Presentation on theme: "Nomenclature and Properties of Alkyl Halides"— Presentation transcript:
1Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions Nomenclature and Properties of Alkyl HalidesSynthesis of Alkyl HalidesReactions of Alkyl HalidesMechanisms of SN1, SN2, E1, and E2 ReactionsNucleophilicity, Substrate, and Leaving Group Effects
2Alkyl Halides Alkyl halide: a compound with a halogen atom bonded to one of the sp3 hybridized carbon atoms of an alkyl groupTwo types of names:IUPAC systemCommon names
3Nomenclature 1-chloropropane bromocyclohexane IUPAC System: Alkyl halides are named as an alkane with a halo-substituent:Review the rules for naming alkanes covered in Unit 21-chloropropanebromocyclohexane
4Nomenclature n-propyl chloride Cyclohexyl bromide Common Names: alkyl group name + haliden-propyl chlorideCyclohexyl bromide
5Nomenclature CH2Cl2 CHCl3 Methylene chloride dichloromethane Special common names:CH2X2 = methylene halideCHX3 = haloformCX4 = carbon tetrahalideCH2Cl2CHCl3Methylene chloridedichloromethanechloroformtrichloromethaneCCl4Carbon tetrachloridetetrachloromethane
6Types of Alkyl HalidesAlkyl halides can be classified by the type of carbon atom the halogen is bonded to:primary halide (1o):halogen attachedto a 1o carbonsecondary halide (2o):halogen attached to a 2o carbontertiary halide (3o):halogen attached to a 3o carbon
7Types of Alkyl Halides Geminal dihalide: 2 halogens bonded to the same carbon atomVicinal dihalide:2 halogens bonded toadjacent carbon atoms
8Other Organic Halides thyroxine benzylic carbon benzylic chloride Aryl halide:halogen is attached directly to an aromatic ringBenzylic halidehalogen is attached to a carbon that is attached to a benzene ringthyroxinebenzylic carbonbenzylic chloride
9Other Organic Halides Allylic carbon Allylic chloride Allylic halide: halogen is attached to a carbon that is attached to a C=CAllylic carbonAllylicchloride
10Other Organic Halides Vinyl Halide: halogen attached to a carbon that is part of a C=CMonomer for PVCMonomer for teflon
11Uses of Alkyl Halides Anesthetics: Chloroform (CHCl3) toxic carcinogenic (causes cancer)Solvents:CCl4formerly used in dry cleaningCH2Cl2formerly used to decaffeinate coffeeliquid CO2 used now
13Physical Properties Boiling Point: Compounds with higher MW’s and greater surface area (more linear) tend to have higher BP.BP increases as size of halogen increasesF < Cl < Br < IBP decreases as branching increases
14Physical Properties More dense than water Density: Alkyl chlorides are common solvents for organic reactions.CH2Cl2CHCl3CCl4More dense than water
15Preparation of Alkyl Halides Alkyl halides can be prepared from a variety of starting materials including alkanes, alkenes, alkynes, alcohols, and other alkyl halides.You are responsible for knowing and applying the synthesis of R-X by:free radical halogenation reactionsfree radical allylic bromination reactions
16Preparation of Alkyl Halides Free Radical Halogenation of Alkanesalkane + X alkyl halide(s) + HXPoor selectivity and moderate yields often limit usefulness.Bromination is more selective and gives the product formed from the most stable free radical.Chlorination is useful when only one type of reactive hydrogen is presenthu or D
17Preparation of Alkyl Halides Useful Examples:hu50 %
18Preparation of Alkyl Halides The following free radical halogenation is doomed to failure!The following addition reaction occurs instead:
20Preparation of Alkyl Halides NBS is used to generate low levels of Br2 in situ.Minimizes addition of bromine across the C=CAllylic bromination is highly selective and occurs in the allylic position due to resonance stabilization of the resulting free radical.
