carbonyl group

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Carbonyl group

Ketones

Properties of the carbonyl group

Other molecules with carbonyl groups

Resources

A carbonyl group in chemistry is a group of atoms that consists of a carbon (C) atom covalently attached to an oxygen (O) atom by a double bond: C = O. The carbon atom, to satisfy its valence of 4, must also be attached by covalent bonds to two other atoms. The simplest type of molecule that contains a carbonyl group is a ketone. Other types of molecules that contain carbonyl groups are aldehydes, acids, esters, and amides.

Ketones

A ketone is a compound whose molecules contain a carbonyl group and have two other groups attached to the carbon atom of the carbonyl group. There are many molecules that belong to this classification, but the simplest one is acetone. A condensed structural formula for acetone looks like the following formula.

In this formula, the C=O represents the carbonyl group, and the two CH3 groups satisfy the carbon atoms valence of 4. In other molecules that contain the carbonyl group, the C=O is still present, but the two CH3 groups are traded for other atoms or groups of atoms.

Sometimes chemists need to talk about the entire class of possible ketone molecules. In this case, one uses a structural formula that looks like this formula.

In this picture, R and R can stand for any hydrocarbon-containing group and as CH3 -, C2 H5 -, etc.

Properties of the carbonyl group

The carbonyl group is somewhat polar. That means that one end of it (the carbon atom) has a slight positive electric charge, and one end of it (the oxygen atom) has a slight negative charge. This makes the entire molecular a polar molecule.

The polar nature of the carbonyl part of the molecule affects the physical properties of the entire molecule. For instance, small ketone molecules, with fewer than six carbon atoms in all, are soluble in water, a very polar solvent. At the same time, small ketone molecules are themselves excellent solvents for other compounds with polar groups. This is in contrast to small hydrocarbon molecules with no carbonyl groupthey are insoluble in water, and they will not dissolve other polar molecules.

A carbonyl group in a molecule is often the most chemically reactive portion. When a molecule containing a carbonyl group undergoes a chemical reaction, it is often this polarity that controls which reaction will take place. Usually a chemical reaction in a molecule containing only a carbonyl group and hydrocarbon groups will take place at the carbonyl group.

KEY TERMS

Covalent bond A chemical bond formed when two atoms share a pair of electrons with each other.

Double bond A covalent bond consisting of two pairs of shared electrons that hold the two atoms together.

Hydrocarbon group A portion of a molecule containing only carbon and hydrogen atoms.

Molecule A single particle of a substance in which the atoms join together by covalent bonds.

Polarity The distribution of electrical forces within a molecule. There are non-polar molecules, in which the electrical forces balance each other out, and polar ones, in which the complete molecule may still be neutral, but the electrical forces within it are not directed evenly throughout the molecule.

Other molecules with carbonyl groups

In many molecules that contain a carbonyl group, the other two groups of atoms are not hydrocarbon groups. Molecules like this are so different chemically that they belong to entirely different classifications. There are four major classes of molecules like this. Again, R stands for any hydrocarbon group. There is more information about these kinds of carbonyl-containing molecules in their entries in this encyclopedia.

See also Acids and bases; Amides; Ester.

Resources

BOOKS

Carey, Francis A. Organic Chemistry. Dubuque, IA: McGraw-Hill, 2006.

Hoffman, Robert V. Organic Chemistry: An Intermediate Text. Hoboken, NJ: Wiley-Interscience, 2004.

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 2002.

Tro, Nivaldo J. Introductory Chemistry. Upper Saddle River, NJ: Pearson Education, 2006.

G. Lynn Carlson

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Carbonyl group

A carbonyl group is a group of atoms that consists of a carbon atom covalently attached to an oxygen atom by a double bond: C = O. The carbon atom, to satisfy its valence of 4, must also be attached by covalent bonds to two other atoms. The simplest type of molecule that contains a carbonyl group is a ketone. Other types of molecules that contain carbonyl groups are aldehydes , acids, esters, and amides .


Ketones

A ketone is a compound whose molecules contain a carbonyl group and have two other groups attached to the carbon atom of the carbonyl group. There are many molecules that belong to this classification, but the simplest one is acetone . A condensed structural formula for acetone looks like this.

In this formula, the C=O represents the carbonyl group, and the two CH3 groups satisfy the carbon atom's valence of 4. In other molecules that contain the carbonyl group, the C=O is still present, but the two CH3 groups are traded for other atoms or groups of atoms.

