Organic chemistry

Chemical reactions
As is true for all hydrocarbons, alkanes burn in air to produce carbon dioxide (CO2) and water (H2O) and release heat. The combustion of 2,2,4-trimethylpentane is expressed by the following chemical equation:

Hydrocarbon. Chemical equation for the combustion of 2,2,4-trimethylpantane.

The fact that all hydrocarbon combustions are exothermic is responsible for their widespread use as fuels. Grades of gasoline are rated by comparing their tendency toward preignition or knocking to reference blends of heptane and 2,2,4-trimethylpentane and assigning octane numbers. Pure heptane (assigned an octane number of 0) has poor ignition characteristics, whereas 2,2,4-trimethylpentane (assigned an octane number of 100) resists knocking even in high-compression engines.

As a class, alkanes are relatively unreactive substances and undergo only a few reactions. An industrial process known as isomerization employs an aluminum chloride (AlCl3) catalyst to convert unbranched alkanes to their branched-chain isomers. In one such application, butane is isomerized to 2-methylpropane for use as a starting material in the preparation of 2,2,4-trimethylpentane (isooctane), which is a component of high-octane gasoline.

Hydrocarbon. butan is isomerized to 1-methylpropane for use as a starting material in the preparation of 2,2,4-trimethylpentane (isooctane), which is a component of high-octane gasoline.

The halogens chlorine (Cl2) and bromine (Br2) react with alkanes and cycloalkanes by replacing one or more hydrogens with a halogen. Although the reactions are exothermic, a source of energy such as ultraviolet light or high temperature is required to initiate the reaction, as, for example, in the chlorination of cyclobutane.

Hydrocarbon. Formula for the chlorination of cyclobutane.

The chlorinated derivatives of methane (CH3Cl, CH2Cl2, CHCl3, and CCl4) are useful industrially and are prepared by various methods, including the reaction of methane with chlorine at temperatures on the order of 450 °C (840 °F).

The most important industrial organic chemical reaction in terms of its scale and economic impact is the dehydrogenation of ethane (obtained from natural gas) to form ethylene and hydrogen (see below Alkenes and alkynes: Natural occurrence and Synthesis). The hydrogen produced is employed in the Haber-Bosch process for the preparation of ammonia from nitrogen.

Hydrocarbon. The hydrogen produced is employed in the Haber-Bosch process for the preparation of ammonia from nitrogen.

The higher alkanes present in petroleum also yield ethylene under similar conditions by reactions that involve both dehydrogenation and the breaking of carbon-carbon bonds. The conversion of high-molecular-weight alkanes to lower ones is called cracking.

Alkenes and alkynes
Alkenes (also called olefins) and alkynes (also called acetylenes) belong to the class of unsaturated aliphatic hydrocarbons. Alkenes are hydrocarbons that contain a carbon-carbon double bond, whereas alkynes have a carbon-carbon triple bond. Alkenes are characterized by the general molecular formula CnH2n, alkynes by CnH2n − 2. Ethene (C2H4) is the simplest alkene and ethyne (C2H2) the simplest alkyne.

Hydrocarbon. Structural formulas for ethene (ethylene) C2H4 and ethyne (acetylene) C2H2.

Ethylene is a planar molecule with a carbon-carbon double bond length (1.34 angstroms) that is significantly shorter than the corresponding single bond length (1.53 angstroms) in ethane. Acetylene has a linear H―C≡C―H geometry, and its carbon-carbon bond distance (1.20 angstroms) is even shorter than that of ethylene.

Bonding in alkenes and alkynes
The generally accepted bonding model for alkenes views the double bond as being composed of a σ (sigma) component and a π (pi) component. In the case of ethylene, each carbon is sp2 hybridized, and each is bonded to two hydrogens and the other carbon by σ bonds. Additionally, each carbon has a half-filled p orbital, the axis of which is perpendicular to the plane of the σ bonds. Side-by-side overlap of these two p orbitals generates a π bond. The pair of electrons in the π bond are equally likely to be found in the regions of space immediately above and below the plane defined by the atoms. Most of the important reactions of alkenes involve the electrons in the π component of the double bond because these are the electrons that are farthest from the positively charged nuclei and therefore the most weakly held.

