Alkyl Group

Alkenes

Alkyl radicals

Alkenes

Resources

An alkyl group is a paraffinic hydrocarbon group that may be derived from an alkane by dropping one hydrogen from the structure. Alkyl groups modify the reactivity of organic molecules, for example, by acting as electron-donating substituents. Such groups are often represented in chemical formulas by the letter R and have the generic name C_n H_2n+1.

Alkenes

Alkanes Aliphatic compounds, of which the alkanes are one example, have an open chain of carbon atoms as a skeleton. This open chain may be straight or branched. Alkanes, also known as paraffins, are composed of carbon and hydrogen only; they have the generic formula C_n H_2n+2, and are the simplest and least reactive of the aliphatic compounds. Alkanes with straight chains are known as normal alkanes. (Branched chain alkanes are treated as alkyl derivatives of the straight chain compounds.) The first four members of the normal alkane series are methane, ethane, propane, and butane (see below). The names of the remaining normal alkanes are composed of a prefix that indicates the number of carbon atoms in the compound, followed by the termination -ane. Thus, n-hexane is the name given to the normal alkane having a chain of six carbon atoms.

The n-alkanes exist on a continuum that extends from simple gases to molecules of very high molecular weights. The physical properties and uses of the n-alkane change with the number of repeating CH₂ units in the chain. For example, compounds with one to four CH₂ units in the chain are simple gases (e.g., cooking gas) at room temperature; compounds with five to 11 CH₂ units in the chain are simple liquids (e.g., gasoline) at room temperature; compounds with 16 to 25 CH₂ units in the chain are high viscosity liquids (e.g., oil) at room temperature; and compounds with 1, 000 to 3, 000 CH₂ units in the chain are tough plastic solids (e.g., polyethylene bottles) at room temperature.

Alkyl radicals

Alkanes from which one atom of hydrogen has been removed become monovalent radicals. These radicals, which are molecular fragments having unpaired electrons, are known as alkyl groups. The names of the alkyl groups are formed by substituting the suffix -yl for -ane in the names of the alkanes from which they are derived.

The methyl group (CH₃^-) is formed from methane, CH₄. The ethyl group, C₂ H₅^-, is formed from ethane, C₂ H₆. Two different alkyl groups can be formed from propane, CH₃ CH₂ CH₃. Removal of a hydrogen atom from one of the carbon atoms at the end of the chain forms C₃ H₇^-. This CH₃ CH₂ CH₂^- group is called a normal propyl group (n-propyl group). Removing a hydrogen from the second carbon produces an isopropyl group (i-propyl group).

The next member of the alkanes has the formula C₄ H₁₀. There are four isomers of this molecular formula. Removal of a hydrogen from one of the end carbons in n-butane, CH₃ CH₂ CH₂ CH₃, produces the n-butyl group (CH₃ CH₂ CH₂ CH₂^-). Removing a hydrogen atom from carbon 2 or 3 produces the secondary butyl group (sec-butyl group; CH₃ CH₂ CH(CH₃)^-). Removal of a hydrogen from carbon atoms 1, 3, or 4 forms the isobutyl group (i-butyl group; (CH₃)₂ CHCH₂^-). Finally, removing a hydrogen from carbon 2 gives the tertiary butyl group (t-butyl group; (CH₃)₃ C^-).

Frequently the terms primary, secondary, and tertiary are used to describe carbon atoms in a molecule. A primary carbon atom is one that is attached to only one other carbon atom; a secondary carbon atom is attached to two other carbon atoms; and a tertiary carbon atom is attached to three other carbon atoms in the molecule. Thus, the ethyl group defined above is a primary group, since the carbon atom that has lost the hydrogen is attached to only one other carbon atom in the molecule. The n-propyl group is also a primary group for the same reason. However, the i-propyl group is a secondary group, because the central carbon atom from which the hydrogen has been

KEY TERMS

Alkane— A subset of aliphatic hydrocarbons characterized by having a straight or branched chain, and having the generic formula C_n H_2n+2.

