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Chemistry 2131:
Organic Chemistry for the Life Sciences (3)

The Amines

• the last group of compounds that we will look at is the amines. Amines have nitrogen atoms covalently bonded to carbon.
• let's step back and have a look at nitrogen for a moment. Nitrogen is the smallest member of family V. It sits between carbon and oxygen on the periodic table.
• nitrogen has 5 valence shell electrons. This means that to get an octet it must make at least 3 bonds (and have a lone pair) or make four bonds and have a positive charge
• amines are derivatives of ammonia with one or more hydrogen atoms replaced by alkyl or arly groups
• the classification of amines is quite different from other organic compounds. This classification depends on how many carbon atoms are directly attached to the nitrogen atom. Note that this is very different than the classification of alcohols and alkyl halides
• the base compound is ammonia, :NH. Then we have the primary amines, with one carbon atom attached directly to the nitrogen atom. Similarly, secondary amines have 2 carbon atoms attached to the nitrogen, and tertiary amines have three carbon atoms attached to the nitrogen atom.
• there also exists quaternary ammonium ions where there are four groups on the nitrogen atom. This nitrogen atom necessarily bears a positive charge
• amines can be part of a ring directly. These are called heterocyclic amines. Examples include pyridine, pyrrole...
• how do we name amines? Systematic names for straight chain amines are simple, the "e" is dropped from the parent alkane name and is replaced with the suffix "-amine". A number is given to designate the position of the amino group. For example 2-propanamine, 1,6-hexandiamine
• for aromatic amines, the base name is aniline. For example, 4-methylaniline
• that was the case for simple primary amines. What if there are many groups in the nitrogen atom? These are usually named as N-substituted primary amines. We take the largest group as the base parent amine. The smaller groups are named and listed as attachments to nitrogen with the N- designation
• for example: N-methylaniline, N,N-dimethylcyclopentanamine
• there are many common names that we won't worry about.
• if the amino group is not of highest precedence, the prefix "amino-" is used. For example 2-aminoethanol.

2. Physical Properties of Amines:

• the electronegativity of nitrogen is 3.0. This is relatively high, but not as high as for oxygen (3.5)
• the result of this is that the N-H bond is polarized but not as much as the O-H bond. Thus the possibility for hydrogen bonding exists. Clearly it will not be as extreme as for alcohols.
• to look at the consequence of this let's look at some boiling points. Ethane (30.1 g/mole) has a b.p. of -88.6 degrees, methylamine (31.1 g/mole) has a b.p. of -6.3, and methanol (32.0 g/mole) has a b.p. of 65.0. This shows us that hydrogen bonding markedly affects the physical properties of amines, but not to the extent that alcohols are affected
• amines are also more soluble in water than corresponding alkanes. See Table 10.1 on page 284 of Brown.

3. Basicity of Amines:

• like ammonia, the ammonia derivatives, the amines, are weak bases. This means that aqueous solutions of amines are basic
• if one mixes an amine with water, the lone pair on nitrogen can abstract a proton from water generating a hydroxide ion.
• for methylamine the Kb is 4.37 x 10^-4 giving a pKb of 3.36. We often think of the reverse reaction, the dissociation of the protonated form. This gives us a Ka of 2.29 x 10^-11 or a pKa of 10.64. Of course the pKa and the pKb add up to 14
• regular aliphatic or alkyl amines have pKa's in the 9 - 11 range. Thus, they tend to be protonated at physiological pH's
• aromatic amines such as aniline are much weaker bases with pKa's around 4 -5. Why are aromatic amines weaker bases? Resonance stabilization of the free base occurs. When the amine is deprotonated the lone pair can sit on the nitrogen, but it can also interact with the pi electron ring of the aromatic ring. You can show this with resonance structures, pushing the electron pair around the ring.

4. Reactions of Amines:

• we have already seen a number of reactions where amines act as nucleophiles. These include Schiff base formation and amide formation.
• a key feature of amines as nucleophiles is that they must be deprotonated to act as nucleophiles.