ATAR Notes: Forum

Uni Stuff => Universities - Victoria => University of Melbourne => Topic started by: Belgarion on April 28, 2013, 07:34:57 pm

Title: Chemistry Reactions
Post by: Belgarion on April 28, 2013, 07:34:57 pm: Hi guys. Just had a quick question. Does anyone have a really effective way of telling if a reaction will undergo Sn1, Sn2, E1 or E2? It seems to be something im really struggling with. Right now im basing it on structure( primary, secondary or tertiary) as well as weak/strong nucleophile (for Sn1 and Sn2) and weak/strong base (for E1 and E2). However sometimes even this confuses me as something can be a strong nucleophile and strong base.

Thanks
Title: Re: Chemistry Reactions
Post by: simpak on April 28, 2013, 07:51:29 pm: Mrrrgghhhh you have a thread, post in it!

What textbook are you using? There is a good description of this in that 'Organic Chemistry' textbook that's assigned for first year.
Anyway:

"
Summary: which reaction occurs when?

Primary alkyl halides: SN2 substitution occurs if a good nucleophile is used, E2 elimination occurs if a strong base is used.

Secondary alkyl halides: SN2 substitution occur if a weakly basic nucleophile is used in a polar aprotic solvent. E2 elimination occurs if a strong base is used. SN1 and E1 reactions occur if a weakly basic nucleophile is used in a protic solvent.

Tertiary alkyl halides: E2 occurs when a base is used but SN1 and E1 elimination occur together under neutral conditions eg pure ethanol or water. SN2 is unlikely to occur because of steric hindrance of the substrate."
Title: Re: Chemistry Reactions
Post by: Starlight on April 28, 2013, 11:13:04 pm: Just expanding on mavisgibbons' post:

Primary alkyl halides: E2 is likely to occur if the base is "bulky"
Title: Re: Chemistry Reactions
Post by: simpak on April 29, 2013, 12:05:46 am: Soz if any of it is wrong, I wrote it out two years ago and haven't read over it in a while...haha.
Title: Re: Chemistry Reactions
Post by: Belgarion on April 29, 2013, 06:20:31 pm: Quote from: mavisgibbons on April 28, 2013, 07:51:29 pm
Mrrrgghhhh you have a thread, post in it!

What textbook are you using? There is a good description of this in that 'Organic Chemistry' textbook that's assigned for first year.
Anyway:

"
Summary: which reaction occurs when?

Primary alkyl halides: SN2 substitution occurs if a good nucleophile is used, E2 elimination occurs if a strong base is used.

Secondary alkyl halides: SN2 substitution occur if a weakly basic nucleophile is used in a polar aprotic solvent. E2 elimination occurs if a strong base is used. SN1 and E1 reactions occur if a weakly basic nucleophile is used in a protic solvent.

Tertiary alkyl halides: E2 occurs when a base is used but SN1 and E1 elimination occur together under neutral conditions eg pure ethanol or water. SN2 is unlikely to occur because of steric hindrance of the substrate."
ye we dont need to know about solvents any more. I know most of what you wrote out, but what confuses me is what if something is a strong nucleophile and a strong base and vice versa?
Title: Re: Chemistry Reactions
Post by: simpak on April 29, 2013, 06:53:54 pm: I can post more but if you're going to respond with "I already know what you're saying" then it's not worth my time :) What /do/ you know?
Title: Re: Chemistry Reactions
Post by: Belgarion on April 29, 2013, 07:22:32 pm: i know your trying to help but to be fair, your wasting your time yourself b/c i did say in my first post that i can differ b/w what will occur via : structure( primary, secondary or tertiary) as well as weak/strong nucleophile (for Sn1 and Sn2) and weak/strong base (for E1 and E2).

What im having trouble with is what if something is say a strong base and a strong nucleophile and vice versa? Which i have asked twice already.
Title: Re: Chemistry Reactions
Post by: Starlight on April 29, 2013, 09:02:58 pm: I suppose it has to do with context. A strong base generally refers to one involved in an elimination reaction since it abstracts the hydrogen atom, whilst a strong nucleophille is used to refer to SN2/SN1 reactions..

You don't have to know much else about this stuff than what has been written in the thread already. Do you have a specific question that asks about strong bases/ nucleophiles?
Title: Re: Chemistry Reactions
Post by: simpak on April 29, 2013, 09:16:48 pm: It really would do you well to learn that you don't snap at people offering their help to you. I'm sorry for wasting my own time, sometimes I find it difficult to understand what people are asking when they are expressing their questions quite poorly? When people are having difficulty understanding what you are asking (and I'm not the only one, because other people have read this thread and upvoted my previous post) then it can be beneficial to rephrase your question instead of repeating it in exactly the same way several times.

Strong bases will always be strong nucleophiles. There is no need to decide which rule to apply when you have a strong base - you assume it will undergo an elimination reaction because it is 'too tempted by hydrogens', in the words of our chemistry professors. Nucleophiles will not necessarily be basic (and therefore hydrogen acceptors). If you have a strong base, assume elimination. If you have a strong nucleophile that is not a base, assume substitution.
Title: Re: Chemistry Reactions
Post by: Belgarion on April 29, 2013, 09:26:55 pm: i didnt snap at you, i was trying to be more clear as you basically wrote out in your first post essentially what i had said in my first post and my question was in reference to a strong base/nucleophile and what would happen. Not everything to do with the reactions which as i stated in my first post, i understood. And saying something is not worth your time because i dont fully understand and saying i phrase my question poorly due to that is quite rude of you. If you can't try to help without being rude, i'd rather you not post. As well as, one other person has contributed to this post and i have found El2012's comments a lot more helpful then yours and he did it without being rude.

