Chapter 3 Structure and Stereochemistry of Alkanes. Classification Review. Alkenes: Structure and Stereochem Slide 3-2

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1 hapter 3 Structure and Stereochemistry of Alkanes lassification Review Alkenes: Structure and Stereochem Slide 3-2 1

2 Alkane Structural Formulas All - single bonds Saturated with hydrogens (no pi bonds) Ratio: n 2n+2 ; cycloalkanes will be different! Straight chain. acyclic alkane homologs: 3 ( 2 ) n 3 Same ratio for branched alkanes (alkanes with carbons attached to internal carbons of longest chain) Butane, 4 10 Isobutane, 4 10 Alkenes: Structure and Stereochem Slide 3-3 Alkane Examples Alkenes: Structure and Stereochem Slide 3-4 2

3 IUPA Nomenclature 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. See nomenclature of organic molecules handout Alkenes: Structure and Stereochem Slide 3-5 Longest hain The number of carbons in the longest chain determines the base name: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, undecane. (for 1 to 11 ) If there are two possible chains with the same number of carbons, use the chain with the most substituents Alkenes: Structure and Stereochem Slide 3-6 3

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 (rule of lowest sum of index numbers) vs Alkenes: Structure and Stereochem Slide , methyl 3 2 -, ethyl , n-propyl , n-butyl etc Alkyl Groups 3 3 isopropyl sec-butyl isobutyl tert-butyl Alkenes: Structure and Stereochem Slide 3-8 4

5 5 Alkenes: Structure and Stereochem Slide 3-9 Propyl Groups n-propyl isopropyl A primary carbon A secondary carbon Alkenes: Structure and Stereochem Slide 3-10 Butyl Groups n-butyl sec-butyl A primary carbon A secondary carbon

6 Isobutyl Groups isobutyl A primary carbon tert-butyl A tertiary carbon Alkenes: Structure and Stereochem Slide 3-11 Alphabetize Alphabetize substituents by name. Ignore di-, tri-, etc. for alphabetizing ethyl-2,6-dimethylheptane Alkenes: Structure and Stereochem Slide

7 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 methyl-3-(1,2-dimethylpropyl)cyclohexane 3 Alkenes: Structure and Stereochem Slide 3-13 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). Alkenes: Structure and Stereochem Slide

8 Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces. Alkenes: Structure and Stereochem Slide 3-15 Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p. Alkenes: Structure and Stereochem Slide

9 Branched Alkanes Lower b.p. with increased branching igher m.p. with increased branching Examples: bp 60 mp bp 58 mp bp 50 mp -98 Alkenes: Structure and Stereochem Slide 3-17 Major Uses of Alkanes 1-2 : 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 Alkenes: Structure and Stereochem Slide

10 Reactions of Alkanes ombustion omplete oxidation of an organic molecule in the presence of oxygen: O 2! "! 8 O O + heat racking and hydrocracking (industrial) catalyst long-chain alkanes shorter-chain alkanes Free-Radical alogenation Reaction of an organic molecule with X 2 ; replaces s on sp 3 carbons: heat or light 4 + l 2 3 l + 2 l 2 + l 3 + l 4 Alkenes: Structure and Stereochem Slide 3-19 onformers of Alkanes Isomers that can be interconverted simply by rotations around carbon-carbon bonds are called conformations. All structures result from the free rotation of a - single bond Different conformations may differ in energy. The lowestenergy conformer is most prevalent (highest population). Typical molecules constantly rotate through all the possible conformations, even though the populations of each may differ widely. Alkenes: Structure and Stereochem Slide

11 onformational Analysis Vernacular ow to represent (draw) conformations? Sawhorse and Newman Projections onsider an ethane-like species (G G 2 ) G 2 G 1 G 1 G 2 G 1 c G 1 d b a d b a c d c Eclipsed onformation c b a d b G 2 a G 2 Staggered onformation anti G 3f G 3b G 1b gauche (60 o ) G 2f G 2b gauche G syn anti clinal G periplanar clinal periplanar G 1f Alkenes: Structure and Stereochem Slide 3-21 Ethane onformers Staggered conformer has lowest energy. Dihedral angle = 60 degrees model Newman projection sawhorse Alkenes: Structure and Stereochem Slide

12 Ethane onformers (2) Eclipsed conformer has highest energy Dihedral angle = 0 degrees (drawn as below for convenience) Alkenes: Structure and Stereochem Slide 3-23 onformational Analysis Torsion: energy required to move one group past another in space. Based primarily on sterics. Torsional strain: resistance to rotation. For ethane, only 12.6 kj/mol Alkenes: Structure and Stereochem Slide

13 Propane onformers Note slight increase in torsional strain; due to the more bulky ( sterically demanding ) methyl group. Alkenes: Structure and Stereochem Slide 3-25 Butane onformers 2-3 ighest energy has methyl groups eclipsed. Steric hindrance Dihedral angle = 0 degrees totally eclipsed Alkenes: Structure and Stereochem Slide

14 Butane onformers (2) Lowest energy has methyl groups anti. Dihedral angle = 180 degrees anti Alkenes: Structure and Stereochem Slide 3-27 Butane onformers (3) Methyl groups eclipsed with hydrogens igher energy than staggered conformer Dihedral angle = 120 degrees eclipsed Alkenes: Structure and Stereochem Slide

15 Butane onformers (4) Gauche, staggered conformer Methyl groups closer than in anti conformer Dihedral angle = 60 degrees gauche Alkenes: Structure and Stereochem Slide 3-29 onformational Analysis NOTE: more common to go anti to total eclipse to anti (lowest to highest to lowest Alkenes: Structure and Stereochem Slide

