Most reactions of organic compounds take place at or adjacent to a functional group. In order to establish a baseline of behavior against which these reactions may be ranked, we need to investigate the reactivity of compounds lacking any functional groups. Such compounds are necessarily hydrocarbons, made up of chains and rings of carbon atoms bonded to a full complement of hydrogen atoms (all carbons are sp³ hybridized). From the previous discussion of formula analysis, the formulas for such hydrocarbons will be C_nH_(2n+2–2r) , where n is the number of carbon atoms and r is the number of rings. Hydrocarbons of this kind are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings. Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. Alkanes and cycloalkanes are termed saturated, because they incorporate the maximum number of hydrogens possible without breaking any carbon-carbon bonds. They are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any unsaturated aromatic rings in their molecular structure.

The increasingly large number of organic compounds identified with each passing day, together with the fact that many of these compounds are isomers of other compounds, requires that a systematic nomenclature system be developed. Just as each distinct compound has a unique molecular structure which can be designated by a structural formula, each compound must be given a characteristic and unique name.

      1. Common Names
As organic chemistry grew and developed from its early beginnings, many compounds were given trivial names at the time of their discovery. These names have remained in common use, and are widely recognized. The relationship of these names to each other is usually arbitrary, and no rational or systematic principles underly their assignments. Some examples are:

Common names such as these often have their origin in the history of the science and the natural sources of specific compounds. The relationship of these names to each other is arbitrary, and no rational or systematic principles underly their assignments. Consequently, common names can only be remembered by repeated use, in much the same way we use nicknames.

A rational nomenclature system should do at least two things:

First, it should indicate how the carbon atoms of a given compound are bonded together in a characteristic lattice of chains and rings. Second, it should identify and locate any functional groups present in the compound. Since hydrogen is such a common component of organic compounds, its quantity and location can be assumed from the tetravalency of carbon, and need not be specified in most cases.

      2. The IUPAC Systematic Approach to Nomenclature
The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features:

As an introduction to the IUPAC nomenclature system, we shall first consider the alkanes and cycloalkanes, since these compounds provide the foundation on which the nomenclature of functional groups is built.
A full presentation of the IUPAC Rules has been provided by Advanced Chemistry Development. To use this site Click Here.

The following table lists the IUPAC names assigned to simple continuous-chain alkanes from C-1 to C-10. A common "ane" suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names. Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms.

Some important behavior trends and terminologies:

i.   The formulas and structures of these alkanes increase uniformally by a CH2 increment. ii.   A uniform variation of this kind in a series of compounds is called homologous. iii.   These formulas all fit the CnH2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon. iv.   Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen).

Beginning with butane (C₄H₁₀), and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C₆H₁₄ isomers, shown below as abbreviated line formulas (A through E):

Although these distinct compounds all have the same molecular formula, only one (A) can be called hexane. How then are we to name the others?
The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the "ane" suffix is replaced by "yl" in naming groups. The symbol R is used to designate a generic (unspecified) alkyl group.

IUPAC Rules for Alkane Nomenclature  1.   Find and name the longest continuous carbon chain.  2.   Identify and name groups attached to this chain.  3.   Number the chain consecutively, starting at the end nearest a substituent group.  4.   Designate the location of each substituent group by an appropriate number and name.  5.   Assemble the name, listing groups in alphabetical order.         The prefixes di, tri, tetra etc., used to designate several groups of the same kind,         are not considered when alphabetizing.

For the above isomers of hexane, A, the IUPAC names are:  B  2-methylpentane    C  3-methylpentane    D  2,2-dimethylbutane    E  2,3-dimethylbutane

Halogen substituents are easily accomodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-). For example, (CH₃)₂CHCH₂CH₂Br would be named 1-bromo-3-methylbutane. If the halogen is bonded to a simple alkyl group an alternative "alkyl halide" name may be used. Thus, C₂H₅Cl may be named chloroethane (no locator number is needed for a two carbon chain) or ethyl chloride. For additional examples of how these rules are used in naming branched alkanes, and for some sub-rules of nomenclature   Click Here.

Cycloalkanes have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The IUPAC names of the first five members of this series are given in the following table. The last (yellow shaded) column gives the general formula for a cycloalkane of any size. If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is C_nH_2n.