22Reactions of RXMost reactions of alkyl halides involve breaking the C-X bond.Nucleophilic substitutionEliminationThe halogen serves as a leaving group in these reactions.the halogen leaves as X-, taking the bonding electrons with itd+d-
23Reactions of RX Nucleophilic substitution: reaction in which a nucleophile replaces a leaving groupNucleophile:electron pair donorLeaving group:an atom or group of atoms that are lost during a substitution or elimination reactionretains both electrons from the original bond
24Reactions of RX General Equation for Nucleophilic Substitution The nucleophile can be neutral or negatively charged, but it must have at least one lone pair of electrons.Example:
25Reactions of RX Elimination Reaction: two substituents are lost from adjacent (usually) carbons, forming a new p bondDehydrohalogenation:an elimination reaction in which H+ and X- are lost, forming an alkeneCH3O-
26Reactions of RXThere are two common types of nucleophilic substitution reactions:SN1 reactionssubstitution, nucleophilic, unimolecular3o, allylic, benzylic halidesweak nucleophilesSN2 reactionssubstitution, nucleophilic, bimolecularmethyl and 1o halidesstrong nucleophiles
28Reactions of RX Common strong nucleophiles: hydroxide ion alkoxide ionsmany aminesiodide and bromide ionscyanide ionCommon weak nucleophiles:wateralcoholsfluoride ion
29SN2 ReactionsThe reaction between methyl iodide and hydroxide ion is a concerted reaction that takes places via an SN2 mechanismnucleophileLeaving groupsubstrateproductSubstrate:the compound attacked by a reagent (nucleophile)
30SN2 Reactions Concerted reaction: a reaction that takes place in a single step with bonds breaking and forming simultaneouslySN2:substitution, nucleophilic, bimoleculartransition state of rate-determining step involves collision of 2 molecules2nd order overall rate lawRate = k[RX][Nuc]
31SN2 Reactions SN2 Mechanism: Nucleophile attacks the back side of the electrophilic carbon, donating an e- pair to form a new bondSince carbon can only have 8 valence electrons, the C-X bond begins to break as the C-Nuc bond begins to form
32SN2 ReactionsSN2 Mechanism for the reaction of methyl iodide and hydroxide ion:
33SN2 Reactions Reaction Energy Diagram: large Ea due to 5-coordinate carbon atom in transition stateno intermediatesexothermic
34SN2 ReactionsSN2 reactions occur with inversion of configuration at the electrophilic carbon.The nucleophile attacks from the back side (the side opposite the leaving group).Back-side attack turns the tetrahedron of the carbon atom inside out.
35SN2 Reactions Inversion of configuration: a process in which the groups bonded to a chiral carbon are changed to the opposite spatial configuration:R S or S R
36SN2 ReactionsExample: Predict the product formed by the SN2 reaction between (S)-2-bromobutane and sodium cyanide. Draw the mechanism for the reaction.
37SN2 ReactionsThe SN2 displacement reaction is a stereospecific reactiona reaction in which a specific stereoisomer reacts to give a specific diastereomer of the product
38SN2 Reactions SN2 reactions occur under the following conditions Nucleophile:strong, unhindered nucleophileOH- not H2OCH3O- not CH3OHCH3CH2O- not (CH3)3CO-Substrate:1o or methyl alkyl halide (most favored)2o alkyl halide (sometimes)3o alkyl halides NEVER react via SN2
39methyl > 1o > 2o >>>3o SN2 ReactionsThe relative rate of reactivity of simple alkyl halides in SN2 reactions is:methyl > 1o > 2o >>>3o3o alkyl halides do not react at all via an SN2 mechanism due to steric hinderance.The back side of the electrophilic carbon becomes increasingly hindered as the number or size of its substituents increases
40SN2 ReactionsSteric hinderance at the electrophilic carbon:
41SN2 Reactions KNOW THESE! Be able to apply these! SN2 reactions can be used to convert alkyl halides to other functional groups:RX + I - R-IRX + OH- R-OHRX + R’O- R-OR’RX + NH3 R-NH3+ X-RX + xs NH3 R-NH2RX + CN- R-CNRX + HS- R-SHRX + R’S- R-SR’RX + R’COO- R’CO2RKNOW THESE!Be able to apply these!
42SN2 ReactionsExample: Predict the product of the following reactions:
43SN2 ReactionsExample: What reagent would you use to do the following reactions:??