Organic Chemistry William H Brown Christopher S Foote
Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson 2 -1
Alkanes and Cycloalkanes Chapter 2 2 -2
Structure u Hydrocarbon: a compound composed only of carbon and hydrogen u Saturated hydrocarbon: a hydrocarbon containing only single bonds u Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain u Aliphatic hydrocarbon: another name for an alkane 2 -3
Hydrocarbons 2 -4
Structure u Shape • tetrahedral about carbon • all bond angles are approximately 109. 5° 2 -5
Drawing Alkanes u Line-angle formulas • an abbreviated way to draw structural formulas • each vertex and line ending represents a carbon 2 -6
Constitutional Isomerism u Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms • example: C 4 H 10 2 -7
Constitutional Isomerism • do these formulas represent constitutional isomers? • find the longest carbon chain • number each chain from the end nearest the first branch • compare chain lengths as well the identity and location of branches 2 -8
Constitutional Isomerism World population is about 6, 000, 000 2 -9
Nomenclature - IUPAC u Suffix -ane specifies an alkane u Prefix tells the number of carbon atoms 2 -10
Nomenclature - IUPAC u Parent name: the longest carbon chain u Substituent: a group bonded to the parent chain • alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R- 2 -11
Nomenclature - IUPAC 1. The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. The parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number 2 -12
Nomenclature - IUPAC 5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first • indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc. • use commas to separate position numbers 2 -13
Nomenclature - I�UPAC 6. If there are two or more different substituents, • list them in alphabetical order • number from the end of the chain that gives the substituent encountered first the lower number 2 -14
Nomenclature - IUPAC 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization • alphabetize the names of substituents first and then insert these prefixes 2 -15
Nomenclature - IUPAC u Alkyl groups 2 -16
Nomenclature - Common u The number of carbons in the alkane determines the name • all alkanes with four carbons are butanes, those with five carbons are pentanes, etc. • iso- indicates the chain terminates in -CH(CH 3)2; neothat it terminates in -C(CH 3)3 2 -17
Classification of C & H u Primary (1°) C: a carbon bonded to one other carbon • 1° H: a hydrogen bonded to a 1° carbon u Secondary (2°) C: a carbon bonded to two other carbons • 2° H: a hydrogen bonded to a 2° carbon u Tertiary (3°) C: a carbon bonded to three other carbons • 3° H: a hydrogen bonded to a 3° carbon u Quaternary (4°) C: C a carbon bonded to four other carbons 2 -18
Cycloalkanes u General formula Cn. H 2 n • five- and six-membered rings are the most common u Structure and nomenclature • to name, prefix the name of the corresponding openchain alkane with cyclo-, and name each substituent on the ring • if only one substituent, no need to give it a number • if two substituents, number from the substituent of lower alphabetical order • if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order 2 -19
Cycloalkanes u Line-angle drawings • each line represents a C-C bond • each vertex and line ending represents a C 2 -20
Cycloalkanes u Example: name these cycloalkanes 2 -21
Bicycloalkanes u Bicycloalkane: an alkane that contains two rings that share two carbons Bicyclo[4. 4. 0]decane (Decalin) Bicyclo[4. 3. 0]nonane (Hydrindane) Bicyclo[2. 2. 1]heptane (Norbornane) 2 -22
Bicycloalkanes u Nomenclature • parent is the alkane of the same number of carbons as are in the rings • number from a bridgehead, alongest bridge back to the bridgehead, then along the next longest bridge, etc. • show the lengths of bridges in brackets, from longest to shortest 2 -23
IUPAC - General u prefix-infix-suffix • prefix tells the number of carbon atoms in the parent • infix tells the nature of the carbon-carbon bonds • suffix tells the class of compound Infix -an-en-yn- Nature of Carbon-Carbon Bonds in the Parent Chain Suffix Class hydrocarbon all single bonds -e -ol alcohol -al aldehyde -amine -one ketone -oic acid carboxylic acid one or more double bonds one or more triple bonds 2 -24
IUPAC - General prop-en-e en = propene eth-an-ol an = ethanol but-an-one = butanone an but-an-al an = butanal pent-an-oic acid = pentanoic acid an cyclohex-an-ol an = cyclohexanol eth-yn-e yn = ethyne eth-an-amine = ethanamine an 2 -25
Conformations u Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond u Newman projection: a way to view a molecule by looking along a carbon-carbon single bond 2 -26
Conformations u Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon 2 -27
Conformations u Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon 2 -28
Conformations u Torsional strain • also called eclipsed interaction strain • strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation • the torsional strain between eclipsed and staggered ethane is approximately 12. 6 k. J (3. 0 kcal)/mol 2 -29
Conformations angle (Q): (Q) the angle created by two intersecting planes u Dihedral 2 -30
Conformations u Ethane as a function of dihedral angle 2 -31
Conformations u The origin of torsional strain in ethane • originally thought to be caused by repulsion between eclipsed hydrogen nuclei • alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds • theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation • this stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation 2 -32
