Organic Chemistry Lecture 5 PDF
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This document provides lecture notes on organic chemistry, specifically focusing on alkenes, including bonding, isomerism, nomenclature, preparation, and mechanisms. The lecture covers various aspects of alkene chemistry like geometric isomerism, the E/Z system, and different synthesis methods.
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Chem 0901 Organic Chemistry LECTURE 5 Lecture Outline ► Bonding in alkenes ► Geometric isomerism (alkenes/ cycloalkanes) ► Nomenclature ► Preparation ► From 10 alkyl halides ► From 20/ 30 alkyl halides ► Mechanism of elimination Structure/Bonding in Ethene Alkenes ( H2C=CH2, an a...
Chem 0901 Organic Chemistry LECTURE 5 Lecture Outline ► Bonding in alkenes ► Geometric isomerism (alkenes/ cycloalkanes) ► Nomenclature ► Preparation ► From 10 alkyl halides ► From 20/ 30 alkyl halides ► Mechanism of elimination Structure/Bonding in Ethene Alkenes ( H2C=CH2, an alkene) ► Bonding (sp2 hybridization) Geometric isomerism For an alkene to exist as geometric isomers, neither of the double bond carbons can be bonded to two identical groups. If A=D or B=C, no geometric isomerism exists Cis-trans Isomerism Cis-trans isomers differ from one another in the way the atoms/ groups are positioned in space – Cis –same, trans –across – They have different physical and chemical properties – They interact differently with enzymes/ receptor sites – They cannot be interconverted by rotation around C-C bonds Rotation is restricted by double bond or cyclic structure The E/Z System The E/Z system is based on an assignment of priorities to the atoms or groups attached to each carbon of the double bond – If the higher-priority atoms or groups are on opposite sides of the pi bond, the isomer is (E) [E= across]. – If the higher-priority groups are on the same side, the isomer is (Z) [Z = together]. E/Z Sequence rules – The atom with the higher atomic number is of higher priority – If two atoms are identical, the atomic numbers of the next atoms are used to assign the priority. Priority is determined at the first point of difference along the chain if the atoms have the same atoms directly attached Atoms attached by double or triple bonds are given single-bond equivalencies – they are treated as single- bonded groups for priority assignments. a b c d ► On carbon a, the methyl substituent is of higher priority than H ► On carbon b, the bromine substituent is of higher priority than H ► IUPAC name is (E)-1-bromopropene ► On carbon c, the chloroethyl substituent is of higher priority than methyl ► On carbon d, the bromine substituent is of higher priority than chlorine ► IUPAC name is (Z)-1-bromo-1,4-dichloro-2-methylbut-1-ene E/Z Nomenclature Other priority assignments Example Alkenes in Technology ► Besides, alkenes also used for artificial ripening of fruits, as a general anesthetic, for making poisonous mustard gas (War gas) ► General Anesthesia12/13/2010http://www.nucleusinc.c om/medical-animation General anesthesia is used to block pain, put the body to sleep, and regulate bodily functions during surg. Watch Video... http://www.metacafe.com/watch/5645638/gener al_anesthesia/ ► and ethylene-oxygen flame. METHOHEXITAL - ANESTHETIC Preparation of alkenes ► From alkyl halides ► Elimination reaction Synthesis by dehydrohalogenation -from primary alkyl halides Bases: NaOH, KOH, NaOC2H5, KOC(CH3)3 Dehydrohalogenation from secondary/ tertiary alkyl halides ► Saytzeff’s rule ► states that if more than one alkene can be formed by an elimination reaction, the more stable alkene is the major product. ► the compound that has a more highly substituted olefin is more stable due to the electron donating properties of the alkyl group. Dehydrohalogenation from secondary/ tertiary alkyl halides Hoffman product – Less substituted product formed (with bulky base) Zaitsev [Saytzeff] product – More substituted product formed Dehydrohalogenation from secondary/ tertiary alkyl halides Past paper question ► December 2001 ► In the reaction shown below there are two possible elimination products. ► Draw the structures of the two elimination products. (3) ► State which of the two products would be expected to predominate in the reaction mixture and explain why. (4) ► Show the mechanism which occurs as the starting material is converted to the major product. (4) ► Briefly discuss the formation of possible side products in the above reaction. (4) Preparation from alcohols - Dehydration FEATURES Occurs with heat under acidic conditions An elimination reaction – water is lost Some primary and secondary alcohols undergo rearrangements Mechanism (E2) Reaction with secondary and primary alcohols Protonation with acid (H2SO4 or H3PO4) Loss of water catalysed by a base/ molecule of alcohol Mechanism (E1) Reaction with secondary and tertiary alcohols Protonation with acid Loss of water – formation of a carbocation Deprotonation – formation of alkene The carbocation ► Relative stability ► A carbocation will be more stable when the charge can be spread (delocalized) over several atoms