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©2012 Gregory R Cook
Chapter 02Alkanes and Cycloalkanes: Introduction to Hydrocarbons
CHEM 341: Spring 2012
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem341
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©2012 Gregory R Cook
Hydrocarbons
• Aliphatic hydrocarbons
• Alkanes: C-C single bondsAlkenes: C-C double bondsAlkynes: C-C triple bonds
• Aromatic hydrocarbons
• Arenes
2
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©2012 Gregory R Cook
Electron Waves and Chemical Bonds
Section 2.2
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©2012 Gregory R Cook
Bonding Models
• Valence Bond Theory
• Atomic orbitals combine (overlap) to form chemical bonds
• Molecular Orbital Theory
• Electrons not assigned to individual bonds but are calculated over the whole molecule
4
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©2012 Gregory R Cook
Bonding in H2
5
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©2012 Gregory R Cook
Wave Property of Electrons
• Electrons are in orbitals (calculated wave functions)
• As two nuclei come together their waves can either reinforce or cancel each other
6
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©2012 Gregory R Cook
Valence Bond vs MO Theory
• Both valence bond theory and molecular orbital theory take into account the wave functions of electrons
• Valence Bond theory treats them as atomic orbitals that overlap
• MO theory mathematically combines all the atomic orbitals and to produce new molecular orbitals
7
+
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©2012 Gregory R Cook
Bonding in H2: The Valence Bond Model
Section 2.3
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©2012 Gregory R Cook
Valence Bond Model
• The electron pair can be shared (bond) when the half-filled orbitals overlap in phase with each other
9
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©2012 Gregory R Cook
Valence Bond Model
• As the atoms with 1 electron each approach each other the 1s orbitals overlap forming a new orbital encompassing the atoms
• Electron density is greatest in the region between the nuclei
• Sigma bond (σ) - end to end overlap of atomic orbitals
10
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©2012 Gregory R Cook
Bonding in H2: The Molecular Orbital Model
Section 2.4
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©2012 Gregory R Cook
Main ideas
• Electrons in a molecule occupy Molecular Orbitals just like they occupy Atomic Orbitals in atoms
• Each MO can contain only two electrons
• MO’s are expressed as a combination of AO’s
• Two AO’s will produce two MO’sCombination produces a bonding and antibonding orbital
𝜓MO = 𝜓(H)1s + 𝜓(H')1s 𝜓'MO = 𝜓(H)1s - 𝜓(H')1s
12
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©2012 Gregory R Cook
MO Bonding in H2
13
𝜓MO = 𝜓(H)1s + 𝜓(H')1s
𝜓'MO = 𝜓(H)1s - 𝜓(H')1s
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©2012 Gregory R Cook
MO Bonding in H2
13
𝜓MO = 𝜓(H)1s + 𝜓(H')1s
𝜓'MO = 𝜓(H)1s - 𝜓(H')1s
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©2012 Gregory R Cook
MO Bonding in H2
13
𝜓MO = 𝜓(H)1s + 𝜓(H')1s
𝜓'MO = 𝜓(H)1s - 𝜓(H')1s
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©2012 Gregory R Cook
Introduction to Alkanes: Methane, Ethane and Propane
Section 2.5
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©2012 Gregory R Cook
Alkanes
15
Alkanes CnH2n+2
MF CondensedName
Methane CH4CH4
Ethane C2H6
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
CH3CH3
C3H8 CH3CH2CH3
C4H10 CH3CH2CH2CH3
C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
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©2012 Gregory R Cook
sp3 Hybridization and Bonding in Methane, Ethane and Propane
Section 2.6-2.7
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©2012 Gregory R Cook
Methane Structure
17
H
CH H
H
109.5°
C-H bond length1.1Å (110 pm)
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©2012 Gregory R Cook
sp3 Hybridization
18
s
p
Carbon with 4 valence electronsatomic electron configuration
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©2012 Gregory R Cook
sp3 Hybridization
18
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©2012 Gregory R Cook
sp3 Hybridization
18
sp3
sp3 Hybrid electron configuration
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©2012 Gregory R Cook
Bonding in Methane
19
C H
H
HH
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©2012 Gregory R Cook
Bonding in Methane
20
C H
H
HH
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©2012 Gregory R Cook
Bonding in Ethane
21
C C
H
H
HH
H
H
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©2012 Gregory R Cook
Hybridization in Water?