Examples of Simple Cycloalkanes

Name	Cyclopropane	Cyclobutane	Cyclopentane	Cyclohexane	Cycloheptane	Cycloalkane
Molecular Formula	C3H6	C4H8	C5H10	C6H12	C7H14	CnH2n
Structural Formula						(CH2)n
Line Formula

Substituted cycloalkanes are named in a fashion very similar to that used for naming branched alkanes. The chief difference in the rules and procedures occurs in the numbering system. Since all the carbons of a ring are equivalent (a ring has no ends like a chain does), the numbering starts at a substituted ring atom.

For examples of how these rules are used in naming substituted cycloalkanes   Click Here

Small rings, such as three and four membered rings, have significant angle strain resulting from the distortion of the sp³ carbon bond angles from the ideal 109.5º to 60º and 90º respectively. This angle strain often enhances the chemical reactivity of such compounds, leading to ring cleavage products. It is also important to recognize that, with the exception of cyclopropane, cycloalkyl rings are not planar (flat). The three dimensional shapes assumed by the common rings (especially cyclohexane and larger rings) are described and discussed in the Conformational Analysis Section.

Hydrocarbons having more than one ring are common, and are referred to as bicyclic (two rings), tricyclic (three rings) and in general, polycyclic compounds. The molecular formulas of such compounds have H/C ratios that decrease with the number of rings. In general, for a hydrocarbon composed of n carbon atoms associated with r rings the formula is: C_nH_{(2n + 2 - 2r)}. The structural relationship of rings in a polycyclic compound can vary. They may be separate and independent, or they may share one or two common atoms. Some examples of these possible arrangements are shown in the following table.

Examples of Isomeric C₈H₁₄ Bicycloalkanes

Isolated Rings	Spiro Rings	Fused Rings	Bridged Rings
No common atoms	One common atom	One common bond	Two common atoms

Hydrocarbons incorporating double or triple carbon-carbon bonds are called unsaturated because hydrogen can be added to the multiple bond, converting the compound to an alkane or cycloalkane. Such compounds are called alkenes and alkynes respectively. Because they are isomeric with cycloalkanes or bicycloalkanes, their names must clearly convey the presence of functional unsaturation. This is done by changing the ane suffix in the name of an alkane to ene for a double bond, or yne for a triple bond, as illustrated by the following examples. The location of a multiple bond in a chain is designated by a number, just as is done for substituents on a chain. A complete treatment of alkene and alkyne nomenclature is presented below.

Alkenes and alkynes are hydrocarbons which respectively have carbon-carbon double bond and carbon-carbon triple bond functional groups. The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:

As noted earlier in the Analysis of Molecular Formulas section, the molecular formula of a hydrocarbon provides information about the possible structural types it may represent. For example, consider compounds having the formula C₅H₈. The formula of the five-carbon alkane pentane is C₅H₁₂ so the difference in hydrogen content is 4. This difference suggests such compounds may have a triple bond, two double bonds, a ring plus a double bond, or two rings. Some examples were shown above, and there are at least fourteen others!

      Alkene and Alkyne Nomenclature

For examples of how these rules are used in naming alkenes, alkynes and cyclic analogs   Click Here.

The alkanes and cycloalkanes, with the exception of cyclopropane, are probably the least chemically reactive class of organic compounds. Despite their relative inertness, alkanes undergo several important reactions that are discussed in the following section.

      1. Combustion
The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history. The practical importance of this reaction cannot be denied, but the massive and uncontrolled chemical changes that take place in combustion make it difficult to deduce mechanistic paths. Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. No other common reaction involves such a profound and pervasive change, and the mechanism of combustion is so complex that chemists are just beginning to explore and understand some of its elementary features.

Two points concerning this reaction are important:

1. Since all the covalent bonds in the reactant molecules are broken, the quantity of heat evolved in this reaction is related to the strength of these bonds (and, of course, the strength of the bonds formed in the products). Precise heats of combustion measurements can therefore provide useful iinformation about the structure of molecules. 2. The stoichiometry of the reactants is important. If insufficient oxygen is supplied some of the products will consist of carbon monoxide, a highly toxic gas.

