Chapter 4: Alkanes. Hydrocarbons contain only carbon and hydrogen; they consist of saturated and unsaturated compounds:

Transcription

1 Chapter 4 Outline: Alkanes 1. Structure of Alkanes & Cycloalkanes (cover mostly on your own) 2. Physical Properties of Alkanes & Cycloalkanes (cover on your own) 3. Nomenclature overview 4. Newman Projections Conformations of Alkanes in 3 D space 5. Chair Conformations Conformations of Cycloalkanes in 3 D space Everyone should work at ALL the problems in the chapter at the end, but below is a list of the minimum problems you should do at the end of the chapter: , 4.48, 4.49, , 4.57, , 4.66, 4.68, 4.69, 4.70 Hydrocarbons contain only carbon and hydrogen; they consist of saturated and unsaturated compounds: Page 1 of 30

2 1. Structures of Alkanes Alkanes are hydrocarbons containing only single C C bonds, with a formula of C n H 2n+2. Cycloalkanes are hydrocarbons containing only single C C bonds, with one site of unsaturation, with a formula of C n H 2n. Alkanes are also called saturated compounds = containing as much hydrogen as possible (i.e. all carbons have 4 bonds to hydrogens). Identify types of carbons: Page 2 of 30

3 2. Nomenclature (i.e. naming compounds in a standard format) There is are two types of naming: common & IUPAC Rules Established by IUPAC = International Union of Pure and Applied Chemistry Page 3 of 30

4 You will need to know these names and substituents names: Prefixes are NOT used for alphabetical purposes (except iso). Page 4 of 30

5 Page 5 of 30

6 Practice (do the nomenclature worksheet posted online too): a. b. c. Page 6 of 30

7 d. e f. Page 7 of 30

8 g. 1 tert butyl 2 ethyl 4 methyl 3 cyclopentylcyclohexane h. 7,7 dimethyl 4 propylnon 2 ene i. 1,3 pentadiene Page 8 of 30

9 Naming cycloalkanes: Page 9 of 30

10 Bicycloalkanes bicyclo[m.n.o]alkane where m, n and o are the numbers of the carbons in each bridge, in decreasing order of number. Page 10 of 30

11 Name: Provide structures for the following: Bicyclo[3.2.1]octane Bicyclo[2.2.2]octane Bicyclo[4.3.0]nonane Page 11 of 30

12 Biologically important polycyclics: All steroids are made in cells either from the sterols lanosterol (animals and fungi) or from cycloartenol (plants). C D A B Page 12 of 30

13 Relative Stability of Constitutional Isomers Compare the heats of combustion for three octane isomers. Which is most stable? Which is the best fuel? o Octane rating measure of how smoothly a fuel burns in an engine (lower octane gasoline leads to knocking) o Octane is measured relative to a mixture of 2,2,4 trimethylpentane (100) and n heptane (0). o e.g. 87 octane gasoline, for example, has the same knock resistance as a mixture of 87 vol % isooctane and 13 vol % n heptane. Hep tane (octane rating 0) 2,2,4-Trimeth ylp entane (octane rating 100) Page 13 of 30

14 Why do we care about alkanes? Fossil fuels are hydrocarbons that are excavated from the ground. Fossil fuels encompass coal, oil & natural gas. Petroleum, which literally means rock oil is the main source of alkanes. Petroleum is a mixture of hundreds of hydrocarbons, mostly alkanes, with varying numbers of carbons and varying degrees of branching. The alkanes in petroleum with 5 to 12 carbons per molecule are most valuable, and they can be separated from the rest of the oil by distillation. World reserves of fossil fuels (as of 2010; all exhausting by 2220): o Natural gas, 70 year supply (90 95% methane/5 10 ethane) o Petroleum, 45 year supply (thick, viscous liquid; > 1000 compounds) o Coal, 250 year supply? Types of Coal Carbon content BTUs per pound Comments anthracite 86 98% 15,500 Smallest amount of available coal bituminous 45 86% 10,500 15,500 Most plentiful and most used in US for generating electricity subbituminous 35 45% 8,300 10,500 Lower sulfur content lignite 25 35% 4,000 8,300 Geologically young, but available Page 14 of 30

15 Why do we care about alkanes? The gasoline fraction of crude oil only makes up about 19%, which is not enough to meet demand. Page 15 of 30

16 3. Newman Projections conformations of alkanes in 3 D space Line drawings do not show the three dimensional structure of the compound as it really exists. σ bonds rotate in most compounds at all moments above 0 Kelvin. When the bonds in an alkane rotate, the compounds is said to change its conformation (they are NOT constitutional isomers). Due to this strain (12 kj/mol), bond rotation occurs times/second instead of times/second if this strain did not exist! Page 16 of 30

17 Newman Projection allows one to write on paper a three dimensional structure without models. Rotation through an entire 360 o bond rotation Page 17 of 30

18 Potential Energy Diagram for Butane: Page 18 of 30

19 Q: Look at the following set of Newman projections and determine which configurations have the highest and lowest stability. H Me Et H Me Me Et Et HEt Et Et Me H Me H A B C H Me HEt Et Et H Et Et Me Me Et Me Me H Me H H D E F Me H Which of the following structures above appears the most stable? Which is appears the least stable? What compound is this?? Can you draw the structure from what is given? Page 19 of 30

20 Work with a neighbor to draw a Newman projection for the highest and lowest energy conformations for 2,2,3,5,5 pentamethylhexane. View the Newman projection down the C 3 C 4 axis. Page 20 of 30

21 4. Conformations of Cycloalkanes in 3 dimensional space Carbon atoms in alkanes are sp 3 hybridized. What bond angles are optimal for such carbons? If cycloalkanes were flat, what bond angles would be expected? To optimize the bond angles, most cycloalkanes are NOT flat in their most stable conformation. Ring strain = Destabilization of a cyclic molecule due to a high energy spatial orientation of atoms. Page 21 of 30

22 Cyclopropane is 44 kj/mol less stable than cyclohexane per CH 2 group. It is highly strained and very reactive due to: Angle strain Torsional strain Page 22 of 30

23 o The twist boat alleviates some of the torsional strain in the boat. o Cyclohexane undergoes a chair flip at room temperature o Bonds rotate during the chair flip, but they never rotate a full 360 o o (Axial hydrogens become equatorial hydrogens after a chair flip) Page 23 of 30

24 Drawing cyclohexanes: Page 24 of 30

25 Cyclohexane chair flips Compare: Page 25 of 30

26 Disubstituted Cyclohexanes Evaluate each molecule as cis or trans (stereoisomers) and then draw its chair, perform the chair flip and find the more stable conformer. Page 26 of 30

27 Energy Considerations during Chair Flips Page 27 of 30

28 Let s practice drawing chair flips together: t Bu OH Page 28 of 30

29 Work with your neighbor(s) to draw both chairs and evaluate the best conformer: Br Page 29 of 30

30 Applications of six membered rings Page 30 of 30