Ammonia
Aminaare organic compounds
Aminaare organic compounds containing and often based on one or more nitrogen atoms. Structurally, amines are similar to ammonia in that nitrogen can bond up to three hydrogen atoms, but amines also have additional properties based on their carbon connectivity. In an amine, one or more ammonia hydrogen atoms are replaced by organic substituents such as alkyl (alkane chain) and aryl (aromatic ring) groups.
Another type of organic molecule contains nitrogen without being strictly ain: Derivatives of carboxylic acids containing a trivalent ammonia (three bonds) in the ground stateAmideinstead of amines. Amides and amines have different structures and properties, so the distinction is really important. Nitrogenous organic compounds containing metals are also mentioned.AmideSo if you see a molecule that has a nitrogen and a carbonyl group or a metal next to that nitrogen, then you know the molecule must be an amide rather than an amine.
types of amines
The amines can beprimary,secondaryoTertiary-, depending on the number of carbon-containing groups attached to it. If there is only one carbon-containing group (as in the molecule CH3N.H2), then this amine is considered primary. Two carbon-containing groups make an amine secondary, and three groups make it tertiary. Using nitrogen's lone pair of electrons, it is sometimes energetically preferable to use nitrogen as the nucleophile and thus attach a fourth carbon-containing group to the amine. In this case you can callammonium quarters.
primary amine
secondary amine
tertiary amine
An organic compound with multiple amine groups is calleddiamine,triamine,tetraamineand so on depending on the number of amine groups (also calledamino groups) attached to the molecule. For example, the chemical formula of methylenediamine (also called diaminomethane) would be as follows: H2N-CH2-N.H2
aromatic amines
In aromatic amines, the nitrogen atom is attached directly to an aromatic ring structure. because of youelectron deprivationProperties The aromatic ring greatly reduces the basicity of the amine, and this effect can be enhanced or offset depending on the substituents on the ring and on the nitrogen. The presence of the lone pair of electrons on nitrogen has the opposite effect on the aromatic ring itself; Because the nitrogen atom can "borrow" electron density from the ring, the ring itself becomes much more reactive for other types of chemistry.
naming rules
The prefix "amino-" is used for primary amines where the amine is not the characteristic main group. For example: 4-aminobenzoic acid, where carboxylic acid is the main feature. Otherwise, the suffix "-amine" is used with the name of the parent hybrid substituent or R group. Example: Ethanamine or Ethylamine. Alternatively, the suffix "-azan" can be added to the name of the R group substituent: Example: propylazan.
For secondary, tertiary, and quaternary amines, the naming convention is slightly different, but the suffixes are the same. For symmetrical amines, the prefix "di" or "tri" is used depending on whether there are 2 or 3 substituents. For example, dipropylamine is a secondary amine and triphenylamine is a tertiary amine. Asymmetric amines have the backbone attached with "-amine". This name is then preceded by an "N-" (indicating a nitrogen bond) and the substituent group name for each substituent, using the alphabetical order of tertiary amides. For example, N-ethyl-N-methyl-propylamine, not N-methyl-N-ethyl-propylamine.
In summary:
- as prefix: "amino-"
- als Suffix: "-amine"
- the prefix "N-" indicates substitution on the nitrogen atom (for secondary, tertiary and quaternary amines)
Systematic names for some common amines:
Methylamine
- Primary amines:ethanamineof ethanol.
- Secondary Amines:dimethylamine
- Tertiary amines:trimethylamine
physical properties
As you can imagine, the inclusion of a heteroatom like nitrogen in the carbon and hydrogen molecules has a big impact on the properties of amines compared to alkanes.
General properties
Hydrogen bonding significantly affects the properties of primary and secondary amines, as well as the protonated derivatives of all amines. For example, the boiling point of amines is higher than that of the corresponding phosphines (phosphorus-containing compounds) but generally lower than that of the corresponding alcohols. Alcohols or alkanols resemble amines but have a -OH group instead of NR2. Since oxygen is more electronegative than nitrogen, RO-His typically more acidic than the related R2NORTE-Hcompound.
The methyl, dimethyl, trimethyl, and ethylamines are gases under standard conditions. The most common alkylamines are liquid and the high molecular weight amines are intrinsically solid at standard temperatures. In addition, gaseous amines have a characteristic ammonia odor, while liquid amines have a characteristic "fishy" odor.
Most aliphatic amines exhibit some solubility in water, reflecting their ability to form hydrogen bonds. Solubility decreases relatively proportionally as the number of carbon atoms in the molecule increases, especially when the number of carbon atoms is greater than six. Aliphatic amines also show significant solubility in organic solvents, especially polar organic solvents. Primary amines readily react with ketone compounds (such asAcetone), and most amines are incompatible with chloroform and also with carbon tetrachloride as solvent solutions.
