Organic Chemistry Lecture 2-3 (5) PDF

CHEM 0901 Organic Chemistry LECTURE # 2  Past Paper review question Which statement about sp3 orbital hybridization (e.g. in methane) is false? – (a) It results from combination of one s orbital and three p orbitals. – (b) It forms three new identical orbitals – (c) Each new or...

CHEM 0901 Organic Chemistry LECTURE # 2  Past Paper review question Which statement about sp3 orbital hybridization (e.g. in methane) is false? – (a) It results from combination of one s orbital and three p orbitals. – (b) It forms three new identical orbitals – (c) Each new orbital possesses an unpaired electron – (d) The angle between each orbital is 109.50 – (e) There is the same probability of finding an electron in any of these new orbitals.  Lecture Outline Introduction to reaction mechanisms – Homolytic and heterolytic cleavage – Bond dissociation energy – Reaction intermediates Relative stability of carbocations and radicals – Nucleophiles and electrophiles Alkanes – Homologous series – Nomenclature Alkanes (incl. cyclic alkanes) Alkenes – Structural isomerism Skeletal Positional Functional group  Introduction to reaction mechanisms  A reaction mechanism is a step-by-step description of the bond-breaking and bond-making processes that occur when reagents react to form products  Homolytic cleavage A bond consists of two electrons Movement of one electron is denoted by a fishhook arrow – Homolytic cleavage occurs Radicals are formed H H H + H H H H H H C C H H C + C H H H H H  Heterolytic cleavage + Cl- H O H Cl H O H + H H  Movement of an electron pair (two electrons) is denoted with a curved arrow – this shows the direction of electron flow in a reaction  Reaction Intermediates Relative stability of carbocations  The stability of a carbocation is increased by Relative stabilities of carbocations elements that can disperse the positive charge (through the inductive effect) CH3 CH3CH2 (CH3)2CH (CH3)3C methyl 1o 2o 3o H H H H C + +C + H3C CH3 H C H C+ C + H H CH3 H Relative stability of radicals Nucleophiles and electrophiles _ E+ + Nu E N Arrows move from a region of high electron density to a region of lower electron density * Electrophile – electron-lovers (electron- poor reagents/ seeking electrons) – Nucleophile – nucleus lovers (electron- rich reagents/ donate electrons to electrophile) Who is the electrophile?  Bond dissociation energy Bond breakage is endothermic The energy required for homolytic bond cleavage is equivalent to the energy released when the atoms combine to form molecules – Homolytic cleavage is useful in determining energies required for bond dissociation APPLICATIONS Used to calculate enthalpy changes for reactions Can provide an estimate of relative stabilities of radicals – Multiple bonds require more energy for dissociation when compared with single bonds H3C H CH3 + H (H3C)3C H (CH3)3C + H  Selected bond dissociation energies (homolytic) Bond Energy (kJ/ mol) CH3-H 440 (CH3)2CH-H 413 (CH3)3C-H 400 C6H5CH2-H 375 CH3CH2-I 233 CH3CH2-OH 393 CH3CH2-CH3 371  Alkanes Natural sources – Petroleum – Natural gas (methane & ethane)  Homologous series General formula – CnH2n+2 A series of compounds in which the members are built up in a regular, repetitive way is called a homologous series. – Similar chemical and physical properties  Homologous series - Differ by methylene (CH2)groups Name # of Mol. Structural # of carbons formula formula Structural isomers methane 1 CH4 CH4 1 ethane 2 C 2H 6 CH3CH3 1 Propane 3 C 3H 8 CH3CH2CH3 1.. octane 8 C8H18 CH3(CH2)6CH3 18 decane 10 C10H22 CH3(CH2)8CH3 75  Nomenclature What’s in a name? Hubert Blaine Wolfe­schlegel­stein­hausen­berger­- dorff, Sr. Talula Does The Hula From Hawaii  Nomenclature IUPAC [International Union of Pure and Applied Chemistry] The –ane ending is used for all saturated hydrocarbons Unbranched alkanes are named according to the number of carbon atoms in the compound H H H H H H H H H C C C C C C