8Chapter 02

8Chapter 02 - Chapter 2 Hydrocarbon Frameworks: Alkanes Dr....

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Unformatted text preview: Chapter 2 Hydrocarbon Frameworks: Alkanes Dr. Wolf's CHM 201 & 202 2- 1 2.1 Classes of Hydrocarbons Dr. Wolf's CHM 201 & 202 2- 2 Hydrocarbons Hydrocarbons Aliphatic Aliphatic Dr. Wolf's CHM 201 & 202 Aromatic Aromatic 2- 3 Hydrocarbons Hydrocarbons Aliphatic Aliphatic Alkanes Alkanes Dr. Wolf's CHM 201 & 202 Aromatic Aromatic Alkenes Alkenes Alkynes Alkynes 2- 4 Hydrocarbons Hydrocarbons Aliphatic Aliphatic Alkanes are Alkanes hydrocarbons in which all of the bonds are single bonds. single H Dr. Wolf's CHM 201 & 202 H C C H Alkanes Alkanes H H H 2- 5 Hydrocarbons Hydrocarbons Aliphatic Aliphatic Alkenes are Alkenes hydrocarbons that contain a carboncontain carbon double bond. carbon double H H C Alkenes Alkenes H Dr. Wolf's CHM 201 & 202 C H 2- 6 Hydrocarbons Hydrocarbons Aliphatic Aliphatic Alkynes are Alkynes hydrocarbons that contain a carboncontain carbon triple bond. carbon triple Alkynes Alkynes Dr. Wolf's CHM 201 & 202 HC CH 2- 7 Hydrocarbons Hydrocarbons The most common The aromatic hydrocarbons are those that contain a benzene ring. benzene Aromatic Aromatic H H H Dr. Wolf's CHM 201 & 202 H H H 2- 8 2.2 Electron Waves and Chemical Bonds Dr. Wolf's CHM 201 & 202 2- 9 Models for Chemical Bonding Models for Chemical Bonding The Lewis model of chemical bonding predates the idea that electrons have wave properties. There are two other widely used theories of bonding that are based on the wave nature of an electron. Valence Bond Theory Molecular Orbital Theory Dr. Wolf's CHM 201 & 202 2- 10 Formation of H22 ffrom Two Hydrogen Atoms Formation of H rom Two Hydrogen Atoms + e– + e– Examine how the electrostatic forces change Examine as two hydrogen atoms are brought together. as These electrostatic forces are: attractions between the electrons and attractions the nuclei the repulsions between the two nuclei repulsions repulsions between the two electrons repulsions Dr. Wolf's CHM 201 & 202 2- 11 Figure 2.1 Figure 2.1 weak net attraction at long distances Potential energy Dr. Wolf's CHM 201 & 202 H• + H• H Internuclear distance H 2- 12 Figure 2.1 Figure 2.1 attractive forces increase faster than repulsive forces as atoms approach each other Potential energy H• + H• H H H Dr. Wolf's CHM 201 & 202 H H H Internuclear distance 2- 13 Figure 2.1 Figure 2.1 maximum net attraction maximum (minimum potential energy) (minimum at 74 pm internuclear distance Potential energy 74 pm H• + H• H H H H H H -436 kJ/mol H2 Dr. Wolf's CHM 201 & 202 Internuclear distance 2- 14 Figure 2.1 Figure 2.1 1s H H 1s 2 H atoms: each electron "feels" atoms: attractive force of one proton attractive H H H2 molecule: each electron "feels" attractive force of both protons a Dr. Wolf's CHM 201 & 202 ttractive 2- 15 Figure 2.1 Figure 2.1 repulsive forces increase faster than attractive forces at distances closer than 74 pm Potential energy 74 pm H• + H• H H H H H H -436 kJ/mol H2 Dr. Wolf's CHM 201 & 202 Internuclear distance 2- 16 Models for Chemical Bonding Models for Chemical Bonding Valence Bond Theory constructive interference between electron waves of two half-filled atomic orbitals is basis waves of shared-electron bond of Molecular Orbital Theory derive wave functions of molecules by combining wave functions of atoms Dr. Wolf's CHM 201 & 202 2- 17 2.3 Bonding in H2: The Valence Bond Model Dr. Wolf's CHM 201 & 202 2- 18 Valence Bond Model Valence Bond Model Valence Electron pair can be shared when half-filled orbital of one atom overlaps in phase with orbital half-filled orbital of another. half-filled Dr. Wolf's CHM 201 & 202 2- 19 Valence Bond Model Valence Bond Model 1s H H 1s in-phase overlap of two half-filled hydrogen 1s orbitals H H σ bond of H2 bond Dr. Wolf's CHM 201 & 202 2- 20 Valence Bond Model