Aromatic amines have their conjugated ("shared") lone pairs of electrons on the benzene ring, so their propensity to form hydrogen bonds is somewhat reduced. Therefore, the boiling points of these molecules are generally somewhat higher than those of other smaller amines due to their typically larger size.
They also have relatively low solubility in water, although they retain their solubility in other organic solvents.
Conjugated aromatic amines are usually quite toxic and can be easily absorbed through the skin, so they should always be treated as "hazardous".
chirality
Tertiary amines of the type NHRR' and NRR'R" are not chiral: although the nitrogen atom has four different substituents, counting the lone pair of electrons, the lone pair of electrons can "hop" across the nitrogen atom and reverse the other molecules.The energy barrier for such a Walden inversion of the stereocenter with a lone pair of electrons is relatively low, e.g. ~7 kcal/mol for a trialkylamine, therefore it is difficult to obtain reliable chiral products with tertiary amines Due to this low barrier, amines such as NHRR' cannot be resolved optically and NRR'R" can only be resolved if the R, R' and R" groups are restricted in cyclic structures. Quaternary amine structures, eg H3CN+-RR'R", are chiral and can be easily resolved optically.
properties as base
Like ammonia, amines act like bases and are quite strong (see the table for some examples of the conjugate acid KAValues). The basicity of amines varies depending on the molecule and is highly dependent on:
- The availability of the lone pair of electrons in nitrogen.
- The electronic properties of the pendant substituent groups (for example, alkyl groups increase basicity, aryl groups decrease it, etc.)
- The degree of solvation of the protonated amine depends mainly on the solvent used in the reaction.
The nitrogen atom of a typical amine has a lone pair of electrons that can bond to a hydrogen ion (H+) to form an ammonium ion - R3N.H+. The water solubility of simple amines is due in large part to the hydrogen bonding capacity that can occur between the protons in water molecules and these lone pairs of electrons.
Basicity of nitrogen groups
In this section we consider the relative basicity of various nitrogen-containing functional groups: amines, amides, anilines, imines, and nitriles. When evaluating the basicity of a nitrogen-containing organic functional group, we must ask the key question: how reactive (and thus how basic) is the lone pair of electrons on nitrogen? In other words, how badly does this lone pair want to break away from the nitrogen nucleus and form a new hydrogen bond?
Comparison of the basicity of alkylamines with ammonia
Because alkyl groups donate electrons to the more electronegative nitrogen. The inductive effect causes the electron density to be higher in the alkylamine nitrogen than in the ammonium nitrogen. Consequently, primary, secondary, and tertiary alkylamines are more basic than ammonia.
Comparison of the basicities of alkylamines with amides
In an alkylamine, the lone pair of electrons is on nitrogen. However, the lone pair of electrons in an amide is delocalized by resonance between nitrogen and oxygen. This makes amides much less basic compared to alkylamines.
In fact, when an amide reacts with an acid, protonation occurs on the carbonyl oxygen instead of nitrogen. This is because the cation resulting from the protonation of oxygen is stabilized by resonance. The cation resulting from the protonation of nitrogen is not stabilized by resonance.
Basicity of heterocyclic amines
When a nitrogen atom is directly attached to an aromatic ring, its basicity depends on the context of the bond. For example, in a pyridine ring, the nitrogen pair occupies a sp2-hybrid orbital, and isNOPart of the aromatic sextet: it is essentially an imine nitrogen. Its pair of electrons are available to form a bond with a proton, and therefore the nitrogen atom of pyridine is somewhat basic.
In a pyrrole ring, on the other hand, the nitrogen pairEsPart of the aromatic sextet. This means that these electrons are very stable where they are (in the aromatic system) and are much less available to attach to a proton (and when they are).doaccepts a proton, the aromatic system is destroyed). For these reasons, the pyrrole nitrogens are not strongly basic.
The examples of aniline, pyridine, and pyrrole are good models for predicting the reactivity of nitrogen atoms in more complex ring systems (of which a great variety occurs in nature). For example, the side chain of tryptophan contains a "pyrrole-like" non-basic nitrogen, while adenine (a DNA/RNA base) contains all three types.
The lone pair of electrons in the nitrogen of anitrileare included in aspOrbital hybrids. die 50%Scharacter onespHybrid orbital means that the electrons are close to the nucleus and therefore not essentially basic.
A review of the basics.Acid-base conceptsshould be useful for the following discussion. Like ammonia, most amines are Brønsted and Lewis bases, but substituents can greatly alter their base strength. It is common to quantitatively compare the basicity with thatpackageAof their conjugate acidsinstead of your pKB's. the PKA+ pKB= 14,the higher the pKAThe stronger the base, in contrast to the usual inverse relationship of pKAwith acid Most simple alkylamines have pKAthey oscillate between 9.5 and 11.0 and their aqueous solutions are basic (with a pH of 11 to 12 depending on the concentration). The first four compounds in the following table, including ammonia, fall into this category.