22
O
HH
105°
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©2012 Gregory R Cook
Structures of Methane, Ethane and Propane
23
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©2012 Gregory R Cook
Isomeric Alkanes: The Butanes and more
Section 2.8-2.10
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©2012 Gregory R Cook
Alkanes
25
Alkanes CnH2n+2
MF CondensedName
Methane CH4CH4
Ethane C2H6
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
CH3CH3
C3H8 CH3CH2CH3
C4H10 CH3CH2CH2CH3
C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
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©2012 Gregory R Cook
Alkanes
26
• Linear Alkanes - carbons linked in a straight chain
• Branched Alkanes - some carbons are attached as a branch off the main chain
Constitutional Isomers of Butane C4H10
CH3 CH CH3
CH3
CH3 CH2 CH2 CH3
normal butanebp -0.4°C
isobutanebp -10.2°C
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©2012 Gregory R Cook
Isomers of Hexane
27
Constitutional Isomers of Hexane C6H14
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©2012 Gregory R Cook
Isomers of Pentane
28
• Pentane - C5H12
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Alkane Isomers
29
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©2012 Gregory R Cook
Naming Alkanes
30
• With all these isomers possible, how can we distinguish them?
• We need to have a systematic method to name all these isomers
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©2012 Gregory R Cook
IUPAC Nomenclature for Alkanes
Section 2.11-2.15
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©2012 Gregory R Cook
IUPAC Nomenclature for Linear Alkanes
32
• International Union of Pure and Applied Chemistry
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
33
• Substituted derivatives of the unbranched alkane
• STEP 1 - Find the longest continuous chain of carbons. If there is more than one possibility, choose the chain that is more branched. This is reference to as the PARENT chain.
• STEP 2 - Identify the substituent groups attached to the parent chain (-ane ending changed to -yl)
• STEP 3 - Number the chain beginning at the end of the nearest substituent. If there are substituents equal distance from either end, look for the next nearest branch.
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
34
• STEP 4 - Write the name for the molecule using hyphens between prefixes and commas between numbers.
• If there are more than one substituent on the same carbon, they would each have the same number.
• If there is more than one substituent with the same name, indicate the number of them using di, tri, tetra, etc.
• Prefixes are arranged in alphabetical order according to the substituent name.
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
35
Constitutional Isomers of Hexane C6H14
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
36
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
36
identify longest chain
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
36
identify longest chain
identify the substituents
methyl
methyl
ethyl
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
36
identify longest chain
identify the substituents
methyl
methyl
ethyl
12
3 45
6
78
3-methyl
4-methyl
6-ethyl
Number the chain
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©2012 Gregory R Cook
IUPAC Nomenclature for Branched Alkanes
36
identify longest chain
identify the substituents
methyl
methyl
ethyl
12
3 45
6
78
3-methyl
4-methyl
6-ethyl
Number the chain
6-ethyl-3,4-dimethyloctane
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©2012 Gregory R Cook
Alkyl Groups
37
H3C Cl OHmethyl chloride ethyl alcohol
ClCl
ClCl
n-butyl chloride sec-butyl chloride iso-butyl chloride tert-butyl chloride
1-chlorobutane 2-chlorobutane 1-chloro-2-methylpropane 2-chloro-2-methylpropane
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©2012 Gregory R Cook
Degree of Alkyl Substitution
38
primary carbon1°
C
H
R
H
H
R = any carbon alkyl group
C
H
R
R
H
C
H
R
R
R C
R
R
R
R
secondary carbon2°
tertiary carbon3°
quaternary carbon4°
• We designate a kind of carbon (or kind of functional group attached to that carbon) according to how many other alkyl groups are attached to it.
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©2012 Gregory R Cook
Complex Substituents
39
C C C
H
HH
H
H
H
H
propyln-propyl
C C C
HH
H
H H
H
H
1-methylethyliso-propyl
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©2012 Gregory R Cook
Complex Substituents
39
C C C
H
HH
H
H
H
H
propyln-propyl
C C C
HH
H
H H
H
H
1-methylethyliso-propyl
• Start at the point of attachment and find the longest continuous carbon chain. This is the parent substituent. Number and name any branching groups according to this.
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©2012 Gregory R Cook
Complex Substituents
40
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©2012 Gregory R Cook
Cycloalkanes
41
• Alkanes can be connected in a circle
cyclopropane
H2C
CH2
CH2 H2C
H2C CH2
CH2CH2H2C
H2CCH2
CH2
CH2H2C
H2C
H2C CH2
CH2
cyclobutane cyclopentane cyclohexane
methylcyclopentane1,3-dimethylcyclohexane
3-ethyl-1,1-dimethylcyclohexane
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