C C H H H H H H H H H  Nomenclature cont’d For branched alkanes, the root name is that of the longest continuous chain of carbon atoms CH3 CH3 CH3 CH3 H2 H2 H3C CH CH C CH3 H3C CH CH C CH3 Groups attached to the main chain are termed substituents (e.g. alkyl groups) – Alkyl groups are named by taking the name of the alkane and changing the –ane to -yl H 2 CH3 C CH3 methyl group ethyl group Some common substituents CH3 H3C CH2 CH2 H3C CH propyl isopropyl H2 H3C CH2 CH2 CH2 C CH3 CH butyl or n-butyl CH3 isobutyl CH3 Br bromo CH2 CH3 CH C CH3 Cl chloro CH3 CH3 F fluoro sec-butyl tert-butyl I iodo  Nomenclature cont’d The main chain is numbered so that the first substituent along the chain receives the lowest number – The terms di, tri, tetra, penta etc. are used when two or more identical groups are attached to the main chain CH3 H2 CH3 CH3 CH3 CH3 H3C C C CH3 1 2 3 4 H2 H2 H3C CH CH C CH3 H3C CH CH C CH3 CH3 5 4 3 2 1 1 2 3 4 5 2,2-dimethylbutane 2,3-dimethylpentane NOT 2,2-methylbutane or 2-dimethylbutane  Nomenclature cont’d IUPAC names for hydrocarbons are written as one word – Numbers are separated from each other by commas and are separated from letters by hyphens – There is no space between the last named substituent and the name of the parent alkane that follows it.  Nomenclature cont’d When a noncyclic carbon chain contains more carbons than an attached ring, the ring is treated as a substituent. If there is a branch equidistant from each end of the longest chain, begin numbering nearest to the third branch If there is no third branch, begin numbering nearest to the substituent with alphabetic priority The numerical prefixes di, tri etc are ignored in alphabetizing The prefixes iso- and cyclo- are considered in alphabetizing The prefixes tert- and sec- are ignored in alphabetizing  CH2CH3 2 1 CH2CH3 H2 H2 H2 H2 H3C CH C C CH3 H3C CH C C CH3 3 4 5 6 3-methylhexane H2 H2 H H2 H2 H Cl C C C CH3 Cl C C C CH3 1 2 3 4 Br Br 3-bromo-1-chlorobutane NOT 1-chloro-3-bromobutane or 2-bromo-4-chlorobutane  Testing, testing …. H2 C CH3 H3C 3-methyl-4-propyloctane CH H2 H2 C CH C CH3 H3C C C C H2 H2 H2 F F Cl F H 2-bromo-2-chloro-1,1,1-trifluoroethane Br  Naming cyclic alkanes Cycloalkanes are saturated hydrocarbons that have at least one ring of carbon atoms – General formula CnH2n – They are named by placing the prefix cyclo before the alkane name corresponding to the number of carbon atoms in the ring cyclopropane cyclopentane cycloheptane  Naming cyclic alkanes CH3 CH Alkyl or halogen substituents attached H2C CH2 to the rings are named similarly to H2C CH2 alkanes methylcyclopentane If there is only one substituent present, no number is required to locate it Br – With several substituents, numbers are CH required to identify their positions H2C CH2 H2C CH2 bromocyclopentane  Cyclic alkanes The ring carbons are CH3 CH3 numbered to give the 5 CH 1 CH3 2 CH 1 CH3 H2C H2C substituents the lowest CH CH 2 5 possible numbers H2C CH2 H2C CH2 4 3 3 4 – One substituent is denoted CH3 CH3 carbon number 1 H2C H2C 1 With different substituents, 5 H2C CH 1 CH CH3 2 H2C CH CH CH3 2 5 the one with the highest H2C 4 CH2 H2C CH2 4 3 3 alphabetic priority is located 1-ethyl-2-methylcyclopentane, at carbon 1 NOT 2-ethyl-1-methylcyclopentane  Representation of alkanes Dash Condensed Bond-line H H H H H H C C C C C H CH3CH2CH2CH2CH3 H H H H H H H H H H C H H C C C C H CH3CH2CH(CH3)CH3 H H H H  Representation of alkenes Dash Condensed Bond-line H H H H C C C C C H CH3CH2CH=CHCH3 H H H H H H H H H H C C C C C H CH3CH2CH=CHCH3 H H H H H H H C H H CH2 H C C C C H CH3CCH2CH3 H H H  Representation of Alkynes Dash Condensed Bond-line H H H H C C C C C H H3CH2CC CCH3 H H H  Nomenclature priorities of selected Partial structure functional groups Name CO2H -oic acid O -al O H -one Increasing priority OH -ol NR2 -amine C C -ene C C -yne R-, C6H5-, Cl-, Br-, -NO2  Nomenclature in Alkenes The ending ene is used to designate the