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

Common Elements in Organic Compounds

Classification of Hydrocarbons Hydrocarbons are made up of only hydrogen and carbon.

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

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

24.1 How many structural isomers can be identified for pentane, C5H12?

24.1 Strategy For small hydrocarbon molecules (eight or fewer C atoms), it is relatively easy to determine the number of structural isomers by trial and error.

24.1 Solution The first step is to write the straight-chain structure: The second structure, by necessity, must be a branched chain:

24.1 Yet another branched-chain structure is possible: We can draw no other structure for an alkane having the molecular formula C5H12. Thus, pentane has three structural isomers, in which the numbers of carbon and hydrogen atoms remain unchanged despite the differences in structure.

Physical Properties of Some Alkanes

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 heptane 1 2 3 4 5 6 7 An alkane less one hydrogen atom is an alkyl group. CH4 methane CH3 methyl

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

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 CH2 C 1 2 3 4 5 6 3,3-dimethylhexane

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 Cl 1 2 3 4 F 1-chloro-3-fluorobutane

24.2 Give the IUPAC name of the following compound:

24.2 Strategy We follow the IUPAC rules and use the information in Table 24.2 to name the compound. How many C atoms are there in the longest chain?

24.2 Solution The longest chain has six C atoms so the parent compound is called hexane. Note that there are two methyl groups attached to carbon number 2 and one methyl group attached to carbon number 4. Therefore, we call the compound 2,2,4-trimethylhexane.

24.3 Write the structural formula of 3-ethyl-2,2-dimethylpentane.

24.3 Strategy We follow the preceding procedure and the information in Table 24.2 to write the structural formula of the compound. How many C atoms are there in the longest chain? Solution The parent compound is pentane, so the longest chain has five C atoms. There are two methyl groups attached to carbon number 2 and one ethyl group attached to carbon number 3. Therefore, the structure of the compound is

Alkane Reactions Combustion CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH0 = -890.4 kJ/mol 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•

Optical Isomerism achiral chiral

24.4 Is the following molecule chiral?

24.4 Strategy Recall the condition for chirality. Is the central C atom asymmetric; that is, does it have four different atoms or different groups attached to it? Solution We note that the central carbon atom is bonded to a hydrogen atom, a chlorine atom, a –CH3 group, and a –CH2 – CH3 group. Therefore, the central carbon atom is asymmetric and the molecule is chiral.

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

Geometry of Cycloalkanes more stable

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 2-butene 1-butene C Cl H C Cl H cis-dichloroethylene trans-dichloroethylene

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)

and/or Markovnikov’s rule: in the addition of unsymmetrical (that is, polar) reagents to alkenes, the positive portion of the reagent (usually hydrogen) adds to the carbon atom that already has the most hydrogen atoms.

Cis-Trans Isomerization in the Vision Process electron micrograph of retinal rods

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)

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)

Chemistry In Action: Ice That Burns methane hydrate

Aromatic Hydrocarbons Electron micrograph of benzene

Other Representations of Benzene

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

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

Polycyclic Aromatic Hydrocarbons

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

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

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

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

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

Properties and Reactions of Carboxylic Acids Generally weak acids React with alcohols to form esters Neutralization Production of acid halides (intermediates in syntheses)

Functional Group Chemistry Esters have the general formula R′COOR, where R is a hydrocarbon group. Characteristic odors and flavors Hydrolysis Alkaline hydrolysis (saponification)

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

Some Aromatic Amines

24.5 Cholesterol is a major component of gallstones, and it is believed that the cholesterol level in the blood is a contributing factor in certain types of heart disease. From the following structure of the compound, predict its reaction with (a) Br2, (b) H2 (in the presence of a Pt catalyst), (c) CH3COOH.

24.5 Strategy To predict the type of reactions a molecule may undergo, we must first identify the functional groups present (see Table 24.4). Solution There are two functional groups in cholesterol: the hydroxyl group and the carbon-carbon double bond. The reaction with bromine results in the addition of bromine to the double-bonded carbons, which become single-bonded. (b) This is a hydrogenation reaction. Again, the carbon-carbon double bond is converted to a carbon-carbon single bond.

24.5 (c) The acid reacts with the hydroxyl group to form an ester and water. Figure 24.10 shows the products of these reactions.

Chemistry In Action: The Petroleum Industry Crude Oil