Chapter Twelve Introduction to Organic Chemistry: Alkanes James E. Mayhugh Oklahoma City University  2007 Prentice Hall, Inc. Fundamentals of General, Organic, and Biological Chemistry 5th Edition

Prentice Hall © 2007 Chapter Twelve 2 Outline ►12.1 The Nature of Organic Molecules ►12.2 Families of Organic Molecules: Functional Groups ►12.3 The Structure of Organic Molecules: Alkanes and Their Isomers ►12.4 Drawing Organic Structures ►12.5 The Shapes of Organic Molecules ►12.6 Naming Alkanes ►12.7 Properties of Alkanes ►12.8 Reactions of Alkanes ►12.9 Cycloalkanes ►12.10 Drawing and Naming Cycloalkanes

Prentice Hall © 2007 Chapter Twelve The Nature of Organic Molecules ►Organic chemistry: The study of carbon compounds. ►Carbon is tetravalent; it always forms four bonds. In methane, carbon is connected to four hydrogen atoms.

Prentice Hall © 2007 Chapter Twelve 4 Organic molecules have covalent bonds. In ethane, the bonds result from the sharing of two electrons, either between C and C atoms or between C and H atoms.

Prentice Hall © 2007 Chapter Twelve 5 ►Organic molecules contain polar covalent bonds when carbon bonds to an electronegative element on the right side of the periodic table. ►In chloromethane, the Cl atom attracts electrons more strongly than C, resulting in a polar bond. The C has a partial (+) charge, and Cl has a partial (-) charge.

Prentice Hall © 2007 Chapter Twelve 6 ►Carbon forms multiple covalent bonds by sharing more than two electrons with a neighboring atom. ►In ethylene, the two carbon atoms share four electrons in a double bond; in acetylene the two carbons share six electrons in a triple bond.

Prentice Hall © 2007 Chapter Twelve 7 ►When carbon is bonded to four atoms, the bonds are oriented toward the four corners of a regular tetrahedron with carbon in the center. ►This is shown using normal lines for bonds in the plane of the page, dashed lines for bonds receding behind the page, and wedged lines for bonds coming out of the page:

Prentice Hall © 2007 Chapter Twelve Families of Organic Molecules: Functional Groups ►The structural features that allow us to class compounds together are called functional groups. A functional group is a group of atoms within a larger molecule that has a characteristic structure and chemical behavior. ►The chemistry of an organic molecule is primarily determined not by its size and complexity, but by the functional groups it contains. A given functional group tends to undergo the same reactions in every molecule of which it is a part.

Prentice Hall © 2007 Chapter Twelve 9 ►The first four families in Table 12.1 are hydrocarbons, organic compounds that contain only C and H. ►Alkanes have only single bonds. Alkenes contain a C-C double-bond; alkynes contain a C-C triple-bond; and aromatic compounds contain a six-membered ring of carbon atoms with three alternating double bonds. ►The next four families contain only single bonds and have a carbon atom bonded to an electronegative atom. ►Alkyl halides have a carbon–halogen bond; alcohols have a C-O bond; ethers have two C’s bonded to the same O; and amines have a C-N bond. ►Aldehydes, ketones, carboxylic acids, anhydrides, esters, and amides all contain a C=O double bond.

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Prentice Hall © 2007 Chapter Twelve The Structure of Organic Molecules: Alkanes and Their Isomers ►Alkane: A hydrocarbon that has only single bonds. ►The rule for hydrocarbons is that each carbon must be bonded to at least one other carbon forming the “backbone” of the compound. The hydrogens are on the periphery. There is only one possible combination of three carbons with eight hydrogens.

Prentice Hall © 2007 Chapter Twelve 13 ►Straight-chain alkane: An alkane that has all its carbons connected in a row. ►Branched-chain alkane: An alkane that has a branching connection of carbons.

Prentice Hall © 2007 Chapter Twelve 14 Constitutional isomers: Compounds with the same molecular formula but different connections among their atoms.