Hydrocarbon. double bond model for alkenes. (sigma) component and a (pi) component in ethylene.

The triple bond of an alkyne consists of one σ and two π components linking two sp hybridized carbons. In the case of acetylene, the molecule itself is linear with σ bonds between the two carbons and to each hydrogen. Each carbon has two p orbitals, the axes of which are perpendicular to each other. Overlap of two p orbitals, suitably aligned and on adjacent carbons, gives two π bonds.

Hydrocarbon. Triple bond of an alkyne consists of one (sigma) component and two (pi) components. Example: acetylene.

Nomenclature of alkenes and alkynes
Ethylene and acetylene are synonyms in the IUPAC nomenclature system for ethene and ethyne, respectively. Higher alkenes and alkynes are named by counting the number of carbons in the longest continuous chain that includes the double or triple bond and appending an -ene (alkene) or -yne (alkyne) suffix to the stem name of the unbranched alkane having that number of carbons. The chain is numbered in the direction that gives the lowest number to the first multiply bonded carbon, and adding it as a prefix to the name. Once the chain is numbered with respect to the multiple bond, substituents attached to the parent chain are listed in alphabetical order and their positions identified by number.

Hydrocarbon. Structural formulas for 1-pentene, 2-methyl-2-hexene, and 7-methyl-3-octyne.

Compounds that contain two double bonds are classified as dienes, those with three as trienes, and so forth. Dienes are named by replacing the -ane suffix of the corresponding alkane by -adiene and identifying the positions of the double bonds by numerical locants. Dienes are classified as cumulated, conjugated, or isolated according to whether the double bonds constitute a C=C=C unit, a C=C―C=C unit, or a C=C―(CXY)n―C=C unit, respectively.

Hydrocarbon. examples of diene compounds: 2,3-pentadiene (cumulated), 1,3-pentadiene (conjugated), and 1,4-pentadiene (isolated).

Double bonds can be incorporated into rings of all sizes, resulting in cycloalkenes. In naming substituted derivatives of cycloalkenes, numbering begins at and continues through the double bond.

Hydrocarbon. Structural formulas for cyclopropene, 1-methylcyclopentene, and 3-methylcyclopentene.

Unlike rotation about carbon-carbon single bonds, which is exceedingly rapid, rotation about carbon-carbon double bonds does not occur under normal circumstances. Stereoisomerism is therefore possible in those alkenes in which neither carbon atom bears two identical substituents. In most cases, the names of stereoisomeric alkenes are distinguished by cis-trans notation. (An alternative method, based on the Cahn-Ingold-Prelog system and using E and Z prefixes, is also used.) Cycloalkenes in which the ring has eight or more carbons are capable of existing as cis or trans stereoisomers. trans-Cycloalkenes are too unstable to isolate when the ring has seven or fewer carbons.

Hydrocarbon. Structural formulas for cis-2-butene, trans-2-butene, cis-cyclooctene, and trans-cyclooctene.

Because the C―C≡C―C unit of an alkyne is linear, cycloalkynes are possible only when the number of carbon atoms in the ring is large enough to confer the flexibility necessary to accommodate this geometry. Cyclooctyne (C8H12) is the smallest cycloalkyne capable of being isolated and stored as a stable compound.

Natural occurrence
Ethylene is formed in small amounts as a plant hormone. The biosynthesis of ethylene involves an enzyme-catalyzed decomposition of a novel amino acid, and, once formed, ethylene stimulates the ripening of fruits.

Hydrocarbon. The biosynthesis of ethylene involves an enzyme-catalyzed deomposition of a novel amino acid, and, once formed, ethylene stimulates the ripening of fruits.

Alkenes are abundant in the essential oils of trees and other plants. (Essential oils are responsible for the characteristic odour, or “essence,” of the plant from which they are obtained.) Myrcene and limonene, for example, are alkenes found in bayberry and lime oil, respectively. Oil of turpentine, obtained by distilling the exudate from pine trees, is a mixture of hydrocarbons rich in α-pinene. α-Pinene is used as a paint thinner as well as a starting material for the preparation of synthetic camphor, drugs, and other chemicals.

