Alkanes and Cycloalkanes
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Transcript of Alkanes and Cycloalkanes
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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