Prentice Hall © 2007 Chapter Twelve 15 ► ►The number of possible alkane isomers grows rapidly as the number of C atoms increases. ► ►Different constitutional isomers are completely different chemical compounds with different structures, different physical properties, and potentially different physiological properties.

Prentice Hall © 2007 Chapter Twelve Drawing Organic Structures Condensed structure: A shorthand way of drawing structures in which C-C and C-H bonds are understood rather than shown.

Prentice Hall © 2007 Chapter Twelve The Shapes of Organic Molecules The two parts of a molecule joined by a C-C single bond are free to spin around the bond, giving rise to an infinite number of possible three-dimensional geometries, or conformations.

Prentice Hall © 2007 Chapter Twelve 18 ►As long as any two structures show identical connections between atoms, they represent identical compounds no matter how the structures are drawn. Such molecular structures are called conformers. ►To see that the following two structures represent the same compound rather than two isomers, picture one of them flipped right to left so that the red groups are on the same side.

Prentice Hall © 2007 Chapter Twelve Naming Alkanes ►The system of naming now used is one devised by the International Union of Pure and Applied Chemistry, IUPAC. ►In the IUPAC system for organic compounds, a chemical name has three parts: prefix, parent, and suffix.

Prentice Hall © 2007 Chapter Twelve 20 ►Straight-chain alkanes are named by counting the number of C atoms and adding the family suffix -ane. ►The first four parent names have historical origins, the rest are named from Greek numbers.

Prentice Hall © 2007 Chapter Twelve 21 ►Alkyl group: The part of an alkane that remains when a hydrogen atom is removed.

Prentice Hall © 2007 Chapter Twelve 22 ►The situation is more complex for larger alkanes. ►There are two different three carbon alkyl groups, there are four different four carbon alkyl groups.

Prentice Hall © 2007 Chapter Twelve 23 ►The butyl groups are n-butyl, sec-butyl, isobutyl, and tert-butyl. The prefix sec- stands for secondary, and the prefix tert- stands for tertiary, referring to the number of other carbon atoms attached to the branch point. ►There are four possible substitution patterns, called primary, secondary, tertiary, and quaternary.

Prentice Hall © 2007 Chapter Twelve 24 ►Branched-chain alkanes can be named by following four steps: ►STEP 1: Name the main chain. Find the longest continuous chain of carbons, and name the chain according to the number of carbon atoms it contains. The longest chain may not be immediately obvious because it is not always written on one line; you may have to “turn corners” to find it.

Prentice Hall © 2007 Chapter Twelve 25 ►STEP 2: Number the carbon atoms in the main chain. Begin at the end nearer the first branch point: ► ►STEP 3: Identify the branching substituents, and number each according to its point of attachment to the main chain:

Prentice Hall © 2007 Chapter Twelve 26 ►STEP 4: Write the name as a single word, using hyphens to separate the numbers from the different prefixes and commas to separate numbers if necessary. If two or more different substituent groups are present, cite them in alphabetical order. ► ►If there are two substituents on the same carbon, assign the same number to both. There must always be as many numbers in the name as there are substituents.

Prentice Hall © 2007 Chapter Twelve 27 If two or more identical substituents are present, use one of the prefixes di-, tri-, tetra-, and so forth, but do not use these prefixes for alphabetizing purposes.

Prentice Hall © 2007 Chapter Twelve Properties of Alkanes ►Odorless or mild odor; colorless; tasteless; nontoxic ►Nonpolar; insoluble in water but soluble in nonpolar organic solvents; less dense than water ►Flammable; otherwise not very reactive ►The first four alkanes are gases at room temperature and pressure, alkanes with 5–15 carbon atoms are liquids; those with 16 or more carbon atoms are generally low-melting, waxy solids.

Prentice Hall © 2007 Chapter Twelve 29 The boiling and melting points for the straight-chain alkanes increase with molecular size.

