TOPIC 3. ALKANES (chapter 4 and parts of chapters 7 and 10)

OBJECTIVES 1. Develop rules for systematic nomenclature of alkanes alkenes and alkynes 2. Describe the geometry of hydrocarbons 3. Describe the rotation around C-C single bonds (conformation) 4. Describe the geometry of cyclic alkanes

ALKANES S:4.1-2,7 C1: Methane C2: Ethane C3: Propane

C4: Butane Isobutane C5: Pentane Isopentane Neopentane 7

All acyclic (non-cyclic) alkanes have the formula: CnH2n+2 Number of isomers of CnH2n+2 C3 1 C4 2 C5 3 C6 5 C7 9 C8 18 C40 62,481,801,147,341! 8

Physical Properties of Alkanes, CnH2n+2 C1-C4: gas C5-C15: liquid >C16: solid large n: polyethylene Branching usually decreases m.p.: - branches impede crystal packing. Highly symmetical molecules have higher m.p.: C(CH3)4, m.p.=-16 °C; CH3(CH2)3CH3, m.p.=-130 °C. Branching decreases b.p.: - More compact structure gives less surface area: weaker van der Waals interactions Non-polar: immiscible with water Less dense than water Dissolve other non-polar organic molecules Flammable! Chemically inert Uses: fuels, solvents,waxes

SYSTEMATIC IUPAC NOMENCLATURE S:4.3 Prob 4.37 Linear (Unbranched) Alkanes CH4 CH3CH3 CH3CH2CH3 CH3(CH2)2CH3 CH3(CH2)3CH3 CH3(CH2)4CH3 CH3(CH2)5CH3 C1 2 3 4 5 6 7 8 9 10 methane ethane propane butane pentane C11 12 13 14 15 16 17 18 19 20 9

Branched alkanes octane e.g., find longest continuous chain of carbons base name = alkane List substituents as prefixes in alphabetical order ((ignoring di, tri, sec, tert; but do not ignore “iso”) -CH3 methyl Me -CH2CH2CH2CH3 butyl Bu -CH2CH3 ethyl Et -CH2CH(CH3)2 isobutyl i-Bu -CH2CH2CH3 propyl Pr -CH(CH3)CH2CH3 sec-butyl s-Bu -CH(CH3)2 isopropyl i-Pr -C(CH3)3 tert-butyl t-Bu octane 10

Number longest continuous chain from the end that has the substituents at the lowest possible number ethyl dimethyloctane 11

Problem: Name the following. e.g., Problem: Name the following. 12

What is the IUPAC systematic name of isooctane? Problem: Isooctane is used as a standard to measure the rate of gas combustion. Isooctane, C8H18, has five 1° carbons, one 2° carbon, one 3° carbon and one 4° carbon. There are two types of methyl groups. What is the IUPAC systematic name of isooctane? 18

Problem: A female tiger moth pheromone, C18H38, has three 1° carbons, fourteen 2° carbons and one 3° carbon; there are two different types of methyl groups. What are the IUPAC names of the two compounds which fit this description? 19

Naming Alcohols and Alkyl Halides Alcohols e.g. Alkyl Halides e.g.

CYCLIC COMPOUNDS Cyclic Alkanes Alkanes: CnH2n+2 S:4.4 Prob: 4.22,27,33, 38,39,41 Cyclic Alkanes Alkanes: CnH2n+2 Cyclic Alkanes: CnH2n (one ring) 22

Bicyclic Compounds Problems:

For each ring in a molecule there are two fewer hydrogens than expected for a non-cyclic alkane, so: Number of rings = (2#C + 2 – #H) / 2 Problems: How many rings are there in C9H14? C10H17?

Problem: Draw all of the constitutional isomers of monocyclic alkanes with the molecular formula C6H12. [There are eight possible structures] 23

ALKENES AND ALKYNES: NOMENCLATURE AND FORMULAS S:4.5-6 Prob: 4.25 e.g., Find longest continuous chain of carbons which contains the C=C (or CC): base name = alkene (alkyne) Number longest continuous chain of carbons to assign an sp2 (sp) carbon to as low a number as possible: #-alkene (#-alkyne) heptene Identify substituents and position of substitutents. Substituents are listed alphabetically 3-heptene

Cyclic alkenes: define the sp2 carbons of the alkene as C-1 and C-2 such that the first encountered substituent occupies the lowest possible number (this is sometimes called the first point of difference rule). Problem: What about compounds with alcohol and double (or triple) bonds? Answer: Consider them alcohols first (whether cyclic or acyclic).