Hydrocarbon. Structural formulas for essential oils, myrcene, limonene, and (alpha)-pinene.

Other naturally occurring hydrocarbons with double bonds include plant pigments such as lycopene, which is responsible for the red colour of ripe tomatoes and watermelon. Lycopene is a polyene (meaning many double bonds) that belongs to a family of 40-carbon hydrocarbons known as carotenes.

Hydrocarbon. Structural formula for lycopene.

The sequence of alternating single and double bonds in lycopene is an example of a conjugated system. The degree of conjugation affects the light-absorption properties of unsaturated compounds. Simple alkenes absorb ultraviolet light and appear colourless. The wavelength of the light absorbed by unsaturated compounds becomes longer as the number of double bonds in conjugation with one another increases, with the result that polyenes containing regions of extended conjugation absorb visible light and appear yellow to red.

The hydrocarbon fraction of natural rubber (roughly 98 percent) is made up of a collection of polymer molecules, each of which contains approximately 20,000 C5H8 structural units joined together in a regular repeating pattern.

Hydrocarbon. The hydrocarbon fraction of natural rubber is made up of a collection of polymer molecules, each of which contains approximately 20,000 C5H8 structural units joined together in a regular repeating pattern.

Natural products that contain carbon-carbon triple bonds, while numerous in plants and fungi, are far less abundant than those that contain double bonds and are much less frequently encountered.

Synthesis
The lower alkenes (through four-carbon alkenes) are produced commercially by cracking and dehydrogenation of the hydrocarbons present in natural gas and petroleum (see above Alkanes: Chemical reactions). The annual global production of ethylene averages around 75 million metric tons. Analogous processes yield approximately 2 million metric tons per year of 1,3-butadiene (CH2=CHCH=CH2). Approximately one-half of the ethylene is used to prepare polyethylene. Most of the remainder is utilized to make ethylene oxide (for the manufacture of ethylene glycol antifreeze and other products), vinyl chloride (for polymerization to polyvinyl chloride), and styrene (for polymerization to polystyrene). The principal application of propylene is in the preparation of polypropylene. 1,3-Butadiene is a starting material in the manufacture of synthetic rubber (see below Polymerization).

Higher alkenes and cycloalkenes are normally prepared by reactions in which a double bond is introduced into a saturated precursor by elimination (i.e., a reaction in which atoms or ions are lost from a molecule).

Hydrocarbon. Higher alkenes and cycloalkenes are normally prepared by reactions in which a double bond is introduced into a saturated precursor by elimination.

Examples include the dehydration of alcohols
Hydrocarbon. Dehydration of alcohol (2-methyl-2-butanol to 2-methyl-2-butane + water by the use of sulfuric acid + heat
and the dehydrohalogenation (loss of a hydrogen atom and a halogen atom) of alkyl halides.

Hydrocarbon. Dehydrohalogenation (loss of a hydrogen atom and a halogen atom) of alkyl halides. Bromocyclohexane + sodium hydroxide yields cyclohexene + sodium bromide + water.

These usually are laboratory rather than commercial methods. Alkenes also can be prepared by partial hydrogenation of alkynes (see below Chemical properties).
Acetylene is prepared industrially by cracking and dehydrogenation of hydrocarbons as described for ethylene (see above Alkanes: Chemical reactions). Temperatures of about 800 °C (1,500 °F) produce ethylene; temperatures of roughly 1,150 °C (2,100 °F) yield acetylene. Acetylene, relative to ethylene, is an unimportant industrial chemical. Most of the compounds capable of being derived from acetylene are prepared more economically from ethylene, which is a less expensive starting material. Higher alkynes can be made from acetylene (see below Chemical properties) or by double elimination of a dihaloalkane (i.e., removal of both halogen atoms from a disubstituted alkane).
Hydrocarbon. Formula for the reaction: 1,2-dibromobutane + sodium amide yields 1-butyne + sodium bromide + ammonia.

