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

Transcription

1 L 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

2 SYSTEMATIC IUPAC NOMENCLATURE OF ALKANES S:4.1-2 Prob 4.37 Linear (Unbranched) Alkanes C 4 C 3 C 3 C 3 C 2 C 3 C 3 (C 2 ) 2 C 3 C 3 (C 2 ) 3 C 3 C 3 (C 2 ) 4 C 3 C 3 (C 2 ) 5 C 3 C 3 (C 2 ) 6 C 3 C 3 (C 2 ) 7 C 3 C 3 (C 2 ) 8 C 3 C methane ethane propane butane pentane pentadecane 16 hexadecane 17 heptadecane 18 octadecane 19 nondecane 20 eicosane Branched alkanes 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 ) octane

3 Number longest continuous chain from the end that places a substituent at the lowest possible number ethyl dimethyloctane e.g., Problem: Name the following.

4 Problem: Isooctane is used as a standard to measure the rate of gas combustion. Isooctane, C 8 18, has five 1 carbons, one 2 carbon, one 3 carbon and one 4 carbon. Three of the methyl groups are attached to a 4 carbon atom, and two methyl groups are attached to a 3 carbon. What is the IUPAC systematic name of isooctane? Problem: A female tiger moth pheromone, C 18 38, has three 1 carbons, fourteen 2 carbons and one 3 carbon. Two of the methyl groups are attached to a 3 carbon atom, and one methyl group is attached to a 2 carbon. What are the IUPAC names of the compounds which fit this description?

5 NAMING OTER COMPOUNDS S:4.3 Alkyl alides e.g. Cl Named as haloalkanes longest chain substitutents (incl. halogen) named Lowest possible position of substituent defines positions Alcohols e.g. O Named as alkanols longest chain numbered to put O in lowest possible position substitutents named and positions given Cyclic Alkanes Named as (substituted-)cycloalkanes compare formulas: alkanes (C n 2n+2 ) with cyclic alkanes (C n 2n - one ring) S:4.4 Prob: 4.22,27,33, 38,39,41

6 Problem: Draw all of the constitutional isomers of monocyclic alkanes with the molecular formula C [There are twelve possible structures] Bicyclic Alkanes Named as bicyclo[x,y,z]alkanes S:4.15 Problem. Name the following bicyclic compounds.

7 L Alkenes e.g., S:4.5-6 Prob: 4.25 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 sp 2 (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 sp 2 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). Cl What about compounds with alcohol and double (or triple) bonds? Consider them alcohols first, and name them as (substituted alkanols) O O

8 MOLECULAR FORMULA S:4.18 Alkanes : C n 2n Alkenes: C n 2n Cyclic alkanes : C n 2n Dialkenes: C n 2n-2 Alkynes: C n 2n-2 For each ring or pi-bond in a molecule there are two fewer hydrogen atoms than expected for a non-cyclic alkane, so: Number of rings = (2#C + 2 #) / 2 Problems: ow many pi bonds or rings are there in C 9 14? C 10 17? The same equation is true in the presence of divalent oxygen or sulfur atoms: For C,,O,S: SODAR = (2#C + 2 #) / 2 In the presence of halogens (monovalent) or nitrogen (trivalent): For C,,O,N,S, al: SODAR = (2#C + 2 # #al + #N) / 2 (each al replaces a ; each N adds an extra ; and S,O have no effect on the calculation)

9 Problem: Determine the value of SODAR for compounds with the following molecular formulas. SODAR C 6 6 C 3 Br C 5 N C 3 8 N Differentiating alkenes and cycloalkanes Catalytic hydrogenation 2 /Pt 2 /Pt Problem. Compound A, C 4 6, undergoes hydrogenation to afford C (a) ow many rings are present in A? (b) ow many pi bonds? (c) Draw all of the possible structures of A.

