Bioorganic Chemistry Chem 250 Chapter 11 Alkanes Student 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 Prof Rachid Skouta Department of Chemistry rskouta umass edu Lecture time Tue Thu 10 00 11 15 am Location ISB221 Book reading Introduction to General Organic and Biochemistry Pages 324 351 Group review sessions are created to support student learning Send your owl questions to Ritam Das Email ritamdas umass edu 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 4134047234 4 Group activity in class 1 Student individual attempted answers 2 Group discussion Alkanes Molecular formula structural formula line angle formula Nomenclature Isomers Cycloalkanes constitutional isomers structural isomers Reactions of Alkanes and Cylcoalkanes Learning goals From formula to constitutional isomers The general formula of linear alkanes is CnH2n 2 Recall Chain with all carbons 1 Chain with n 1 carbons Chain with n 2 carbons C6H14 Requirement n 6 International Union of Pure and Applied Chemistry IUPAC system Recall Branched 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 name Common alkyl substituents F fluoro Cl chloro Br bromo I iodo substituent group derived from hydrocarbons by removal of a H Alkyl Groups Recall IUPAC name one substituent More than one of the same substituent IUPAC name different substituents Parent chain Substituent groups obey alphabetic order Number Low alphabetic order lower number IUPAC name different substituents OWL question OWL question Cycloalkanes A hydrocarbon what contains atoms joined to form a ring The general formula of cycloalkanes is CnH2n Unbranched cycloalkanes Cyclo prefix ane 345678910PropButPentHexHeptOctNonDec Name cycloalkanes If the substituents is longer than the ring Single substitution then the parent name is the substituent If the ring is longer then the ring is the parent 1 Methylcyclopentane 1 Cyclopropylbutane CH3CH2CH2CH2CH3 Name cycloalkanes More than one substituent groups Make sure that the substituent have the lowest numbers If two or more alkyl chains can be the same number then number alphabetically CH3CH3123456CH3CH31234561 3 Dimethylcyclohexane1 5 DimethylcyclohexaneRightWrongCH3CH2CH3CH3CH2CH312345123452 Ethyl 1 methylcyclopentane1 Ethyl 2 methylcyclopentaneRightWrong Conformations 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 conformations CH3CH2CH2CH3 Least crowd most crowd The conformation of each molecule constantly changes At any give time the most stable fully extended conformation dominates the population Conformations of cycloalkanes Cyclopantane Cyclopropane Cyclobutane Cyclohexane Chair Boat Cis Trans Isomers 1 2 dimethylcyclopropane Cis Isomers Trans Isomers Cis 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 HHHHHHHH Which molecule shows cis trans isomerism A Methylcyclopentane B 1 1 Dimethylcyclopentane C 1 3 Demethylcyclopentane H3CH3CH3CH3CCH3 Physical properties of Alkanes Melting and Boiling point Solubility not soluble in water like dissolves like Density Polar dissolves polar non polar dissolves non polar Melting and Boiling point Unbranched Alkanes have higher boiling point than branched ones More contacting surface Less contacting surface Intermolecular Forces The 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 nonpolar Factors Affecting London Forces 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 molecules This is due to the increased surface area in n pentane Intermolecular Forces Affect Many Physical Properties If 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 Alkanes Combustion CH4 2O2 CO2 2H2O 212 kcal mol Products CH3 CH2 CH3 5O2 3 CO2 4H2O 530 kcal mol Many alkanes are used in this way Methane natural gas Propane used in gas grilles Butane lighters Gasoline mixtures of hydrocarbons C5 C12 Reactions of Alkanes Halogenation reaction with halogens CH4 Cl2 CH3Cl CH2Cl2 CH2Cl3 Dichloromethane Chloromethane Cl2 heat Cl2 heat Important haloalkanes Freon 11 CCl3F Trichlorofluoromethane Summary Name the alkanes and cycloalkanes Write the structual form of the alkanes and cycloalkanes Physical properties of the alkanes Intermolecule force Branched vs Unbranched one Constitutional isomers Cis Trans isomers Chemical reaction of alkanes Useful Links Naming alkane https www khanacademy org science organic chemistry bond line structures alkanes cycloalkanes naming alkanes v naming simple alkanes Naming cycloalkane https www khanacademy org science organic chemistry bond line structures alkanes cycloalkanes naming alkanes cycloalkanes bicyclic v alkane and cycloalkane nomenclature ii Constitutional isomers https www khanacademy org science organic chemistry gen chem review bond line structures v structural constitutional isomers new Cis Trans isomers https www khanacademy org science organic chemistry alkenes alkynes naming alkenes jay v cis trans isomerism End of Chapter 11