查看完整版本 : reaction of benzyl halide, SN1 or SN2?

190303458 2013-5-10 10:08 PM

reaction of benzyl halide, SN1 or SN2?

I read the reference book, it says the reaction is SN2 (picture 1). But my lecturer taught me that it is SN1 (picture 2:my lecture notes) because the carbocation formed is stabilised by resonance although the substrate is a primary halide. Which one is correct?

jmlo 2013-5-10 11:07 PM

回覆 1# 的帖子

In most cases primary benzylic halides undergo SN2 reactions. However, under some conditions they could also undergo SN1 reactions since the resulting carbocations are stabilized by resonance. Therefore, to determine which mechanism is more favored, you have to check carefully the reaction conditions. If a very good nucleophile is present (e.g. OH-), then the SN2 mechanism is more favored. If solvolysis is possible, then the SN1 mechanism will be more favored. For example C6H5-CH2Cl could be converted to C6H5-CH2-O-CH3 by reacting with either CH3OH or CH3ONa but undergoing via different mechanisms.

So, I think both reactions you showed in the post are correct. In NaOH, benzylic chloride will be converted to alcohol via SN2 since OH- is a very good nucleophile. In the second example, since NaCN is usually used in conjunction with alcohol, it will make the solvolysis more feasible, thus favoring the initial dissociation of Cl-.

190303458 2013-5-11 01:17 AM

回覆 2# 的帖子

But cyanide ion is a stronger nucleophile than hydroxide ion, why the reaction in picture 2 doesn't undergo SN2 mechanism?

jmlo 2013-5-11 04:42 AM

回覆 3# 的帖子

I think the second reaction can proceed via either SN1 or SN2. It is common to benzyl halides.

The factor that determine the relative importance of mechanism is the reaction environment. If alcohol is present in the mixture (e.g. alcoholic NaCN), then carbocation formation is favored.

190303458 2013-5-11 02:20 PM

回覆 4# 的帖子

I have checked that solvolysis is the reaction that the solvent acts as the nucleophile. But if alcoholic NaCN is used, both of alcohol and NaCN can act as a nucleophile, how can NaCN be the dominant nucleophile and the reaction undergoes SN1 mechanism?

jmlo 2013-5-12 01:48 AM

回覆 5# 的帖子

Initially benzyl chloride is dissolved in alcohol; in this environment, the dissociation to form carbocation is favored. Then there will be competition between CN- and alcohol attacking the carbocation, and CN- will win as it is a stronger nucleophile.

190303458 2013-5-12 02:44 AM

回覆 6# 的帖子

Whether there is a possibility that some alcohol molecules also undergo nucleophlic substitution and the reaction will yield a mixture products?

190303458 2013-5-12 02:57 AM

[font=Times New Roman]Besides, I would like to ask 3 questions about mechanisms and 1 question about synthesis.[/font]

[font=Times New Roman]Q1: In mechanisms of acid-catalysed hydrolysis of ester and amide, the first step is the protonation of the carbonyl group. Why the lone pair electrons on O atom is used to attract the H+ ion (the red part of picture 1 and 2)? However, when I studied AL, my teacher taught me that it should be the pi electrons are used to attract the H+ ion because the pi electrons are more diffused than the lone pair electrons on O atom so that it should be the carbonyl C atom bears the positive charge. Which one is correct?[/font]
[font=Times New Roman][/font]
Q2: [font=Times New Roman]In the mechanism of acid-catalysed hydrolysis of amide, the amine group is protonated first before the carbon-oxygen double bond is reformed (the blue part of picture 2) while in the mechanism of base-catalysed hydration of amide, no such kind of protonation of the amine group when the [/font][font=Times New Roman]carbon-oxygen double bond is reformed (the blue part of picture 3). Why?[/font]
[font=Times New Roman][/font]
[font=Times New Roman]Q3: In the mechanism of aldol condensation (the green part of picture 4), hydroxide is a poor leaving group, protonation of the hydroxy group is required before ejecting it. However, as the reaction condition is basic, protonation cannot be achieved. Why the hydroxide still can be ejected even though it is a poor leaving group?[/font]
[font=Times New Roman][/font]
[font=Times New Roman]Q4: When converting an alcohol to a alkyl halide, e.g.ethanol to 1-bromoethane, can I just simply suggest HBr instead of PBr3, PBr5 or NaBr+conc. H2SO4?[/font]
[font=Times New Roman][/font]

[[i] 本帖最後由 190303458 於 2013-5-12 03:39 AM 編輯 [/i]]

jmlo 2013-5-12 09:14 AM

回覆 7# 的帖子

Not likely as CN- is a much stronger nucleophile than alcohol.

jmlo 2013-5-12 09:52 AM

回覆 8# 的帖子

[quote] Q1: In mechanisms of acid-catalysed hydrolysis of ester and amide, the first step is the protonation of the carbonyl group. Why the lone pair electrons on O atom is used to attract the H+ ion (the red part of picture 1 and 2)? However, when I studied AL, my teacher taught me that it should be the pi electrons are used to attract the H+ ion because the pi electrons are more diffused than the lone pair electrons on O atom so that it should be the carbonyl C atom bears the positive charge. Which one is correct? [/quote]
The protonation occurs through the lone pair of O but not the pi orbital of C=O. The carbonyl pi orbital is not nucleophilic enough to directly attract H+. Recall the hydrogen bond between C=O and other species; it is formed only between the H-donor and a lone pair of O but not the whole C=O functional group.

The presence of positive charge on C can be accounted for in terms of resonance:
[R2C=O-H]+   <---->   [R2C-O-H]+
The structure on LHS has the positive charge on O while the structure on RHS has the positive charge on C. Both structures are equally likely.

[quote] Q2: In the mechanism of acid-catalysed hydrolysis of amide, the amine group is protonated first before the carbon-oxygen double bond is reformed (the blue part of picture 2) while in the mechanism of base-catalysed hydration of amide, no such kind of protonation of the amine group when the carbon-oxygen double bond is reformed (the blue part of picture 3). Why? [/quote]
Under basic conditions, there is no H+ available to protonate C=O. Therefore, the process starts directly with nucleophilic attack of OH- on C=O.

[quote] Q3: In the mechanism of aldol condensation (the green part of picture 4), hydroxide is a poor leaving group, protonation of the hydroxy group is required before ejecting it. However, as the reaction condition is basic, protonation cannot be achieved. Why the hydroxide still can be ejected even though it is a poor leaving group? [/quote]
OH- by itself is not a good leaving group. However, the process is driven by thermodynamics; the conjugated ketone product is much more stable than the intermediate.

[quote] Q4: When converting an alcohol to a alkyl halide, e.g.ethanol to 1-bromoethane, can I just simply suggest HBr instead of PBr3, PBr5 or NaBr+conc. H2SO4? [/quote]
HBr may work, but better in aqueous form, i.e., HBr/H2O to ensure there is sufficient amount of H+ and Br- ions. The synthesis using PBr3 is indeed more preferred as it will give a better yield.
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查看完整版本: reaction of benzyl halide, SN1 or SN2?