|Lecture Notes: 29 June||
Note the strategy suggested by your text: work backwards from the product to the starting materials. As an example let's look at the synthesis of ortho -Chlorobenzoic acid from benzene.
Nomenclature: Amines are organic derivatives of ammonia. Thus they are the nitrogen analogues of alcohols and ethers, which are in turn organic derivatives of water. As we have seen before, there will be differences since nitrogen supports three bonds, while oxygen supports only two.
A difference in nomenclature is the way the terms primary, secondary, and tertiary are used. In alcohols primary-tertiary refer to the number of carbon substituents on the alcoholic carbon. In contrast, in amines primary-tertiary refers to the number of substituents on the nitrogen: primary (RNH2), secondary (R2NH), tertiary (R3N). Nitrogen can also have a fourth group attached via its extra pair of electrons. This give a positively charged compound known as a quaternary amine, or a quaternary ammonium salt.
Examples: CH3CH2NH2 (1°), (CH3)2NH (2°), (CH3)2CH3CH2N (3°), (CH3)2(CH3CH2)2N+Cl- (4° salt).
The nitrogen in amines can be attached to aromatic rings to give aryl amines or alkyl groups to give alkyl amines.
IUPAC Naming: There are two different IUPAC naming conventions for primary amines.
Secondary and tertiary amines with the same substituents (symmetrical) are named as di- and tri- alkyl amines:
When different substituents are attached to the same nitrogen, the largest substituent is chosen as the parent, and the others are named as N -substituents:
There are a number of important heterocyclic amines, each with its own name. In each case the nitrogen is assigned the number 1 in the ring:
There are only a few common names for amines, some of which are of biological interest:
Structure and Properties: The nitrogen in amines is sp3 hybridized, with bond angles very close to the 109° ideal (108°). The N-C bond length is about 0.15 nm. C-N bonds are highly polarized, and amines with fewer than 5 carbons are usually water soluble.
The nitrogen in amines is also a good H-bond acceptor. Primary and secondary amines are also H-bond donors and will form H-bonds in the pure compound. Thus low MW primary and secondary amines have relatively high melting and boiling points compared to alkanes of similar MW. However, due to the relatively weaker H-bond formed by nitrogen compared to oxygen, the boiling points of amines are still lower than comparable alcohols and acids.
Lo MW amines have a characteristic fish-like odor, while diamines such as putricine and cadaverine have disagreeable odors as suggested by their names.
Amines are significantly more basic than water or its organic derivatives. Thus amines dissolved in water result in a basic solution with the production of some hydroxide ion in an equilibria as shown below:
We can then define an equilibrium constant, Kb to describe basicity analogous to the acid constant, Ka we have seen before:
Note that most amines have pKb's in the range of 3-4, and for alkyls the particular substituent or number of substituents has little effect. On the other hand, aryl substituents have a significant impact, decreasing the basicity by about 6 orders of magnitude (pKb goes from 3.3 for alkyl to 9.4 for aryl; pKa goes from 10.7 for alkyl to 4.6 for aryl). This shift is readily understood as as a result of the delocalization of the amine electrons over the benzene ring - they are thus not as available for binding to a proton:
For similar reasons amide are not basic at all. In this case the electron delocalization is so effective that there is little electron density and thus attractiveness to protons, on the nitrogen.
From a practical point of view we can use the differing behaviors of acids and amides to separate them from non-polar compounds by simple extractions in acid and base, as shown in lab and the flow-diagrams at the end of the chapter in your text (went over flow diagram to isolation of amine).
Last modified 29 June 2004