carbon-carbon double bond The longest chain containing both carbons of the double bond is selected as the parent structure – For more than one double bond, the ending is diene, triene – Compounds with a double and triple bond are -enynes  Nomenclature in Alkenes cont’d The chain is numbered beginning at the end of the chain nearer the double bond to assign it the lowest possible number – The number of the first atom of the double bond is used as the prefix CH3CH2CH=CH2 CH3CH=CH(CH2)3CH3 1-butene 2-heptene The positions of substituents are indicated by the numbers of the carbon atom to which they are attached CH3 CH 3 5 4 3 2 1 1-bromo-3,4,4-trimethyl-2-pentene H3C C C CHCH2Br H3C C C CHCH2Br or 1-bromo-3,4,4-trimethylpent-2-ene CH3 CH3 CH3 CH3  If more than one double bond is present in the molecule, numbering begins from the end closer to the first double bond. H2C CH CH CH CH3 H2C CH CH CH CH3 1 2 3 4 5 5 4 3 2 1 1,3-pentadiene 2,4-pentadiene  Naming Cyclic alkenes Cycloalkenes are designated in CH2CH3 such a way as to assign: – the 1-and 2- positions to the carbon atoms of the double bond 1-ethylcyclopentene – The lowest combination of numbers to substituents CH3 H3C CH3 3,3,5-trimethylcyclohexene  What’s in a name? Br O OH O Cl O 18-bromo-12-butyl-11-chloro-4,8-diethyl-5-hydroxy-15-methoxytricos-6,13-diene-19-yne-3,9-dione  Past paper question What is the IUPAC name of the molecule shown below? CH3 CH2 CH2 CH3 H2 H H3C C CH C C CH3 CH3H2C CH3 (a) 2,4-diethyl-3,3-dimethylheptane (b) 2-ethyl-3,3-dimethyl-4-propylhexane (c) 5-ethyl-3,4,4-trimethyloctane (d) 4-ethyl-5,5,6-trimethyloctane  Structural Isomerism Structural isomers are compounds that have the same molecular formula but different structural formulas – Skeletal isomers CH3 H2 H2 H2 C C CH3 C CH H3C C C H3C C CH3 H2 H2 H2 C7H14 C7H14  Structural isomers Positional isomers H H H2 H2 H2 C C C C C CH3 H3C C C H H3C C C H2 H2 H H H2 H2 H2 C C C CH3 H3C C OH H3C CH H2 OH  Structural isomers Functional group isomers H2 H2 H2 C C C O H3C C OH H3C C CH3 H2 H2 H2 H2 N H2 H H C C C C C C H3C C C H2C C C NH2 H2 H2 H H2 Similar but different…  Physical properties of alkanes Alkanes are insoluble in water They have low boiling points – Weak attractive forces occur between the molecules (intermolecular forces) – Branched compounds have lower boiling points – Non-polar, covalent bonds  Preparation of Alkanes – Fractional distillation of crude petroleum – From alkenes – From carbonyl compounds Wolff-Kishner and Clemmensen Reduction – From alkyl halides  Fractional distillation http://science.howstuffworks.com/oil-refining4.htm  The Process Catalytic cracking Alkane Smaller alkanes + alkenes + H2 400 - 7000C 2 CH3CH2CH3 CH4 + CH3CH=CH2 + CH2=CH2 + H2  Preparation from alkenes Catalytic hydrogenation of alkenes/ alkynes Pt, Ni or Pd + H H pressure H H C C + 2 H H C C H H H H 1mol H2 1mol H2 Pt, Ni or Pd Pt, Ni or Pd C C C C pressure H H H H H H  Clemmensen reduction Zinc amalgam, Conc. HCl Used for cpds which are stable in acids O H2 C C Zn/ Hg, CH3 HCl CH3  Reaction Preparation of with Grignard alkanes from alkyl halides reagents Grignard reagents are organomagnesium compounds – Carbon-metal bond François Auguste Victor Grignard The Nobel Prize in Chemistry 1912  Preparation of alkanes from Reaction with Grignard alkyl halides reagents R X + Mg Diethyl ether or - R + Mg X Tetrahydrofuran [THF] [reflux] H2 H2 O C C H3C O CH3 H2C CH2 H2C CH2 diethyl ether Tetrahydrofuran  Grignard reagents will react with compounds containing acidic hydrogen atoms – Hydrogen atoms attached to electronegative atoms (e.g. S, N, O) Reaction of Grignard reagent with water - + - 2+ H3C Mg Br + H O H3C H + Mg + OH- + Br- H  Reaction of Grignard reagent with alcohols (H-OR) H3C Mg Br H3C H CH CH 2+ + H O + Mg + OR- + Br- H3C R H3C  Reaction of Grignard reagent with 1- alkynes CH3 CH3 H H2 CH C H CH C H + H2 H3C C Mg CI + H Mg CH 3 I H H CH3 H3C C H2 + H H2 H Br Mg CH 3 + Br Mg CH3