Valence Bond Model σ Bond: orbitals overlap along internuclear axis internuclear Cross section of orbital perpendicular to Cross internuclear axis is a circle. internuclear H Dr. Wolf's CHM 201 & 202 H 2- 21 Valence Bond Model of H22 Valence Bond Model of H The 1s orbitals of two separated hydrogen atoms are far apart. Essentially no interaction. Each electron is associated with a single proton. associated Dr. Wolf's CHM 201 & 202 2- 22 Valence Bond Model of H22 Valence Bond Model of H As the hydrogen atoms approach each other, their 1s orbitals begin to overlap and each electron begins to feel the attractive force of both protons. attractive Dr. Wolf's CHM 201 & 202 2- 23 Valence Bond Model of H22 Valence Bond Model of H The hydrogen atoms are close enough so that appreciable The overlap of the the two 1s orbitals occurs. The concentration overlap of electron density in the region between the two protons is more readily apparent. more Dr. Wolf's CHM 201 & 202 2- 24 Valence Bond Model of H22 Valence Bond Model of H A molecule of H2. The two hydrogen 1s orbitals have been replaced by a new orbital that encompasses both hydrogens and contains both electrons. and Dr. Wolf's CHM 201 & 202 2- 25 2.4 Bonding in H2: The Molecular Orbital Model Dr. Wolf's CHM 201 & 202 2- 26 Main Ideas Main Ideas Electrons in a molecule occupy molecular Electrons orbitals (MOs) just as electrons in an atom occupy atomic orbitals (AOs). Two electrons per MO, just as two electrons per AO. per Express MOs as combinations of AOs. Dr. Wolf's CHM 201 & 202 2- 27 MO Picture of Bonding in H22 MO Picture of Bonding in H Linear combination of atomic orbitals method expresses wave functions of molecular orbitals as sums and differences of wave functions of atomic orbitals. Two AOs yield two MOs Bonding combination Antibonding combination ψMO = ψ(H)1s + ψ(H')1s ψ'MO = ψ(H)1s - ψ(H')1s MO 1s MO Dr. Wolf's CHM 201 & 202 2- 28 Fig. 2.6: Energy-Level Diagram for H22 MOs Fig. 2.6: Energy-Level Diagram for H MOs 1s 1s AO Dr. Wolf's CHM 201 & 202 AO 2- 29 Fig. 2.6: Energy-Level Diagram for H22 MOs Fig. 2.6: Energy-Level Diagram for H MOs MO σ* antibonding 1s 1s AO AO σ bonding MO Dr. Wolf's CHM 201 & 202 2- 30 CnH2n+2 2.5 Introduction to Alkanes: Methane, Ethane, and Propane Dr. Wolf's CHM 201 & 202 2- 31 The Simplest Alkanes The Simplest Alkanes Methane (CH4) CH4 CH Ethane Ethane (C2H6) CH3CH3 Propane (C3H8) CH3CH2CH3 bp -160°C Dr. Wolf's CHM 201 & 202 bp -89°C bp -42°C 2- 32 2.6 sp3 Hybridization and Bonding in Methane Dr. Wolf's CHM 201 & 202 2- 33 Structure of Methane Structure of Methane tetrahedral bond angles = 109.5° bond distances = 110 pm but structure seems inconsistent with electron configuration of carbon Dr. Wolf's CHM 201 & 202 2- 34 Electron configuration of carbon Electron configuration of carbon only two unpaired only electrons electrons 2p should form σ should bonds to only two hydrogen atoms hydrogen 2s Dr. Wolf's CHM 201 & 202 bonds should be bonds at right angles to one another one 2- 35 sp33 Orbital Hybridization sp Orbital Hybridization sp 2p Promote an electron from the 2s to the 2p orbital to 2s Dr. Wolf's CHM 201 & 202 2- 36 sp33 Orbital Hybridization sp Orbital Hybridization 2p 2p 2s Dr. Wolf's CHM 201 & 202 2s 2- 37 sp33 Orbital Hybridization sp Orbital Hybridization 2p Mix together (hybridize) the 2s orbital and the three 2p orbitals orbital 2s Dr. Wolf's CHM 201 & 202 2- 38 sp33 Orbital Hybridization sp Orbital Hybridization 2p 2 sp3 sp 4 equivalent half-filled equivalent orbitals are consistent with four bonds and tetrahedral geometry tetrahedral Dr. Wolf's CHM 201 & 202 2s 2- 39 sp33 Orbital Hybridization sp Orbital Hybridization Dr. Wolf's CHM 201 & 202 2- 40 Nodal properties of orbitals Nodal properties of orbitals p + s Dr. Wolf's CHM 201 & 202 – + 2- 41 Shape of sp33 hybrid orbitals Shape of sp hybrid orbitals Shape sp Shape sp p + – take the