As noted in the reaction energetics section isomers may have different potential energies, reflecting the bond energies and strain in each. Since isomeric hydrocarbons must give the same mixture of CO₂ and H₂O on complete combustion, differences in their potential energy will be revealed by their heats of combustion. Thus, the heat of combustion of pentane is –782 kcal/mole, but that of its 2,2-dimethylpropane (neopentane) isomer is –777 kcal/mole. Differences such as this reflect subtle structural variations, including the greater bond energy of 1º-C–H versus 2º-C–H bonds and steric crowding of neighboring groups.
The diagram on the left below demonstrates how the heat of combustion of isomeric hydrocarbons (in this case C₆H₁₂ compounds) provides information about their thermodynamic stability. Cyclohexane is clearly the most stable (lower potential energy) of the four isomers depicted. In small-ring cyclic compounds ring strain can be a major contributor to thermodynamic instability and chemical reactivity. The table on the right lists heat of combustion data for some simple cycloalkanes, and compares the heat derived per CH₂ unit with that of a long chain alkane. The strain induced by the structural constraint of a small ring is evident.
Chemists recognize several kinds of internal molecular strain. Among these, angle strain and eclipsing strain (bond orientation) are severe in small rings such as cyclopropane and cyclobutane. A third strain, steric hindrance (crowding) is common in branched or substituted chains and rings. These strain factors will be discussed in the next chapter.

Cycloalkane (CH2)n CH2 Units n ΔH25º kcal/mole ΔH25º per CH2 Unit Ring Strain kcal/mole Cyclopropane n = 3 468.7 156.2 27.6 Cyclobutane n = 4 614.3 153.6 26.4 Cyclopentane n = 5 741.5 148.3 6.5 Cyclohexane n = 6 882.1 147.0 0.0 Cycloheptane n = 7 1035.4 147.9 6.3 Cyclooctane n = 8 1186.0 148.2 9.6 Cyclononane n = 9 1335.0 148.3 11.7 Cyclodecane n = 10 1481 148.1 11.0 CH3(CH2)mCH3 m = large — 147.0 0.0

Changes in chemical reactivity as a consequence of angle strain are dramatic in the case of cyclopropane, and are also evident for cyclobutane. Some examples are shown in the following diagram. The cyclopropane reactions are additions, many of which are initiated by electrophilic attack. The pyrolytic conversion of β-pinene to myrcene probably takes place by an initial rupture of the 1:6 bond, giving an allylic 3º-diradical, followed immediately by breaking of the 5:7 bond.

Reactions due to Ring Strain

Unstrained hydrocarbons require much higher temperatures to effect this kind of bond cleavage, which generally leads to mixtures of lower molecular weight alkanes and alkenes. Such structural degadation or cracking is used by the petrochemical industry to convert the abundant high-molecular weight components of petroleum into the simple hydrocarbons needed as feedstocks and fuels.

      2. Halogenation
Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, shown below, provides a simple example of this reaction.

Since only two covalent bonds are broken (C-H & Cl-Cl) and two covalent bonds are formed (C-Cl & H-Cl), this reaction seems to be an ideal case for mechanistic investigation and speculation. However, one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products, as shown in the following unbalanced equation. The relative amounts of the various products depend on the proportion of the two reactants used. In the case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the chief product; whereas, an excess of chlorine favors formation of chloroform and carbon tetrachloride.

The following facts must be accomodated by any reasonable mechanism for the halogenation reaction.

The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. The weakest covalent bond in the reactants is the halogen-halogen bond (Cl-Cl = 58 kcal/mole; Br-Br = 46 kcal/mole) so the initiating step is the homolytic cleavage of this bond by heat or light, note that chlorine and bromine both absorb visible light (they are colored). A chain reaction mechanism for the chlorination of methane was described earlier.
Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.

To see an animated model of the bromination free radical chain reaction  

When alkanes larger than ethane are halogenated, isomeric products are formed. Thus chlorination of propane gives both 1-chloropropane and 2-chloropropane as mono-chlorinated products. Four constitutionally isomeric dichlorinated products are possible, and five constitutional isomers exist for the trichlorinated propanes. Can you write structural formulas for the four dichlorinated isomers?