Thanks El2012, exactly what i needed to know. There's no specific question, but i just wanted to know. Thanks
Title: Re: Chemistry Reactions
Post by: Peedles on April 29, 2013, 10:17:43 pm: Mavisgibbons did not personally attack you for your lack of knowledge in Chemistry. The whole purpose of the forum is to help one another. This assumption that you put forth and the fact that you have the audacity to allude to discrediting advice completely from someone who has had more experience/knowledge with this subject than you by saying her advice is not helpful (or not as helpful as someone else) is extremely rude. If you are going to ask for advice, and someone has gone out of their way to give advice, have the respect to accept that not everyone will interpret what you are saying in the same way, she was not personally attacking you. Take it or leave it. This is no place for you to have an ego trip; you can't retaliate (which you have, my post included) everytime someone says something that you don't want to hear. No one here is questioning or judging your academic capabilities, we are just here to help. So chillax.
Title: Re: Chemistry Reactions
Post by: Belgarion on April 29, 2013, 11:30:59 pm: im fine ad id appreciate it if you'd stop dragging this post on. Just because someone has experience and s well liked doesnt give them an excuse to be rude in the first place. And it was clearly said that my question was not worth the time. So please, just let it go, i am.
Title: Re: Chemistry Reactions
Post by: Thu Thu Train on April 30, 2013, 11:39:14 am: Quote from: 2smart4u on April 29, 2013, 09:26:55 pm
i didnt snap at you, i was trying to be more clear as you basically wrote out in your first post essentially what i had said in my first post and my question was in reference to a strong base/nucleophile and what would happen. Not everything to do with the reactions which as i stated in my first post, i understood. And saying something is not worth your time because i dont fully understand and saying i phrase my question poorly due to that is quite rude of you. If you can't try to help without being rude, i'd rather you not post. As well as, one other person has contributed to this post and i have found El2012's comments a lot more helpful then yours and he did it without being rude.

Thanks El2012, exactly what i needed to know. There's no specific question, but i just wanted to know. Thanks

M8 u said that u alredi new the stuff mavisgibbons answered wif so if u already knew y wud she waste her time answering ur qn agen.

Guess ur just 2smart4us m8
Title: Re: Chemistry Reactions
Post by: Belgarion on April 30, 2013, 03:00:04 pm: like i said in my first post, i wanted to know what would happen if it was a strong nucleophile and base, this was the question i wanted answered as i have stated repeatedly. I said in my first post i understood the stuff about structure and strong/weak bases and nucleophiles and what i needed answering was if it was a strong base and nucleophile. Yet mavis had repeated what i said i understood in my very first post and when i repeated again i already understood it, this whole thing started. I let this post rest last night and now you are starting it up again. Im sick of having to justify myself. Just stop. If a mod reads this, please delete this entire thread b/c people cant learn to leave things alone.
Title: Re: Chemistry Reactions
Post by: thushan on April 30, 2013, 03:48:30 pm: Locking this, but not deleting - the chemistry information here can be quite valuable.

Sounds like a misunderstanding that went out of hand here. Let's let this go.
Title: Re: Chemistry Reactions
Post by: Mao on April 30, 2013, 04:32:20 pm: /posting in locked thread.

I don't think the question has been answered properly yet.

In deciding whether or not a reaction is SN or E, we have to think about the leaving group, the incoming group (if any), and the stability of the final product. The requirements are very different.

E:
- a group (typically halide) leaves
- a strong base abstracts an alpha proton (or another leaving group)
- forms double bond

If your initial compound cannot form a double bond like this (e.g. t-Butyl chloride), you don't get elimination reactions.

SN:
- a group (typically halide) leaves
- a group (typically a strong nucleophile) enters
- remains aliphatic

In determining whether a reaction is *1 or *2, it comes down to how well the original compound can support charge. Tertiary carbons can stabilise a cationic charge quite well (via short-range distortions of $\sigma$ orbitals), and thus we tend to have 1st order reactions (dissociation: rate $\propto$ [A] --> subsequent steps). For less substituted carbons, carbocations tend to be much less stable, so dissociation is unlikely. In these cases, the nucleophile/base do their thing first (rate $\propto$ [R][A], hence 2nd order).

This is a very crude way to interpret it, the reality is there is no clear-cut distinction between these mechanisms, or a critical 'strength' of nucleophile or base, or a critical structural feature that will determine SN is favoured over E or vice versa. The only predictive method I have seen is via quantum-chemical calculation of the free-energy landscape of the reaction, and even then it's often a competition between different mechanisms.

TL;DR, SN1, SN2, E1, E2, they all take place to a certain extent. Look at the stability of products to see whether it is SN or E. Look at the stability of intermediates/transition state to see whether it is *1 or *2. If you don't see a clear-cut answer, the reality probably isn't so clear cut either.