s orbital and place it on take top of the p orbital s Dr. Wolf's CHM 201 & 202 + 2- 42 Shape of sp33 hybrid orbitals Shape of sp hybrid orbitals Shape sp Shape sp s+p + + – reinforcement of electron wave in reinforcement regions where sign is the same destructive interference in regions of opposite sign Dr. Wolf's CHM 201 & 202 2- 43 Shape of sp33 hybrid orbitals Shape of sp hybrid orbitals Shape sp Shape sp sp hybrid sp hybrid + – orbital shown is sp hybrid orbital sp analogous procedure using three p orbitals analogous and one s orbital gives sp3 hybrid sp shape of sp3 hybrid is similar shape sp Dr. Wolf's CHM 201 & 202 2- 44 Shape of sp33 hybrid orbitals Shape of sp hybrid orbitals Shape sp Shape sp sp hybrid sp hybrid + – hybrid orbital is not symmetrical higher probability of finding an electron on higher one side of the nucleus than the other one leads to stronger bonds Dr. Wolf's CHM 201 & 202 2- 45 The C—H σ Bond in Methane The In-phase overlap of a half-filled 1s orbital of hydrogen with a half-filled sp3 hybrid orbital of sp carbon: + H s + C– sp3 gives a σ bond. gives + H—C σ H—C Dr. Wolf's CHM 201 & 202 H C– 2- 46 Justification for Orbital Hybridization Justification consistent with structure of methane consistent allows for formation of 4 bonds rather than 2 bonds involving sp3 hybrid orbitals are stronger bonds sp than those involving s-s overlap or p-p overlap Dr. Wolf's CHM 201 & 202 2- 47 2.7 Bonding in Ethane Dr. Wolf's CHM 201 & 202 2- 48 Structure of Ethane C2H6 CH3CH3 tetrahedral geometry at each carbon C—H bond distance = 110 pm C—C bond distance = 153 pm Dr. Wolf's CHM 201 & 202 2- 49 The C—C σ Bond in Ethane The In-phase overlap of half-filled sp3 hybrid In-phase sp orbital of one carbon with half-filled sp3 orbital sp hybrid orbital of another. Overlap is along internuclear axis to give a σ bond. Overlap Dr. Wolf's CHM 201 & 202 2- 50 The C—C σ Bond in Ethane The In-phase overlap of half-filled sp3 hybrid In-phase sp orbital of one carbon with half-filled sp3 orbital sp hybrid orbital of another. Overlap is along internuclear axis to give a σ bond. Overlap Dr. Wolf's CHM 201 & 202 2- 51 C4H10 2.8 Isomeric Alkanes: The Butanes Dr. Wolf's CHM 201 & 202 2- 52 n-Butane CH3CH2CH2CH3 Isobutane (CH3)3CH bp -0.4°C Dr. Wolf's CHM 201 & 202 bp -10.2°C 2- 53 2.9 Higher n-Alkanes Higher Dr. Wolf's CHM 201 & 202 2- 54 CH3CH2CH2CH2CH3 n-Pentane CH3CH2CH2CH2CH2CH3 CH n-Hexane CH3CH2CH2CH2CH2CH2CH3 Dr. Wolf's CHM 201 & 202 n-Heptane 2- 55 2.10 The C5H12 Isomers Dr. Wolf's CHM 201 & 202 2- 56 C5H12 CH3CH2CH2CH2CH3 (CH3)2CHCH2CH3 n-Pentane Isopentane (CH3)4C Neopentane Neopentane Dr. Wolf's CHM 201 & 202 2- 57 How many isomers? How many isomers? The number of isomeric alkanes increases as The the number of carbons increase. the There is no simple way to predict how many There isomers there are for a particular molecular formula. formula. Dr. Wolf's CHM 201 & 202 2- 58 Number of Constitutionally Isomeric Alkanes Number of Constitutionally Isomeric Alkanes CH4 1 C2H6 1 C3H8 1 C4H10 2 C5H12 3 C6H14 5 C7H16 9 Dr. Wolf's CHM 201 & 202 2- 59 Number of Constitutionally Isomeric Alkanes Number of Constitutionally Isomeric Alkanes CH4 1 C8H18 18 C2H6 1 C9H20 35 C3H8 1 C10H22 75 C4H10 2 C15H32 4,347 C5H12 3 C20H42 366,319 C6H14 5 C40H82 62,491,178,805,831 C7H16 9 Dr. Wolf's CHM 201 & 202 2- 60 2.11 IUPAC Nomenclature of Unbranched Alkanes Unbranched Dr. Wolf's CHM 201 & 202 2- 61 IUPAC Names of Unbranched Alkanes IUPAC Names of Unbranched Alkanes Retained: methane CH4 ethane CH3CH3 propane CH3CH2CH3 butane CH3CH2CH2CH3 Dr. Wolf's CHM 201 & 202 2- 62 IUPAC Names of Unbranched Alkanes IUPAC Names of Unbranched Alkanes Note: n-prefix is not part of IUPAC name of any alkane. For example: n-butane is "common name" For -butane for CH3CH2CH2CH3; butane is "IUPAC name." for butane Others: Latin or Greek prefix for number of Latin carbons + ane