Organic Chemistry Chapter 24 Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

Common Elements in Organic Compounds 24.1

The Bonding of Carbon Organic chemistry is the chemistry of compounds containing carbon. Because carbon can form single, double, and triple bonds, the following bonding combinations are possible for carbon:

Three types of hydrocarbons exist. Saturated hydrocarbons: contain only carbon–carbon single bonds. They may be cyclic or acyclic. Unsaturated hydrocarbons: contain at least one carbon–carbon double or triple bonds. Aromatic hydrocarbons: contain benzene rings or similar features. Hydrocarbons The simplest organic compounds are made of carbon and hydrogen. They are called hydrocarbons.

Classification of Hydrocarbons 24.1

Alkanes Alkanes have the general formula CnH2n+2 where n = 1,2,3,… only single covalent bonds saturated hydrocarbons because they contain the maximum number of hydrogen atoms that can bond with the number of carbon atoms in the molecule CH4 C2H6 C3H8 methane ethane propane 24.2

Structural isomers are molecules that have the same molecular formula but different structures 24.2

How many structural isomers does pentane, C5H12, have? n-pentane C H CH3 2,2-dimethylpropane 2-methylbutane 24.2

Alkane Nomenclature The parent name of the hydrocarbon is that given to the longest continuous chain of carbon atoms in the molecule. CH3 CH2 CH 4-methylheptane 1 2 3 4 5 6 7 An alkane less one hydrogen atom is an alkyl group. CH4 methane CH3 methyl 24.2

The alkanes constitute a homologous series, a series of compounds in which one compound differs from a preceding one by a fixed group of atoms. Members of a homologous series have similar properties that vary systematically, as shown in Table 24.1.

Alkane Nomenclature 24.2

Alkane Nomenclature When one or more hydrogen atoms are replaced by other groups, the name of the compound must indicate the locations of carbon atoms where replacements are made. Number in the direction that gives the smaller numbers for the locations of the branches. CH3 CH CH2 1 2 3 4 5 2-methylpentane CH3 CH2 CH 1 2 3 4 5 4-methylpentane 24.2

Alkane Nomenclature Use prefixes di-, tri-, tetra-, when there is more than one alkyl branch of the same kind. CH3 CH CH2 1 2 3 4 5 6 2,3-dimethylhexane CH3 CH C CH2 1 2 3 4 5 6 3,3-dimethylhexane 24.2

Alkane Nomenclature Use previous rules for other types of substituents. CH3 CH Br 1 2 3 4 NO2 2-bromo-3-nitrobutane CH2 CH CH3 Br 1 2 3 4 NO2 1-bromo-3-nitrobutane 24.2

What is the IUPAC name of the following compound? CH3 CH CH2 C2H5 1 2 3 4 5 6 7 8 2-methyl-4-ethyloctane What is the structure of 2-propyl-4-methylhexane? 1 2 3 4 5 6 CH3 CH CH2 C2H5 24.2

Give the IUPAC name for each of the following. 1 2 6 5 4 3 7 2 3 a. b. 1 4 5 6 a. 2-methylhexane b. 2,3-dimethyl-4-t-butylheptane

Write the condensed structural formula of 2,3,5-trimethylhexane. Hexane is a six-carbon chain. 2,3,5-Trimethyl means three methyl groups: one each at the 2, 3, and 5 carbons.

Alkane Reactions Combustion CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH0 = -890.4 kJ Halogenation CH4 (g) + Cl2 (g) CH3Cl (g) + HCl (g) light Cl2 + energy Cl• + Cl• Cl• + C H C H • + HCl C H • C Cl H + Cl Cl + Cl• 24.2

achiral chiral 24.2

Cycloalkanes Alkanes whose carbon atoms are joined in rings are called cycloalkanes. They have the general formula CnH2n where n = 3,4,… 24.2

Sources of Alkanes and Cycloalkanes Petroleum, or crude oil, is a mixture of alkanes and cycloalkanes with small amount of aromatic hydrocarbons. Crude oil from different regions differ in their compositions of these compounds. 1. Oil from the Canadian north slope contain molecules composed of 20 to 40 carbon atoms. 2. Oil from Saudi Arabia contain molecules composed of 5 to 20 carbon atoms. This oil is more desirable as a source because it is easier to transport crude oil containing low molecular mass molecules. Hydrocarbon mixtures in crude oil are separated by a process known as fractional distillation. This process separates molecules according to molecular mass.

