Organic Chemistry

PROBLEM 36

The organoborane used in a Suzuki reaction is prepared by the reaction of catecholborane

with an alkene or an alkyne.

What hydrocarbon would you use to prepare the organoborane of Problem 35?

PROBLEM 37

Give two sets of alkyl bromide and alkene that could be used in a Heck reaction to prepare

the following compound:

CH 3 C CH CH OCH 3

O

catecholborane

RCH 2 CH 2 B

O

O

RCH CH 2 + H B

O

O

Tutorial:

Common terms

Summary 473

Summary

Alcoholsand ethershave leaving groups that are stronger

bases than halide ions, so alcohols and ethers are less reac-

tive than alkyl halides and have to be “activated”before they

can undergo a substitution or an elimination reaction.

Epoxidesdo not have to be activated, because ring strain in-

creases their reactivity. Sulfonate esters and sulfonium

saltshave weakly basic leaving groups, so they undergo sub-

stitution reactions with ease. is such a strong base that

amines cannot undergo substitution or elimination reactions.

Primary, secondary, and tertiary alcohols undergo nucleo-

philic substitution reactions with HI, HBr, and HCl to form

alkyl halides. These are reactions in the case of primary

alcohols and reactions in the case of secondary and ter-

tiary alcohols. An alcohol can also be converted into an alkyl

halide by phosphorus trihalides or thionyl chloride. These

reagents convert the alcohol into an intermediate with a

leaving group that is readily displaced by a halide ion.

Conversion to a sulfonate esteris another way to activate

an alcohol for subsequent reaction with a nucleophile. Be-

cause a sulfonic acid is a strong acid, its conjugate base is

weak. Activating an alcohol by converting it to a sulfonate

ester forms a substitution product with a configuration oppo-

site to that of the alcohol, whereas activating an alcohol by

converting it to an alkyl halide forms a substitution product

with the same configuration as the alcohol.

An alcohol can be dehydrated if heated with an acid cat-

alyst; dehydrationis an E2 reaction in the case of primary

alcohols and an E1 reaction in the case of secondary and

tertiary alcohols. Tertiary alcohols are the easiest to dehy-

drate and primary alcohols are the hardest. E1 reactions

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• NH
  2

form carbocation intermediates, so carbocation and ring-

expansion rearrangementscan occur. The major product

is the more substituted alkene. Both the Eand Zisomers are

obtained, but the isomer with the bulky groups on opposite

sides of the double bond predominates.

Ethers can undergo nucleophilic substitution reactions

with HBr or HI; if departure of the leaving group creates a

relatively stable carbocation, an reaction occurs; other-

wise an reaction occurs.

Epoxidesundergo ring-opening reactions. Under basic

conditions, the least sterically hindered carbon is attacked;

under acidic conditions, the most substituted carbon is at-

tacked. Arene oxidesundergo rearrangment to form phe-

nols or nucleophilic attack to form addition products. An

arene oxide’s cancer-causing potential depends on the sta-

bility of the carbocation formed during rearrangement.

A crown etherspecifically binds certain metal ions or

organic molecules, depending on the size of its cavity, form-

ing an inclusion compound. The ability of a host to bind

only certain guests is an example of molecular recognition.

The crown ether can act as a phase-transfer catalyst.

Thiolsare sulfur analogs of alcohols. They are stronger

acids and have lower boiling points than alcohols. Thiolate

ions are weaker bases and better nucleophiles in protic sol-

vents than alkoxide ions. Sulfur analogs of ethers are called

sulfidesor thioethers. Sulfides react with alkyl halides to

form sulfonium salts.

Grignard reagentsand organolithium compoundsare

the most common organometallic compounds—compounds

that contain a carbon–metal bond. They cannot be prepared

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a. b. c.

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