Stereoisomerism
Certain substituted derivatives of cycloalkanes exhibit a type of isomerism called stereoisomerism in which two substances have the same molecular formula and the same constitution but differ in the arrangement of their atoms in space. Methyl groups in 1,2-dimethylcyclopropane, for example, may be on the same (cis) or opposite (trans) sides of the plane defined by the ring. The resulting two substances are different compounds, each having its own properties such as boiling point (abbreviated bp here):

Hydrocarbon, Isomerism. Stereoisomerism: comparing compounds cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane

oil refinery
READ MORE ON THIS TOPIC
petroleum refining: Hydrocarbon chemistry
Petroleum crude oils are complex mixtures of hydrocarbons, chemical compounds composed only of carbon (C) and hydrogen...
Cis-trans isomers belong to a class of stereoisomers known as diastereomers and are often referred to as geometric isomers, although this is an obsolete term. Cis-trans stereoisomers normally cannot be interconverted at room temperature, because to do so requires the breaking and reforming of chemical bonds.

Physical properties
Alkanes and cycloalkanes are nonpolar substances. Attractive forces between alkane molecules are dictated by London forces (or dispersion forces, arising from electron fluctuations in molecules; see chemical bonding: Intermolecular forces) and are weak. Thus, alkanes have relatively low boiling points compared with polar molecules of comparable molecular weight. The boiling points of alkanes increase with increasing number of carbons. This is because the intermolecular attractive forces, although individually weak, become cumulatively more significant as the number of atoms and electrons in the molecule increases.

Physical properties of unbranched alkanes
name formula boiling point (°C) melting point (°C)
methane CH4 −164 −182.5
ethane CH3CH3 −88.6 −183.3
propane CH3CH2CH3 −42 −189.7
butane CH3(CH2)2CH3 −0.5 −138.35
pentane CH3(CH2)3CH3 +36.1 −129.7
hexane CH3(CH2)4CH3 +68.9 −95.0
heptane CH3(CH2)5CH3 +98.4 −90.6
octane CH3(CH2)6CH3 +125.6 −56.8
nonane CH3(CH2)7CH3 +150.8 −51.0
decane CH3(CH2)8CH3 +174.1 −29.7
pentadecane CH3(CH2)13CH3 +270 +10
octadecane CH3(CH2)16CH3 +316.1 +28.2
icosane CH3(CH2)18CH3 +343 +36.8
triacontane CH3(CH2)28CH3 +449.7 +65.8
tetracontane CH3(CH2)38CH3 — +81
pentacontane CH3(CH2)48CH3 — +92
For a given number of carbon atoms, an unbranched alkane has a higher boiling point than any of its branched-chain isomers. This effect is evident upon comparing the boiling points (bp) of selected C8H18 isomers. An unbranched alkane has a more extended shape, thereby increasing the number of intermolecular attractive forces that must be broken in order to go from the liquid state to the gaseous state. On the other hand, the relatively compact ellipsoidal shape of 2,2,3,3-tetramethylbutane permits it to pack into a crystal lattice more effectively than octane and so raises its melting point (mp).

Hydrocarbon. Structural formula for unbranced alkanes, octane, 2-methylheptane, 2,2,3,3-tetramethylbutane.

In general, solid alkanes do not often have high melting points. Unbranched alkanes tend toward a maximum in that the melting point of CH3(CH2)98CH3 (115 °C [239 °F]) is not much different from that of CH3(CH2)148CH3 (123 °C [253 °F]).

The viscosity of liquid alkanes increases with the number of carbons. Increased intermolecular attractive forces, as well as an increase in the extent to which nearby molecules become entangled when they have an extended shape, cause unbranched alkanes to be more viscous than their branched-chain isomers.

The densities of liquid hydrocarbons are all less than that of water, which is quite polar and possesses strong intermolecular attractive forces. All hydrocarbons are insoluble in water and, being less dense than water, float on its surface. Hydrocarbons are, however, usually soluble in one another as well as in organic solvents such as diethyl ether (CH3CH2OCH2CH3).

Sources and occurrence
The most abundant sources of alkanes are natural gas and petroleum deposits, formed over a period of millions of years by the decay of organic matter in the absence of oxygen. Natural gas contains 60–80 percent methane, 5–9 percent ethane, 3–18 percent propane, and 2–14 percent higher hydrocarbons. Petroleum is a complex liquid mixture of hundreds of substances—including 150 or more hydrocarbons, approximately half of which are saturated.

