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Chapter 2: Alkanes and Cycloalkanes

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2.1 Hydrocarbons• Hydrocarbons are compounds that only contain H

and C atoms, but they can be subdivided according to the following tree diagram depending on the bond types that are present

Hydrocarbons

Alkanes Alkenes Alkynes Arenes

Generic formula ：CnH2n+2CnH2nCnH2n-2

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• Generic formula of CnH2n+2

2.2 Structure of alkanes

C

H

H

HH

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2.3 Nomenclature and Conformational Analysis of alkane• Structure: Alkanes are the simplest organic compounds,

comprised of only sp3 hybridized C and H atoms connected by s bonds

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• Isomeric Alkanes (Constitutional isomer)Generic formula of CnH2n+2

When N>3, constitution isomerisomers can have different chemical, physical properties and biological properties.

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ＩＵＰＡＣ（International Union of Pure and Applied Chemistry

Systematic name

Common name

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• Nomenclature: Functional group suffix = -aneFunctional group prefix = alkyl-

•

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Rules Summary:• First find the longest continuous carbon chain, the parent

chain, and name it

• Determine the names of any side chains.• If there are side chains, number the carbon chain using a

numbering system that will result in the smallest numbers for the side chains

• If there are multiple side chains of the same length, use the appropriate prefix: di, tri, tetra, etc.

• Lastly, if there are multiple side chains to list, list them in alphabetical order. When listing in alphabetical order, ignore any prefixes you may have added. For example, "dipropyl" would be listed after "ethyl".

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Punctuation Rules

In general:

Letters that are placed before the words (i.e. "n") are

generally italicized.

There are no spaces between most words.

Hyphens "-" are put between numbers and words.

Commas "," are put between numbers.

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• Special BranchesSome side chains have special names: n-, iso, sec-, tert- (common)

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CH3 CH C CH2 CH3

CH3 CH3

CH31

O

1O

1O

1O

1O

2

3

4

O

OO

primary secondary tertiary

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2,2,6,6,7-Pentamethyloctane

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3

4

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78 1

2 34

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2,3,3,7,7-Pentamethyloctane

Incorrect!

2=2

2<3

Locants are not added together, but examined one by one!

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5-(2-methylpropyl)nonane

1

2

3

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6 7

8

9

1 2

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• Conformational analysis

Conformations: The different spatial arrangements that a molecule can adopt due to rotation about sigma bonds are called conformations and hence Rotamers or conformers.

conformational analysis: The study of the energy changes that occur during these rotations is called conformational analysis

Concept:Staggered conformation: GAUCHE, ANTIEclipsed conformation: SYN

The simplest Example: Ethane

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• Different diagram for conformation analysis: WedgeWedge--dash dash diagramsdiagrams (perspective formulas); Sawhorse projectionSawhorse projection; Newman ProjectionsNewman Projections

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Diagram: The conformation analysis of ethane

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• What about butane

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Diagram: The conformation analysis of butane

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2.4 Nomenclature and Conformational Analysis of cycloalkane

• Simple cycloalkanes

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CH3

CH3

CH3CH3CH2 CH3

1,3-二甲基环己烷 1-甲基4-乙基环己烷 3-甲基环己烯

1,3-Dimethylcyclohexane 4-Ethyl-1-methylcyclohexane 3-Methylcyclohexene

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NomenclaturePolycyclic rings: spiro and bicyclo

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• Ring strain of cycloalkanes Baeyer，A.

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Type of StrainThe reason: : Electron pairs repel each other / as far apart from each

other as possible1) Angle strain 2) Steric strain

3) Torsional（扭） strain (dihedral angle ) 4) Ring straina cyclic structure related non-cyclic

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• The energy change in the ring-flip of cyclohexane (chair-chair)

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• Conformation analysis of cyclohexane

Let’s see the ring-flip of cyclohexane (chair - chair) Pay attention to axial(vertical), equatorial-bond(parallel)!

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• Mono-substituted

Equatorial Substituent

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• Why methyl- group on a-bond position is less stable?

• Di-substituted (relative position: 1,2-; 1,3-; 1,4)1,3-disubstituted

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Equilibrium Constants

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• Pay attention to cis- and trans- concept (describing the relatively position of the substituted groups)

You must remember :the plane relates ring projection

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• Let’s see trans-1,4-dimethylcyclohexane

What about stable conformation?

Less stable

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• Quiz: why trans-decalin(十氢化萘)is more stable than cis-decalin?

Reason: 1) e,e-bond position 2) Less steric strain

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2.4 Physical Properties and Intermolecular Forces

• Intermolecular ForcesVan der Waals Forces :The forces that exist between neutral

molecules are known as the van der Waals forces

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• In order of increasing strength these are: (ref. CD) – induced dipole - induced dipole (London or dispersion

forces) – dipole - induced dipole – dipole – dipole(which includes hydrogen bonds)

Dipole-Dipole Interactions

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• Hydrogen Bonding

Intermolecular Forces influence the properties (physical, chemical and biological) of molecules such as melting point, boiling point, solubility and reactivity

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• Melting Points of Alkanes

Hydrocarbon Physical Properties: low melting and boiling points and very low solubility in polar solvents such as water (hydrophobic)

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2.5 Free Radical Reactions• Reaction: bond cleavage and forming, the reaction

mechanisms is very important for us to predicted the products

• Reactive intermediate: short-lived species that reacts quickly as it is formed , include: free radical, cation(carbocation), anion

