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CONFORMATIONAL AND GEOMETRIC ISOMERISM

Alkanes and Cycloalkanes

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Hydrocarbons

PetroleumSaturated

Single-bonded C’s Alkanes or cycloalkanes

Unsaturated At least 1 double- or triple-bonded pair of C’s Alkenes—double Alkynes—triple Can be cyclic

Aromatic Structures like benzene

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Alkane Structure

SimplestBond angleCnH2n + 2

Normal v. branched n-alkanes

Methylene group —CH2

Homologous series Consecutive building with predictable chemical and

physical properties

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Nomenclature of Organic Compounds

Common Sources, structure, or uses Limonene Cubane

IUPAC Systematic method Unique name Write structures from names 1-methyl-4-(1-methylethenyl)-cyclohexene pentacyclo[4.2.0.02,5.03,8.04,7]octane

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First 10 Unbranched Alkanes

Name # of C

Molcular Formula

Structural Formula # Structural Isomers

methane 1 CH4 CH4 1ethane 2 C2H6 CH3CH3 1propane 3 C3H8 CH3CH2CH3 1butane 4 C4H10 CH3CH2CH2CH3 2pentane 5 C5H12 CH3(CH2)3CH3 3hexane 6 C6H14 CH3(CH2)4CH3 5heptane 7 C7H16 CH3(CH2)5CH3 9octane 8 C8H18 CH3(CH2)6CH3 18nonane 9 C9H20 CH3(CH2)7CH3 35decane 10 C10H22 CH3(CH2)8CH3 75

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IUPAC Rules for Alkanes

1. The –ane ending will be used for all saturated

hydrocarbons.2. Alkanes without branches are named according to

the number of C’s3. Branched alkanes use a root or parent name for the

longest continuous chain of C’s

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IUPAC Rules for Alkanes

4. Anything not on the root chain is known as a substituent. Saturated substituents are known as alkyl groups, and names come from # of C’s.

methyl

ethyl

propyl

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IUPAC Rules for Alkanes

n-propyl

isopropyl

n-butyl

isobutyl

sec-butyl

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IUPAC Rules for Alkanes

5. The root chain is numbered such that the first substituent along the chain receives the lowest possible number.• Any other substituents are also located by name and

location. Every substituent must be named and numbered.

• If two or more of the same substituent are present, prefixes such as di, tri, tetra, penta, etc. are used

• If two or more different types of substituents are used, they are listed alphabetically, ignoring prefixes such as di, tri, n, iso, sec, tert unless necessary to alphabetize

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IUPAC Rules for Alkanes

5. Separate numbers from words with a hyphen; separate numbers from numbers with a comma. There are no spaces within an IUPAC name.

Let’s use some C skeletons as our first examples:

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IUPAC Rules for Alkanes

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IUPAC Rules for Alkanes

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Halogen Substituents

• The letter R represents alkyl group. R-H means any alkane.

• The halogens that may take the place of the H: F, Cl, Br, and I

• The name of halogen loses ine suffix and becomes o: fluoromethane, chloromethane, bromomethane, iodomethane

• Common names treat R as substituent and halogen as root chain

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Alkane Sources

Petroleum Mixture of alkanes and cycloalkanes Gasoline v. diesel LPG—1° propane butane

Natural Gas ~80% methane and 5-10% ethane Liquefied for transportation (-160°C, 1 m3 l

= 600 m3 g)

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Alkane Properties and Intermolecular Forces

Insoluble in water H-bonding v. LDF

Lower bp for MW than other organics bp rises as chain length increases bp falls as chain becomes more branched

Name Boiling Point (°C)

n-pentane 36

2-methylbutane 28

2,2-dimethylpropane

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Alkane Conformations

Stereoisomers Atoms connected in same order but arranged

differently in space Sigma bonds

Single bonds allow rotation Staggered Eclipsed Rotamers or Conformers

Dash-wedge Sawhorse Newman projections

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Staggered Conformer of Ethane

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Eclipsed Conformer of Ethane

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Cylcoalkane Nomenclature

Ring of C’scyclo is prefix on alkane nameWith 1 substituent, no number neededMore than 1 substituent, numbers neededAlphabetic priority gets lowest number

cylcopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctanebp = -32.7°C 12°C 49.3°C 80.7°C 118.5°C 149°C

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Cycloalkane Nomenclature

CH3 CH3 CH3

CH3 CH2CH3

methylcyclooctane

NOT…

1,2-dimethylcyclooctane

NOT…

1-ethyl-2-methylcyclooctane

NOT…

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Cylcoalkane Conformations

Cyclopropane Planar with 60° C-C-C (only cycloalkane that is planar)

Cyclobutane 88° C-C-C (predicted 90°)

Cyclopentane 105° C-C-C (predicted 108°)

Cyclohexane 109.5° C-C-C Chair conformation

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Cyclohexane Chair Conformations

Axial…3 up, 3 downEquatorial, 6 in mean plane of C’s

Flipping occurs easily at RT; slows as T decreases

Important in biomolecules like glucose

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Cyclohexane Boat Conformation

Why is this conformation less stable than the chair conformation?

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Cis-Trans Isomerism in Cycloalkanes

Type of stereoisomers or geometric stereoisomers in which substituents are on the same side (cis) or the opposite side (trans) of a ring

Different physical and chemical propertiesNot readily converted like conformers are

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Isomer Review

Interconvertible?

Bond Pattern?

Isomers

Stereoiosomers

Conformers (Rotamers)

Configurational

Structural Isomers

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Reactions of Alkanes

Relatively inertOxidation (Combustion)

Excess O2 produces CO2, most oxidized form of C (ox # is +4)

Insufficient O2 means lower ox # of C CO C CH2O CH3CO2H

Halogenation (substitution rxn) Excess halogen results in more substitution…even mixtures Cycloalkanes make pure organic products

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Oxidation of Alkanes

CH4 + 2O2 CO2 + 2H2O

2CH4 + 3O2 2CO + 4H2O

CH4 + O2 C + 2H2O

CH4 + O2 CH2O + H2O

2C2H6 + 3O2 2 CH3CO2H + 2H2O

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Halogenation of Alkanes

CH4 + Cl2 CH3Cl + HCl

Process is known as chlorination (others are fluorination, bromination, and iodination)

Excess of halogen, more H’s are substitutedCCl4 can be produced with enough chlorine.

+ F2 + HF

energy

lightF

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Halogenation: Free Radical Chain Mechanism

Rxn Mechanism Most rxns have multiple steps

Chain-initiating step Breaking of halogen molecule

Chain-propagating step Radical is formed (odd # of unshared e-) Each step: radical is consumed, but another radical is

producedChain-terminating step

No new radicals are formed and all radicals are combined with some other radical

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Halogenation: Free Radical Chain Mechanism

R-H + Cl-Cl R-Cl + H-ClChain-initiating step

Chain-propagating steps

Possible chain-terminating steps

light

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Reaction Summary

Combustion

Halogenation

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Reaction Mechanism Summary

Halogenation