Approximately two billion tons of methane are produced annually by the bacteria that live in termites and in the digestive systems of plant-eating animals. Smaller quantities of alkanes also can be found in a variety of natural materials. The so-called aggregation pheromone whereby Blaberus craniifer cockroaches attract others of the same species is a 1:1 mixture of the volatile but relatively high-boiling liquid alkanes undecane, CH3(CH2)9CH3, and tetradecane, CH3(CH2)12CH3. Hentriacontane, CH3(CH2)29CH3, is a solid alkane present to the extent of 8–9 percent in beeswax, where its stability and impermeability to water contribute to the role it plays as a structural component.

With the exception of the alkanes that are readily available from petroleum, alkanes are synthesized in the laboratory and in industry by the hydrogenation of alkenes. Only a few methods are available in which a carbon-carbon bond-forming operation gives an alkane directly, and these tend to be suitable only for syntheses carried out on a small scale.

Aliphatic hydrocarbons
Alkanes
Alkanes, hydrocarbons in which all the bonds are single, have molecular formulas that satisfy the general expression CnH2n + 2 (where n is an integer). Carbon is sp3 hybridized (three electron pairs are involved in bonding, forming a tetrahedral complex), and each C—C and C—H bond is a sigma (σ) bond (see chemical bonding). In order of increasing number of carbon atoms, methane (CH4), ethane (C2H6), and propane (C3H8) are the first three members of the series.

Hydrocarbon; Isomerism. Structural formulas for methane (CH4), ethane (C2H6) and propane (C3H8).

Methane, ethane, and propane are the only alkanes uniquely defined by their molecular formula. For C4H10 two different alkanes satisfy the rules of chemical bonding (namely, that carbon has four bonds and hydrogen has one in neutral molecules). One compound, called n-butane, where the prefix n- represents normal, has its four carbon atoms bonded in a continuous chain. The other, called isobutane, has a branched chain.

Hydrocarbon, Isomerism. Structural formulas for n-butane (CH3CH2CH2CH3) and isobutane (CH3)3CH

Different compounds that have the same molecular formula are called isomers. Isomers that differ in the order in which the atoms are connected are said to have different constitutions and are referred to as constitutional isomers. (An older name is structural isomers.) The compounds n-butane and isobutane are constitutional isomers and are the only ones possible for the formula C4H10. Because isomers are different compounds, they can have different physical and chemical properties. For example, n-butane has a higher boiling point (−0.5 °C [31.1 °F]) than isobutane (−11.7 °C [10.9 °F]).

There is no simple arithmetic relationship between the number of carbon atoms in a formula and the number of isomers. Graph theory has been used to calculate the number of constitutionally isomeric alkanes possible for values of n in CnH2n + 2 from 1 through 400. The number of constitutional isomers increases sharply as the number of carbon atoms increases. There is probably no upper limit to the number of carbon atoms possible in hydrocarbons. The alkane CH3(CH2)388CH3, in which 390 carbon atoms are bonded in a continuous chain, has been synthesized as an example of a so-called superlong alkane. Several thousand carbon atoms are joined together in molecules of hydrocarbon polymers such as polyethylene, polypropylene, and polystyrene.

Number of possible alkane isomers
molecular formula number of constitutional isomers
C3H8 1
C4H10 2
C5H12 3
C6H14 5
C7H16 9
C8H18 18
C9H20 35
C10H22 75
C15H32 4,347
C20H42 366,319
C30H62 4,111,846,763
Nomenclature
The need to give each compound a unique name requires a richer variety of terms than is available with descriptive prefixes such as n- and iso-. The naming of organic compounds is facilitated through the use of formal systems of nomenclature. Nomenclature in organic chemistry is of two types: common and systematic. Common names originate in many different ways but share the feature that there is no necessary connection between name and structure. The name that corresponds to a specific structure must simply be memorized, much like learning the name of a person. Systematic names, on the other hand, are keyed directly to molecular structure according to a generally agreed upon set of rules. The most widely used standards for organic nomenclature evolved from suggestions made by a group of chemists assembled for that purpose in Geneva in 1892 and have been revised on a regular basis by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC rules govern all classes of organic compounds but are ultimately based on alkane names. Compounds in other families are viewed as derived from alkanes by appending functional groups to, or otherwise modifying, the carbon skeleton.