suffix carbons Dr. Wolf's CHM 201 & 202 2- 63 IUPAC Names of Unbranched Alkanes IUPAC Names of Unbranched Alkanes Number of carbons Name Number Name Structure 5 pentane CH3(CH2)3CH3 6 hexane CH3(CH2)4CH3 7 heptane CH3(CH2)5CH3 8 octane CH3(CH2)6CH3 9 nonane CH3(CH2)7CH3 10 decane CH3(CH2)8CH3 Dr. Wolf's CHM 201 & 202 2- 64 IUPAC Names of Unbranched Alkanes IUPAC Names of Unbranched Alkanes Number of carbons Name Number Name Structure 11 undecane CH3(CH2)9CH3 12 dodecane CH3(CH2)10CH3 13 tridecane CH3(CH2)11CH3 14 tetradecane CH3(CH2)12CH3 15 pentadecane CH3(CH2)7CH3 16 hexadecane CH3(CH2)8CH3 Dr. Wolf's CHM 201 & 202 2- 65 2.12 Applying the IUPAC Rules: The Names of the C6H14 Isomers Dr. Wolf's CHM 201 & 202 2- 66 The C66H14 IIsomers The C H14 somers CH3CH2CH2CH2CH2CH3 CH (CH3)2CHCH2CH2CH3 (CH3CH2)2CHCH3 (CH3)2CHCH(CH3)2 (CH3)3CCH2CH3 Dr. Wolf's CHM 201 & 202 2- 67 The C66H14 IIsomers The C H14 somers CH3CH2CH2CH2CH2CH3 CH Hexane The IUPAC name of the unbranched alkane with a chain of 6 carbons is hexane. Dr. Wolf's CHM 201 & 202 2- 68 IIUPAC Nomenclature of Branched UPAC IUPAC Nomenclature of Branched IUPAC Alkanes Alkanes Alkanes Alkanes Step 1) Find the longest continuous carbon Step Find chain and use the IUPAC name of the unbranched alkane as the basis. Step 2) Add name of substituent as a prefix. Step Add Step 3) Number the chain from the end nearest Step Number the substituent, and identify the carbon to the which the substituent is attached by number. which Dr. Wolf's CHM 201 & 202 2- 69 The C66H14 IIsomers The C H14 somers (CH3)2CHCH2CH2CH3 (CH 2-Methylpentane (CH3CH2)2CHCH3 3-Methylpentane Dr. Wolf's CHM 201 & 202 2- 70 The C66H14 IIsomers The C H14 somers (CH3)2CHCH(CH3)2 (CH 2,3-Dimethylbutane (CH3)3CCH2CH3 2,2-Dimethylbutane Use replicating prefixes (di-, tri-, tetra-, etc.) Use according to the number of identical substituents according attached Dr. Wolf's CHM 201 & 202 to the main chain. 2- 71 2.13 Alkyl Groups Dr. Wolf's CHM 201 & 202 2- 72 Methyl and Ethyl Groups Methyl and Ethyl Groups H Methyl or C H CH3 or CH3CH2 H H Dr. Wolf's CHM 201 & 202 H C C H Ethyl H H 2- 73 Unbranched Alkyl Groups Unbranched Alkyl Groups R H R If potential point of attachment is at the end of the chain, take the IUPAC name of the corresponding unbranched alkane and replace the -ane ending by -yl. Dr. Wolf's CHM 201 & 202 2- 74 Unbranched Alkyl Groups Unbranched Alkyl Groups R H R If potential point of attachment is at the end of the chain, take the IUPAC name of the corresponding unbranched alkane and replace the -ane ending by -yl. H H H C C C C H H H H H H Dr. Wolf's CHM 201 & 202 or Butyl CH3CH2CH2CH2 2- 75 Unbranched Alkyl Groups Unbranched Alkyl Groups CH3(CH2)4CH2 Hexyl CH3(CH2)5CH2 Heptyl CH3(CH2)16CH2 Octadecyl Dr. Wolf's CHM 201 & 202 2- 76 The C33H7 Alkyl Groups The C H7 Alkyl Groups H H C C C H H H H H or CH3CH2CH2 and H H H H C C C H Dr. Wolf's CHM 201 & 202 H or CH3CHCH3 H 2- 77 The C33H7 Alkyl Groups The C H7 Alkyl Groups H H C C C H H H H H or CH3CH2CH2 IUPAC name: Propyl Common name: n-Propyl Common Dr. Wolf's CHM 201 & 202 2- 78 Naming Alkyl Groups (Table 2.7) Step 1: Identify longest continuous chain starting at Identify point of attachment. point Step 2: Drop -ane ending from name of Step Drop unbranched alkane having same number of carbons as longest continuous chain and replace by -yl. Step 3: Identify substituents on longest continuous Identify chain. chain. Dr. Wolf's CHM 201 & 202 2- 79 The C33H7 Alkyl Groups The C H7 Alkyl Groups H H H H C C C H H or CH3CHCH3 H IUPAC name: 1-Methylethyl Common name: Isopropyl Dr. Wolf's CHM 201 & 202 2- 80 The C33H7 Alkyl Groups The C H7 Alkyl Groups H H C C C H H H H H or CH3CH2CH2 Classification: Primary alkyl group Alkyl groups are classified according to the degree of substitution at the carbon that bears the point of attachment. A carbon that is directly attached to one other