Chapter 12 Alkanes Based on Material Prepared by Andrea D. Leonard University of Louisiana at Lafayette

Transcription

1 Chapter 12 Alkanes Based on Material Prepared by Andrea D. Leonard University of Louisiana at Lafayette Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2 Introduction Alkanes are hydrocarbons having only C C and C H single bonds. Alkanes that contain chains of C atoms but no rings are acyclic alkanes and have the general formula C n H 2n+2. Acyclic alkanes are called saturated alkanes because they have the maximum number of H atoms per C atom. 2

3 Introduction Cycloalkanes contain C atoms joined in a way that form one or more rings. They have the general formula C n H 2n. All alkane molecules have names that end in the suffix -ane. 3

4 Naming Alkanes: Prefixes According to Number of C Atoms # C Prefix 1 meth 2 eth 3 prop 4 but 5 pent # C Prefix 6 hex 7 hept 8 oct 9 non 10 dec For Cycloalkanes add prefix Cyclo to the name 4

5 Simple Alkanes A. Acyclic Alkanes Having Fewer than Five Carbons Methane is a one-carbon alkane. Ethane is a two-carbon alkane. 5

6 Simple Alkanes A. Acyclic Alkanes Having Fewer than Five Carbons Propane is a three-carbon alkane. The following two representations of propane are equivalent: The bends in a carbon chain don t matter when it comes to identifying different compounds. 6

7 Simple Alkanes A. Acyclic Alkanes Having Fewer than Five Carbons Butane is a four-carbon alkane. Four carbons can be a straight-chain or branched-chain alkane. Butane and isobutane are isomers of each other. Isomers are two different compounds with the same molecular formula. Constitutional isomers differ in the way the atoms are connected to each other. 7

8 Simple Alkanes B. Acyclic Alkanes Having Five or More Carbons As the number of C atoms increases, the number of possible isomers increases. Pentane is a five-carbon alkane with three isomers: 8

9 Simple Alkanes B. Acyclic Alkanes Having Five or More Carbons After pentane, the following names apply: # of C s Name Structure hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 heptane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 octane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 nonane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 10 decane CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 9

10 Simple Alkanes C. Classifying Carbon Atoms A primary carbon (1 o C) is bonded to one other C. A secondary carbon (2 o C) is bonded to two other C. A tertiary carbon (3 o C) is bonded to three other C. A quarternary carbon (4 o C) is bonded to four other C.

11 Simple Alkanes D. Bond Rotation in Acyclic Alkanes Rotation can occur around carbon carbon single bonds. The zigzag arrangement of atoms is the most stable, because it avoids crowding. 11

12 Simple Alkanes E. Bond Rotation and Skeletal Structures for Acyclic Alkanes The skeletal structures of alkanes follow the same zigzag pattern. 12

13 An Introduction to Nomenclature The IUPAC System IUPAC stands for International Union of Pure and Applied Chemistry. The IUPAC system of nomenclature provides a system of naming organic compounds. Using the IUPAC system, each organic compound gets a unique and unambiguous name. 13

14 An Introduction to Nomenclature Naming New Drugs Most drugs have three names: Systematic: The IUPAC name e.g., 2-[4-(2-methylpropyl)phenyl]propanoic acid Generic: The official, internationally approved name of the drug e.g., ibuprofen Trade: The name assigned by the company that manufactures the drug e.g., Motrin or Advil 14

15 Alkane Nomenclature The names of alkanes with substituents have three parts: The parent name indicates the number of C s in the longest continuous carbon chain in the molecule. The suffix indicates what functional group is present. It is -ane for alkanes. The prefix tells the identity, location, and number of substituents attached to the carbon chain. 15

16 Alkane Nomenclature A. Naming Substituents e.g., Alkyl Groups To name an alkyl group, change the -ane ending of the parent alkane to -yl. 16

17 Alkane Nomenclature A. Naming Substituents e.g., Alkyl Groups 17

18 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System Step [1] Find the parent carbon chain and add the suffix. Find the longest continuous carbon chain, and name it with an -ane ending. 18

19 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System The longest chain may not be written horizontally It does not matter if the chain is straight or has bends. All three examples below have 6 C s in their longest chain: 19

20 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System Step [2] Number the atoms in the carbon chain to give the first substituent the lower number. CORRECT INCORRECT 20

21 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System Step [3] Name and number the substituents. Name the substituents as alkyl groups. Use the numbers from step [2] to designate their location. 21

22 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System If two or more substituents are identical, use prefixes to indicate how many. # of Substituents Prefix 2 di- 3 tri- 4 tetra- 22