Alkyl— In chemistry an alkyl group is made from an alkane (a hydrocarbon without multiple bonds) through the removal or loss of a hydrogen atom.

removed is attached to two other carbon atoms. The n-butyl and i-butyl groups are both primary groups; the secondary butyl group is a secondary group; and the tertiary butyl group is tertiary, because the central atom is attached to the three other carbon atoms in the molecule. The methyl group, however, cannot be defined using this classification scheme.

In general, the letter R is used to designate any alkyl group (R = CH₃, C₂ H₅, etc.). With this convention, the alkanes are represented by the general formula R-H, and the alkyl halides by R-X, where X is a halogen.

Alkenes

The series of compounds derived from the alkanes by removing one hydrogen atom from each of two adjacent carbon atoms, thereby introducing a double bond into the molecule, bears the name olefin. The systematic names are formed by substituting the suffix -ene for -ane in the name of the alkane from which they are derived. Thus, the series as a whole is called the alkenes. Some common alkenes are methylene (CH₂^-), ethylene (CH₂ = CH₂), and propylene (CH₃ CH = CH₂).

Resources

BOOKS

Loudon, G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.

Sperling, L. H. Introduction to Physical Polymer Science. 3rd ed. New York: John Wiley and Sons, 2001.

Randall Frost

Alkyl group

An alkyl group is a paraffinic hydrocarbon group that may be derived from an alkane by dropping one hydrogen from the structure. Such groups are often represented in chemical formulas by the letter R and have the generic name CnH2n+1.

Alkanes

Aliphatic compounds, of which the alkanes are one example, have an open chain of carbon atoms as a skeleton. This open chain may be straight or branched. Alkanes, also known as paraffins, are composed of carbon and hydrogen only; they have the generic formula CnH2n+2, and are the simplest and least reactive of the aliphatic compounds. Alkanes with straight chains are known as normal alkanes. (Branched chain alkanes are treated as alkyl derivatives of the straight chain compounds.) The first four members of the normal alkane series are methane, ethane, propane, and butane (see below). The names of the remaining normal alkanes are composed of a prefix that indicates the number of carbon atoms in the compound, followed by the termination - ane. Thus, n-hexane is the name given to the normal alkane having a chain of six carbon atoms.

The n-alkanes exist on a continuum that extends from simple gases to molecules of very high molecular weights. The physical properties and uses of the n-alkane change with the number of repeating CH2 units in the chain. For example, compounds with one to four CH2 units in the chain are simple gases (e.g. cooking gas) at room temperature ; compounds with five to 11 CH2 units in the chain are simple liquids (e.g. gasoline) at room temperature; compounds with 16 to 25 CH2 units in the chain are high viscosity liquids (e.g. oil) at room temperature; and compounds with 1,000 to 3,000 CH2 units in the chain are tough plastic solids (e.g. polyethylene bottles) at room temperature.

Alkyl radicals

Alkanes from which one atom of hydrogen has been removed become monovalent radicals. These radicals, which are molecular fragments having an unpaired electron , are known as alkyl groups. The names of the alkyl groups are formed by substituting the suffix -yl for -ane in the names of the alkanes from which they are derived.

The methyl group (CH3 ) is formed from methane, CH4. The ethyl group , C2H5 , is formed from ethane, C2H6. Two different alkyl groups can be formed from propane, CH3CH2CH3. Removal of a hydrogen atom from one of the carbon atoms at the end of the chain forms C3H7 . This CH3CH2CH2 group is called a normal propyl group (n-propyl group). Removing a hydrogen from the second carbon produces an isopropyl group (i-propyl group).

The next member of the alkanes has the formula C4H10. There are four isomers of this molecular formula . Removal of a hydrogen from one of the end carbons in n-butane, CH3CH2CH2CH3, produces the n-butyl group (CH3CH2CH2CH2 ). Removing a hydrogen atom from carbon 2 or 3 produces the secondary butyl group (sec-butyl group; CH CH 3 2CH(CH3) ). Removal of a hydrogen from carbon atoms 1, 3, or 4 forms the isobutyl group (i-butyl group; (CH3)2CHCH2 ). Finally, removing a hydrogen from carbon 2 gives the tertiary butyl group (t-butyl group; (CH )3 3C ).