carbon is a primary carbon. Dr. Wolf's CHM 201 & 202 2- 81 The C33H7 Alkyl Groups The C H7 Alkyl Groups H H H H C C C H H or CH3CHCH3 H Classification: Secondary alkyl group Alkyl groups are classified according to the degree of substitution at the carbon that bears the point of attachment. A carbon that is directly attached to two other carbons is a secondary carbon. Dr. Wolf's CHM 201 & 202 2- 82 The C44H9 Alkyl Groups The C H9 Alkyl Groups H H H C C C C H H H H H H or CH3CH2CH2CH2 IUPAC name: Butyl Common name: n-Butyl Common Classification: Primary alkyl group Dr. Wolf's CHM 201 & 202 2- 83 The C44H9 Alkyl Groups The C H9 Alkyl Groups H H C H C H H C H H C H 1 2 3 or CH3CHCH2CH3 H IUPAC name: 1-Methylpropyl Common name: sec-Butyl Common sec Classification: Secondary alkyl group Dr. Wolf's CHM 201 & 202 2- 84 The C44H9 Alkyl Groups The C H9 Alkyl Groups CH3 3 CH3 2 C 1 CH2 H IUPAC name: 2-Methylpropyl Common name: Isobutyl Classification: Primary alkyl group Dr. Wolf's CHM 201 & 202 2- 85 The C44H9 Alkyl Groups The C H9 Alkyl Groups CH3 CH3 1 C 2 CH3 IUPAC name: 1,1-Dimethylethyl Common name: tert-Butyl Common tert Classification: Tertiary alkyl group Dr. Wolf's CHM 201 & 202 2- 86 2.14 IUPAC Names of Highly Branched Alkanes Dr. Wolf's CHM 201 & 202 2- 87 Branched alkanes Branched alkanes Octane Octane Dr. Wolf's CHM 201 & 202 2- 88 Branched alkanes Branched alkanes Branched 4-Ethyloctane Dr. Wolf's CHM 201 & 202 2- 89 Branched alkanes Branched alkanes 4-Ethyl-3-methyloctane List substituents in alphabetical order. List Dr. Wolf's CHM 201 & 202 2- 90 Branched alkanes Branched alkanes 4-Ethyl-3,5-dimethyloctane List substituents in alphabetical order. List But don't alphabetize di-, tri-, tetra-, etc. Dr. Wolf's CHM 201 & 202 2- 91 First Point of Difference Rule First Point of Difference Rule 2 1 3 4 5 6 7 8 7 8 6 5 4 3 2 1 What is correct name? 2,3,3,7,7-Pentamethyloctane? 2,2,6,6,7-Pentamethyloctane? 2,2,6,6,7-Pentamethyloctane? The chain is numbered in the direction that The gives the lower locant to the substituent at the first point of difference in the names. first Don't add locants! Dr. Wolf's CHM 201 & 202 2- 92 First Point of Difference Rule First Point of Difference Rule 7 8 6 5 4 3 2 1 What is correct name? 2,2,6,6,7-Pentamethyloctane? The chain is numbered in the direction that The gives the lower locant to the substituent at the first point of difference in the names. first Don't add locants! Dr. Wolf's CHM 201 & 202 2- 93 CnH2n 2.15 Cycloalkane Nomenclature Dr. Wolf's CHM 201 & 202 2- 94 Cycloalkanes Cycloalkanes Cycloalkanes are alkanes that contain a ring of Cycloalkanes three or more carbons. three Count the number of carbons in the ring, and Count add the prefix cyclo to the IUPAC name of the cyclo unbranched alkane that has that number of carbons. carbons. Cyclopentane Dr. Wolf's CHM 201 & 202 Cyclohexane 2- 95 Cycloalkanes Cycloalkanes Name any alkyl groups on the ring in the usual way. way. CH2CH3 Ethylcyclopentane Dr. Wolf's CHM 201 & 202 2- 96 Cycloalkanes Cycloalkanes Name any alkyl groups on the ring in the usual Name way. way. List substituents in alphabetical order and count in the direction that gives the lowest numerical locant at the first point of difference. H3C CH3 CH2CH3 3-Ethyl-1,1-dimethylcyclohexane Dr. Wolf's CHM 201 & 202 2- 97 2.16 Sources of Alkanes and Cycloalkanes Dr. Wolf's CHM 201 & 202 2- 98 Crude oil Dr. Wolf's CHM 201 & 202 2- 99 Naphtha Naphtha (bp 95-150 °C) (bp 95-150 °C) Kerosene Kerosene (bp: 150-230 °C) (bp: 150-230 °C) C5-C12 Light gasoline Light gasoline (bp: 25-95 °C) (bp: 25-95 °C) C12-C15 Crude oil Gas oil (Diesel) Gas oil (Diesel) (bp: 230-340 °C) (bp: 230-340 °C) Refinery gas Refinery gas C1-C4 Dr. Wolf's CHM 201 & 202 C15-C25 Residue Residue 2- 100 Petroleum Refining Petroleum Refining Cracking converts high molecular weight hydrocarbons converts to more useful, low molecular