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amide

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Did you mean: amide (in chemistry), amidation (organic chemistry)

 
Dictionary: am·ide   (ăm'īd', -ĭd) pronunciation
 
n.
  1. An organic compound, such as acetamide, containing the CONH2 radical.
  2. A compound with a metal replacing hydrogen in ammonia, such as sodium amide, NaNH2.

[AM(MONIA) + –IDE.]

amidic a·mid'ic (ə-mĭd'ĭk, ă-mĭd'-) adj.
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A derivative of a carboxylic acid with general formula RCONH2, where R is hydrogen or an alkyl or aryl radical. Amides are divided into subclasses, depending on the number of substituents on nitrogen. The simple, or primary, amides are considered to be derivatives formed by replacement of the carboxylic hydroxyl group by the amino group, NH2. They are named by dropping the “-ic acid” or “-oic acid” from the name of the parent carboxylic acid and replacing it with the suffix “amide.” In the secondary and tertiary amides, one or both hydrogens are replaced by other groups. The presence of such groups is designated by the prefix capital N (for nitrogen).

Except for formamide, all simple amides are relatively low-melting solids, stable, and weakly acidic. They are strongly associated through hydrogen bonding, and hence soluble in hydroxylic solvents, such as water and alcohol. Because of ease of formation and sharp melting points, amides are frequently used for the identification of organic acids and, conversely, for the identification of amines.

Commercial preparation of amides involves thermal dehydration of ammonium salts of carboxylic acids. Thus, slow pyrolysis of ammonium acetate forms water and acetamide. N,N-dimethylacetamide may be similarly prepared from dimethylammonium acetate.

Amides are important chemical intermediates since they can be hydrolyzed to acids, dehydrated to nitriles, and degraded to amines containing one less carbon atom by the Hofmann reaction. In pharmacology, acetophenetidin is a popular analgesic. However, the most important commercial application of amides is in the preparation of polyamide resins, also called nylons. See also Acid anhydride; Polyamide resins.


 

Any member of either of two classes of nitrogen-containing organic compounds related to ammonia and amines and containing a carbonyl group (-C=O; see functional group). The first class, covalent amides are formed by replacing the hydroxyl group (-OH) of an acid with an amino group (-NR2, in which R may represent a hydrogen atom or an organic combining group, such as methyl). Amides formed from carboxylic acids, called carboxamides, are solids except for the simplest, formamide, a liquid. They do not conduct electricity, have high boiling points, and (when liquid) are good solvents. There are no practical natural sources of simple covalent amides, but the peptides and proteins in living systems are long chains (polymers) with peptide bonds (see covalent bond), which are amide linkages. Urea is an amide with two amino groups. Commercially important covalent amides include several used as solvents; others are the sulfa drugs and nylon. The second class, ionic (salt-like) amides (see ionic bond), are made by treating a covalent amide, an amine, or ammonia with a reactive metal (e.g., sodium) and are strongly alkaline.

For more information on amide, visit Britannica.com.

 
amide (ăm'īd) , organic compound formed by reaction of an acid chloride, acid anhydride, or ester with an amine. Under strong acidic conditions an amide can be hydrolyzed to yield an amine and a carboxylic acid. The reverse of this process results in the loss of water and is used in nature to link amino acids to form proteins. See amino group; carboxyl group.


 

Any compound derived from ammonia by substitution of an acid radical for hydrogen, or from an acid by replacing the −OH group by −NH2.

  • a. compound herbicides — diphenamid and CDAA may cause poisoning if given in large doses. Signs include depression, weight loss and muscular weakness of the hindquarters.
 
Wikipedia: Amide
Top
Amide functional group
Amides possess a conjugated system spread over the O, C and N atoms, consisting of molecular orbitals occupied by delocalized electrons. One of the π molecular orbitals in formamide is shown above.