Frequently the terms primary, secondary, and tertiary are used to describe carbon atoms in a molecule . A primary carbon atom is one that is attached to only one other carbon atom; a secondary carbon atom is attached to two other carbon atoms; and a tertiary carbon atom is attached to three other carbon atoms in the molecule. Thus the ethyl group defined above is a primary group, since the carbon atom that has lost the hydrogen is attached to only one other carbon atom in the molecule. The n-propyl group is also a primary group for the same reason. But the i-propyl group is a secondary group, because the central carbon atom from which the hydrogen has been removed is attached to two other carbon atoms. The n-butyl and i-butyl groups are both primary groups; the secondary butyl group is a secondary group; and the tertiary butyl group is tertiary, because the central atom is attached to the three other carbon atoms in the molecule. The methyl group, however, cannot be defined using this classification scheme.

In general, the letter R is used to designate any alkyl group (R = CH3, C2H5, etc.). With this convention, the alkanes are represented by the general formula R-H, and the alkyl halides by R-X, where X is a halogen.

Alkenes

The series of compounds derived from the alkanes by removing one hydrogen atom from each of two adjacent carbon atoms, thereby introducing a double bond into the molecule, bears the name olefin. The systematic names are formed by substituting the suffix -ene for -ane in the name of the alkane from which they are derived. Thus the series as a whole is called the alkenes. Some common alkenes are methylene (-CH2 ), ethylene (CH2=CH2), and propylene (CH3CH=CH2).

Resources

books

Loudon, G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.

Sperling, L.H. Introduction to Physical Polymer Science. 3rd ed. New York: John Wiley and Sons, 2001.

Randall Frost

KEY TERMS

Alkane

—A subset of aliphatic hydrocarbons characterized by having a straight or branched chain, and having the generic formula CnH2n+2.

Alkyl

—A paraffinic hydrocarbon group that may be obtained from an alkane by removing a hydrogen atom from the latter.

Ketones

Ketones belong to a class of organic compounds known as carbonyls. They contain a carbon atom linked to an oxygen atom with a double bond (C=O). Acetone (dimethyl ketone) is a ketone commonly used in industrial applications. Other ketones include methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), methyl amyl ketone (MAK), isophorone, and diacetone alcohol.

As solvents, ketones have the ability to dissolve other materials or substances, particularly polymers and adhesives. They are ingredients in lacquers, epoxies, polyurethane, nail polish remover, degreasers, and cleaning solvents. Ketones are also used in industry for the manufacture of plastics and composites and in pharmaceutical and photographic film manufacturing. Because they have high evaporation rates and dry quickly, they are sometimes employed in drying applications.

Some types of ketones used in industry, such as methyl isobutyl ketone and methyl ethyl ketone, are considered both hazardous air pollutants (HAP) and volatile organic compounds (VOC) by the EPA. As such, the Clean Air Act regulates their use.

In addition to these industrial sources, ketones are released into the atmosphere in cigarette smoke and car and truck exhaust. More "natural" environmental sources such as forest fires and volcanoes also emit ketones. Acetone, in particular is readily produced in the atmosphere during the oxidation of organic pollutants or natural emissions. Ketones (in the form of acetone, beta-hydroxybutyric acid , and acetoacetic acid) also occur in the human body as a byproduct of the metabolism , or break down, of fat.

[Paula Anne Ford-Martin ]

RESOURCES

PERIODICALS

Wood, Andrew. "Cleaner Ketone Oxidation." Chemical Week (Aug 1, 2001).

OTHER

U.S. National Library of Medicine. Hazardous Substances Data Bank. [cited May 2002]. <http://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?HSDB>.

ORGANIZATIONS

American Chemical Society, 1155 Sixteenth St. NW, Washington, D.C. USA 20036 (202) 872-4600, Fax: (202) 872-4615, Toll Free: (800) 227-5558, Email: help@acs.org, <http://www.chemistry.org>

ketone (kee-tohn) n. any member of a group of organic compounds consisting of a carbonyl group (=CO) flanked by two alkyl groups. k. bodies (acetone bodies) the ketones acetoacetic acid, acetone, and β-hydroxybutyrate, produced during the metabolism of fats. See also ketosis.

ketones Chemical compounds containing a carbonyl group (C=O), with two alkyl groups attached to the same carbon; the simplest ketone is acetone (dimethylketone, (CH₃)₂−C=O).