The halogenation of propane discloses an interesting feature of these reactions. All the hydrogens in a complex alkane do not exhibit equal reactivity. For example, propane has eight hydrogens, six of them being structurally equivalent primary, and the other two being secondary. If all these hydrogen atoms were equally reactive, halogenation should give a 3:1 ratio of 1-halopropane to 2-halopropane mono-halogenated products, reflecting the primary/secondary numbers. This is not what we observe. Light-induced gas phase chlorination at 25 ºC gives 45% 1-chloropropane and 55% 2-chloropropane.

The results of bromination ( light-induced at 25 ºC ) are even more suprising, with 2-bromopropane accounting for 97% of the mono-bromo product.

These results suggest strongly that 2º-hydrogens are inherently more reactive than 1º-hydrogens, by a factor of about 3:1. Further experiments showed that 3º-hydrogens are even more reactive toward halogen atoms. Thus, light-induced chlorination of 2-methylpropane gave predominantly (65%) 2-chloro-2-methylpropane, the substitution product of the sole 3º-hydrogen, despite the presence of nine 1º-hydrogens in the molecule.

It should be clear from a review of the two steps that make up the free radical chain reaction for halogenation that the first step (hydrogen abstraction) is the product determining step. Once a carbon radical is formed, subsequent bonding to a halogen atom (in the second step) can only occur at the radical site. Consequently, an understanding of the preference for substitution at 2º and 3º-carbon atoms must come from an analysis of this first step.

Since the H-X product is common to all possible reactions, differences in reactivity can only be attributed to differences in C-H bond dissociation energies. In our previous discussion of bond energy we assumed average values for all bonds of a given kind, but now we see that this is not strictly true. In the case of carbon-hydrogen bonds, there are significant differences, and the specific dissociation energies (energy required to break a bond homolytically) for various kinds of C-H bonds have been measured. These values are given in the following table.

The covalent bond homolyses that define the bond dissociation energies listed above may are described by the general equation:

The difference in C-H bond dissociation energy reported for primary (1º), secondary (2º) and tertiary (3º) sites agrees with the halogenation observations reported above, in that we would expect weaker bonds to be broken more easily than are strong bonds. By this reasoning we would expect benzylic and allylic sites to be exceptionally reactive in free radical halogenation, as experiments have shown. The methyl group of toluene, C₆H₅CH₃, is readily chlorinated or brominated in the presence of free radical initiators (usually peroxides), and ethylbenzene is similarly chlorinated at the benzylic location exclusively. The hydrogens bonded to the aromatic ring (referred to as phenyl hydrogens above) have relatively high bond dissociation energies and are not substituted.

Since the hydrogen atom is common to all the cases cited here, we can attribute the differences in bond dissociation energies to differences in the stability of the alkyl radicals (R·) as the carbon substitution changes. This leads us to the conclusion that:

alkyl radical stability increases in the order:  phenyl < primary (1º) < secondary (2º) < tertiary (3º) < allyl ≈ benzyl.

Because alkyl radicals are important intermediates in many reactions, this stability relationship will prove to be very useful in future discussions. The enhanced stability of allyl and benzyl radicals may be attributed to resonance stabilization. If you wish to review the principles of resonance Click Here.
Formulas for the allyl and benzyl radicals are shown below. Sructural formulas for the chief canonical forms in the resonance hybrid iwill be shown by clicking the button.

The poor stability of phenyl radicals, C₆H₅•, may in turn be attributed to the different hybridization state of the carbon bearing the unpaired electron (sp² vs. sp³). A caveat regarding allylic halogenation must be noted. Since carbon-carbon double bonds add chlorine and bromine rapidly in liquid phase solutions, free radical halogenation reactions of allylic sites must be carried out in the gas phase, or by specialized halogenating reagents, as noted in a subsequent chapter.

Bond Dissociation Energy and Radical Stability The stability of alkyl radicals cannot be simply derived from R-H bond dissociation energies. For a brief explanation of this point, and a reference to a more complete discussion Click Here.

Three problems concerning the identification of structurally equivalent groupings and various alkane reactions are presented here.