• Radical: species with an unpaired electron (usually just draw the unpaired electron)

Here we first discuss free radical reaction:

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Reactive intermediate : The structure of alkyl radical, cation, anion• RadicalRadical : sp2 hybridization, a electron in p-orbit, electron deficient

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• Cation (carbocationcarbocation) sp2 hybridization

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•• Anion Anion sp3 hybridization

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Background of knowledgeBackground of knowledge

• Reaction MechanismsReaction mechanism: describes how a reaction occurs, it

includes – which bonds are broken and which new ones are

formed– the order in which bond-breaking and bond-forming

steps take place– the role of the catalyst (if any is present)– the energy of the entire system during the reaction

First: How to describe the energy change in a reaction?• Potential energy (PE) diagram:

a graph showing the changes in energy that occur during a chemical reaction

• Reaction coordinate:a measure of the change in position of atoms during a

reaction

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1. Potential energy (PE) diagram (or Reaction coordinate digram)

Transition state: an energy maximum on a PE diagram– represents an unstable species of maximum PE formed during

the course of a reaction

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瞬间的

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Potential energy diagram for two-step reaction• Reaction intermediate： an unstable species that lies in a

potentialenergy minimum between two transition states

• Rate-limiting step：the step in a reaction sequence that crosses the highest potential energy barrier; the slow step in a multistep reaction

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3. Activation Energy, Ea: The difference in potential energy between reactants and the transition state (Relate toKinetics, describes Rate)

– determines the rate of reaction– if Ea is large, only a few molecular collisions occur

with sufficient energy to reach the transition state, and the reaction is slow

– if Ea is small, many collisions generate sufficient energy to reach the transition state, and the reaction is fast

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4.4. Thermodynamic ControlThermodynamic Control and Kinetic ControlKinetic ControlIf more than one product is formed in a reaction, what determines the

ratio of products? It depends on the reaction!

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Radical Monochlorination of Methane

• To answer these questions, we need to:1. Understand the mechanism of the reaction (arrow-

pushing!).2. Use thermodynamics and kinetics to analyze the reaction.

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A. Mechanism for the monochlorination of MethaneFree-radical chain reactions have three distinct mechanistic

steps:Initiation step, Propagation steps, and Termination stepI. Initiation Step: generates reactive intermediate

Cl2 absorbs energy and the bond is homolytically cleaved（均裂）

(Remember to use fishhook arrows!)

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II. Propagation Steps: The Meat of the Reaction Reactive intermediates react with stable molecules to generate other reactive

intermediates (allows chain to continue)

Reaction a) Chlorine radical abstracts a hydrogen from methane to form a methyl radical. b) Methyl radical abstracts a chlorine from Cl2 to form product and another chlorine radical

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III. Termination Steps: Unproductive StepsConsumption of reactive intermediates (radicals) without generation of

new ones

• Termination steps slow down and eventually stop the chain reaction• Termination reactions become most important at the end of a

reaction, when there are very few reactant molecules left

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B. Thermodynamics and Kinetics to Analyze • Thermodynamics: energy changes that accompany

chemical and physical transformationsThermodynamics are used to describe systems that are in

their Equilibrium State

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• Kinetics: Reaction Rates– Reaction rate: how fast products appear and

reactants disappear (measured experimentally)– rate equation: relationship between concentration of

reactants and observed reaction– rate (determined experimentally and used to deduce

reaction mechanisms)

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C. Reaction-Energy Diagram for Radical Chorination of Methane

Bond dissociation energies:

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The Radical Product Formation for Butane (Mono-Substituted!)

1) Alkyl Radical Formation Rates

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2) Alkyl Radical Stabilities (lack of electron)

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3) Calculating the amounts of products CH3CH2CH2CH3

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• Quiz: what about the monochlorination of 2,2,5-Trimethylhexane?Pay attention: How many kinds of hydrogen in the molecule?

Answer

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In conclusion:• It is important that the initiation step involves cleavage of a fairly

weak bond like Cl–Cl.

• Other than that, the initiation step is not important in the overall thermodynamics and kinetics of the reaction. This is because oneinitiation step starts a chain that can go through thousands of propagation cycles.

• The termination steps are also unimportant in the overall energetic picture of the reaction, so let's focus on the propagation steps.

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• Bromine Radical Formation (mono-substituted)

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• Rate-limiting step

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• The difference between chlorination and bromination in potential energy diagram

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• The stabilities of some free radicals

Why? Electron Effect (inductive effect, resonance effect)Radical Stabilization

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• Hyperconjugation

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• Two important Substituted ProductBenzylic substituted product and allylic substituted product

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• Other important reagents for radical halogenations reaction:

NBS and NCS Always used for the sythesis of benzylic substituted product

and allylic substituted product

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General Selectivity of Radical Halogenations

Radical brominations are highly selective when one type of hydrogen is particularly reactive (resonance-stabilized).

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2.6 Anti-oxidation Reagents

• In general, free raicals are unstable and active.

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• Some nature products act as radical capture

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Summary

• Isomer (constitutional isomer构造异构)

• Nomenclature and Conformational Analysis (IUPAC nomenclature, diagrams--include Newman projection, Wedge-dash, Sawhorsediagrams; Alkanes and Cyclohexanesconformation, Ring strain)

• Physical Properties

• Free Radical Reactions (Reactive intermediate/ Transition state Reaction Mechanisms)