Presentation on theme: "Nucleophilic Substitution and Elimination"— Presentation transcript:
1Nucleophilic Substitution and Elimination Alkyl HalidesNucleophilic Substitution and Elimination
2Nomenclature of Alkyl Halides Name halogen as substituent on alkane or cylcoalkane.Learn common names for some of the simple structures. e.g. chloroform, methylene chloride.Note degree of substitution - name as type of C it is bonded to (i.e. 10, 20, 30).Geminal (gem-) dihalide has two halogen atoms bonded to the same carbon.Vicinal (vic-) dihalide has two halogens bonded to adjacent carbons.Do problem 6-1, 6-2 and 6-3 of the text.
4Uses and General Chracteristics BOND DIPOLE (): + at C, - at Xall reactions based on this.The bond dipole moments increase in the order: C—I < C—Br < C—F < C—ClPhysical propertiesgenerally, trends are similar to those seen in alkanes.bp affected by London forces and dipole-dipole attractions.Common uses: solvents, anesthetics, freons (refrigerants), pesticides.
5Preparation of alkyl & allylic halides Free radical halogenation of alkanes (Chpt 4)You are expected to know the mechanism by which this transformation takes place.
6Free radical halogenation of alkenes at allylic position Preparation of Alkyl HalidesFree radical halogenation of alkenes at allylic positionneed to know resonance structures for intermediate & predict major/minor productSee pages of the text. Do problems 6-8 and 6-9.
81. Identify electrophilic carbon in substrate 2. Identify nucleophilic electrons in nucleophile3. Identify leaving group in substrate
9Then draw product(s) 4. Draw substrate without LG but with bond 5. Add Nuc to bond where LG used to be
10Factors influencing what products are formed Substrate/steric effectsStrength of nucleophile vs. basicity of nucleophileStability of leaving groupReaction conditionsPolarity of solventacidic/neutral/basic
11Substitution Mechanisms Continuum of possible mechanismsMechanism determined primarily by substrate steric effectsSN2 - methyl, 1º & unhindered 2ºSN º, hindered 2º
12Bimolecular (SN2) Nucleophilic Substitution concerted reaction; Nuc attacks, LG leavespentacoordinate carbon in transition staterate depends on conc. of both reactantsMe = methyl group; Et = ethyl group
13Reaction is “stereospecific” 100 % inversion of configurationYou should know how to represent thismechanism in an energy diagram!!
14Factors that Affect SN2 Reaction Rates Strength of Nucleophile: species with negative charge is a stronger nuc than an analogous neutral species (e.g. -OH > H2O; -NH2 > NH3).Nucleophilicity increases from left to right across the periodic chart (e.g. -OH > -F).Nucleophilicity increased down the periodic table (I- > Br- > Cl- > F-) or (-SeH > -SH > -OH).Solvent: Polar protic solvents (e.g. ethanol, ammonia decrease nucleophilicity. Polar aprotic solvents e.g. (acetonitrile, DMSO, acetone) increase nucleophilicity.
15Factors that affect SN2 reaction rates Steric Effects: When bulky groups interfere with a rxn. because of their size, this is called steric hindrance. Steric hindrance affects nucleophilicity, not basicity. (e.g. ethoxide ion is a stronger base than t-butoxide ion). Also, alkyl halide reactivity decreases from methyl to 10 to 20 to 30. In fact, 30 alkyl halides do not react by SN2.Leaving group: The substrate should have a good leaving group. A good leaving group should be electron withdrawing, relatively stable, and polarizable. They are weak bases. Examples are Cl-, Br-, I-, RSO3-, RSO4-, RPO4-, and neutral molecules such as water, alcohols and amines. Strong bases (OH-, RO-, H2N-) are not good leaving groups!
16Unimolecular (SN1) Nucleophilic Substitution Two-step reactionLG leaves, then Nuc: attacksTricoordinate carbocation intermediateSolvolysis (when solvent is also the nucleophile = SN1 reactionRate depends on substrate conc. only
17Mechanism of SN1 reaction You must be able to represent this on an energy diagram!
18Reaction Not Stereoselective Unless Steric Factors Apply Racemization - not always exactly 50/50. Carbocation can be attacked from the top or bottom face giving both enantiomers.Steric hindrance gives attack at one side preferentiallyLonger-lived carbocations give more racemization, shorter-lived give more inversion
19Factors Influencing SN1 Reaction Rates Stability of the carbocation*Allylic 3° >> 3° allylic 2° > 2° allylic 1° >> 1° > Me Carbocations are stabilized by alkyl groups (through hyperconjugation and the inductive effect) and by resonance.Leaving group stability: the better the leaving group, the faster the reaction.Solvent polarity: the reaction is favored in polar protic solvents.* must have neutral to acidic conditions to form carbocation
20Rearrangement of Carbocations Large difference in energy (stability) of 3° vs. 2° C+H- (hydride) or R- will shift (migrate) to adjacent position to form more stable carbocation. E.g. when neopentyl bromide is boiled in methanol, only rearranged product is formed.
21Elimination Reactions May proceed by a unimolecular (E1) or bimolecular (E2) mechanism.In an alkyl halide, when a halide ion leaves with another atom or ion, the reaction is an elimination.If the halide ion leaves with H+, the reaction is called a dehydrohalogenation.
22Elimination Mechanisms Mechanism determined primarily by substrate steric effects
30E1 E2 Comparison of SN1 and SN2 Base strength unimportant Substrates: reactivity order is 3o > 2o > 1oSolvent: good ionizing solvent requiredRate: depends on substrate conc. onlyStereochemistry: no particular geometry required for slow step; Saytzeff rule followedRearrangements: very commonStrong bases requiredSubstrates: reactivity order is 3o > 2o > 1oSolvent polarity is not so importantRate: depends on conc. of substrate and base.Stereochemistry: coplanar arrangement required in transition state; Saytzeff rule followedRearrangements: not possible