Cycloalkanes 24.2

Alkenes and Alkynes Alkenes and alkynes are unsaturated hydrocarbons. Alkenes contain carbon–carbon double bonds. They have the general formula CnH2n. Alkynes contain carbon–carbon triple bonds. They have the general formula CnH2n-2.

Alkenes and Geometric Isomers In an alkene, all of the atoms connected to the double-bonded carbons are in one plane. Rotation around the carbon–carbon double bond is restricted due to the side-to-side overlap of the p orbitals to form the p bond. This makes geometric isomers—that is, cis and trans isomers—possible.

This is illustrated below for 2-pentene.

Alkenes Alkenes have the general formula CnH2n where n = 2,3,… contain at least one carbon-carbon double bond also called olefins CH2 CH CH3 CH3 CH 1-butene 2-butene C Cl H C Cl H cis-dichloroethylene trans-dichloroethylene 24.2

Cis-Trans Isomerization in the Vision Process 24.2

Addition Reactions of Alkenes Alkenes are more reactive than alkanes owing to the presence of the double bond. Many reactions add to the double bond. In an addition reaction, parts of a reactant are added to each carbon atom of a carbon–carbon double bond, which converts it to a carbon–carbon single bond.

Alkene Reactions Cracking C2H6 (g) CH2 CH2 (g) + H2 (g) Pt catalyst Addition Reactions CH2 CH2 (g) + HBr (g) CH3 CH2Br (g) CH2 CH2 (g) + Br2 (g) CH2Br CH2Br (g) 24.2

2-bromopropane 1-bromopropane Unsymmetrical reactants such as HBr and HCl add to unsymmetrical alkenes to give two products. 2-bromopropane 1-bromopropane

The two products are not formed in equal amounts The two products are not formed in equal amounts. Markovnikoff’s rule is a generalization that states that the major product formed by the addition of an unsymmetrical reactant, such as H—Cl, H—Br, or H—OH, is the one obtained when the hydrogen atom of the reactant adds to the carbon atom of the multiple bond that has the greater number of hydrogen atoms attached to it. The major product is then 2-bromopropane.

Alkynes Alkynes have the general formula CnH2n-2 where n = 2,3,4,… contain at least one carbon-carbon triple bond CH C CH2 CH3 CH3 C 1-butyne 2-butyne Production of acetylene CaC2 (s) + 2H2O (l) C2H2 (g) + Ca(OH)2 (aq) 24.2

Alkynes Alkynes are unsaturated hydrocarbons containing a carbon–carbon triple bond. The general formula is CnH2n-2. The simplest alkyne is acetylene, C2H2. It is very reactive with oxygen, so it is used in torches for welding to give a very hot flame (~3000°C).

Here acetylene is prepared by the reaction of water with calcium carbide, CaC2. The acetylene produced burns with a sooty flame.

