Introduction to Organic Chemistry: Alkanes Fundamentals of General, Organic and Biological Chemistry 6th Edition Chapter Twelve Introduction to Organic Chemistry: Alkanes James E. Mayhugh Copyright © 2010 Pearson Education, Inc.

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

Goals 1. What are the basic properties of organic compounds? Be able to identify organic compounds and the types of bonds contained in them. 2. What are functional groups, and how are they used to classify organic molecules? Be able to classify organic molecules into families by functional group. 3. What are isomers? Be able to recognize and draw constitutional isomers.

Goals Contd. 4. How are organic molecules drawn? Be able to convert between structural formulas and condensed or line structures. 5. What are alkanes and cycloalkanes, and how are they named? Be able to name an alkane or cycloalkane from its structure, or write the structure, given the name. 6. What are the general properties and chemical reactions of alkanes? Be able to describe the physical properties of alkanes and the products formed in the combustion and halogenation reactions of alkanes.

12.1 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.

Organic molecules have covalent bonds 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.

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.

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.

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:

12.2 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.

The first four families in Table 12 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.

Identify the functional groups in adrenaline, shown below.

12.3 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.

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.

Constitutional isomers: Compounds with the same molecular formula but different connections among their atoms.

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.

Which of the alkanes shown below is straight chain and which is branched?

12.4 Drawing Organic Structures Condensed structure: A shorthand way of drawing structures in which C-C and C-H bonds are understood rather than shown.

12.5 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.

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.

Which of the structures shown below represent the same compound in different conformations?

12.6 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.

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.

Alkyl group: The part of an alkane that remains when a hydrogen atom is removed.

The situation is more complex for larger alkanes. There are two different three carbon alkyl groups, there are four different four carbon alkyl groups.

The butyl groups are n-butyl, sec-butyl, isobutyl, and tert-butyl 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.

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.

STEP 2: Number the carbon atoms in the main chain 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:

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. 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 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.

The red carbon atom in the structure below is a CH3 | CH3CCH3 | H

What is the IUPAC name of the alkane shown below What is the IUPAC name of the alkane shown below? Gray spheres represent carbon atoms and blue spheres represent hydrogen atoms. Decane 2,4-dimethyloctane 5,7-dimethyloctane 1,1,3-trimethylheptane

12.7 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.

The boiling and melting points for the straight-chain alkanes increase with molecular size.

12.8 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.

Discussion on balancing reaction equation When the chemical equation for the complete combustion of butane is balanced using whole numbers, what is the coefficient in front of the O2? 5 8 13 26

12.9 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.

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.

12.10 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.

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:

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.

STEP 2: Identify and number the substituents 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.

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.

Chapter Summary Contd. 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.

Chapter Summary Contd. 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.

Key Words Alkane Alkyl group Branched-chain alkane Combustion Condensed structure Conformation Conformer Constitutional isomer Cycloalkane Ethyl group Functional group Hydrocarbon Isomer

Key Words Contd. Isopropyl group Line structure Methyl group Methylene group Propyl group Organic chemistry Primary carbon atom Quaternary carbon atom Secondary carbon atom Straight-chain alkane Substituent Tertiary carbon atom

How many hydrogen atoms are in the molecule represented by the line drawing below? 8 10 12 14

What is the condensed formula that corresponds to the line drawing below? CH3CH2CH2CH3 CH3CH2CH=CH2 CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH=CHCH3

End of Chapter 12