Carbonyl group

Carbonyl group

General structure of a carbonyl compound (carbonyl group shown in blue ): Ketone  : A, B = organic residue, aldehyde  : A = H , B = organic residue or H carboxylic acid  : A = OH, B = organic residue or H ester  : A = OR, B = organic residue or H amide  : A = NH 2 , NHR, NR 1 R 2 , B = organic residue or H ureas : A, B = NH 2 , NHR, NR 1 R 2 urethanes : A = OR, B = NH 2 , NHR, NR 1 R 2

The carbonyl group , also known as the CO group , is part of many organic chemical compounds . It is characterized by a carbon atom ( carbonyl carbon ) that carries a double bonded oxygen atom ( carbonyl oxygen ); the similar-sounding carboxy group also has a hydroxyl group on the same carbon atom , which creates a carboxylic acid . If a molecule contains a carbonyl group, it is also known as a carbonyl compound . If the carbonyl compound only bears alkyl radicals , the grouping is also referred to as a keto group . The single-bond organic residue, which formally consists of z. B. an organic acid after cleavage of the hydroxyl group is called acyl group. An example of an acyl group that is derived from acetic acid is the acetyl group . The electrophilic power of the carbonyl C atom and thus the reactivity of the compound are decisively influenced by the properties of the substituent (A). Substituents with the −I effect increase the reactivity, + I and + M substituents decrease it.

The carbonyl group is (formally) included in:

1. Carboxylic acids , undissociated (R – COOH)
2. Carboxylates , dissociated carboxylic acids or carboxylic acid salts (R – COO ^- )
3. Carboxamides (R – CO – NHR or R – CO – NR ₂ )
4. Imides (R – CO – NR – CO – R)
5. Carboxylic acid anhydrides (R – CO – O – CO – R)
6. Carboxylic acid esters (R – CO – O – R)
7. Carboxylic acid halides (R – CO – X where X stands for the halogen, i.e. F, Cl, Br, I, At)
8. Aldehydes / alkanals (R – CHO)
9. Ketones / alkanones (R – CO – R)
10. Ureas (R ₂ N – CO – NR ₂ )
11. Urethanes (R – NH – CO – O – R)
12. Carboxylic acid azides (R – CO – N ₃ )
13. Ketenes (R ₂ C = C = O).

The carbonyl group is similar to the sulfonic group , there are many analogues , e.g. B. sulfonic acids or sulfonamides .

Carbonyl complexes

In inorganic complex compounds (see metal carbonyls ), carbonyl denotes a carbon monoxide ligand that is coordinated to the central atom via the carbon atom. Examples are nickel tetracarbonyl , systematically tetracarbonylnickel (0), Ni (CO) ₄ , and the carbonyl ligand can also act as bridging, for example in dicobalt octacarbonyl , Co ₂ (CO) ₈ . The carbonyl ligand is a strongly splitting ligand and binds synergistically to many metal ions, since it is a σ donor of two electrons and a π acceptor. It splits the ligand field even more than the cyanide ligand CN ^- .

Reactivity of the carbonyl compounds

Carbonyl activity

Because of its polarity (the carbonyl carbon atom has a positive partial charge , the carbonyl oxygen atom, on the other hand, a negative partial charge) the carbonyl group is often responsible for the physical and chemical behavior of the molecules it contains. Due to the electronegativity difference , the LUMO ( lowest unoccupied molecular orbital ) is relatively low in energy and can be more easily attacked by nucleophiles . In addition, this means that carbonyl compounds usually have a higher melting and boiling point than their corresponding compound free of carbonyl groups.

The respective reactivity of the carbonyl compounds can be taken from a fixed series of the carbonyl activity.

The following compounds are arranged according to increasing carbonyl activity. This can be derived from the M and I effects .

• Carboxylates
• Carboxylic acids
• Carboxamides
• Carboxylic acid ester
• Esters of thiocarboxylic acids
• Ketones
• Aldehydes
• Carboxylic acid anhydrides
• Carboxylic acid halides

Reactions (selection)

Chemical conversions of carbonyl groups ( carbonyl chemistry ) are among the most common and important chemical reactions:

• Nucleophilic addition . The addition of hydrogen cyanide produces cyanohydrins
• Reduction (chemistry) to primary or secondary alcohols
• Condensation reaction
• Michael addition
• Enolate chemistry
• Alkylation by reaction with organometallic reagents (example: Grignard compounds )
• Reaction with primary amines to form imines
• Reaction with hydroxylamine to form oximes
• Reaction with hydrazine to form hydrazones
• Reductive amination with the formation of primary or secondary amines .

Keto-enol tautomerism

An important aspect of carbonyl chemistry is the so-called keto-enol tautomerism . Carbonyl compounds are in chemical equilibrium with their “enol form”, provided that they have hydrogen atoms in the alpha.

Examples of carbonyl compounds

Some examples of compounds with carbonyl groups (substance class, occurrence):

• Acetone ( ketone , a solvent contained in nail polish remover);
• Formaldehyde ( aldehyde , a preservative for medicinal preparations);
• Acetic acid ( carboxylic acid contained in table vinegar);
• Phosgene ( acid chloride of carbonic acid , a highly reactive product of the chemical industry);
• Carbonic acid (dissolved form of carbon dioxide in sparkling water);
• Glycine (and all other amino acids , the building blocks of proteins );
• Polycarbonates ( polymers , a material that CDs are made of);
• Flavones (flower pigments).

Individual evidence

1. ↑ G. Blumenthal, D. Linke, S. Vieth: Chemistry: Basic knowledge for engineers , Teubner Verlag , 2006, ISBN 3-519-03551-0 .
2. ^ Hans P. Latscha, Uli Kazmaier, Helmut Alfons Klein: Chemistry for Pharmacists . Springer DE, 2002, ISBN 3-540-42755-4 ( limited preview in Google book search).

literature

• Organikum , Wiley-VCH Verlag GmbH, 23rd edition, 2009, ISBN 978-3-527-32292-3 .
• Karl Schwister : Taschenbuch der Chemie , Hanser Verlag, 2004.

Web links

Commons : Carbonyl Group  - Collection of images, videos, and audio files

Wikibooks: Organic Chemistry for Students / Carbonyl Compounds  - Learning and Teaching Materials