Molecular Formula of Alkenes and Alkynes (with no rings) Alkenes: CnH2n Dialkenes: CnH2n-2 Alkynes: CnH2n-2 e.g., C4H6

INDEX OF HYDROGEN DEFICIENCY (SODAR) There are two fewer hydrogen atoms than expected for a saturated alkane for each pi-bond or ring in a molecule So, the number of double bonds or rings, or the “Sum of Double Bonds and Rings” (SODAR), or the “Index of Hydrogen Deficiency”, of a hydrocarbon is given by: (2#C + 2 – #H) / 2 The same equation is true in the presence of divalent oxygen or sulfur atoms: For C,H,O,S: SODAR = (2#C + 2 – #H) / 2 In the presence of halogens (monovalent) or nitrogen (trivalent): For C,H,O,N,S, Hal: SODAR = (2#C + 2– #H – #Hal + #N) / 2 Therefore: each Hal replaces a H; each N adds an extra H; and S,O have no effect on the calculation

Problem: Determine the value of SODAR for compounds with the following molecular formulas. C6H6 CH3Br CH5N C3H8N

Differentiating alkenes and cycloalkanes Catalytic hydrogenation e.g., C4H6 undergoes hydrogenation to afford C4H10.

Problem: a-Pinene, C10H16, found in cedar wood oil and a major constituent of oil of turpentine, undergoes catalytic hydrogenation to give a compound with formula C10H18. How many double bonds does a-pinene have? How many rings?

Problem: Zingiberene, C15H24, isolated from ginger root, undergoes catalytic hydrogenation to give a compound with formula C15H30. How many double bonds does zingiberene have? How many rings?

CONFORMATIONAL ANALYSIS: ROTATION AROUND C-C SINGLE BONDS S:4.8-9 Prob: 4.36 Rotation around C-C single bonds leads to formation of different conformers. The energy required to interconvert conformers is small: Conformers can rarely be isolated at room temperature.

The eclipsing conformation of ethane is destabilized by torsional strain. The eclipsing conformations of butane are further destabilized by steric strain

Ethane F= Dihedral angle: Energy / kJ.mol-1 60 120 180 240 300 360(0) 60 120 180 240 300 360(0) F - °

Butane F= Dihedral angle: Energy / kJ.mol-1 60 120 180 240 300 360(0) 19 16 Energy / kJ.mol-1 4 60 120 180 240 300 360(0) F - °

RELATIVE STABILITY OF CYCLOALKANES Ring strain: Heat of combustion per methylene unit in cycloalkane n DHc/n - kJ/mol/carbon 3 -697 4 -686 5 -664 6 -659 7 -662

CONFORMATIONAL ANALYSIS OF SUBSTITUTED CYCLOHEXANES The Shape of Cyclohexane

cis and trans Stereoisomers Problem: Draw chair conformations of the following

Ring Inversion (“Ring Flip”)

Conformational Preferences of Substituted Cyclohexanes 7.5 kJ mol-1 more stable

Conformational Preferences of Disubstituted Cyclohexanes: 1,2-Dimethylcyclohexanes

1,3-Dimethylcyclohexanes Problem: Determine the preferred conformation of the cis and trans 1,4-dimethylcyclohexane

Conformational Preferences of Disubstituted Cyclohexanes Bearing Different Substituents e.g., Larger substituents have a stronger preference for the equatorial position

FREE-RADICAL HALOGENATION OF ALKANES S:4.17A and 10.3,4,6 Overall Reaction Alkane + Halogen Alkyl Halide + Hydrogen Halide Reaction is selective for substitution of hydrogen atoms on 3° over those on 1° and 2° positions. e.g., Bromination is more selective than chlorination BOOM! R-H + F2 R-H + I2 http://www.duke.edu/vertices/update/win95/snail.gif89

Mechanism Step 1: Initiation - generation of radicals

Step 2: Propagation - formation of product and regeneration of radicals

Step 3: Termination - consumption of radicals

Regioselectivity

Radical bromination of alkanes is only useful when replacing a hydrogen on a 3o carbon. Otherwise the reaction is unselective. e.g., or when there is only one possible monobrominated product

Origin of Regioselectivity Bond Dissociation Energies (R-H  R. + H.) kJ/mol 435 410 395 381 Radical (R.) Stability < < <

Energetics Fluorination Chlorination BOOM! Bromination Iodination (a) F. is very reactive and reacts unselectively with 1, 2 and 3 C-H (b) R. + F2  R-F + F. Is very exothermic (uncontrolable!) Chlorination Cl. is quite reactive and provides some selectivity in reaction with 1, 2 and 3 C-H quite exothermic Bromination Br. is less reactive than Cl. and provides more selectivity in reaction with 1, 2 and 3 C-H exothermic Iodination I. is unreactive and reacts very slowly with C-H; I. + R3C-H  H-I + R3C. endothermic BOOM! http://www.duke.edu/vertices/update/win95/snail.gif89

SYNTHESES OF ALKANES Hydrogenation of Alkenes and Alkynes Hydrogenation is stereospecific

TOPIC 3 ON EXAM 2 Types of Questions Preparing for Exam 2 S:4.1-4.17A S:10.3,4,6 Types of Questions - Describe structure and bonding in alkanes, alkenes, alkynes and cycloalkenes. - Provide systematic names for hydrocarbons - Analyze conformational preferences of alkanes and substituted cyclohexanes. - Describe reactions of alkenes. The problems in the book are good examples of the types of problems on the exam. Preparing for Exam 2 - Work as many problems as possible. - Work in groups. - Do the “Learning Group Problem” at the end of the chapter. - Work through the practice exam