The IUPAC rules assign names to unbranched alkanes according to the number of their carbon atoms. Methane, ethane, and propane are retained for CH4, CH3CH3, and CH3CH2CH3, respectively. The n- prefix is not used for unbranched alkanes in systematic IUPAC nomenclature; therefore, CH3CH2CH2CH3 is defined as butane, not n-butane. Beginning with five-carbon chains, the names of unbranched alkanes consist of a Latin or Greek stem corresponding to the number of carbons in the chain followed by the suffix -ane. A group of compounds such as the unbranched alkanes that differ from one another by successive introduction of CH2 groups constitute a homologous series.

IUPAC names of unbranched alkanes
alkane formula name alkane formula name
CH4 methane CH3(CH2)6CH3 octane
CH3CH3 ethane CH3(CH2)7CH3 nonane
CH3CH2CH3 propane CH3(CH2)8CH3 decane
CH3CH2CH2CH3 butane CH3(CH2)13CH3 pentadecane
CH3(CH2)3CH3 pentane CH3(CH2)18CH3 icosane
CH3(CH2)4CH3 hexane CH3(CH2)28CH3 triacontane
CH3(CH2)5CH3 heptane CH3(CH2)98CH3 hectane
Alkanes with branched chains are named on the basis of the name of the longest chain of carbon atoms in the molecule, called the parent. The alkane shown has seven carbons in its longest chain and is therefore named as a derivative of heptane, the unbranched alkane that contains seven carbon atoms. The position of the CH3 (methyl) substituent on the seven-carbon chain is specified by a number (3-), called a locant, obtained by successively numbering the carbons in the parent chain starting at the end nearer the branch. The compound is therefore called 3-methylheptane.

Hydrocarbon. formula for the compound 3-methylheptane.

When there are two or more identical substituents, replicating prefixes (di-, tri-, tetra-, etc.) are used, along with a separate locant for each substituent. Different substituents, such as ethyl (―CH2CH3) and methyl (―CH3) groups, are cited in alphabetical order. Replicating prefixes are ignored when alphabetizing. In alkanes, numbering begins at the end nearest the substituent that appears first on the chain so that the carbon to which it is attached has as low a number as possible.

Hydrocarbon. Formula for the compound 4-ethyl-2,4-dimethyloctane.

Methyl and ethyl are examples of alkyl groups. An alkyl group is derived from an alkane by deleting one of its hydrogens, thereby leaving a potential point of attachment. Methyl is the only alkyl group derivable from methane and ethyl the only one from ethane. There are two C3H7 and four C4H9 alkyl groups. The IUPAC rules for naming alkanes and alkyl groups cover even very complex structures and are regularly updated. They are unambiguous in the sense that, although a single compound may have more than one correct IUPAC name, there is no possibility that two different compounds will have the same name.

Three-dimensional structures
Most organic molecules, including all alkanes, are not planar but are instead characterized by three-dimensional structures. Methane, for example, has the shape of a regular tetrahedron with carbon at the centre and a hydrogen atom at each corner. Each H―C―H angle in methane is 109.5°, and each C―H bond distance is 1.09 angstroms (Å; 1Å = 1 × 10−10 metre). Higher alkanes such as butane have bonds that are tetrahedrally disposed on each carbon except that the resulting C―C―C and H―C―H angles are slightly larger and smaller, respectively, than the ideal value of 109.5° characteristic of a perfectly symmetrical tetrahedron. Carbon-carbon bond distances in alkanes are normally close to 1.53 angstroms.

