1559Tch08132-14710/30/0511:59Page135 EQA Keyso the Chapter·135 examples that you will see of nucleophilic additions to the electrophilic carbons of carbonyl groups.This is to aldehydes and ketones.the reverse of reduction.Alcohol oxidation is very useful in that it produces a carbonyl group.the most important Cr(vD: icrom which ondary alcohols to ketones,but ov 8-7.Organometallic Reagents 0=N Alcohol Ether Nitrile dIf we were to seek a logicalns which od proce s.we would try to take find bine with ment to electronegative atoms,then carbons should result from attachment to electropositive atoms Metals are the most mntsso the way toget ophilic)carbon would be to attac tions are easy to do,and the reagents you get are very useful in synthesis.The R groups in RLi and RMgX R”+ H →RH Weak acid 8-8.Organometallic Reagents in the Synthesis of Alcohols We now come to the primary value of these reagents:their ability to react as nucleophiles toward the el trophilic carbon in carbonyl compounds (C=O).In a reaction mechanistically analogous to the hydride you 8-9.An Introduction to Synthetic Strategy In order to learn how to levise sen e the pro n systen 1.Reactions CH:I+NaOH→CHOH+NalKeys to the Chapter • 135 examples that you will see of nucleophilic additions to the electrophilic carbons of carbonyl groups. This is one of the most important classes of reactions in organic chemistry. The second part of this text section introduces the oxidation of alcohols to aldehydes and ketones, the reverse of reduction. Alcohol oxidation is very useful in that it produces a carbonyl group, the most important functional group of all. Note the two types of reagents based on Cr(VI): PCC (pyridinium chlorochromate, pyH CrO3Cl
), which is specifically intended for oxidation of primary alcohols to aldehydes, and aqueous dichromate, which oxidizes secondary alcohols to ketones, but overoxidizes 1° alcohols to carboxylic acids. With these aspects of alcohol preparation and chemistry as background, we now turn to a discussion of molecules that will greatly expand our ability to make bonds: compounds with nucleophilic carbon atoms. 8-7. Organometallic Reagents So far the only kind of polarized carbons we’ve looked at in any detail is the (electrophilic) carbon that results from its attachment to a very electronegative atom: If we were to seek a logical way to link two carbon atoms in a synthetic process, we would try to take advantage of electrostatics and find molecules with
carbons, which could combine with the carbons above. That’s very nice, but where can we find
carbon atoms? Logically, if carbons result from attach￾ment to electronegative atoms, then
carbons should result from attachment to electropositive atoms. Metals are the most electropositive elements, so the way to get a
(nucleophilic) carbon would be to attach it to a metal. Compounds with carbon–metal bonds are called organometallic compounds and are sources of nucleophilic carbons. This section describes the preparations of some organometallic compounds. These reac￾tions are easy to do, and the reagents you get are very useful in synthesis. The R groups in RLi and RMgX also act as very strong bases. They are protonated by even weak acids like water or ammonia, giving the hydrocarbon RH as the product: 8-8. Organometallic Reagents in the Synthesis of Alcohols We now come to the primary value of these reagents: their ability to react as nucleophiles toward the elec￾trophilic carbon in carbonyl compounds In a reaction mechanistically analogous to the hydride additions of Section 8-6, organometallic reagents add nucleophilic carbon to aldehydes and ketones, resulting in alcohols, and making a new carbon–carbon bond in the process. At the end of the next section, you will find a summary chart of the major types of reactions that convert carbonyl compounds to alcohols. 8-9. An Introduction to Synthetic Strategy In order to learn how to devise sensible ways to make large organic molecules from small ones (a typical task of synthesis), you need to approach the problem systematically. First, note that the reactions you are learning can be classified into two categories: 1. Reactions that exchange one functional group for another but do not make or break any carbon–carbon bonds. These are called functional group interconversions, and a simple example is CH3I NaOH n CH3OH NaI C O). ( “H “R ”
” RH From even weakly Weak acid acidic molecules (e.g., H2O, ROH, NH3) Strong base (as in ROM) C X C O Haloalkane C OH Alcohol C OR Ether Carbonyl C N Nitrile 1559T_ch08_132-147 10/30/05 11:59 Page 135