I understand the mechanisms of E1, E2, and dehydrohalogenation. I just don't know how to find out whether an alkyl halide undergoing dehydrohalogenation will follow an E1 or an E2 reaction.
I have an OLD copy of Organic Chemistry by Morrison and Boyd which discusses E1 and E2 reactions. That book confirms what you have read and stated that E1 dehydrohalogenations occur among tertiary and secondary alkyl halides while E2 reactions occur with all three kinds of alkyl halides. As to which mechanism, E2 or E1, I quote from Morrison and Boyd on page 314 (4th edition), "For a given substrate, then, the more concentrated the base, or the stronger the base, the more E2 is favored or E1. Under the conditions typically used to bring about dehydrohalogenation--a concentrated solution of a strong base--the E2 mechanism is the path taken by elimination. In general, the E1 mechanism is encountered only with secondary or tertiary substrates, and in solutions where the base is either in low concentration or weak--typically where the base is the solvent."
A strong base would be concentrated KOH in alcohol or a similar alkoxide. A weak base would be water or alcohol (as a solvent as stated above).
My conclusions would be as follows: If dealing with a primary alkyl halide, pick E2 over E1. If dealing with secondary or tertiary alkyl halides, pick E2 when the reaction has a strong base and pick E1 when the reaction has a weak base.
You are correct that I answer very few organic chemistry questions; howwever, DrRuss checks in now and then. Perhaps he will see this and comment further.
I made a typo in which I typed the word or but I intended to type over. Here is the correction in bold.
"For a given substrate, then, the more concentrated the base, or the stronger the base, the more E2 is favored over E1.
DrBob has essentially covered it. It is not that easy to generalise, you need to consider each halide looking for steric influences, stabilisation of a carbocation and stability of the product. Some general points:
E1 is a two-step process of elimination ionization and deprotonation.
1. Ionization, Carbon-halogen breaks to give a carbocation intermediate.
2. Deprotonation of the carbocation.
Typical of tertiary and some secondary substituted alkyl halides.
The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. Therefore first order kinetics apply.
Reaction mostly occurs in complete absence of base or presence of only weak base.
E1 reactions are in competition with SN1 reactions because both involve a carbocation.
E2 is a one-step process of elimination with a single transition state.
Typical of secondary or tertiary substituted alkyl halides. It is also observed with primary alkyl halides if a hindered base is used.
The reaction rate, influenced by both the alkyl halide and the base, is second order.
The reaction rate is influenced by halogens reactivity; iodide and bromide being favoured. There can be competition between elimination reaction and nucleophilic substitution. I.e. there is competition between E2 and SN2 and also between E1 and SN1. Generally, elimination is favoured over substitution when:
steric hindrance increases
the steric bulk of the base increases for example potassium tert-butoxide
the nucleophile is poor
Hope this helps
THANKS SO MUCH GUYS! (sorry for the delayed response; I've been pretty busy)
It's nice to know that this forum/website is filled with intelligent people!
Answer this Question
English - Hey guys, I was just wondering if you could help me find or explain ...
For All Your Help! - I just wanted to say thanks for all of the help that you ...
Math - If you pass general chemistry, then you can take organic chemistry. You ...
QUESTION ABOUT JISHKA - Do you guys have a tutor that specialized in computer ...
physics - hey you guys , i reay appreciate it if you guys help me out . Q: a ...
Chemistry (Organic) - What is the molecular formula for an alkane with 12 carbon...
english - hey people!! im new here and i just got an essay to do on english ...
Chemistry - Hey guys I was wondering if you guys know the answer to this: Which...
english - Hmm I'm just wondering why no one is helping me and skipping my ...
student civics - why are gay guys given less of a chance then strate guys in a ...