Sometimes we need to talk about the entire class of possible ketone molecules, and then we use a structural formula that looks like this.

In this picture, R and R' can stand for any hydrocarbon-containing group and as CH3-,C2H5-, etc.


Properties of the carbonyl group

The carbonyl group is somewhat polar. That means that one end of it (the carbon atom) has a slight positive electric charge , and one end of it (the oxygen atom) has a slight negative charge. This makes the entire molecular a polar molecule.

The polar nature of the carbonyl part of the molecule affects the physical properties of the entire molecule. For instance, small ketone molecules, with fewer than six carbon atoms in all, are soluble in water , a very polar solvent. At the same time, small ketone molecules are themselves often good solvents for other compounds with polar groups. This is in contrast to small hydrocarbon molecules with no carbonyl group—they are insoluble in water, and they won't dissolve other polar molecules.

A carbonyl group in a molecule is often the most chemically reactive portion. When a molecule containing a carbonyl group undergoes a chemical reaction, it is often this polarity that controls which reaction will take place. Usually a chemical reaction in a molecule containing only a carbonyl group and hydrocarbon groups will take place at the carbonyl group.


Other molecules with carbonyl groups

In many molecules that contain a carbonyl group, the other two groups of atoms are not hydrocarbon groups. Molecules like this are so different chemically that they belong to entirely different classifications. There are four major classes of molecules like this. Again, R stands for any hydrocarbon group. There is more information about these kinds of carbonyl-containing molecules in their entries in this encyclopedia.

See also Acids and bases; Amides; Ester.

Resources

books

Mark, Herman F. From Small Organic Chemicals to Large: a Century of Progress. Washington DC: American Chemical Society, 1993.

Mauskopf, Seymour H. Chemical Sciences in the Modern World. Pennsylvania: University of Pennsylvania Press, 1993.


G. Lynn Carlson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Covalent bond

—A chemical bond formed when two atoms share a pair of electrons with each other.

Double bond

—A covalent bond consisting of two pairs of shared electrons that hold the two atoms together.

Hydrocarbon group

—A portion of a molecule containing only carbon and hydrogen atoms.

Molecule

—A single particle of a substance in which the atoms join together by covalent bonds.

Polarity

—The distribution of electrical forces within a molecule. There are nonpolar molecules, in which the electrical forces balance each other out, and polar ones, in which the complete molecule may still be neutral, but the electrical forces within it are not directed evenly throughout the molecule.

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carbonyl group (kär´bənĬl), in chemistry, functional group that consists of an oxygen atom joined by a double bond to a carbon atom. The carbon atom is joined to the remainder of the molecule by two single bonds or one double bond. If the carbonyl group is joined only to alkyl groups or aryl groups, the compound is a ketone; if it is joined to at least one hydrogen atom, the compound is an aldehyde. The chemical reactivity of aldehydes and ketones is primarily due to the difference in electronegativity between carbon and oxygen. Because oxygen has the greater affinity for electrons, it acquires a partial negative charge, becoming electron-rich; the carbon atom of the carbonyl group thus becomes electron-deficient, acquiring a partial positive charge. One major type of reaction of aldehydes and ketones involves the addition of an electron-rich chemical species to the electron-deficient carbon atom of the carbonyl group. Another type of reaction is due to the tendency of the electron-deficient carbon atom of the carbonyl group to partially attract electrons from carbon atoms adjacent to it in the molecule, thus increasing the acidity of hydrogen atoms that are bonded to the adjacent carbon.

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car·bon·yl / ˈkärbəˌnil/ • n. [as adj.] Chem. of or denoting the divalent radical =C=O, present in such organic compounds as aldehydes, ketones, amides, and esters, and in organic acids as part of the carboxyl group: carbonyl compounds. ∎  a coordination compound in which one or more carbon monoxide molecules are bonded as neutral ligands to a central metal atom: nickel carbonyl.

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ketone Any one of a group of organic compounds that contain the carbonyl group (>C=O) linked to two hydrocarbon groups. The ketone group is a carbonyl group with two single bonds to other carbon atoms (–CO–). Examples are acetone (propanone), CH3COCH3, and methyl ethyl ketone (butanone), CH3COC2H5. See also ketone body.