CHAPTEr EIGHTEEN Enols and enolates As an electron-withdrawing group on a carbon-carbon double bond, a carbonyl group renders the double bond susceptible to nucleophilic attack: R,C=CHCR Normally, carbon-carbon double bonds are attacked by electrophiles; a carbon-carbon double bond that is conjugated to a carbonyl group is attacked by nucleophiles. The presence of a carbonyl group in a molecule makes possible a number of chem- ical reactions that are of great synthetic and mechanistic importance. This chapter is com- plementary to the preceding one; the two chapters taken together demonstrate the extra- ordinary range of chemical reactions available to aldehydes and ketones. 18.1 THE Q-CARBON ATOM AND ITS HYDROGENS It is convenient to use the Greek letters a, B, Y, and so forth, to locate the carbons in a molecule in relation to the carbonyl group. The carbon atom adjacent to the carbonyl is the a-carbon atom, the next one down the chain is the B carbon, and so on. Butanal, for example, has an a carbon, a p carbon, and a y carbon Carbonyl group CH3CH,CH,CH no greek letter assigned to Hydrogens take the same Greek letter as the carbon atom to which they are attached. A hydrogen connected to the a-carbon atom is an a hydrogen. Butanal has two a protons, two B protons, and three y protons. No Greek letter is assigned to the hydro- gen attached directly to the carbonyl group of an aldehyde PROBLEM 18.1 How many a hydrogens are there in each of the following? (a)3, 3-Dimethyl-2-butanon (c) Benzyl methyl ketone (b)2, 2-Dimethylpropanal SAMPLE SOLUTION (a) This ketone has two different a carbons, but only one of them has hydrogen substituents. There are three equivalent a hydrogens. the other nine hydrogens are attached to B-carbon atoms CH3-C--C--CH3 3, 3-Dimethyl-2-butanone her than nucleophilic addition to the carbonyl group, the most important reac- tions of aldehydes and ketones involve substitution of an a hydrogen. A particularly well studied example is halogenation of aldehydes and ketones Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAs an electron-withdrawing group on a carbon–carbon double bond, a carbonyl group renders the double bond susceptible to nucleophilic attack: The presence of a carbonyl group in a molecule makes possible a number of chem￾ical reactions that are of great synthetic and mechanistic importance. This chapter is com￾plementary to the preceding one; the two chapters taken together demonstrate the extra￾ordinary range of chemical reactions available to aldehydes and ketones. 18.1 THE -CARBON ATOM AND ITS HYDROGENS It is convenient to use the Greek letters , , , and so forth, to locate the carbons in a molecule in relation to the carbonyl group. The carbon atom adjacent to the carbonyl is the -carbon atom, the next one down the chain is the carbon, and so on. Butanal, for example, has an carbon, a carbon, and a  carbon. Hydrogens take the same Greek letter as the carbon atom to which they are attached. A hydrogen connected to the -carbon atom is an hydrogen. Butanal has two protons, two protons, and three  protons. No Greek letter is assigned to the hydro￾gen attached directly to the carbonyl group of an aldehyde. PROBLEM 18.1 How many hydrogens are there in each of the following? (a) 3,3-Dimethyl-2-butanone (c) Benzyl methyl ketone (b) 2,2-Dimethylpropanal (d) Cyclohexanone SAMPLE SOLUTION (a) This ketone has two different carbons, but only one of them has hydrogen substituents. There are three equivalent hydrogens. The other nine hydrogens are attached to -carbon atoms. Other than nucleophilic addition to the carbonyl group, the most important reac￾tions of aldehydes and ketones involve substitution of an hydrogen. A particularly well studied example is halogenation of aldehydes and ketones. 3,3-Dimethyl-2-butanone CH3±C±C±CH3   CH3  CH3 O X W W Carbonyl group is reference point; no Greek letter assigned to it. O CH3CH2CH2CH  Normally, carbon–carbon double bonds are attacked by electrophiles; a carbon–carbon double bond that is conjugated to a carbonyl group is attacked by nucleophiles. O R2C CHCR 702 CHAPTER EIGHTEEN Enols and Enolates Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website