Basicity.pptx

Basicity

1. Basicity

2. A base is a substance that can accept a proton by donating a pair of electrons . how can we measure a base’s strength? To what extent does a base attract a proton

3. What factors affect how basic a compound is?

4. What factors affect how basic a compound is? The more ‘ stable’ the base, the weaker it is . The more accessible the electrons are, the stronger the base is . negatively charged base is more likely to pick up a proton than a neutral one a compound in which the negative charge is delocalized is going to be less basic than one with a more concentrated, localized charge, and so on.

5. There are two main factors that determine the strength of a neutral base. how accessible is the lone pair to what extent can the resultant positive charge formed be stabilized either by delocalization or by the solvent

6. how accessible is the lone pair The accessibility of the lone pair depends on its energy. it is usually the HOMO of the molecule and so, the higher its energy, the more reactive it is and hence the stronger the base. The lone pair is lowered in energy if it is on a very electronegative element or if it can be delocalized in some manner.

7. Neutral nitrogen bases Any substituents: increasing the electron density on the nitrogen => raises the energy of the lone pair => more available for protonation => increasing the basicity of the amine (larger p K aH ). Conversely , any substituent that withdraws electron density from the nitrogen makes it less basic (smaller p K aH ).

8. Effects that increase the electron density on nitrogen attaching an electron-releasing group by conjugating the nitrogen with an electron-donating group.

9. attaching an electron-releasing group The simplest example of an electron-releasing group is an alkyl group All the amines have p K aHs greater than that of ammonia (9.24) • All the primary amines have approximately the same p K aH (about 10.7) • All the secondary amines have p K aHs that are slightly higher • Most of the tertiary amines have p K aHs lower than those of the primary amines Why? =>> the stabilization of the resultant positive charge formed on protonation, is also important

10. II. to what extent can the resultant positive charge formed be stabilized either by delocalization or by the solvent Each successive alkyl group does help stabilize the positive charge because it is electron-releasing but there is another stabilizing effect—the solvent . Every hydrogen attached directly to nitrogen will be hydrogen bonded with solvent water and this also helps to stabilize the charge : =>> the more hydrogen bonding, the more stabilization a combination of effects: (1) the increased availability of the lone pair and the stabilization of the resultant positive charge, which increases with successive replacement of hydrogen atoms by alkyl groups; and (2) the stabilization due to solvation, an important part of which is due to hydrogen bonding and this effect decreases with increasing numbers of alkyl groups.

11. lone pair: in an sp2 or an sp orbital, it is more tightly held (the orbital is lower in energy) and therefore much harder to protonate. This explains why the lone pair of the nitrile group is not at all basic and needs a strong acid to protonate it.

12. The low p K aH of aniline (PhNH2), 4.6, due to: the nitrogen being attached to an sp2 carbon the lone pair can be delocalized into the benzene ring .  when protonated, the positive charge cannot be delocalized over the benzene ring and any stabilization derived from the lone pair in un-protonated aniline being delocalized into the ring is lost.

13. Amides are weak bases protonated on oxygen In contrast to aromatic amines, the amide group is completely planar (p. 165) with the nitrogen sp2 hybridized and its lone pair in the p orbital, thereby enabling it to overlap effectively with the carbonyl group This delocalization ‘ties up’ the lone pair and makes it much less basic :  Because of the delocalization amides are not protonated on nitrogen.

14. Amidines are stronger bases than amides or amines protonation occurs at the sp2 nitrogen atom . This happens because we have the same situation as with an amide only if we protonate on the sp2 nitrogen can the positive charge be delocalized over both nitrogens .

15. Imidazoline is a simple cyclic amidine and its p K aH value is just what we expect, around 11. Imidazole, on the other hand, is less basic ( p K aH 7.1) because both nitrogens are attached to an electron-withdrawing sp2 carbon. However, imidazole, with its two nitrogen atoms, is more basic than pyridine ( p K aH 5.2) because pyridine only has one nitrogen on which to stabilize the positive charge .

16. Both imidazole and pyridine are aromatic—they are flat, cyclic molecules with 6 π electrons in the conjugated system (p. 177). Imidazole has one lone pair that is and one that is not involved in the aromaticity (Chapter 43 ).  Protonation occurs on the nitrogen atom having the sp2 lone pair because both lone pairs contribute and the resulting delocalized cation is still aromatic

17. Guanidines are very strong bases Even more basic is guanidine, p K aH 13.6, nearly as strong a base as NaOH ! On protonation, the positive charge can be delocalized over three nitrogen atoms to give a very stable cation.

18. Conjugation with an electron-donating group produces even stronger bases (p. 202) but we could also increase the electron density by using elements such as silicon. Silicon is more