chemical structure of methane
chemical structure of methane
Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)
Encyclopædia Britannica, Inc.
An important aspect of the three-dimensional shape of alkanes and other organic molecules is their conformations, the nonidentical arrangements of atoms that are generated by rotation about single bonds. Of the infinite number of conformations possible for ethane—which are related by tiny increments of rotation of one CH3 group with respect to the other—the eclipsed conformation is the least stable, and the staggered conformation is the most stable. The eclipsed conformation is said to suffer torsional strain because of repulsive forces between electron pairs in the C―H bonds of adjacent carbons. These repulsive forces are minimized in the staggered conformation since all C―H bonds are as far from one another as possible. Although rotation about the C―C bond of ethane is exceedingly rapid (millions of times per second at room temperature), at any instant most of the molecules exist in the staggered conformation.

eclipsed conformation of ethane
eclipsed conformation of ethane
Eclipsed conformation is the least stable of all ethane conformations because the repulsive forces between electron pairs in the C―H bonds of adjacent carbons are maximized.
Encyclopædia Britannica, Inc.
staggered conformation of ethane
staggered conformation of ethane
Staggered conformation is the most stable of all ethane conformations because the repulsive forces between electron pairs in the C―H bonds of adjacent carbons are minimized.
Encyclopædia Britannica, Inc.
For butane, two different staggered conformations, called anti and gauche, are possible. Methyl is a larger substituent than hydrogen, and the greater separation between methyl groups in the anti conformation makes it slightly more stable than the gauche.

Hydrocarbon. The two different staggered conformations (anti and gauche) for butane.

The three-dimensional structures of higher alkanes are governed by the tetrahedral disposition of the four bonds to each carbon atom, by the preference for staggered conformations, and by the greater stability of anti C―C―C―C arrangements over gauche.

Cycloalkanes
Countless organic compounds are known in which a sequence of carbon atoms, rather than being connected in a chain, closes to form a ring. Saturated hydrocarbons that contain one ring are referred to as cycloalkanes. With a general formula of CnH2n (n is an integer greater than 2), they have two fewer hydrogen atoms than an alkane with the same number of carbon atoms. Cyclopropane (C3H6) is the smallest cycloalkane, whereas cyclohexane (C6H12) is the most studied, best understood, and most important. It is customary to represent cycloalkane rings as polygons, with the understanding that each corner corresponds to a carbon atom to which is attached the requisite number of hydrogen atoms to bring its total number of bonds to four.

Hydrocarbon, Isomerism. Structural formulas showing cycloalkane rings as polygons (each corner corresponds to a carbon atom). Cyclopropane and Cyclohexane.

In naming cycloalkanes, alkyl groups attached to the ring are indicated explicitly and listed in alphabetical order, and the ring is numbered so as to give the lowest locant to the first-appearing substituent. If two different directions yield equivalent locants, the direction is chosen that gives the lower number to the substituent appearing first in the name.

Hydrocarbon. Structural formulas for 1,1,3-trimethylcyclopentane and 1-ethyl-3-methylcycloheptane.

The three carbon atoms of cyclopropane define the corners of an equilateral triangle, a geometry that requires the C―C―C angles to be 60°. This 60° angle is much smaller than the normal tetrahedral bond angle of 109.5° and imposes considerable strain (called angle strain) on cyclopropane. Cyclopropane is further destabilized by the torsional strain that results from having three eclipsed C―H bonds above the plane of the ring and three below.

Cyclopropane is the only cycloalkane that is planar. Cyclobutane (C4H8) and higher cycloalkanes adopt nonplanar conformations in order to minimize the eclipsing of bonds on adjacent atoms. The angle strain in cyclobutane is less than in cyclopropane, whereas cyclopentane and higher cycloalkanes are virtually free of angle strain. With the exception of cyclopropane, all cycloalkanes undergo rapid internal motion involving interconversion of nonplanar “puckered” conformations.

Hydrocarbon. Structural formulas for cyclopropane, cyclobutane, and cyclopentane.

Many of the most important principles of conformational analysis have been developed by examining cyclohexane. Three conformations of cyclohexane, designated as chair, boat, and skew (or twist), are essentially free of angle strain. Of these three the chair is the most stable, mainly because it has a staggered arrangement of all its bonds. The boat and skew conformations lack perfect staggering of bonds and are destabilized by torsional strain. The boat conformation is further destabilized by the mutual crowding of hydrogen atoms at carbons one and four. The shape of the boat brings its two “flagpole” hydrogen atoms to within 1.80 angstroms of each other, far closer than the 2.20-angstrom distance at which repulsive forces between hydrogen atoms become significant. At room temperature, 999 of every 1,000 cyclohexane molecules exist in the chair form (the other being skew).

