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Prof. Rachid SkoutaLecture time: Tue/Thu: 10:00-11:15 amLocation: ISB221Department of [email protected] Chemistry (Chem 250)Chapter 11: AlkanesStudent’s questions are welcome during, before or after class: I will stay 15-30 minutes after class for questions as I am happy to assist.Book reading• Introduction to General, Organic and Biochemistry • Pages 324-3514Group review sessions are created to support student learning:Send your owl questions toRitam Das; Email: [email protected];1) Wed, 6 pm -7 pm in the conference room in PSB 283.2) Sun, 5 pm - 6 pm via Zoom.Zoom link: https://umass-amherst.zoom.us/j/4134047234Group activity in class• 1) Student individual attempted answers▪ 2) Group discussionAlkanes✓Nomenclature✓Isomersconstitutional isomers, structural isomers✓Cycloalkanes✓Reactions of Alkanes and CylcoalkanesLearning goals✓Molecular formula, structural formula, line-angle formulaFrom formula to constitutional isomersC6H14Chain with all carbons: 1Chain with (n-1) carbons: Chain with (n-2) carbons: Requirement: n<6 The general formula of linear alkanes is CnH2n+2RecallInternational Union of Pure and Applied Chemistry (IUPAC) systemBranched alkanes:▪Find the longest chain that is the parent chain giving the root name▪Give each substituent group on the parent chain a name and a number▪Use a hyphen to connect the number to the nameCommon alkyl substituents: substituent (group): derived from hydrocarbons by removal of a H F-: fluoroCl-: chloroBr-: bromoI- : iodoRecallAlkyl GroupsRecall▪one substituent▪More than one of the same substituentIUPAC namedifferent substituents▪Parent chain▪Substituent groups obey alphabetic order▪Number/Low alphabetic order, lower numberIUPAC namedifferent substituentsIUPAC nameOWL questionOWL question3 4 567 8 9 10Prop But Pent Hex Hept Oct Non DecCycloalkanes▪A hydrocarbon what contains atoms joined to form a ring▪The general formula of cycloalkanes is CnH2nUnbranched cycloalkanesCyclo+prefix+aneName cycloalkanesSingle substitution: If the substituents is longer than the ringthen the parent name is the substituent. If the ring is longer then the ringis the parent.CH3CH2CH2CH2CH31-Methylcyclopentane 1-CyclopropylbutaneMore than one substituent groups:Name cycloalkanesMake sure that the substituent have the lowest numbersIf two or more alkyl chains can be the same number then number alphabeticallyCH3CH3123456CH3CH31234561,3-Dimethylcyclohexane1,5-DimethylcyclohexaneRightWrongCH3CH2CH3CH3CH2CH312345123452-Ethyl-1-methylcyclopentane1-Ethyl-2-methylcyclopentaneRightWrongConformations of alkanes ▪Free rotation about each carbon-carbon bond in an alkane. ▪A molecule can have an infinite number of possible three-dimensional shapes, or conformationsCH3CH2CH2CH3Least crowdmost crowd▪The conformation of each molecule constantly changes. ▪At any give time, the most stable fully extended conformation dominates the populationConformations of cycloalkanes CyclopropaneCyclobutaneCyclopantaneCyclohexaneChairBoatCis-Trans Isomers1, 2-dimethylcyclopropane HHHHHHHHCis IsomersTrans IsomersCis Isomers are when two substituents are on the same side of a ring. Trans Isomers are when two substituents are on different side of a ring.Which molecule shows cis-trans isomerism?A. MethylcyclopentaneB. 1,1-DimethylcyclopentaneC.1,3-DemethylcyclopentaneH3CH3CH3CH3C CH3Physical properties of Alkanes▪Melting and Boiling point▪Solubility: not soluble in water▪Densitylike dissolves like! Polar dissolves polar, non-polar dissolves non-polarMelting and Boiling pointUnbranched Alkanes have higher boiling point than branched onesMore contacting surfaceLess contacting surfaceIntermolecular ForcesThe attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.• These forces are present in all molecules, whether they are polar or nonpolarFactors Affecting London ForcesThis is due to the increased surface area in n-pentane• The strength of dispersion forces tends to increase with increased molecular weight• The shape of the molecule affects the strength of dispersion forces: long, skinny moleculesIntermolecular Forces Affect Many Physical PropertiesIf the intermolecular forces between molecules are:✓relatively strong, the boiling point will be relatively high.✓relatively weak, the boiling point will be relatively low.Reactions of AlkanesCombustionMany alkanes are used in this way: Methane: natural gasPropane: used in gas grillesButane: lightersGasoline: mixtures of hydrocarbons(C5-C12)CH4+ 2O2CO2+ 2H2O + 212 kcal/molProductsCH3CH2CH3+ 5O23 CO2+ 4H2O + 530 kcal/molHalogenation: reaction with halogensCH4+ Cl2CH3Cl CH2Cl2 CH2Cl3Reactions of AlkanesChloromethane DichloromethaneCl2Cl2heatheatImportant haloalkanes:Freon-11: CCl3F (Trichlorofluoromethane)Summary✓Name the alkanes and cycloalkanes✓Constitutional isomers✓Cis-Trans isomers✓Physical properties of the alkanes✓Write the structual form of the alkanes and cycloalkanes▪Branched vs Unbranched one▪Intermolecule force✓Chemical reaction of alkanesUseful LinksEnd of Chapter 11Naming alkanehttps://www.khanacademy.org/science/organic-chemistry/bond-line-structures-alkanes-cycloalkanes/naming-alkanes/v/naming-simple-alkanesNaming cycloalkanehttps://www.khanacademy.org/science/organic-chemistry/bond-line-structures-alkanes-cycloalkanes/naming-alkanes-cycloalkanes-bicyclic/v/alkane-and-cycloalkane-nomenclature-iiConstitutional isomershttps://www.khanacademy.org/science/organic-chemistry/gen-chem-review/bond-line-structures/v/structural-constitutional-isomers-newCis-Trans

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UMaine BMB 221 - Chapter 11: Alkanes

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