[acuben]

2nd semester Organic Chemistry
not a HW problem, I just wanted to get this right...

(note: by primary/secondary/tertiary I mean carbon atom bonded to 1,2,3 carbons respectively
if they are actually called 1st degree/2nd degree or anything else, please correct me)
When I add H20 to primary Carbon. 1-bromobutane will it react?
How about with 2-bromobutane (2ndary carbon)
how about with t-butyl bromine (tertiary carbon)

Also...
When I add HBr to 1-butanol, 2-butanol, and t-butylol
will they react? or not react?

----predictions
1.
H2O + 1-bromobutane > NR

H2O is a weak nucleophile and weak base, and we have primary carbon so NR

2.
H2O + 2-bromobutane > 2-butanol + (E) 2-butene and other minor butene

H2O is a weak nucleophile and weak base, and we have secondary carbon so SN1+E1 reaction

3.
H2O + t-butyl bromine > t-butanol

Classical SN1+E1 reaction, Br will leave and deprotinate H+ , leaving OH- which then attachs to t-butoxide

4.
HBr + 1-butanol > 1-bromobutane

HBr is a strong acid, so we have Br(-1) ion, which attacks 1-butanol and forces OH group to leave
going through SN2 reaction

5.
HBr + 2-butanol > 2-bromobutane + (E) 2-butene and other minor butene

answer 1
HBr is a strong acid, so we have Br(-1) ion, which attacks 2-butanol and goes through
SN2 reaction.

or... (tell me which one is right ^^)

answer 2
HBr is a strong acid, so H+ will be added to butanol so OH group will be OH2 (+1 charge)
instead make it very good leaving group. Now Br- will attack (errr SN2?) and OH2+ will leave


6.
HBr + t-butanol > 1-methyl-2-propene

tertiary carbon so no SN2, but E2 occurs so it forms alkene



I really hate using HBr and H2O in these cases in synthesis due to their uncertainty in reaction,
nevertheless I thought I should know these in case I can't use other reagents.


Am I right? Instead of simple right or wrong, would you also add a note to each one how
strong the reaction would be?
like number 1 and 4 (2and5, and 3 and 6) can't both be right because they are opposite reaction.

as for number 5, I got the wrong in test during synthesis, but I can't understand why because I saw this reaction in my notes as well.
or could it because E2 is major in this reaction (and that notes were only explaining SN2 because E2 wasn't being covered at that time)

also for HBr Nucleophilic attack, does HBr has unique nucleophilic attack by adding proton to the OH group and making it OH2 (+1charge) group and making it good leaving group? Or dose that never happen? (or only happen in certain situations?)

[adianadiadi]

H₂O is weak nucleophile.

HBr is stronger acid and can form Br^- easily. The reactivity increases from primary to tertiary.

Primary react by SN2 and tertiary react by SN1.

[zolarpwr]

#1 looks correct. For #2 I would also expect no reaction... or maybe E2 to give the butene if you boil it for a LONG time. With just plain water I wouldn't expect much. You would need OH^- to get any real reaction.

Answer 2 for #5 is the best. First OH is protonated and leaves to give the carbocation which reacts by Sn1 and E1 to give bromobutane and butene. I don't expect a one step Sn2 or E2 reaction here. That would happen more in base than in acid.

When cooled in an ice bath #6 should also give you tert-butyl bromide by Sn1.

In all reactive cases you should expect to see alkene formed by elimination. From my personal experience, I attempted to form the alkyl chloride from HCl and a large tertiary alcohol and got only an internal alkene as product.