[kriggy]

Hi there, I run into several mechanism problems (my notes and my textbook say the oposite)

a) What is the mechanism of hydrolysis of 2-bromopropionic acid in acidic and basic media
- basic: its double Sn2 because the carboxylate ion will have the intermolecular interaction with 2-carbon (neighbor interaction), then the hydroxide comes from the same side as Br left - BUT my book says its Sn1
-acidic: my notes say its Sn1 while the book says Sn2. Sn1 doesnt seem plausible since it would require to find a secondary carbocation in the presence of COOH group in alpha position.

b) if I have vicinal dibromo compound how can I predict if the compound undergoes dehalogenation or double dehydrohalogenation? I was thinking that the erythro isomer then i get the dehalogenation since the halogens are in antiperiplanar configuration while the threo isomer forms the triple bond by double dehydrohalogenation but the book says that both compound form the dehalogenation product via E2a. Is there a way to force one or the another reaction? I was thinking that using base I would get dehydrohalogenation and by using Zn I get dehalogenation product.

I understand that Sn1 and Sn2 are borderline examples and the reaction could react with mechanism in between sn1 and sn2 but those two examples seem like something that will be asked at the exam so I would like to understand it.
THanks 

[pgk]

You mix many different issues in one. Nevertheless:
1). Substitution reactions are always in competition with elimination ones. Therefore, substitution reactions are often accompagnied by elimination side products
2). Primary halides undergo Sn2 and tertiary halides undergo Sn1. But secondary halides may undergo both mechanisms and sometimes simultaneously, depending on case.
3). Hydrolysis of 2-bromopropionic acid in basic medium, occurs by Sn2 mechanism, regardless the neighboring of the carboxylic group. E2 is not very favored in aqueous medium
4). Hydrolysis of 2-bromopropionic acid in acidic medium, occurs mainly by Sn2 mechanism. Sn1 leads to a carbocation that favors E1 elimination and obtaining of acrylic acid that is stabilized by conjugation, before hydrolysis occurs.
5). Double dehydrohalogenation would lead to formation of alkynes or allenes that are very difficult to obtain due to thermodynamic reasons. Probably, dehydrohalogenation will stop to the vinyl bromide derivative. Except, if the double dehydrohalogenation is favored by specific conditions, e.g. neighboring of carboxylic groups or an enol formation that leads to a conjugated carbonyl. Contrary and according the literature, double dehalogenation is feasible with Zn and vic- dihalides.
6). I hope that such really complicated issues that apparently seem simple, will not be asked at the exam.

[kriggy]

Thank you, its very clear now.

I hope the same, But I would probably prefer this than the reactions of cyclohexanes