Hydrocarbon, Isomerism. Three conformations of cyclohexane, designated as chair, boat, and skew (or twist).

There are two orientations of carbon-hydrogen bonds in the chair conformation of cyclohexane. Six bonds are parallel to a vertical axis passing through the centre of the ring and are called axial (a) bonds. The directions of these six axial bonds alternate up and down from one carbon to the next around the ring; thus, the axial hydrogens at carbons one, three, and five lie on one side of the ring and those at carbons two, four, and six on the other. The remaining six bonds are referred to as equatorial (e) because they lie in a region corresponding to the approximate “equator” of the molecule. The shortest distances between nonbonded atoms are those involving axial hydrogens on the same side of the molecule.

A rapid process of chair-chair interconversion (called ring-flipping) interconverts the six axial and six equatorial hydrogen atoms in cyclohexane. Chair-chair interconversion is a complicated process brought about by successive conformational changes within the molecule. It is different from simple whole-molecule motions, such as spinning and tumbling, and because it is a conformational change only, it does not require any bonds to be broken.

Hydrocarbon, Isomerism. Chair-chair interconversion (called ring-flipping) interconverts the six axial and six equatorial hydrogen atoms in cyclohexane.

Chair-chair interconversion is especially important in substituted derivatives of cyclohexane. Any substituent is more stable when it occupies an equatorial rather than an axial site on the ring, since equatorial substituents are less crowded than axial ones. In methylcyclohexane, the chair conformation in which the large methyl group is equatorial is the most stable and, therefore, the most populated of all possible conformations. At any instant, almost all the methylcyclohexane molecules in a given sample exist in chair conformations, and about 95 percent of these have the methyl group in an equatorial orientation.

Hydrocarbon, Isomerism: chair-chair interconversion in methylcyclohexane.

The highly branched tert-butyl group (CH3)3C― (tert-butyl) is even more spatially demanding than the methyl group, and more than 99.99 percent of tert-butylcyclohexane molecules adopt chair conformations in which the (CH3)3C― group is equatorial.

Conformational analysis of six-membered rings, especially the greater stability of chair conformations with equatorial substituents, not only is important in the area of hydrocarbons but also is essential to an understanding of the properties of biologically important molecules, especially steroids and carbohydrates. Odd Hassel of Norway and Derek H.R. Barton of England shared the Nobel Prize for Chemistry in 1969 for their important discoveries in this area. Hassel’s studies dealt with structure, while Barton showed how conformational effects influence chemical reactivity.

The most stable structures of cycloalkanes and compounds based on them have been determined by a number of experimental techniques, including X-ray diffraction and electron diffraction analyses and infrared, nuclear magnetic resonance, and microwave spectroscopies. These experimental techniques have been joined by advances in computational methods such as molecular mechanics, whereby the total strain energies of various conformations are calculated and compared (see also chemical bonding: Computational approaches to molecular structure). The structure with the lowest total energy is the most stable and corresponds to the best combination of bond distances, bond angles, and conformation. One benefit of such calculations is that unstable conformations, which are difficult to study experimentally, can be examined. The quality of molecular mechanics calculations is such that it is claimed that many structural features of hydrocarbons can be computed more accurately than they can be measured.

The conformations of rings with 7–12 carbons have been special targets for study by molecular mechanics. Unlike cyclohexane, in which one conformation (the chair) is much more stable than any other, cycloalkanes with 7–12 carbons are generally populated by several conformations of similar energy. Rings with more than 12 carbons are sufficiently flexible to adopt conformations that are essentially strain-free.

Polycyclic hydrocarbons are hydrocarbons that contain more than one ring. They are classified as bicyclic, tricyclic, tetracyclic, and so forth, according to the number of formal bond disconnections necessary to produce a noncyclic carbon chain. Examples include trans-decalin and adamantane—both of which are present in small amounts in petroleum—and cubane, a compound synthesized for the purpose of studying the effects of strain on chemical reactivity.