In chemistry, an amide is one of three kinds of compounds:

  • (sometimes called acid amide) the organic functional group characterized by a carbonyl group (C=O) linked to a nitrogen atom (N), or a compound that contains this functional group (pictured to the right); or
  • a kind of anion or
  • any organic compound derived by the replacement of a hydroxyl group by an amino group.

Amides are the most stable of all the carbonyl functional groups.

Many chemists make a pronunciation distinction between the two, saying /əˈmiːd/ for the carbonyl-nitrogen compound and /ˈæmaɪd/  ( listen) for the anion. Others substitute one of these with /ˈæmɨd/, while still others pronounce both /ˈæmɨd/, making them homonyms.

In the first sense referred to above, an amide is an amine where one of the nitrogen substituents is an acyl group; it is generally represented by the formula: R1(CO)NR2R3 , where either or both R2 and R3 may be hydrogen. Specifically, an amide can also be regarded as a derivative of a carboxylic acid in which the hydroxyl group has been replaced by an amine or ammonia.
Compounds in which a hydrogen atom on nitrogen from ammonia or an amine is replaced by a metal cation are also known as amides or azanides.

The second sense of the word amide is the amide (or amino) anion NH2-, which is a deprotonated form of ammonia (NH3) or an amine. It is generally represented by the formula: [R1NR2]-, and is an extremely strong base, due to the extreme weakness of ammonia and its analogues as Brønsted acids.

The remainder of this article is about the carbonyl-nitrogen sense of amide. For examples of the anionic amide, see the articles Sodium amide and Lithium diisopropylamide.

Contents

Amide synthesis

Amide bond formation
Chapman rearrangement
The reaction mechanism is based on a nucleophilic aromatic substitution.[4]
  • The seemingly simple direct reaction between an alcohol and an amine to an amide was not tried until 2007 when a special ruthenium-based catalyst was reported to be effective in a so-called dehydrogenative acylation:[5]
Synthesis of Amides from Alcohols and Amines with Liberation of H2
The generation of hydrogen gas compensates for unfavorable thermodynamics. The reaction is believed to proceed by one dehydrogenation of the alcohol to the aldehyde followed by formation of a hemiaminal and the after a second dehydrogenation to the amide. Elimination of water in the hemiaminal to the imine is not observed.

Amide reactions

  • Amide breakdown is possible via amide hydrolysis. Such hydrolysis can occur under basic or acidic conditions. Acidic conditions yield the carboxylic acid and the ammonium ion while basic hydrolysis yield the carboxylate ion and ammonia.
  • In the Vilsmeier-Haack reaction an amide is converted into an imine.
  • Hofmann rearrangement of primary amides to primary amines.

Owing to their resonance stabilization, amides are relatively unreactive under physiological conditions, even less than similar compounds such as esters. Nevertheless, amides can undergo chemical reactions, usually through an attack of an electronegative atom on the carbonyl carbon, breaking the carbonyl double bond and forming a tetrahedral intermediate. When the functional group attacking the amide is a thiol, hydroxyl or amine, the resulting molecule may be called a cyclol or, more specifically, a thiacyclol, an oxacyclol or an azacyclol, respectively.

The proton of an amide does not dissociate readily under normal conditions; its pKa is usually well above 15. However, under extremely acidic conditions, the carbonyl oxygen can become protonated with a pKa of roughly -1.

Amides will react with nitrous acid (HONO) forming the carboxylic acid and yielding nitrogen. Nitrous acid is formed by addition of a strong acid to a nitrite salt in solution at temperatures of between 0 and 10 degrees.[citation needed]

Amides undergo the Hofmann rearrangement in which an amine with one less carbon atom is produced upon reaction with bromine and sodium hydroxide. On the other hand, reacting the amide with the strong reducing agent lithium aluminium hydride yields an amine with the same number of carbon atoms.

Amides are dehydrated with phosphorus (V) oxide forming the nitrile. Care should be taken when performing such a reaction since phosphorus (V) oxide smoulders when in contact with organic matter.