The last five connections (colored cells) are significantly weaker bases due to three factors. The first of these is hybridization with nitrogen. In pyridine the nitrogen is sp2hybrida, and in nitriles (last entry) an sp hybrid nitrogen forms part of the triple bond. In each of these compounds (shaded red), the nonbonding pair of electrons is on the nitrogen atom, but increasing the s character brings it closer to the nitrogen nucleus, reducing its tendency to bind to a proton.
compound |  |  |  | N.H3 |  |  |  |  |  | CH3C≡N |
|---|---|---|---|---|---|---|---|---|---|---|
| packageA | 11,0 | 10.7 | 10.7 | 9.3 | 5.2 | 4.6 | 1,0 | 0,0 | -1,0 | -10. |
Finally, the very low basicity of pyrrole (shaded blue) reflects the unique electron pair delocalization of nitrogen associated with its incorporation into ascent ring. Indole (pKA= -2) e Imidazole (pKA= 7,0),look for, also have similar heterocyclic aromatic rings. Imidazole is more than a million times more basic than pyrrole because the sp2the nitrogen that is part of a double bond is structurally similar to pyridine and has comparable basicity.
Although resonance delocalization generally reduces the basicity of amines, a dramatic example of the reverse effect is found in the compound guanidine (pKA= 13.6). Here, as shown below, the resonance stabilization of the base due to charge separation is small, while the conjugate acid is strongly stabilized by charge delocalization. Consequently, aqueous solutions of guanidine are almost as basic as caustic soda.
The relationship between the basicity of the amine and the acidity of the corresponding conjugate acids can be summarized analogouslymentioned above for acids:
Strong bases have weak conjugate acids, and weak bases have strong conjugate acids.
Extraction of amines in the laboratory
Extraction is often used in organic chemistry to purify compounds. Liquid-liquid extractions use the difference in solubility of a substance in two immiscible liquids (for example, ether and water). The two immiscible liquids used in an extraction process are (1) the solvent in which the solids are dissolved and (2) the extraction solvent. The two immiscible liquids can be easily separated with a separatory funnel. The basicity of amines can be exploited using the protonated salt (RNH2+Kl−), which is soluble in water. The salt is extracted to the aqueous phase, leaving neutral compounds in the non-aqueous phase. The aqueous layer is then treated with base (NaOH) to regenerate the amine and NaCl. A second stripping is then performed to isolate the amine in the non-aqueous layer and leave the NaCl in the aqueous layer.
amino acids
We usually think of amines as bases, but it should be remembered that amines 1 and 2 (not amines 3 that lack N-H protons) are also very weak acids (Ammonia has a pKaA= 34). In this context, it should be notedpackageAit is used as a measure of the acidity of the amine itself and not of its conjugate acid, as in the previous section. For ammonia this is expressed by the following hypothetical equation:
N.H3+H2o____>N.H2(-)+H2OH(+)
The same factors that decrease the basicity of amines increase their acidity. This is illustrated by the following examples, presented in order of increasing acidity. It should be noted that the first four examples show the same order and degree of increased acidity as the decreased basicity in the table above. The first compound is a typical second amine, and the next three are characterized by varying degrees of delocalization of the nitrogen electron pairs. The last two compounds (shaded in blue) show the influence of neighboring sulfonyl and carbonyl groups on the acidity of NÀH. From the above discussion it should be clear that the basicity of these nitrogens is correspondingly reduced.
| compound | | | | | C6H5SO2N.H2 |  |
|---|---|---|---|---|---|---|
| packageA | 33 | 27 | 19 | 15 | 10 | 9.6 |
The acids shown here can be converted to their conjugate bases by reaction with bases derived from weaker acids (stronger bases). Three examples of such reactions are shown below, with the acid hydrogen colored red in each case. About a million times stronger reagent base is required for complete conversion to the conjugate base, as shown.
C6H5SO2N.H2+ YES C6H5SO2N.H(-)k(+)+H2o | a sulfonamide base |
| (CH3)3COH + NaH(CH3)3CO(-)Of(+)+H2 | an alkoxide base |
| (C2H5)2NH + ALT4H9li(C2H5)2norte(-)li(+)+C4H10 | an amide base |
Important Basic Concepts About Reagents
The importance of all these acid-base relationships in practical organic chemistry lies in the need for organic bases of different strengths as reagents tailored to the needs of particular reactions. Common base sodium hydroxide is not soluble in many organic solvents and is therefore not widely used as a reagent in organic reactions. Most of the basic reagents are alkoxide salts, amines, or amide salts. Because alcohols are much stronger acids than amines, their conjugate bases are weaker than amide bases, bridging the gap in base strength between amines and amide salts. In the following pK tableAagain refers to the conjugate acid of the base drawn above.
