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Home , Alkyl

Chapter 10: Halides X X R C X C C X = F, Cl, Br, I

Alkyl halide halide Vinyl halide

10.1 Naming alkyl halides- Read 10.2 Structure of alkyl halides Table 10.1

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

H3C-F 139 452 1.85 H3C-Cl 178 351 1.87 H3C-Br 193 293 1.81 H3C-I 214 234 1.62

The significant dipole moment of alkyl d - X halides make then good electrophiles C d + for substitution reactions

_ R THF R R C C + C X R C C C + NaX H H Na + H H

Preparation of Alkyl Halides:

Electrophilic addition of HX and X2 to (Chapter 6)

Br2 HBr H Br Br Br CH 3 CH3 CH3

1 Free halogenation Mechanism of free radical halogenation (Chapter 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

H H H H Cl2, hn H H H H H H Cl H H C C C C H H C C C C Cl + H C C C C H H H H H H H H H H H H H four 2° (30%) (70%) six 1° hydrogens

H H H H C H H C H H C H H H Cl2, hn H H + H H H C C C H H C C C Cl H C C C H H H H H H H H Cl H one 3° hydrogens nine 1° hydrogens (65%) (35%)

Relative reactivity of hydrogens toward free radical chlorination

H H R R C H R C H R C H H R R Primary (1°) Secondary (2°) Tertiary (3°) hydrogens < hydrogens < hydrogens 1.0 3.5 5.0 Reactivity is reflective of radical stability

H H R • R C R C • R C • H R R Primary (1°) < Secondary (2°) < Tertiary (3°)

H H R R C H R C H R C H H R R DH° = 420 KJ/mol 401 KJ/mol 390 KJ/mol

2 Free radical bromination is much more selective for the most stable radical intermediate.

H H H H C H H C H H C H H H X2, hn H H + H H H C C C H H C C C X H C C C H H H H H H H H X H

X = Cl 65 : 35 X = Br 1 : 99

The propagation step for free radical bromination is endergonic, as opposed to chlorination which is exergonic. According to the Hammond 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

R3C-H + X• R3C• + HX

X = Cl DH° = -50 KJ/mol X = Br DH° = +12 KJ/mol

3 Allylic Bromination of Alkenes allylic position is the next to a double bond

allylic C H C C allylic

Br

NBS, hn

CCl4

Allylic bromination of an takes place through a free radical mechanism.

Radical Stability

H H R • C • C H C • C R C • R C • R C • C C • H H R R Benzylic _~ Allylic Vinylic < Methyl < Primary (1°) < Secondary (2°) < Tertiary (3°) <

Increasing stability

H H H R C C H H C H R C H R C H R C H C H H C C H H R R

DH° (KJ/mol) = 444 438 420 401 390 361 368

Radicals are also stablized by

4 Allylic radical is resonance stabilized

Recall (and please review) from Chapter 2:

resonance forms- atoms remain fixed in all resonance forms. Resonance 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. Resonance 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 • C 1/2 • C C • • C C C C C C

1 1/2 bonds

Map of the electron density due to the unpaired electron (the “spin”)

5 NBS, hn R R R • •

R + R Br Br + (17 %) Br

R

(83 %)

Br KOH NBS, hn

1,3-cyclohexadiene (conjugate diene)

Alkyl halides from the most general method of preparing alkyl halides

1. of alcohols with HX

R-OH + HX R-X + H2O

Works better with more substituted alcohols

H H H R H C OH > R C OH > R C OH > R C OH H H R R Methyl Primary (1°) Secondary (2°) Tertiary (3°)

increasing reactivity

Polar mechanism with a intermediate. Reactivity reflects the stability of the carbocation intermediate (Chapter 11). One drawback to this method is that can rearrange.

6 2. Prepartion of alkyl chlorides by the treatment of alcohols

with thionyl chloride (SOCl2)

R-OH + SOCl2 R-Cl + SO2 + HCl

3. Prepartion of alkyl bromides by the treatment of alcohols

with Phosphorous tribromide (PBr3)

R-OH + PBr3 R-Br + P(OH)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 (M0) in or THF to form organometallic reagents

Grignard reagent- organomagnesium

ether R-X + Mg(0) or THF R-Mg(II)-X X= Cl, Br, I

R can be a variety of organic groups: 1°-, 2°-, 3°-alkyl, aryl or vinyl

d- d+ _ C MgX C carbon nuccleophile (recall acetylide anion) : nucleophile react with electrophile

7 H O R-X R-MgX 2 R-H

Grignard reagents are most commonly used in reactions with carbonyl compounds (Chapter 19).

Lab: Br MgBr _ Mg(0)

ether

_ O O O OH _ O + C H3O O

Organometallic coupling reactions: organolithium reagents Li(0) R-X R-Li + LiX

- + d d _ very strong bases C Li C very strong nucleophiles

organolithium reagents are most commonly used as very strong bases and in reactions with carbonyl compounds

Cuprates (Gilman’s reagent)

ether H3C _ + 2 CH3Li + CuI Cu Li + LiI H3C Gilman's reagent (dimethylcuprate, dimethylcopper lithium)

8 - R2CuLi = R strong nucleophiles

Nucleophilic substitution reaction with alkyl halides ()

ether CH3(CH2)8CH2-I + (CH3)2CuLi CH3(CH2)8CH2-CH3 + CH3Cu + LiI

Reaction with vinyl and aryl halides

H H (CH3)2CuLi I CH3 H H

double bond geometry is preserved

Br CH3 (CH3)2CuLi

Oxidation [O]: the loss of electrons.

Decrease in electron density on carbon by forming a C-O, C-N, C-X bonds or by breaking C-H bonds

Increase in O, N, X content or decrease in H content

Reduction [H]: the gain of electrons

Increase in electron density on carbin by forming C-H bonds or breaking C-O, C-N, C-X bonds

increase on H content or decrease in O, N, X content

9 CH4 + Cl2 H3C-Cl oxidation

1) Mg H3C-Cl + CH4 reduction 2) H3O

H H Br Br C C + Br2 C C H H Oxidation H H H H

H H Pd H H + H C C Reduction C C 2 H H H H H H

H H H Br + HBr C C C C Neither [O] or [H] H H H H H H

H H H OH + H2O C C Neither [O] or [H] C C H H (hydration) H H H H

Table 10.5: Increasing oxidation state

C C C C C C

O C OH C O C CO2 OR

C NH2 C NH C N

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