Orgo Prelim 3

Basic equation for alcohols and thiols

RCH2OH —> RCH2X

                        carboxylic       alkenes

                            acids

 

 

(where X=halide or other LG)

 

What is the SN2 reaction for direct substitution/elimination reactions on alcohols
RCH2OH + HX <—> RCH2X + H2O
How do you track redox changes?
use oxidation numbers, which are formal charges that result from electronegativity differences in atoms in a bond
dehydration
a reaction where the elements of water are lost from the starting material
why is alcohol dehydration not promoted by strong bases?

  1. a strong base will remove the O-H proton, which is more acidic than a C-H beta-proton
  2. **the OH group is a very poor leaving group because it is a relatively strong base**

what is te role of the acid in alcohol dehydration
to convert the -OH, the poor leaving group, into the +OH2 group, the good leaving group.
what the three step mechanism that alcohol dehydration occurs by

short version: leaving group–>lewis acid-base dissociation rxns–>bronsted acid-base rxns

 

long version:

  1. the -OH is activated as a leaving group by acting as a Bronsted base to accept a proton from the catalyzing acid
  2. the carbon-oxygen bond breaks (in a Lewis acid-base rxn) to make water and a carbocation
  3. water (conjugate base of the catalyzing acid H3O+) removes a beta-proton from the carbocation in another Bronsted acid-base rxn

What is true about alcohol dehydration

  1. alcohol dehydration is an E1 rxn
  2. alkene hydration and alcohol dehydration are opposite rxns
  3. alkene hydration and alcohol dehydration are both catalyzed by acids

why can’t; hydroxide be the base in the Bronsted acid-base reaction of alcohol dehydration
hydroxide cannot be the base because the reaction is carried out in strongly aciding solution, and hydroxide can’t survive in strong acid because it protonates instantaneously to form water. (hydroxy is also not necessary as a base bc the carbocation is a strong acid)
what is the order of the relative rates of alcohol dehydration
tertiary>secondary>>primary
What do tertiary carbocations do for the dehydration rxn?
because they are the most stable, teritary carbocations should be faster
what can you expect if the alcohol has more than one type of beta-hydrogen
a mixture of alkene products can be expected
In the reactions of tertiary alcohols, the protonation of the alcohol oxygen is followed by what?

carbocation formation, which is formed by the ionization of strong acid HCl.

 

Once protonated, the reaction is an SN1 reaction with H2O as the leaving group

When a primary alcohol is the starting material, what kind of reaction occurs?

thereaction occurs as a concerted displacement of water from the protonated alcohol by halide ions.

 

the reaction is an SN2 reaction in which water is the leaving group.

 what do substitution and elimination reactions of alcohols do to the -OH group?
convert into a better leaving group
sulfonic acids
compounds of the form R-SO3H
sulfonate ester
a compound in which the acidic hydrogen of the sulfonic acid is replaced by an alkyl or aryl group
x

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