| basename | pyridine | trityl in | Base de Hünig | Bartons Base | Potassium t-butóxido | Sodium-HMDS | LDA |
|---|---|---|---|---|---|---|---|
| Formula | | (C2H5)3norte |  |  | (CH3)3CO(-)k(+) | [(CH3)3MI]2norte(-)Of(+) | [(CH3)2CH]2norte(-)li(+) |
| packageA | 5.3 | 10.7 | 11.4 | 14 | 19 | 26 | 35,7 |
Pyridine is commonly used as an acid scavenger in reactions that produce mineral acid byproducts. Its basicity and nucleophilicity can be modified by steric hindrance, as in the case of 2,6-dimethylpyridine (pKA=6.7) or resonance stabilization as in the case of 4-dimethylaminopyridine (pKA=9.7). Hünig's base is relatively non-nucleophilic (due to steric hindrance) and, like DBU, is commonly used as a base in E2 elimination reactions performed in nonpolar solvents. Barton's base is a weakly nucleophilic, strong neutral base that serves where electrophilic substitution of DBU or other amine bases is a problem. Alcoholates are stronger bases that are often used in the corresponding alcohol as a solvent or to increase reactivity in DMSO. Finally, the two amide bases are commonly used to generate enolate bases from carbonyl compounds and other weak carboxylic acids.
exercises
Questions
Q24.3.1
Choose the most basic amine of each of the following pairs of compounds.
(A)
(B)
(C)
Q24.3.2
The 4-methylbenzylammonium ion has a pKa of 9.51 and the butylammonium ion has a pKa of 10.59. What is more basic? What is the pKb of each compound?
solutions
S24.3.1
(A)
(B)
(C)
S24.3.2
Butylammonium is more basic. The pKb of butylammonium is 3.41, the pKb of 4-methylbenzylammonium is 4.49.
taxpayers
- dr Dietmar KennepohlFCIC (Chemistry Professor,athabasca university)
- Profesor Steven Farmer (Sonoma State University)
- William Reusch, Professor Emeritus (Michigan u.),Virtual Book of Organic Chemistry
- Organic chemistry with a biological approach.Vontim soderberg(University of Minnesota, Morris)
aniline basicity
Aniline is significantly less basic than methylamine, as indicated by the pKa value.AValues for their respective conjugated ammonium acids (remember that the lower the pKa of the conjugated acid, the weaker the base).
This difference in basicity can be explained by the observation that in aniline the lone base pair on nitrogen is bound to some extent and stabilized by the aromatic pair system.
This effect is enhanced by the addition of an electron-withdrawing group, such as carbonyl, and reversed to some extent by the addition of an electron-donating group, such as methoxide.
In the case of 4-methoxyaniline (the molecule on the left in the figure above), the lone pair of electrons on the methoxy group donates electron density to the aromatic system, and a resonance contribution can be drawn into which is placed a negative charge. on the one adjacent to carbon nitrogen, making the nitrogen lone pair more reactive. In fact, the methoxy group "pushes" the electron density towards nitrogen. On the other hand, the aldehyde group in the molecule on the right "pulls" the electron density of nitrogen and reduces its basicity.
At this point, you should draw resonance structures to convince yourself that these resonance effects are possible when the substituent in question (methoxy or carbonyl) is in theHuertaoforposition, but not withinMetaPosition. an imine functional group is denoted by a sp2Hybridized nitrogen doubly bonded to a carbon. The imines are somewhat basic, with pKAValues for the protonated forms hover around 7. Note that this is significantly less basic than the amine groups (for example, pKA= 10.6 for methylammonium), where the nitrogen sp3-hybridized. This phenomenon can be explained with the orbital theory and the inductive effect: The sp2The orbitals of an imine nitrogen are partSand two partsPAG, which means that they are about 67%SCharacter. the SP3The orbitals of an amine nitrogen, on the other hand, are only 25%Scharacter (a partS, three partsPAG). That's whySThe atomic orbital contains electrons in a spherical shape, closer to the nucleus than aPAGorbital,sp2Hybridization implies higher electronegativity than sp3hybridization. Finally, remember the inductive effect from Section 7.3C: electronegative atoms accept electron density more readily and are therefore more acidic. Conclusion: The nitrogens of protonated imines are more acidic than protonated amines, so imines are less basic than amines.
taxpayers
- dr Dietmar KennepohlFCIC (Chemistry Professor,athabasca university)
- Profesor Steven Farmer (Sonoma State University)
- William Reusch, Professor Emeritus (Michigan u.),Virtual Book of Organic Chemistry
- Organic chemistry with a biological approach.Vontim soderberg(University of Minnesota, Morris)
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