Orgo 11.6-11.8

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  1. The reduction of alcohols to alkanes is not common because it removes a __, leaving fewer options for further reactions. 

    We can reduce an alcohol in two steps: __

    Another method for reducing an alcohol involves __. This reactionw orks with most primary and secondary alcohols.
    • functional group
    • 1) dehydrating it to an alkene
    • 2) hydrogenating the alkene

    converting hte alcohol to the tosylate ester, then using a hydride reducing agent to displace the tosylate leaving group
  2. Tosylation of an alcohol, followed by displacement of the tosylate by a halide ion, does what
    What is a simpler one step reaction that converts alcohols to alkyl halides?
    • converts an alcohol to an alkyl halide
    • treating it with a hydrohalic acid, either HBr, HCl, or HI
  3. In acidic solution, an alcohol is in __ with its protonated form. Protonation converts the hydroxyl from a __ to a __. Once the alcohol is protonated, all the usual substitution and elimination reactions are feasible, depending on the structure of the alcohol.
    • equilibrium
    • poor LG
    • good LG
  4. Most good nucleophiles are __, becoming protonated and losing their __ in acidic solutions. __ are exceptions, however. Halides are anions of strong acids, so they are weak bases. Solutions of HBr, HCl, or HI contain __ ions. These acids are commonly used to convert __ to the corresponding __.
    • basic
    • nucleophilicity
    • Halide ions
    • nuceophilic Br-, Cl-, or I- 
    • alcohols 
    • alkyl halides
  5. Reactions with Hydrobromic Acid

    Concentrated hydrobromic acid rapidly converts __ to __. The strong acid protonates the __, converting it to a good leaving group. The hindered tertiary carbon atom cannot undergo __, but it can ionize to a __. Attack by bromide gives the __. The mechanism is similar to other __, except that __ is the leaving group from the protonated acid.
    • tert-butyl alcohol
    • tert-butyl bromide
    • hydroxyl group
    • Sn2 displacement
    • tertiary carbocation
    • alkyl bromide
    • Sn1 mechanisms
    • water
  6. Reaction of a Tertiary Alcohol with HBr (Sn1)

    A __ reacts with HBr by the Sn1 mechanism
    tertiary alcohol

    • 1: protonation converts the hydroxyl group to a good LG
    • 2: water leaves, forming the carbocatioin
    • 3: Bromide ion attacks the carbocation
  7. Many other alcohols react with HBr, with the reaction mechanism depending on __. 

    Protonaton converts the hydroxyl group to a good leaving group, but __ to a __ is unfavorable. The protonated primary alcohol is well suited for the __ displacement. __ by bromide ion gives 1-bromobutane.
    • the structure of the alcohol
    • ionization
    • primary carbocatioin
    • Sn2
    • backside attack
  8. Reaction of a Primary Alcohol with HBr (Sn2)

    A __ reacts with HBr by the Sn2 mechanism.

    • primary alcohol
    • 1) protonation converts the hydroxyl group to a good LG
    • 2) bromide displaces water to give the alkylbromide
  9. __ also react with HBr to form alkyl bromides, usually by the __ mechanism. 

    Reactions with HCl

    HCl reacts with alcohols in much the same way that hydromromic acid does. 

    __ is a weaker nucleoophile than __ because it is smaller and less polarizable. An additional __ is sometimes necessary to promote the reaction of HCl with primary and secondary alcohols. __ coordinates with the oxgen of the alcohol in the same way a proton does-- except that __ coordinates more strongly.
    • Chloride ion
    • bromide ion
    • Lewis acid (like ZnCl2)
    • Zinc chloride x2
  10. The reagent compposed of HCl and ZnCl2 is called the __. Seconday and tertiary alcohols react with the __ by the __.
    • Lucas reagent
    • Lucas reagent
    • Sn1 mechanism
  11. Sn1 reaction with the Lucas reagent (__)
    Sn2 reaction with the Lucas reagent (__)

    When a primary alcohol reacts with the __, ionization is not possible--the __ is too unstable. Primary substrates react by an __, which is slower than a __ of secodnary and tertiary substrates.
    • fast 
    • slow
    • Lucas reagent
    • primary carbocation
    • Sn2 mechanism
    • Sn1 reaction
  12. The __ reacts with primary, secondary, and tertiary alcohols at predictable rates, and these rates can distinguish among the three types of alcohols. When the reagent is first added to the alcohol, the mixture forms a single __: The concentrated HCl solution is very polar and the polar alcohol-zinc chloride complex dissolves. Once the alcohol has reacted to form the alkyl halide, the relatively nonpolar halide separates into a second phase.
    • Lucas reagent
    • homogeneous phase
  13. What is the Lucas test?

