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Text Related to Segment 10.01 ©2002 Claude E. Wintner The possibility for structural variation and combination in the realm of organic molecules is effectively endless, and this fact can at first present a daunting prospect to anyone who approaches the field. The categorization of molecules in terms of their functional groups, that is, the specific atomic groupings or arrangements they may contain, turns out to provide an extraordinarily useful organizational principle for breaking up the realm into more manageable domains. For example, any molecule containing the grouping —O—H is termed an . Once one knows something about in general, on the basis of analogy one hopes to be able to make at least rough predictions about the likely physical and chemical properties of a particular new alcohol that one may have in hand. Indeed, the proper use of analogy is of exceptional importance to chemists in their endeavor. To the extent that what is true of one alcohol may be true of another, it should be unnecessary for us to keep in our heads many individual facts about this or that particular alcohol, while still having at our disposal enough practical working knowledge to be able to carry on our work with efficiency. In some respects great uniformity of behavior is observed. For example, virtually all alcohols show a characteristic infrared absorption, in terms of both median frequency and shape, that is quite unmistakable, and that confirms the presence of the alcohol functionality if oxygen is known to be present in the molecule. On the other hand, in the area of chemical reactivity somewhat finer classification may be needed. To introduce another functionality, we soon shall see that bromides (molecules in which a hydrogen atom in a hydrocarbon molecule has been replaced by a bromine atom and, in turn, a subset of the family of alkyl halides) undergo characteristic transformations. To know that bromoethane (CH3CH2Br) reacts with hydroxide ion to form ethanol via a substitution reaction is, in effect, to know also that

1-bromopropane (CH3CH2CH2Br) reacts with the same reagent to produce 1- propanol. On the other hand, we also shall see that these primary alkyl bromides (of the form R—CH2—Br, where R is a hydrocarbon moiety) may react quite differently from tertiary alkyl bromides (of the form R3C—Br), and that secondary alkyl bromides (of the form R2CH—Br) may show intermediate reactivity. In fact, when treated with hydroxide ion, 2-bromo-2-methylpropane ((CH3)3C—Br) generally does not yield 2-methyl-2-propanol by a substitution reaction, but rather 2- methylpropene as a result of an elimination reaction in the course of which the hydroxide ion removes the components of hydrogen bromide from the molecule. Our goal will be to gain some understanding about how functionality and structural environment, in conjunction with what is commonly referred to as the mechanism of a chemical reaction — a detailed picture of how it proceeds — determine the products that are obtained. H H OH CH3CH2 C Br CH3CH2 C OH substitution H H 1-bromopropane 1-propanol a primary alkyl bromide a primary alcohol (RCH OH, where R = Et) (RCH2Br, where R = Et) 2

CH 3 CH 3 CH3 OH CH3 C Br CH3 C OH and CH2 C substitution H and H propene H elimination by 2-bromopropane (of HBr) 2-propanol elimination a secondary alkyl bromide a secondary alcohol

(R2CHBr, where R = R = Me) (R2CHOH, where R = R = Me) by substitution

CH3 CH3 OH CH3 C Br CH C elimination 2 (of HBr) CH3 CH3 2-bromo-2-methylpropane 2-methylpropene a tertiary alkyl bromide

(R3CBr, where R = R = R = Me)

whether functionality is primary, secondary, or tertiary can influence the course of chemical reaction

In sum, it is useful to have anchored in one's mind the common functional groups of organic chemistry. The next figure presents a list of some common classes. While far from comprehensive, it does contain many of the functionalities that one is likely to meet at first. Frequently there is great resistance from students to learning such a list. Let it only be said that, just as it may be difficult to deal effectively with individuals among a group of people when one does not know their names or something about what they do, so also is this the case with molecules. Indeed, to be unable to name molecules or recognize their functional group affiliation is almost to doom from the beginning the enterprise of closer acquaintance — that is, understanding — of organic chemistry. Mrs. is not Mr. ; Dr. Ketal is not Professor ! One is well advised, then, to commit such a list to memory! It will be noted that there appear in the figure a number of functionalities derived from the , C=O; the chemistry of this ubiquitous grouping of two atoms will occupy us extensively in the sequel.

primary alcohol R CH2 OH

secondary alcohol R2CH OH

tertiary alcohol R3C OH

R O ether R O C C oxirane () R R R R HO OH

1,2-diol R C C R R R

alkyl halide R X (X = )

Grignard reagent R Mg X (X = halogen)

lithium reagent (alkyl lithium) R Li O

C R OH O

carboxylic acid halide C (X = halogen) R X O O carboxylic acid anhydride C C O R O R

ester C R O R O C O lactone (cyclic ester)

O

C R H O ketone C R R OH

hemiacetal R C H

OH OR

hemiketal R C R

OR OR

R C H

OR OR ketal R C R

O OR

peracid C O R O H primary R NH2 secondary amine R2NH tertiary amine R3N + - quaternary ammonium salt R4N X O primary C H R N O

H C R secondary amide R N O H tertiary amide O C R R N C H (R) N R lactam (cyclic amide) (alkyl cyanide) R C N OH cyanohydrin R C H (R) H (R) N CN C R H (R) NH2 -amino acid R C H O CO2H + N - R O

(thio alcohol, mercaptan) R SH

R S dialkyl (thio ether) R O thiol ester C R R S O R S OH O O sulfonyl halide R S X (X = halogen) O O ester R S OR O O R S R O O sulfinic acid S R OH O S R R monoalkylphosphate ester O HO P OR O dialkylphosphate ester OH HO P OR O

trialkylphosphate ester OR RO P OR

OR

©2002 Claude E. Wintner