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The mechanism of alkyl halides preparation through chlorination and bromination, discussing the relative rates, energy differences, and selectivity of each process. It also covers the problems and solutions related to the synthesis of monochloro isomers and the use of grignard reagents.
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Typology: Lecture notes
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R H + X 2 UVor heat R^ X + HX
heat or UV
Initiation-
Propagation-
Termination-
2
X X 2 X
X + H R X^ H^ + R R +^ X^ X^ R^ X^ + X
2 X X^ X X + R^ R^ X R R R
∆H (^1) ∆H (^2)
Preparation of Alkyl Halides, R-X
Reaction of alkanes with Cl 2 & Br 2 (F 2 is too reactive, I 2 is unreactive):
As we have seen previously, the mechanism involves a chain reaction. A chain reaction involves a series of steps in which a product formed in a later step is a reactant in an earlier step.
∆Ho^ in kJ/mole
R = CH 3 1 o^ R 2 o^ R 3 o^ R ∆H 1 , Cl 2 +6^ -12^ -31^ - ∆H 1 , Br2 +72^ +54^ +35^ + ∆H 2 , Cl 2 -108^ -95^ -96^ - ∆H 2 , Br2 -100^ -92^ -87^ -
Note that the second propagation step (∆H 2 ) is quite
exothermic in all cases. The first step (∆H 1 ) is less
exothermic or endothermic. It is reasonable to conclude that the first step would have the larger ∆G ‡^ and is rate
limiting. It also follows that the rate of reaction for substitution of hydrogens by halogens would be 3 o^ > 2o^ > 1o^ > methane. This is also the order of stability of the alkyl free radicals that are formed: the more stable intermediate radical is formed faster. At room temperature, the relative rate of replacement of hydrogen atoms in alkanes by chlorine is 3 o^ : 2o^ : 1o^ = 5.0 : 3.5 : 1.0 (per hydrogen).
8 0 7 0 6 0 5 0 4 0 3 0 2 0 1 0 0 -1 0 -2 0 -3 0 -4 0 -5 0
M e th y l
M e th y l
P rim a ry
P rim a ry
S e c o n d a ry
S e c o n d a ry
Te rtia ry
Te rtia ry
∆H for R a d ic a l C h lorina tio n a n d B ro m ina tio n
(^) R -H
R e a ctio n P ro g re s s
B ro m in a tio n : Tra n sitio n s ta te is re a c h e d la te in re a c tio na n d re se m b le s a lk y l ra d ic a l in te rm e d ia te. Tra n s itio n s ta te e n e rg y d iffe re n c e sa re la rg e r.
C h lo rin a tio n : Tra n s itio n s ta te is re a c h e d e a rly in re a ctio n a n d re s e m b le sre a c ta n ts. Tra n s itio n s ta te e n e rg y d iffe re n c e s a re s m a lle r.
H 3 C C
CH (^3)
H
CH 2 CH 3 Cl^2 UV
H 3 C C
CH 2 Cl
H
CH 2 CH (^3)
H 3 C C
CH (^3)
Cl
CH 2 CH (^3)
H 3 C C
CH (^3)
H
CH CH (^3) Cl H^3 C^ C
CH (^3)
H
CH 2 CH 2 Cl
I
II III^ IV
Problem: Calculate the amounts of monochloro isomers formed by chlorination of 2-methylbutane.
Partial rates of formation of products – I: 6H x 1.0 = 6. II: 1H x 5.0 = 5. III: 2H x 3.5 = 7. IV: 3H x 1.0 = 3. Total rate = 6+5+7+3 = 21 % of I: (6/21)x100 = 28.6%, % of II: (5/21)x100 = 23.8% % of III: (7/21)x100 = 33.3%, % of IV: (3/21)x100 = 14.3%
Owing to its lack of selectivity, free radical chlorination is of limited usefulness in synthesis.
Two allylic radicals are formed ...
... leading H to four products
Br H
Br
H Br
Br
Complications of allylic bromination in terms of synthesis–
R X + Mg^ etheror THF "^ R^ Mg^ X"
Alkyl Halides from Alcohols
X = Cl, Br, I Reactivity of ROH: 3o^ > 2o^ > 1o Rearrangement of 2o^ R possible; rearrangement of 1o occasionally occurs. Reactivity of HX: HI > HBr > HCl Other reagents that are used: thionyl chloride, SOCl 2 , phosphorous tribromide, PBr 3 , phosphorous + iodine, P + I (^2) Rearrangements are less likely with these reagents.
Formation of Grignard Reagents
R = 1o, 2o, or 3o^ alkyl, vinylic, aromatic; vinylic and aromatic are less reactive. X = Cl, Br, I; usually not F; reactivity: I > Br > Cl.
R Mg X + C O acid^ R^ C^ OH
R X + Li^ R^ Li^ + LiX
R 2 Cu -^ Li +^ + R' X R^ R'^ + Li^ +^ X-+ RCu
A major reason for making Grignard reagents is to synthesize alcohols by reacting the Grignard with carbonyl compounds:
Alkyllithium Reagents
R-Li is chemically similar to a Grignard reagent.
Gilman Reagents – Lithium Diorganocopper Reagents Preparation –
Coupling reaction –
Works best if R & R’ are 1 o^ alkyl, aryl, or vinyl.