Alkyne Reactions Hydrogenation CH CH (g) + H2 (g) CH2 CH2 (g) Addition Reactions CH CH (g) + HBr (g) CH2 CHBr (g) CH CH (g) + Br2 (g) CHBr CHBr (g) CH CH (g) + 2Br2 (g) CHBr2 CHBr2 (g) 24.2

IUPAC Rules for Naming Alkenes and Alkynes Identify the longest chain containing the multiple bonds. Alkenes have contain carbon-carbon double bond(s) and alkynes contain carbon-carbon triple bond(s). The longest chain provides the stem name followed by the ending –ene for alkenes or –yne for alkynes. Number the longest chain from the end neared the multiple bond. The multiple bond is given the number of the first carbon atom involved in the multiple bond. This number is written in front of the alkene or alkyne name, separating the number from the name by a dash. Branched chains are named using the same method used in naming alkanes. If the multiple bond is in the middle of the longest chain, number the chain from the end that gives the branches the lowest number. The simplest alkene, CH2CH2, is ethene and has a common name of ethylene. The simplest alkyne, CHCH, is ethyne and has a common name of acetylene.

Give the IUPAC name for each of the following compounds: a. 2,4-dimethyl-2-hexene b. 3-propyl-1-hexene

Give the IUPAC name for each of the following alkynes: a. 1-propyne b. 3-methyl-1-pentyne

Chemistry In Action: Ice That Burns

Aromatic Hydrocarbons Aromatic hydrocarbons usually contain benzene rings: six-membered rings of carbon atoms with alternating carbon–carbon single and carbon–carbon double bonds. The molecular orbital description of benzene utilizes a p molecular orbital that is delocalized across the entire molecule. The result is that the double bonds in benzene do not behave as isolated double bonds.

Aromatic Hydrocarbons 24.3

Aromatic Compound Nomenclature CH2CH3 ethylbenzene NH2 aminobenzene Cl chlorobenzene NO2 nitrobenzene Br Br 1 2 3 4 5 6 1,2-dibromobenzene 1,3-dibromobenzene 24.3

Aromatic Compound Reactions Substitution reaction H Br + HBr + Br2 FeBr3 catalyst H CH2CH3 + HCl + CH3CH2Cl AlCl3 catalyst 24.3

Polycyclic Aromatic Hydrocarbons 24.3

The term aromatic indicates that compounds containing a benzene ring have aromas. Flavoring agents that can be made in the lab or are found in nature and that contain benzene rings include cinnamaldehyde, which is the source of the cinnamon flavor, and methyl salicylate, which is the source of the wintergreen flavor that is used in candies and gum. Benzene rings are also found in pain relievers such as acetylsalicylic acid (aspirin), acetaminophen (Tylenol), and the illegal drug mescaline.

Functional Group Chemistry Alcohols contain the hydroxyl functional group and have the general formula R-OH. 24.4

Biological production of ethanol C6H12O6 (aq) 2CH3CH2OH (aq) + 2CO2 (g) enzyme Commercial production of ethanol CH2 CH2 (g) + H2O (g) CH3CH2OH (g) H2SO4 Metabolic oxidation of ethanol CH3CH2OH CH3CHO + H2 alcohol dehydrogenase 24.4

Functional Group Chemistry Ethers have the general formula R-O-R’. Condensation Reaction CH3OH + HOCH3 CH3OCH3 + H2O H2SO4 catalyst 24.4

Functional Group Chemistry Aldehydes and ketones contain the carbonyl ( ) functional group. O C R C H O aldehydes have the general formula R C R’ O ketones have the general formula H C H O H C O CH3 C O CH3 H3C formaldehyde acetaldehyde acetone 24.4

Give the IUPAC name of the following compound: 3-ethyl-3-hexanol

Give the common name of each of the following compounds: b. Ethyl methyl ether a. Dimethyl ether

Functional Group Chemistry Carboxylic acids contain the carboxyl ( -COOH ) functional group. 24.4

Functional Group Chemistry Esters have the general formula R’COOR, where R is a hydrocarbon group. CH3COOH + HOCH2CH3 CH3 C O CH2CH3 + H2O O ethyl acetate 24.4

Functional Group Chemistry Amines are organic bases with the general formula R3N. CH3NH2 + H2O RNH3+ + OH- CH3CH2NH2 + HCl CH3CH2NH3+Cl- 24.4

24.4

Chemistry In Action: The Petroleum Industry Crude Oil