Alkanes and Cycloalkanes Conformational and Geometric Isomerism

Hydrocarbons Petroleum Saturated Unsaturated Aromatic 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

Alkane Structure Simplest Bond angle CnH2n + 2 Normal v. branched n-alkanes Methylene group —CH2 Homologous series Consecutive building with predictable chemical and physical properties

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

First 10 Unbranched Alkanes Name # of C Molcular Formula Structural Formula # Structural Isomers methane 1 CH4 ethane 2 C2H6 CH3CH3 propane 3 C3H8 CH3CH2CH3 butane 4 C4H10 CH3CH2CH2CH3 pentane 5 C5H12 CH3(CH2)3CH3 hexane 6 C6H14 CH3(CH2)4CH3 heptane 7 C7H16 CH3(CH2)5CH3 9 octane 8 C8H18 CH3(CH2)6CH3 18 nonane C9H20 CH3(CH2)7CH3 35 decane 10 C10H22 CH3(CH2)8CH3 75

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’s 3. Branched alkanes use a root or parent name for the longest continuous chain of C’s

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

IUPAC Rules for Alkanes n-propyl isopropyl n-butyl isobutyl sec-butyl

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

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:

IUPAC Rules for Alkanes

IUPAC Rules for Alkanes

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

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)

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 10

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

Staggered Conformer of Ethane

Eclipsed Conformer of Ethane

Cylcoalkane Nomenclature Ring of C’s cyclo is prefix on alkane name With 1 substituent, no number needed More than 1 substituent, numbers needed Alphabetic priority gets lowest number cylcopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctane bp = -32.7°C 12°C 49.3°C 80.7°C 118.5°C 149°C

Cycloalkane Nomenclature CH3 CH3 CH3 CH3 CH2CH3 methylcyclooctane NOT… 1,2-dimethylcyclooctane NOT… 1-ethyl-2-methylcyclooctane NOT…

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

Cyclohexane Chair Conformations Axial…3 up, 3 down Equatorial, 6 in mean plane of C’s Flipping occurs easily at RT; slows as T decreases Important in biomolecules like glucose

Cyclohexane Boat Conformation Why is this conformation less stable than the chair conformation?

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 properties Not readily converted like conformers are

Conformers (Rotamers) Isomer Review Interconvertible? Bond Pattern? Isomers Stereoiosomers Conformers (Rotamers) Configurational Structural Isomers

Reactions of Alkanes Relatively inert Oxidation (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

Oxidation of Alkanes CH4 + 2O2  CO2 + 2H2O 2CH4 + 3O2  2CO + 4H2O CH4 + O2  C + 2H2O CH4 + O2  CH2O + H2O 2C2H6 + 3O2  2 CH3CO2H + 2H2O

Halogenation of Alkanes energy CH4 + Cl2  CH3Cl + HCl Process is known as chlorination (others are fluorination, bromination, and iodination) Excess of halogen, more H’s are substituted CCl4 can be produced with enough chlorine. + F2  + HF light F

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 produced Chain-terminating step No new radicals are formed and all radicals are combined with some other radical

Halogenation: Free Radical Chain Mechanism light R-H + Cl-Cl  R-Cl + H-Cl Chain-initiating step Chain-propagating steps Possible chain-terminating steps

Reaction Summary Combustion Halogenation

Reaction Mechanism Summary Halogenation