Amide linkage (peptide bond)

An amide linkage is kinetically stable to hydrolysis. However, it can be hydrolysed in boiling alkali, as well as in strong acidic conditions. Amide linkages in a biochemical context are called peptide linkages. Amide linkages constitute a defining molecular feature of proteins, the secondary structure of which is due in part to the hydrogen bonding abilities of amides.

Amide properties

Amide resonance:

Compared to amines, amides are very weak bases. While the conjugate acid of an amine has a pKa of about 9.5, the conjugate acid of an amide has a pKa around -0.5. Therefore amides don't have as clearly noticeable acid-base properties in water. This lack of basicity is explained by the electron-withdrawing nature of the carbonyl group where the lone pair of electrons on the nitrogen is delocalized by resonance, thus forming a partial double bond with the carbonyl carbon and putting a negative charge on the oxygen. On the other hand, amides are much stronger bases than carboxylic acids, esters, aldehydes, and ketones (conjugated acid pKa between -6 and -10). It is estimated in silico that acetamide is represented by resonance structure A for 62% and by B for 28% [6]. Resonance is largely prevented in the very strained quinuclidone.

Solubility

Amides contain C=O (carbonyl) and N-C dipoles arising from covalent bonding between electronegative oxygen and nitrogen atoms and electro-neutral carbon atoms. Primary and secondary amides also contain two- and one N-H dipoles, respectively. Because of the pi-bonding arrangement of the carbonyl and the greater electronegativity of oxygen, the carbonyl (C=O) is a stronger dipole than the N-C dipole. The presence of a C=O dipole and, to a lesser extent a N-C dipole, allows amides to act as H-bond acceptors. In primary and secondary amides, the presence of N-H dipoles allows amides to function as H-bond donors as well. Thus amides can participate in hydrogen bonding with water and other protic solvents; the oxygen and nitrogen atoms can accept hydrogen bonds from water and the N-H hydrogen atoms can donate H-bonds. As a result of interactions such as these, the water solubility of amides is greater than that of corresponding hydrocarbons

While hydrogen bonding may enhance the water solubility of amides relative to hydrocarbons (alkanes, alkenes, alkynes and aromatic compounds), amides typically are regarded as compounds with low water solubility. They are significantly less water soluble than comparable acids or alcohols due to: 1). their non-ionic character 2). the presence of nonpolar hydrocarbon functionality, and 3). the inability of tertiary amides to donate hydrogen bonds to water (they can only be H-bond acceptors). Thus amides have water solubilities roughly comparable to esters. Typically amides are less soluble than comparable amines and carboxylic acids since these compounds can both donate and accept hydrogen bonds, and can ionize at appropriate pHs to further enhance solubility

Derivatives

Sulfonamides are analogues of amides in which the atom double-bonded to oxygen is sulfur rather than carbon.

Cyclic amides are called lactams.

Naming conventions

References

  1. ^ Bodroux F., Bull. Soc. Chim. France, 1905, 33, 831;
  2. ^ Bodroux reaction at the Institute of Chemistry, Skopje, Macedonia Link
  3. ^ A. W. Chapman, "CCLXIX. - Imino-aryl ethers. Part III. The molecular rearrangement of N-phenylbenziminophenyl ether", Journal of the Chemical Society, Transactions, 127:1992-1998, 1925. doi:10.1039/CT9252701992
  4. ^ Advanced organic Chemistry, Reactions, mechanisms and structure 3ed. Jerry March ISBN 0-471-85472-7
  5. ^ Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2 Chidambaram Gunanathan, Yehoshoa Ben-David, David Milstein Science 10 August 2007: Vol. 317. no. 5839, pp. 790 - 792 doi:10.1126/science.1145295
  6. ^ "Amide Resonance" Correlates with a Breadth of C-N Rotation Barriers Carl R. Kemnitz and Mark J. Loewen J. Am. Chem. Soc.; 2007; 129(9) pp 2521 - 2528; (Article) doi:10.1021/ja0663024

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