    Tertiary alcohols react and show a __ almost instantly because they form relatively stable __. Secondary alcohols react in about 1 to 5 minutes because their secondary carbocations are less stable than tertiary ones. Primary alcohols react very slowly. Since the activated primary alcohol cannot form a __, it simply remains in solution until it is attacked by the chloride ion. With a primary alcohol, the reaction may take from ten minutes to several days
    adding the Lucas reagent to an unknown alcohol and watching for the second phase to separate

    second phase

    tertiary carbocations

  14. Limitations on the Use of Hydrohalic Acids with Alcohols
    The reactions of alcohols with hydrohalic acids do not always give good yields of the expected alkyl halides. Four principal limitations restrict the generality of this technique. What are they?
    • 1) Poor yields of alkyl chlorides from primary and secondary alcohols
    • 2) Eliminations
    • 3) Rearrangements
    • 4) Limited ability to make alkyl halides
  15. 1) Poor yields. 

    __ and __ react with HCl much more slowly than __, even with ZnCl2 added. Under these conditions, __ may prevent good yields of the alkyl halides.
    • Primary and secondary alcohols
    • side reactions
  16. 2) Elimination

    Heating an alcohol in a concentrated acid like HCl or HBr often leads to __. Once the hydroxyl group of the alcohol has been __ and converted to __, it becomes a candidate for __ and __.
    • elimination
    • protonated
    • a good LG
    • sub
    • elim
  17. 3) Rearrangements

    __ are always prne to __. This may occur as the __ leaves or as the __ has formed.

    4) Limited ability

    Many alcohols don't react with __ to give acceptable yields of __ because they are valuable intermediates since __ are most reactive of the alkyl halides.
    • carbocation intermediates
    • rearrangements 
    • LG
    • cation
    • HI
    • alkyl iodides
    • iodides
  18. Seceral __ are useful for converting alcohols to alkyl halides and are commercially available. It's unstable and can be generated in situ by the reaction of __ with __.
    • phosphorus halides
    • phosphorus
    • iodine
  19. __ produce good yields of most primary and secondary alkyl halides, but none works well with tertiary. The two __ used most often are __ and the __ combo. __ is often thebest reagent for converting __ to the __, especially if the alcohol might rearrange in strong acid.
    • phosphorus halides x2
    • PBr3
    • phosphorus/iodine 
    • PBr3
    • primary or secondary alcohol
    • alkyl bromide
  20. A __ is one of the best reagents for converting a primary or secondary alcohol to the __. For the synthesis of __, __ generally gives better yields than __ or __, especially with tertiary alcohols.
    • phosphorus and iodine combo
    • alkyl iodide
    • alkyl chlorides, thionyl chloride
    • PCl3 or PCl5
  21. Mechanism of the Reaction with Phosphorus Trihalides

    The mechanism of the reaction of alcohols with phosphorus trihalides explains why __ are uncommon and why __ work poorly with tertiary alcohols.
    • rearrangments 
    • phosphorus halides
  22. Reaction of Alcohols with PBr3
    • 1: PBr3 is a strong electrophile. An alcohol displaces bromide ion from PBr3 to give an excellent leaving group
    • 2: Bromide displaces the LG to give the alkyl bromide
  23. __ are uncommon because no __ is involved, so there is no opportunity for __. This mechanism also explains the poor yields with __. The final step is an __ where __ attacks the back side of an alkyl group. This attack is hindered if the alkyl group is __. In the case of a __, an __ is needed. It may be slow and it invites __.
    • rearrangements
    • carbocation
    • rearrangement
    • tertiary alcohols
    • Sn2 displacement
    • bromide
    • tertiary
    • ionization to a carbocation
    • side reactions
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Orgo 11.6-11.8
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