Conformations u anti conformation • a conformation about a single bond in which the groups lie at a dihedral angle of 180° 2 -33
Conformations u Steric strain (nonbonded interaction strain): strain) • the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow u Angle strain: • strain that arises when a bond angle is either compressed or expanded compared to its optimal value u The total of all types of strain can be calculated by molecular mechanics programs • such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization 2 -34
Conformations • conformations of butane as a function of dihedral angle 2 -35
Anti Butane u Energy-minimized anti conformation • the C-C-C bond angle is 111. 9° and all H-C-H bond angles are between 107. 4 and 107. 9° • the calculated strain is 9. 2 k. J (2. 2 kcal)/mol 2 -36
Eclipsed Butane • calculated energy difference between (a) the nonenergy-minimized and (b) the energy-minimized eclipsed conformations is 5. 6 k. J (0. 86 kcal)/mol 2 -37
Cyclopropane • angle strain: the C-C-C bond angles are compressed from 109. 5° to 60° • torsional strain: there are 6 sets of eclipsed hydrogen interactions • strain energy is about 116 k. J (27. 7 kcal)/mol 2 -38
Cyclobutane • puckering from planar cyclobutane reduces torsional strain but increases angle strain • the conformation of minimum energy is a puckered “butterfly” conformation • strain energy is about 110 k. J (26. 3 kcal)/mol 2 -39
Cyclopentane • puckering from planar cyclopentane reduces torsional strain, but increases angle stain • the conformation of minimum energy is a puckered “envelope” conformation • strain energy is about 42 k. J (6. 5 kcal)/mol 2 -40
Cyclohexane u Chair conformation: the most stable puckered conformation of a cyclohexane ring • all bond C-C-C bond angles are 110. 9° • all bonds on adjacent carbons are staggered 2 -41
Cyclohexane u In a chair conformation, six H are equatorial and six are axial 2 -42
Cyclohexane u For cyclohexane, there are two equivalent chair conformations • all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa 2 -43
Cyclohexane u Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other • there are four sets of eclipsed C-H interactions and one flagpole interaction • a boat conformation is less stable than a chair conformation by 27 k. J (6. 5 kcal)/mol 2 -44
Cyclohexane u Twist-boat conformation • approximately 41. 8 k. J (5. 5 kcal)/mol less stable than a chair conformation • approximately 6. 3 k. J (1. 5 kcal)/mol more stable than a boat conformation 2 -45
Cyclohexane 2 -46
Methylcyclohexane u Equatorial and axial methyl conformations 2 -47
G 0 axial ---> equatorial • given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of conformations using the following relationship 2 -48
Cis, Trans Isomerism u Stereoisomers: compounds that have • the same molecular formula • the same connectivity • a different orientation of their atoms in space u Cis, trans isomers • stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5) 2 -49
Isomers u relationships among isomers 2 -50
Cis, Trans Isomers u 1, 2 -Dimethylcyclopentane 2 -51
Cis, Trans Isomerism u 1, 4 -Dimethylcyclohexane 2 -52
Cis, Trans Isomerism u trans-1, 4 -Dimethylcyclohexane • the diequatorial-methyl chair conformation is more stable by approximately 2 x (7. 28) = 14. 56 k. J/mol 2 -53
Cis, Trans Isomerism u cis-1, 4 -Dimethylcyclohexane 2 -54
Cis, Trans Isomerism u The decalins 2 -55
Steroids u The steroid nucleus u Cholestanol 2 -56
Bicycloalkanes u Norbornane drawn from three different perspectives 2 -57
Bicycloalkanes u Adamantane 2 -58
Physical Properties u Intermolecular • • forces of attraction (example) ion-ion (Na+ and Cl- in Na. Cl) ion-dipole (Na+ and Cl- solvated in aqueous solution) dipole-dipole and hydrogen bonding dispersion forces (very weak electrostatic attraction between temporary dipoles) 2 -59
Physical Properties u Low-molecular-weight alkanes (methane. . butane) are gases at room temperature u Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature u High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature 2 -60
Physical Properties u Constitutional isomers have different physical properties 2 -61
Oxidation of Alkanes u Oxidation is the basis for their use as energy sources for heat and power • heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water 2 -62
Heat of Combustion u Heat of combustion for constitutional isomers 2 -63
Heats of Combustion u For constitutional isomers [k. J (kcal)/mol] -5470. 6 (-1307. 5) -5465. 6 (-1306. 3) -5458. 4 (1304. 6)-5451. 8 (1303. 0) 8 CO 2 + 9 H 2 O 2 -64
Heat of Combustion • strain in cycloalkane rings as determined by heats of combustion 2 -65
Sources of Alkanes u Natural gas • 90 -95% methane u Petroleum • • • gases (bp below 20°C) naphthas, including gasoline (bp 20 - 200°C) kerosene (bp 175 - 275°C) fuel oil (bp 250 - 400°C) lubricating oils (bp above 350°C) asphalt (residue after distillation) u Coal 2 -66
Gasoline u Octane rating: the percent 2, 2, 4 -trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties 2 -67
Synthesis Gas u. A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced 2 -68
Synthesis Gas u Synthesis gas is a feedstock for the industrial production of methanol and acetic acid • it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed 2 -69
Alkanes and Cycloalkanes End Chapter 2 2 -70
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