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2/12/2012 Alkanes Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) SIMPLE HYDROCARBONS Alkanes are saturated with hydrogen (no more can be added). They are also called aliphatic compounds The formula for an alkane with no rings in it must be CnH2n+2 “LINEAR” HYDROCARBONS SIMPLE HYDROCARBONS Hydrocarbons Benzene ring Aliphatic Alkanes Single bond Aromatic Alkenes Alkynes LEARN THESE CH 4 CH 4 methane C 2H 6 CH 3CH 3 ethane C 3H 8 CH 3CH 2CH 3 propane C 4H 10 CH 3CH 2CH 2CH 3 butane C 5H 12 CH 3CH 2CH 2CH 2CH 3 pentane C 6H 14 CH 3(CH 2) 4CH 3 hexane C 7H 16 CH 3(CH 2) 5CH 3 heptane C 8H 18 CH 3(CH 2) 6CH 3 octane C 9H 20 CH 3(CH 2) 7CH 3 nonane C 10H 22 CH 3(CH 2) 8CH 3 decane CnH2n+2 1 2/12/2012 HYDROCARBONS ARE NOT LINEAR CHAINS CAN BEND (make a model and see for yourself) compare 109o 109o tetrahedral PROPANE ETHANE [ animation ] DECANE H H H H H H H H H H H H C C C C C C C C C C C H H H H H H H H H H H ALL OF THESE ARE THE SAME MOLECULE H CONDENSED STRUCTURES CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 BALL AND STICK MODEL The atom-to-atom connection sequence matters but not the orientation in space. It is important for you to be able to recognize the same molecule when drawn in different orientations. 2 2/12/2012 HYDROCARBONS LET’S START WITH THE HYDROCARBONS CONSTITUTIONAL ISOMERS C1 point C2H6 no isomers C2 line C3 H8 C3 plane (angle) Molecules having the same molecular formula, but a different bonding arrangement for the atoms. CONSTITUTIONAL ISOMERS . CH4 The first three linear hydrocarbons (C1 - C3) have no isomers. No other arrangements of the atoms are possible. At C4, however, we begin to see isomers. ( STRUCTURAL ISOMERS ) C4H10 EXAMPLE C 2 H6 O .. CH3-O-CH 3 .. .. H-O-CH 2-CH3 .. C4 molecular formula constitutional isomers - same molecular formula - different structure 2 isomers C3 + C1 C5H12 3 isomers C5 C4+C1 C3+2(C1) 3 2/12/2012 BE SYSTEMATIC WHEN DRAWING ISOMERS C6H14 5 isomers 1. Draw the “linear” chain first ( no branching ). C6 2. Remove one carbon and place it at different positions on the shortened chain, making sure not to repeat any structure already drawn. C5+C1 3. Remove two carbons from the original chain, place them on the chain, both individually (two one carbon units) and together (one two carbon unit), taking care not to repeat any structure already drawn. C4+2(C1) With increasing numbers of carbon, the number of isomers goes up rapidly. ORIENTATION AND BENDING DO NOT MATTER These are all the same isomer: 4. Continue removing carbons and replacing them individually and in groups until it is no longer possible to create new isomers. CONSTITUTIONAL ISOMERS OF HEPTANE all of these branch at carbon number two C7 C6 + C1 These are all the same isomer: all of these branch at carbon number three The atom-to-atom sequence determines the isomer ……………….. not the shape. C5 + 2 (C1) C5 + C2 C4 + 3 (C1) 4 2/12/2012 CONSTITUTIONAL ISOMERS EXAMPLE Both are C7H16 , but they are different molecules. - They are different molecules. They have the same formula, but they are connected together differently. - You cannot convert one to the other without taking CONSTITUTIONAL ISOMERS WITH A GIVEN FUNCTIONAL GROUP PROBLEM ONE DRAW ALL THE ISOMERS OF C4H10O a) that are alcohols b) that are ethers it apart and rebuilding it ( i.e., without breaking and remaking bonds). - All the physical properties of constitutional As we shall see, there can be isomers both within a class of compounds AND across two or more classes of compounds. isomers are different different melting points and boiling points, different solubilities and densities, etc. Types of Constitutional Isomerism Different carbon skeletons C4H10O CONSTITUTIONAL ISOMERS OH alcohols OH OH C4H10 Different functional groups C2H6O Different position of functional groups hydrogens omitted OH 2-2 O O 3-1 ethers 3-1 O C3H9N TWO DIFFERENT FUNCTIONAL GROUPS - SAME FORMULA 5 2/12/2012 Chapter 2 #10, #11, #12,#13 Chapter 3 #1, #2b. Some of the questions need information from next section. Please remember to come back later. #3. this question is to ask you to draw one bromine-substituted isobutane and two bromine-substituted isobutane. Hint: draw the structure of isobutane; for monobromination derivatives you should replace one hydrogen in the molecule with a Br and you should get two different structures; for dibromination derivatives, you should replace two hydrogens of isobutane with two Br and you may get 3 different structures. 6