Lecture_VI_Day

Lecture_VI_Day - ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER 18 1 ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER 18 We have seen that there is a vast array of reactions that are additions to the polarized C=O of ketones and aldehydes. These reactions are very useful synthetically and very instructive mechanistically. But they represent only part of the story of the chemistry of aldehydes and ketones and carbonyl compounds in general. There is another vast array of reactions in which the point of attack is not at the C=O but on the carbon adjacent to the C=O. This carbon is called the α-carbon. Many of these reactions are examples of the interaction between a functional group and an adjacent reactive center of the type we have seen in Chapter 14. We can also extend the ideas of functional group interactions presented in Chapter 14 to α,β-unsaturated ketones, which are similar to 1,3- dienes except that one of the 4 atoms of the conjugated system is an oxygen. In this chapter we will look at some examples of the reactions that display the influence of the carbonyl of an aldehyde or ketone on nearby portions of the molecule. I. Acidity of Aldehydes and Ketones We have mentioned that the α position of a carbonyl is one of the sites for reaction. The reason is that a hydrogen at this position is relatively acidic.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER 18 2 The main reason for this acidity is electron withdrawal by the carbonyl, both inductively in the acid for and by resonance in the conjugate base. . In fact this H is slightly less acidic than the H in H 2 O, but comparable to the H in ROH. The pK A ’s tend to be between 16 and 20. The very electronegative O of the carbonyl withdraws electrons by induction from the α-position in both aldehydes and ketones. This effect is more important in aldehydes (where the H on the carbonyl carbon is also electron withdrawing) than in ketones (where R is electron donating.) In the conjugate base there is electron withdrawal by resonance as well, resulting in a relatively stable anion. This species, the conjugate base of an aldehyde or ketone is called an enolate ion. Remember, although this proton is acidic enough to be removed by hydroxide, the position of the equilibrium does not usually lie on the side of the anion. Not much of an issue, however, because if we run reactions that remove the enolate the equilibrium keeps shifting over. Or if we want a high concentration of enolate we can use a stronger base. (as we shall see) Also notice that the negative charge in the enolate ion is distributed over two different atoms, a C and an O. This anion, just like almost any other, is a nucleophile looking for an electrophile. It can accept an electrophile at either the C or the O. As we shall see
Background image of page 2
ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER 18 3 where the electrophile is accepted depends in part on the nature of the electrophile. We can see the effects of electron withdrawal on the α proton
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 13

Lecture_VI_Day - ORGANIC CHEMISTRY 308 LECTURE VI CHAPTER...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online