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

Common Elements in Organic Compounds 24.1

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

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

Alkane Reactions Combustion CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH0 = -890.4 kJ Halogenation – the replacement of one or more hydrogen atoms by halogen atoms 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

Optical Isomerism Optical isomers are compounds that are nonsuperimposable mirror images of each other The mirror images of CH2ClBr are superimposable: achiral A molecule is achiral if it is superimposable on its mirror image. Most achiral molecules do have a plane of symmetry or a center of symmetry. The mirror images of CHFClBr are not superimposable, no mater how we rotate the molecules: chiral A molecule is chiral if it is not superimposable on its mirror image. Most chiral molecules can be identified by their lack of a plane of symmetry or a center of symmetry. 

achiral chiral Most simple chiral molecules have at least one asymmetric carbon atom – a carbon atom bonded to four different atoms or groups of atoms 24.2

Are the following molecules chiral or achiral? a) Cl | H-C-CH2-CH3 CH3 b) Br I-C-CH2-CH3 Br

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

Cycloalkanes Chair form is more stable than boat form because the electron densities are further apart 24.2

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

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

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

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

Chemistry In Action: Ice That Burns

Aromaticity Pleasant odor Unusually low reactivity Unusually stable Substitution, not addition Unusually stable Characteristic ring structure with delocalized pi bonding

Aromatic Hydrocarbons 24.3

Benzene - stability C6H6 No typical C=C reactions Unreactive with HX, X2, KMnO4 Reaction requires extreme conditions When reaction does occur, it is substitution, not addition

Benzene - structure All C are sp2 (trigonal planar - 120° angles) Ideal for a planar hexagon All C-C bonds are the same (139 pm) Compare C-C (154 pm) C=C (135 pm) Cyclic conjugated pi bonds unusually stable (resonance)

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

Nomenclature of Aromatics Monosubstituted benzenes Fuctional group name + benzene Ex. Chlorobenzene, nitrobenzene, aminobenzene If more than one substituent is present, we must indicate the location of the second group relative to the first ortho (1,2-), meta (1,3-), para (1,4)

Common names of monosubstituted benzenes

2-chloronitrobenzene, or o-chloronitrobenzene 3-chloronitrobenzene, or m-chloronitrobenzene 4-chloronitrobenzene, or p-chloronitrobenzene

A benzene ring less one H is called a phenyl group: 2-phenylpentane

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

Polycyclic Aromatic Hydrocarbons 24.3

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

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