.2.1]heptane hapter 3 55 hapter

15 is- and Trans-Decalin Fused cyclohexane chair conformers Bridgehead s cis, structure more flexible Bridgehead s trans, no ring flip possible. End of hapter 3 cis-decalin trans-decalin hapter 3 57 hapter