10 Problem: α-pinene, C 10 16, found in cedar wood oil and a major constituent of oil of turpentine, undergoes catalytic hydrogenation to give a compound with formula C ow many double bonds does α-pinene have? ow many rings? Problem: Zingiberene, C 15 24, isolated from ginger root, undergoes catalytic hydrogenation to give a compound with formula C ow many double bonds does zingiberene have? ow many rings?

11 PYSICAL PROPERTIES OF ALKANES S:4.7 C1-C4: gas C5-C15: liquid >C16: solid large n: polyethylene Branching usually decreases m.p.: - branches impede crystal packing. (ighly symmetric molecules have higher m.p. e.g., C(C 3 ) 4, m.p.=-16 C; C 3 (C 2 ) 3 C 3, 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 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. Newnam Projections

12 A B A B C The eclipsing conformation of ethane is destabilized by torsional strain. The eclipsing conformations of butane are further destabilized by steric strain 3 C C 3

13 Ethane Dihedral angle: Φ= Energy / kj.mol Φ - 360(0) Butane Dihedral angle: Φ= 19 Energy / kj.mol Φ - 360(0)

14 L RELATIVE STABILITY OF CYCLOALKANES S:4.10 Ring strain: eat of combustion per methylene unit in cycloalkane n Δ c /n - kj/mol/carbon (C 2 ) n + 3n Δ c / 2 O 2 n CO 2 + n 2 O CONFORMATIONAL ANALYSIS OF SUBSTITUTED CYCLOEXANES S: The Shape of Cyclohexane

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

16 Ring Inversion ( Ring Flip ) Conformational Preferences of Substituted Cyclohexanes C kj mol -1 more stable C 3

17 Conformational Preferences of Disubstituted Cyclohexanes: 1,2-Dimethylcyclohexanes 1,3-Dimethylcyclohexanes Problem: Determine the preferred conformation of the cis and trans 1,4-dimethylcyclohexane [Draw chair conformations to decide]. ow could you experimentally demonstrate this difference in stability?

18 Conformational Preferences of Disubstituted Cyclohexanes Bearing Different Substituent e.g., Larger substituents have a stronger preference for the equatorial position L Overall Reaction FREE-RADICAL ALOGENATION OF ALKANES S:4.16, Alkane + alogen Alkyl alide + ydrogen alide Reaction is selective for substitution of hydrogen atoms on 3 over those on 1 and 2 positions. e.g., R- + F 2 BOOM! Br 2 hν R- + I 2 Bromination is more selective than chlorination.

19 chanism Step 1: Initiation - generation of radicals Br Br hν Br R Step 2: Propagation - formation of product and regeneration of radicals Br Br R

20 Step 3: Termination - consumption of radicals C C C Br Br Br Regioselectivity Br Br Br 2 hν Br Br Br

21 Radical bromination of alkanes is only useful when replacing a hydrogen on a 3 o carbon. Otherwise the reaction is unselective... e.g., Br 2 hν Br 2 hν or when there is only one possible monobrominated product. e.g., Br 2 hν Origin of Regioselectivity Bond Dissociation Energies (R- R. +. ) C 3 C 3 C 2 (C 3 ) 2 C (C 3 ) 3 C kj/mol Radical (R. ) stability C 3 < C 3 C 2 < (C 3 ) 2 C < (C 3 ) 3 C

22 Energetics Fluorination (a) F. is very reactive and reacts unselectively with 1, 2 and 3 C- (b) R. + F 2 R-F + F. Is very, very exothermic (uncontrolable!) BOOM! Chlorination Cl. is quite reactive and provides some selectivity in reaction with 1, 2 and 3 C- highly exothermic Bromination Br. is less reactive than Cl. and provides more selectivity in reaction with 1, 2 and 3 C- exothermic Iodination I. is unreactive and reacts very slowly with C-; I. + R 3 C- -I + R 3 C. endothermic SYNTESES OF ALKANES S:4.17 ydrogenation of Alkenes and Alkynes C C 2 Pt or Pd C C 2 Pt or Pd ydrogenation is stereospecific C 3 C 3 2 Pt or Pd

23 TOPIC 3 ON EXAM 2 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