Sn1 and E1 with alcohols

Question 1

advantages of creating alkyl halides from alcohols

Answer 1

• alcohols are cheaper and easier to work with
• alkyl halides can then be used to make many other types of functional groups

Question 2

disadvantages of creating alkyl halides from alcohols + solutions

Answer 2

• OH- is a stronger base than Cl-, so it’s thermodynamically unfavorable
• we need to make the alcohol a better leaving group by protenating it from an acid!

Question 3

SN1 reaction with alchols

Answer 3

• strong acid protenates OH group, which is then a strong leaving group and leaves
• conjugate base of strong acid has nucleophilic attack of carbocation

*tertiary alcohols are best (most stable carbocation), but secondary and primary can work with stronger acids and heat!

Question 4

dehydration of alcohols

Answer 4

• E1 reaction of alcohol results in removal of water
• needs a “dry acid” that’s conjugate base is a very weak nucleophile (favors elimination)

Question 5

importance of dry acids for alcohol dehydration

Answer 5

• concentrated wet acids like HCl still have lots of water, but dehydration produces water, so this is ba for equilibrium!
• dry acids don’t disturb equilibrium because they have little water
• dry acids (H2SO4, H3PO4) also have smaller atoms, which make the conjugate bases very weak nucleophiles! (won’t do SN1)

Question 6

other ways to make alcohols a better leaving group

Answer 6

thionyl chloride (SOCl2) or phosphorous halides (PBr3, PCl3) can bind to alcohol group and then cause a SN2 reaction, even with tertiary alcohols!!

Question 7

conditions for SN2 reaction with thionyl chloride

Answer 7

• often used with trimethyl amine as solvent to “soak” up acid byproduct
• usually run at cold temps so rxn isn’t too exothermic
• thionyl chloride reactions good for large molecules with many fxl groups that could react with strong acids!
• unlike sulfonates, there is no stable intermediate because of the Cl!

Question 8

replacing alcohols with a different LG for “storage”

Answer 8

• we can replaces alcohols with sulfonates as a good LG and this can be stored!
• -OH turns into -OSO2R
• advantages: stereochemistry conserved, works on all types of alcohols, and sulfonates are great LG (similar to Br)!