16 igher Alkanes Anti conformation is lowest in energy. Straight chain actually is zigzag (line-angle represents this) Alkenes: Structure and Stereochem Slide 3-31 ycloalkanes Rings of carbon atoms (- 2 - groups) Formula: n 2n (same as alkenes) Nonpolar, insoluble in water ompact shape Melting and boiling points similar to branched alkanes with same number of carbons onformational analysis important Alkenes: Structure and Stereochem Slide

17 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 Alkenes: Structure and Stereochem Slide 3-33 is-trans Isomerism Due to restricted rotation caused by ring is: like groups on same side of ring Trans: like groups on opposite sides of ring Alkenes: Structure and Stereochem Slide

18 ycloalkane Stability 5- and 6-membered rings most stable Bond angle closest to Must consider concept of ring strain Torsional strain due to eclipsing interactions Angle (Baeyer) strain Measured by heats of combustion per Alkenes: Structure and Stereochem Slide 3-35 eats of ombustion per 2 (Alkane + O 2 O O) (# 2 s) kj/mol kj Long-chain alkane Alkenes: Structure and Stereochem Slide

19 yclopropane Large ring strain due to angle compression Very reactive, weak bonds Alkenes: Structure and Stereochem Slide 3-37 yclopropane (2) Torsional strain because of eclipsed hydrogens Alkenes: Structure and Stereochem Slide

20 yclobutane Angle strain due to compression Torsional strain partially relieved by ring-puckering Slight increase in angle strain relieves some torsional strain! Alkenes: Structure and Stereochem Slide 3-39 yclopentane If planar, angles would be 108, but all hydrogens would be eclipsed. Puckered conformer reduces torsional strain. Envelope conformation; pseudo-rotation Alkenes: Structure and Stereochem Slide

21 yclohexane ombustion data shows it is 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. OW? Alkenes: Structure and Stereochem Slide 3-41 hair onformer Alkenes: Structure and Stereochem Slide

22 Drawing hair onformations A B D Start by drawing two parallel, but offset lines (A). Put a nose on it (B), followed by a tail (). Finally, add in the axial and equitorial bonds (D). Process starts with offset parallel lines running in other direction for chair flip conformation. When transcribing groups from a cyclohexyl structure to the chair conformation, remember the relationship of axial and equitorial groups (and don t mix this up with cis and trans!!). Practice will make perfect! Alkenes: Structure and Stereochem Slide 3-43 Boat onformer Alkenes: Structure and Stereochem Slide

23 onformational Energy Alkenes: Structure and Stereochem Slide 3-45 Axial and Equatorial Positions Alkenes: Structure and Stereochem Slide

24 Monosubstituted yclohexanes Alkenes: Structure and Stereochem Slide ,3-Diaxial Interactions Alkenes: Structure and Stereochem Slide

25 Disubstituted yclohexanes Alkenes: Structure and Stereochem Slide 3-49 is-trans Isomers Bonds that are cis, alternate axial-equatorial around the ring. 3 One axial, one equatorial 3 e a e e a a a e e a e a Alkenes: Structure and Stereochem Slide

26 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. Alkenes: Structure and Stereochem Slide 3-51 onformational Energies: A Values Used to quantitate 1,3-diaxial interactions; Defined as the the energy released when a group goes FROM an axial position TO an equitorial position; Thermodynamically: A = -ΔG o ; as a result, A values are positive for a thermodynamically-favorable conformational change; Additive; A tot can be used to estimate equilibrium quantities A value (kj) =!G X = RT ln K eq = ( " 10 3 kj/k)t ln II I X K eq X I II Alkenes: Structure and Stereochem Slide

27 Typical A Values X A Value F 0.63 l 1.80 Br 1.59 I 1.80 O 3.64 O OD O OAc 2.51 O(O)F OO 1.13 OTs ONO S 3.77 SN 5.15 S S S SO SO SO SO X A Value -N N NO NS N==N-R 4.18 N N N( 3) N NO P P( 3) Pl P(O 3) P + ( 3) 3 >12.55 P(S)( 3) 2 >12.55 gbr gl 1.26 X A Value F = ! ( 3) OTs 7.32 ( 3) c ( 3) 3 > O O O O O 2( 3) Ol 5.23 O Si( 3) Ge( 3) Sn( 3) Pb( 3) Alkenes: Structure and Stereochem Slide 3-53 Br Determination of A total and K eq I 3 l A Tot = A 3 + ( A l )+ A Br K eq Br II l 3 ( ) = = 3.72 kj At 25 : ln K eq = ln II I = 3.72 ( 8.314! 10 ) ( ) = 1.50 K eq = 0.41 = II I So at equilibrium: 71% I and 29% II are present Note: -A l and -A Br were used because they went from equitorial to axial! Alkenes: Structure and Stereochem Slide

28 Bicyclic Alkanes Fused rings share two adjacent carbons. Bridged rings share two nonadjacent s. bicyclo[3.1.0]hexane bicyclo[2.2.1]heptane Alkenes: Structure and Stereochem Slide 3-55 is- and Trans-Decalin Fused cyclohexane chair conformers Bridgehead s cis, structure more flexible Bridgehead s trans, no ring flip possible. cis-decalin trans-decalin Alkenes: Structure and Stereochem Slide

29 Bicyclo[4.4.0]decane Alkenes: Structure and Stereochem Slide 3-57 hapter 3 omework 34, 35, 37, 39, 42-44, 46 plus: 47) Draw the most stable chair conformation for the each of the following, and calculate the percentages of each present at equilibrium: a) l b) O 3 Br F I Alkenes: Structure and Stereochem Slide