weight ones to Reforming increases branching of hydrocarbon chains branched hydrocarbons have better burning characteristics for automobile engines Dr. Wolf's CHM 201 & 202 2- 101 2.17 Physical Properties of Alkanes and Cycloalkanes Dr. Wolf's CHM 201 & 202 2- 102 Boiling Points of Alkanes Boiling Points of Alkanes Boiling governed by strength of intermolecular attractive forces attractive alkanes are nonpolar, so dipole-dipole and alkanes dipole-induced dipole forces are absent dipole-induced only forces of intermolecular attraction are only induced dipole-induced dipole forces induced Dr. Wolf's CHM 201 & 202 2- 103 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces + – + – two nonpolar molecules center of positive charge and center of negative center charge coincide in each charge Dr. Wolf's CHM 201 & 202 2- 104 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces + – + – movement of electrons creates an movement instantaneous dipole in one molecule (left) instantaneous Dr. Wolf's CHM 201 & 202 2- 105 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces + – + – temporary dipole in one molecule (left) induces temporary a complementary dipole in other molecule (right) (right) Dr. Wolf's CHM 201 & 202 2- 106 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces + – + – temporary dipole in one molecule (left) induces temporary a complementary dipole in other molecule (right) (right) Dr. Wolf's CHM 201 & 202 2- 107 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces + – + – the result is a small attractive force between the the two molecules two Dr. Wolf's CHM 201 & 202 2- 108 Induced dipole-Induced dipole attractive forces Induced dipole-Induced dipole attractive forces – + – + the result is a small attractive force between the the two molecules two Dr. Wolf's CHM 201 & 202 2- 109 Boiling Points Boiling Points Boiling increase with increasing number of carbons more atoms, more electrons, more more opportunities for induced dipole-induced opportunities dipole forces dipole decrease with chain branching decrease branched molecules are more compact with smaller surface area—fewer points of smaller contact contact with other molecules with Dr. Wolf's CHM 201 & 202 2- 110 Boiling Points Boiling Points increase with increasing number of carbons more atoms, more electrons, more more opportunities for induced dipole-induced opportunities dipole forces dipole Heptane Heptane bp 98°C Dr. Wolf's CHM 201 & 202 Octane bp 125°C Nonane bp 150°C 2- 111 Boiling Points Boiling Points decrease with chain branching branched molecules are more compact with smaller surface area—fewer points of smaller contact contact with other molecules with Octane: bp 125°C Octane: 2-Methylheptane: bp 118°C 2-Methylheptane: 2,2,3,3-Tetramethylbutane: bp 107°C 2,2,3,3-Tetramethylbutane: Dr. Wolf's CHM 201 & 202 2- 112 2.18 Chemical Properties: Combustion of Alkanes All alkanes burn in air to give carbon dioxide and water. Dr. Wolf's CHM 201 & 202 2- 113 Heats of Combustion Heats of Combustion increase with increasing number of carbons more moles of O2 consumed, more moles of CO2 and H2O formed Dr. Wolf's CHM 201 & 202 2- 114 Heats of Combustion Heats of Combustion Heptane Heptane 4817 kJ/mol 654 kJ/mol Octane 5471 kJ/mol 654 kJ/mol Nonane Dr. Wolf's CHM 201 & 202 6125 kJ/mol 2- 115 Heats of Combustion Heats of Combustion increase with increasing number of carbons more moles of O2 consumed, more moles of CO2 and H2O formed decrease with chain branching branched molecules are more stable (have less potential energy) than their unbranched isomers Dr. Wolf's CHM 201 & 202 2- 116 Heats of Combustion Heats of Combustion 5471 kJ/mol 5 kJ/mol kJ/mol 5466 kJ/mol 8 kJ/mol 5458 kJ/mol 6 kJ/mol 5452 kJ/mol Dr. Wolf's CHM 201 & 202 2- 117 Important Point Important Point Isomers can differ in respect to their stability. Equivalent statement: Isomers differ in