chapter 02 alkanes and cycloalkanes -...

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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

Tuesday, January 24, 12

http://cook.chem.ndsu.nodak.edu/chem341

http://cook.chem.ndsu.nodak.edu/chem341

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Hydrocarbons

• Aliphatic hydrocarbons

• Alkanes: C-C single bondsAlkenes: C-C double bondsAlkynes: C-C triple bonds

• Aromatic hydrocarbons

• Arenes

2


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Electron Waves and Chemical Bonds

Section 2.2


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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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Bonding in H2

5


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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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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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Bonding in H2: The Valence Bond Model

Section 2.3


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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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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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Bonding in H2: The Molecular Orbital Model

Section 2.4


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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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MO Bonding in H2

13

𝜓MO = 𝜓(H)1s + 𝜓(H')1s

𝜓'MO = 𝜓(H)1s - 𝜓(H')1s


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Introduction to Alkanes: Methane, Ethane and Propane

Section 2.5


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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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sp3 Hybridization and Bonding in Methane, Ethane and Propane

Section 2.6-2.7


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Methane Structure

17

H

CH H

H

109.5°

C-H bond length1.1Å (110 pm)


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sp3 Hybridization

18

s

p

Carbon with 4 valence electronsatomic electron configuration


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sp3 Hybridization

18


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sp3 Hybridization

18

sp3

sp3 Hybrid electron configuration


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Bonding in Methane

19

C H

H

HH


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Bonding in Methane

20

C H

H

HH


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Bonding in Ethane

21

C C

H

H

HH

H

H


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Hybridization in Water?

22

O

HH

105°


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Structures of Methane, Ethane and Propane

23


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Isomeric Alkanes: The Butanes and more

Section 2.8-2.10


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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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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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Isomers of Hexane

27

Constitutional Isomers of Hexane C6H14


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Isomers of Pentane

28

• Pentane - C5H12


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Alkane Isomers

29


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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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IUPAC Nomenclature for Alkanes

Section 2.11-2.15


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IUPAC Nomenclature for Linear Alkanes

32

• International Union of Pure and Applied Chemistry


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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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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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35

Constitutional Isomers of Hexane C6H14


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36


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36

identify longest chain


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36


identify the substituents

methyl

methyl

ethyl


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36



methyl

methyl

ethyl

12

3 45

6

78

3-methyl

4-methyl

6-ethyl

Number the chain


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36



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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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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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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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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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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40


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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


chapter 02 alkanes and cycloalkanes -...

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