Prentice Hall © 2007 Chapter Twelve Reactions of Alkanes ►Alkanes do not react with acids, bases, or most other common laboratory reagents. Their only major reactions are with oxygen (combustion) and with halogens (halogenation). ►Combustion: A chemical reaction that produces a flame, usually because of burning with oxygen. ►Carbon dioxide and water are the products of complete combustion of any hydrocarbon, and a large amount of heat is released.

Prentice Hall © 2007 Chapter Twelve 31 ►Halogenation: The replacement of an alkane hydrogen by a chlorine or bromine in a process initiated by heat or light. ►This reaction, like many organic reactions, yields a mixture of products:

Prentice Hall © 2007 Chapter Twelve Cycloalkanes ►Cycloalkanes contain a ring of carbon atoms. ►Cyclopropane and cyclobutane are less stable and more reactive than other cycloalkanes since their bond angles are considerably compressed.

Prentice Hall © 2007 Chapter Twelve 33 ►Both cyclopentane and cyclohexane rings have nearly ideal bond angles and are therefore stable. ►Cyclic and acyclic alkanes are similar in many of their properties.

Prentice Hall © 2007 Chapter Twelve Drawing and Naming Cycloalkanes ►Even condensed structures become awkward when we work with large molecules that contain rings. ►A more streamlined way of drawing structures is often used in which cycloalkanes are represented simply by polygons.

Prentice Hall © 2007 Chapter Twelve 35 In line structures, a C is located at every intersection, and the number of H atoms necessary to give each C four covalent bonds is understood. Methylcyclohexane, for example, looks like this in a line structure:

Prentice Hall © 2007 Chapter Twelve 36 ►Cycloalkanes are named by a straightforward extension of the rules for naming open-chain alkanes. In most cases, only two steps are needed: ►STEP 1: Use the cycloalkane name as the parent. That is, compounds are named as alkyl-substituted cycloalkanes rather than as cycloalkyl-substituted alkanes. If there is only one substituent on the ring, it is not even necessary to assign a number because all ring positions are identical.

Prentice Hall © 2007 Chapter Twelve 37 STEP 2: Identify and number the substituents. Start numbering at the group that has alphabetical priority, and proceed around the ring in the direction that gives the second substituent the lower possible number.

Prentice Hall © 2007 Chapter Twelve 38 Chapter Summary ►Organic compounds contain C atoms and many are joined in long chains by a combination of single, double or triple bonds. ►Organic compounds can be classified according to their functional groups. A functional group is a group of atoms that has characteristic structure and reactivity. ►Constitutional isomers are compounds with the same formula but different connections among atoms. ►Structural formulas show all atoms and bonds; not all bonds are drawn in condensed structures, the carbon skeleton is represented by lines and the locations of C and H atoms are understood in line structures.

Prentice Hall © 2007 Chapter Twelve 39 Chapter Summary Cont. ►Compounds that contain only C and H are called hydrocarbons, and hydrocarbons that have only single bonds are called alkanes. ►A straight-chain alkane has all its C’s connected in a row, a branched-chain alkane has a branching connection of atoms somewhere along its chain, and a cycloalkane has a ring of C atoms. ►Straight-chain alkanes are named by adding -ane to a parent name that tells how many C atoms are present. ►Branched-chain alkanes are named by using the longest continuous chain of C atoms for the parent and then identifying the alkyl groups present as branches off the main chain.

Prentice Hall © 2007 Chapter Twelve 40 Chapter Summary Cont. ►Substituent groups on the main chain are located by numbering the C’s in the chain so that the substituents have the lowest number. Cycloalkanes are named by adding cyclo- as a prefix to the name of the alkane. ►Alkanes are soluble in nonpolar organic solvents, have weak intermolecular forces, and are nontoxic. ►Their principal chemical reactions are combustion, a reaction with oxygen that gives carbon dioxide and water, and halogenation, a reaction in which hydrogen atoms are replaced by chlorine or bromine.

Prentice Hall © 2007 Chapter Twelve 41 End of Chapter 12