respect to their potential energy. Differences in potential energy can be measured by Differences comparing heats of combustion. comparing Dr. Wolf's CHM 201 & 202 2- 118 Figure 2.14 Figure 5471 kJ/mol 25 + O2 2 5466 kJ/mol 5458 kJ/mol 25 + O2 2 5452 kJ/mol + 25 O2 2 25 + O2 2 8CO2 + 9H2O Dr. Wolf's CHM 201 & 202 2- 119 2.19 2.19 Oxidation-Reduction in Organic Oxidation-Reduction Chemistry Chemistry Oxidation of carbon corresponds to an Oxidation increase in the number of bonds between carbon and oxygen and/or a decrease in the number of carbon-hydrogen bonds. Dr. Wolf's CHM 201 & 202 2- 120 O O iincreasing oxidation ncreasing state of carbon state O H H H C H H H -4 Dr. Wolf's CHM 201 & 202 C H C H C HO C OH OH H OH H -2 0 +2 +4 2- 121 HC iincreasing oxidation ncreasing state of carbon state H H C C H H H C H CH C H H H -3 Dr. Wolf's CHM 201 & 202 H -2 -1 2- 122 But most compounds contain several (or many) carbons, and these can be in different oxidation states. CH3CH2OH Dr. Wolf's CHM 201 & 202 C2H6O 2- 123 But most compounds contain several (or many) carbons, and these can be in different oxidation states. Working from the molecular formula gives Working the average oxidation state. the CH3CH2OH C2H6O Average oxidation state of C = -2 Dr. Wolf's CHM 201 & 202 2- 124 How can we calculate the oxidation state of each carbon in a molecule that contains carbons in different oxidation states? carbons CH3CH2OH CH C2H6O Average oxidation state of C = -2 Dr. Wolf's CHM 201 & 202 2- 125 Table 2.5 How to Calculate Oxidation Numbers Table 2.5 How to Calculate Oxidation Numbers 1. Write the Lewis 1. structure and include unshared electron pairs. electron Dr. Wolf's CHM 201 & 202 H H H C C H •• H O •• H 2- 126 Table 2.5 How to Calculate Oxidation Numbers Table 2.5 How to Calculate Oxidation Numbers 2. Assign the electrons 2. in a covalent bond between two atoms to the more electronegative partner. partner. Dr. Wolf's CHM 201 & 202 H H •• •• H •C •• • H •• C •O • H •• •• • • H 2- 127 Table 2.5 How to Calculate Oxidation Numbers Table 2.5 How to Calculate Oxidation Numbers 3. For a bond between 3. two atoms of the same element, assign the electrons in the bond equally. in Dr. Wolf's CHM 201 & 202 H H •• •• H •C •• • H •• C •O • H •• •• • • H 2- 128 Table 2.5 How to Calculate Oxidation Numbers Table 2.5 How to Calculate Oxidation Numbers 3. For a bond between 3. two atoms of the same element, assign the electrons in the bond equally. in Dr. Wolf's CHM 201 & 202 H H •• •• •• H • C• • C • O • H •• •• •• • • H • H 2- 129 Table 2.5 How to Calculate Oxidation Numbers Table 2.5 How to Calculate Oxidation Numbers 4. Count the number of Count electrons assigned to each atom and subtract that number from the number of valence electrons in the neutral atom; the result is the oxidation number. oxidation Dr. Wolf's CHM 201 & 202 H H •• •• •• H • C• • C • O • H •• •• •• • • H • H Each H = +1 Each C of CH3 = -3 C of CH2O = -1 O = -2 2- 130 Fortunately, we rarely need to calculate the Fortunately, oxidation state of individual carbons in a molecule . We often have to decide whether a process We is an oxidation or a reduction. is Dr. Wolf's CHM 201 & 202 2- 131 Generalization Generalization Oxidation of carbon occurs when a bond between Oxidation carbon and an atom which is less electronegative than carbon is replaced by a bond to an atom that is more electronegative than carbon. The reverse process is reduction. process oxidation C CX Y reduction X less electronegative than carbon Y more electronegative than carbon Dr. Wolf's CHM 201 & 202 2- 132 Examples Examples Oxidation CH4 + Cl2 CH3Cl + HCl Reduction CH3Cl + 2Li Dr. Wolf's CHM 201 & 202 CH3Li + LiCl 2- 133 2.20 sp2 Hybridization and Bonding in Ethylene and Dr. Wolf's CHM 201 & 202 2- 134 Structure of Ethylene Structure of Ethylene C2H4 H2C=CH2 planar bond angles: bond close to 120° close bond distances: C—H = 110 pm bond C—H C=C = 134 pm Dr. Wolf's CHM 201 & 202 2- 135 sp22Orbital Hybridization sp Orbital Hybridization sp Orbital Orbital 2p Promote an electron from the 2s to the 2p orbital to 2s Dr. Wolf's CHM 201 & 202 2- 136 sp22 Orbital Hybridization sp Orbital Hybridization 2p 2p 2s Dr. Wolf's CHM 201 & 202 2s 2- 137 sp22Orbital Hybridization sp Orbital Hybridization Orbital Orbital 2p Mix together (hybridize) the 2s orbital and two of the three 2p orbital orbitals orbitals 2s Dr. Wolf's CHM 201 & 202 2- 138 sp22 Orbital Hybridization sp Orbital Hybridization 2p 2 sp2 sp 3 equivalent half-filled equivalent sp2 hybrid orbitals plus sp 1 p orbital left unhybridized unhybridized Dr. Wolf's CHM 201 & 202 2s 2- 139 sp2 Orbital Hybridization Dr. Wolf's CHM 201 & 202 2- 140 sp22 Orbital Hybridization sp Orbital Hybridization σ p 2 sp2 sp Dr. Wolf's CHM 201 & 202 σ σ σ σ 2- 141 π Bonding in Ethylene π Bonding in Ethylene Bonding Bonding p the unhybridized p orbital of the carbon is involved in π bonding to the other carbon to 2 sp2 sp Dr. Wolf's CHM 201 & 202 2- 142 π Bonding in Ethylene πBonding in Ethylene Bonding Bonding p 2 sp2 sp each carbon has an unhybridized 2p orbital axis of orbital is perpendicular to the plane of the σ bonds Dr. Wolf's CHM 201 & 202 2- 143 π Bonding in Ethylene πBonding in Ethylene Bonding Bonding p 2 sp2 sp side-by-side overlap of half-filled p orbitals gives a π bond bond double bond in ethylene has a double σ component and a π component Dr. Wolf's CHM 201 & 202 2- 144 2.21 sp Hybridization and Bonding in Acetylene and Dr. Wolf's CHM 201 & 202 2- 145 Structure of Acetylene Structure of Acetylene C2H2 HC CH linear bond angles: bond 180° 180° bond distances: C—H = 106 pm bond C—H CC = 120 pm Dr. Wolf's CHM 201 & 202 2- 146 sp Orbital Hybridization spOrbital Hybridization sp Orbital Orbital 2p Promote an electron from the 2s to the 2p orbital to 2s Dr. Wolf's CHM 201 & 202 2- 147 sp Orbital Hybridization sp Orbital Hybridization 2p 2p 2s Dr. Wolf's CHM 201 & 202 2s 2- 148 sp Orbital Hybridization spOrbital Hybridization Orbital Orbital 2p Mix together (hybridize) the 2s orbital and one of the three 2p one orbitals orbitals 2s Dr. Wolf's CHM 201 & 202 2- 149 sp Orbital Hybridization sp Orbital Hybridization 2p 2p 2 sp sp 2 equivalent half-filled equivalent sp hybrid orbitals plus sp 2 p orbitals left unhybridized unhybridized Dr. Wolf's CHM 201 & 202 2s 2- 150 sp Orbital Hybridization Dr. Wolf's CHM 201 & 202 2- 151 sp Orbital Hybridization sp Orbital Hybridization σ 2p σ 2 sp sp Dr. Wolf's CHM 201 & 202 σ 2- 152 π Bonding in Acetylene πBonding in Acetylene Bonding Bonding 2p the unhybridized p orbitals of the carbon are involved in separate carbon π bonds to the other carbon 2 sp sp Dr. Wolf's CHM 201 & 202 2- 153 π Bonding in Acetylene πBonding in Acetylene Bonding Bonding 2p 2 sp sp one π bond involves one of the p orbitals on each carbon one there is a second π bond perpendicular to this one there Dr. Wolf's CHM 201 & 202 2- 154 π Bonding in Acetylene πBonding in Acetylene Bonding Bonding 2p 2 sp sp Dr. Wolf's CHM 201 & 202 2- 155 π Bonding in Acetylene πBonding in Acetylene Bonding Bonding 2p 2 sp sp Dr. Wolf's CHM 201 & 202 2- 156 Which Theory of Chemical Bonding is Best? Dr. Wolf's CHM 201 & 202 2- 157 Three Models Three Models Lewis most familiar—easiest to apply Valence-Bond (Orbital Hybridization) provides more insight than Lewis model ability to connect structure and reactivity ability to hybridization develops with practice to Molecular Orbital potentially the most powerful method but is the most abstract requires the most experience to use requires effectively effectively Dr. Wolf's CHM 201 & 202 2- 158 End of Chapter 2 Dr. Wolf's CHM 201 & 202 2- 159 ...
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This document was uploaded on 01/03/2012.

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