23 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System The following compound contains two methyl groups, so we use the name dimethyl for them. 23

24 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System Step [4] Combine substituent names and numbers + parent + suffix. Alphabetize the substituents, ignoring prefixes. Precede the name of each substituent by the number that indicates its location. There must be one number for each substituent. Separate numbers by commas and separate numbers from letters by dashes. 24

25 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System 25

26 Alkane Nomenclature B. Naming an Acyclic Alkane HOW TO Name an Alkane Using the IUPAC System 26

27 Sample Problem Alkane Nomenclature B. Naming an Acyclic Alkane Give the IUPAC name for the following compound. 27

28 Sample Problem Alkane Nomenclature B. Naming an Acyclic Alkane Answer: 5-ethyl-2,6-dimethyloctane 28

29 Cycloalkanes A. Simple Cycloalkanes 29

30 Cycloalkanes B. Naming Cycloalkanes HOW TO Name a Cycloalkane Using the IUPAC System Step [1] Find the parent cycloalkane. 30

31 Cycloalkanes B. Naming Cycloalkanes HOW TO Name a Cycloalkane Using the IUPAC System Step [2] Name and number the substituents. No number is needed for a cycloalkane with a single substituent. 31

32 Cycloalkanes B. Naming Cycloalkanes HOW TO Name a Cycloalkane Using the IUPAC System For rings with two or more substituents: Begin numbering at one substituent Then, proceed around the ring to give the second substituent the lower number For two different substituents, number the ring to assign the lower number to the substituents alphabetically. 32

33 Cycloalkanes B. Naming Cycloalkanes HOW TO Name a Cycloalkane Using the IUPAC System 33

34 Cycloalkanes B. Naming Cycloalkanes HOW TO Name a Cycloalkane Using the IUPAC System 34

35 Physical Properties Alkanes contain only nonpolar C C and C H bonds. Alkanes exhibit only weak intermolecular forces, so they have low melting points and boiling points. Smaller alkanes are gases at room temperature, whereas larger alkanes are liquids. Alkanes are insoluble in water. Alkanes are less dense than water, meaning that they will float on the surface of water. 35

36 Physical Properties As the number of carbons in an alkane increases, the boiling point increases: 36

37 Focus on the Environment Fossil Fuels Natural gas is composed mostly of methane, which burns in the presence of oxygen, releasing energy for cooking and heating. Petroleum is a complex mixture of compounds that must be refined to separate it into usable fractions. Gasoline (C 5 H 12 C 12 H 26 ), kerosene (C 12 H 26 C 16 H 34 ), and diesel fuel (C 15 H 32 C 18 H 38 ) are some of the products of petroleum refinement. Other portions are used to make plastics, drugs, fabrics, dyes, and pesticides. 37

38 Focus on the Environment Crude Oil The hydrocarbons in crude oil are: Separated by boiling points. Heated to higher temperatures to produce gases that can be removed and cooled. Copyright 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 38

39 Focus on the Environment Combustion Alkanes have no functional group, so they undergo few reactions. Combustion is one of those reactions; It is an oxidation reduction reaction. In the combustion reaction, alkanes burn in the presence of O 2 gas to form CO 2 and H 2 O. The products, CO 2 + H 2 O, are the same, regardless of the identity of the alkane that undergoes combustion. 39

40 Focus on the Environment Combustion 40

41 Focus on the Environment Combustion If there is not enough O 2 to react, incomplete combustion may occur, and carbon monoxide (CO) is formed instead of carbon dioxide (CO 2 ). Carbon monoxide is a poisonous gas that binds to hemoglobin in blood, thereby reducing the amount of O 2 that can be transported to cells. 41

42 Free Radical Halogenation When alkanes react with halogens: One or more hydrogens will be replaced with halogens Halogens prefer to go to the more substituted (location with more surrounding carbons) location Light or heat is required to form radicals Radicals have unpaired electrons and violate the octet rule (reactive) 42

43 Free Radical Halogenation Some examples of free radical halogenation: CH 4 + F 2 H 3 C F HF + + H 3 C CH 3 Cl 2 H 3 C CH 2 Cl HCl + Br Br H 3 C CH 2 + Br 2 H 3 C CH + H 2 C CH 2 + HBr CH 3 CH 3 CH 3 major product minor product I 2 = too unreactive for free radical halogenation of alkanes 43

44 Free Radical Halogenation Mechanism.... Light : X.. X.. : :.. X + :.. X R H + : X R + X H.. : R + :.. X.. X: R X..: + :.. X used to indicate an unpaired electron (free radical) X = shorthand for a halogen atom R = shorthand for an organic compound or group 44