Bond length (pm) Bond strength (KJ/mol)

Transcription

1 hapter 10: Alkyl alides = F, l,, I Alkyl halide Aryl halide Vinyl halide 10.1 Naming alkyl halides- ead 10.2 Structure of alkyl halides Table 10.1 alomethane 3 -F 3 -l 3-3 -I Bond length (pm) Bond strength (KJ/mol) Dipole Moment d - d + The significant dipole moment of alkyl halides make then good electrophiles for substitution reactions _ + Na + TF + Na Preparation of Alkyl alides: Electrophilic addition of and 2 to alkenes (hapter 6)

2 Free radical halogenation Mechanism of free radical halogenation (hapter 5) three distinct steps 1. Initiation 2. Propagation 3. Termination Free radical chlorination is not very useful for making alkyl chlorides polychlorination non-specific chlorination four 2 hydrogens six 1 hydrogens l 2, hn l l + (30%) (70%) one 3 hydrogens nine 1 hydrogens l 2, hn + l l (65%) (35%) elative reactivity of hydrogens toward free radical chlorination Primary (1 ) hydrogens 1.0 Secondary (2 ) hydrogens < < 3.5 Tertiary (3 ) hydrogens 5.0 eactivity is reflective of radical stability Primary (1 ) < Secondary (2 ) < Tertiary (3 ) D = 420 KJ/mol 401 KJ/mol 390 KJ/mol 2

3 Free radical bromination is much more selective for the most stable radical intermediate. 2, hn + = l 65 : 35 = 1 : 99 The propagation step for free radical bromination is endergonic, as opposed to chlorination which is exergonic. According to the ammond postulate the transition state for bromination should resemble the product radical, and therefore be more selective for the product going through the more stable radical intermediate = l = D = -50 KJ/mol D = +12 KJ/mol 3

4 Allylic omination of Alkenes allylic position is the next to a double bond allylic hydrogen allylic carbon NBS, hn l 4 Allylic bromination of an alkene takes place through a free radical mechanism. adical Stability Vinylic Methyl Primary (1 ) Secondary (2 ) Tertiary (3 ) < < < < < Benzylic _~ Allylic Increasing stability D (KJ/mol) = adicals are also stablized by hyperconjugation 4

5 Allylic radical is resonance stabilized ecall (and please review) from hapter 2: resonance forms- atoms remain fixed in all resonance forms. esonance forms differ only by the placement of electrons No one resonance form is entirely accurate. The actual structure is a hybrid of all the resonance forms. esonance forms do not necessarily contribute equally to the resonance hybrid. The greater the number of resonance structures the more stable the resonance hybrid. 1/2 1/2 1 1/2 bonds Map of the electron density due to the unpaired electron (the spin ) 5

6 NBS, hn + (17 %) + (83 %) NBS, hn K 1,3-cyclohexadiene (conjugate diene) Alkyl halides from alcohols the most general method of preparing alkyl halides 1. Substitution reaction of alcohols with Works better with more substituted alcohols > > > Methyl Primary (1 ) Secondary (2 ) Tertiary (3 ) increasing reactivity Polar mechanism with a carbocation intermediate. eactivity reflects the stability of the carbocation intermediate (hapter 11). ne drawback to this method is that carbocations can rearrange. 6

7 2. Prepartion of alkyl chlorides by the treatment of alcohols with thionyl chloride (Sl 2 ) - + Sl 2 -l + S 2 + l 3. Prepartion of alkyl bromides by the treatment of alcohols with Phosphorous tribromide (P 3 ) - + P P() 3 These methods work best on primary and secondary alcohols. They do not work at all for tertiary alcohols... more reactions of alkyl halides: Grignard reagents Alkyl halides will react with some metals (M 0 ) in ether or TF to form organometallic reagents Grignard reagent- organomagnesium ether - + Mg (0) or TF -Mg (II) - = l,, I can be a variety of organic groups: 1 -, 2 -, 3 -alkyl, aryl or vinyl d - d + Mg arbanions: nucleophile react with electrophile _ carbon nuccleophile (recall acetylide anion) 7

8 2 - -Mg - Grignard reagents are most commonly used in reactions with carbonyl compounds (hapter 19). Lab: Mg(0) Mg _ ether 3 + rganometallic coupling reactions: organolithium reagents Li - (0) -Li + Li pentane d - d + Li _ very strong bases very strong nucleophiles organolithium reagents are most commonly used as very strong bases and in reactions with carbonyl compounds uprates (Gilman s reagent) 2 3 Li + ui ether 3 _ u Li LiI Gilman's reagent (dimethylcuprate, dimethylcopper lithium) 8

9 2 uli = - strong nucleophiles Nucleophilic substitution reaction with alkyl halides (alkylation) ether 3 ( 2 ) 8 2 -I + ( 3 ) 2 uli 3 ( 2 ) u + LiI eaction with vinyl and aryl halides I ( 3 ) 2 uli 3 double bond geometry is preserved ( 3 ) 2 uli 3 xidation []: the loss of electrons. Decrease in electron density on carbon by forming a -, -N, - bonds or by breaking - bonds Increase in, N, content or decrease in content eduction []: the gain of electrons Increase in electron density on carbin by forming - bonds or breaking -, -N, - bonds increase on content or decrease in, N, content 9

10 4 + l 2 3 -l oxidation 3 -l 1) Mg 4 reduction 2) xidation + 2 Pd eduction + Neither [] or [] + 2 Neither [] or [] (hydration) Table 10.5: Increasing oxidation state 2 N 2 N N 10