Alkanes are the most basic organic molecules. An alkane is a hydrocarbon containing only single bonds and hydrogen with CnH2n+2 stoichiometry. A saturated hydrocarbon is an organic compound that contains only carbon-carbon single bonds and carbon-hydrogen () bonds. There is no limit to the number of carbons in an alkane, but because every carbon has four single bonds, the generic chemical formula for all alkanes is CnH2n+2, where n represents the number of carbon atoms present. Although many of the short-chain alkanes are known by common names, the International Union of Pure and Applied Chemistry (IUPAC) has adopted a systematic naming system based on some rules:
1. Find the longest continuous chain of bonds. This is called the base chain, or the parent chain. Everything else is a substituent or functional group. Except for the first four alkanes the base name is created by adding the suffix -ane to the Greek root for the number of carbons in the chain. C6H14 is hexane, for example.
Alkane Chain Names
|Number of Carbons||Name||Formula|
2. Number the carbons in the parent chain, making sure the substituents (other functional groups or carbon chains) are bonded to the carbon(s) with the lowest possible numbers.
3. Name the substituents first by lowest by number and then alphabetically. If there are more than one of the same substituent, an atom or group of atoms (functional group) that replaces a bond in an organic compound use the prefixes di-, tri-, tetra-, etc., and repeat the carbon number if necessary. For straight chains (alkanes without branching), use the prefix n- for normal.
4. Use commas to separate numbers from each other; use dashes to separate numbers from the letters.5. List the substituents grouped together alphabetically. When grouping alphabetically, the prefixes (di-, tri-, and so on) are ignored. The compound that has the formula C7H14BrCl with the chlorine and bromine atoms bound to the 5th and 7th carbons from the left is 1-bromo-3-chloroheptane, not 5-chloro-7-bromoheptane. The compond has this name because the third IUPAC rule requires alphabetization, and the second IUPAC rule requires that the carbon atoms be numbered such that the substituents are on the lowest-numbered carbon atoms possible. The numbering of the this molecule, therefore, starts from the right and is 1-bromo-3-chloroheptane. Notice also that two compounds can share the same chemical formula: C7H15Cl, but have different structural formulas, meaning they are structural isomers. An isomer is one of two or more molecules that have the same chemical formula but different molecular structures. Structural isomers of alkanes also arise from different arrangements of carbon-carbon bonds. The huge number of organic molecules mentioned above is largely because of the number of structural isomers available to any given alkane. An alkane with the formula C6H14 has five possible structural isomers, and the number of possible isomers increases exponentially with increasing methyl groups. C8H18 has eighteen structural isomers; C10H22 has 75, and C20H42 has 366,319.
Common Alkyl Groups
|Alkyl Group Common Name (Alternative Name)||Molecular Structure|
|Isopropyl (iso-propyl; 2-propyl-)|
|Isobutyl (iso-butyl; 2-methylethyl-)|
|Sec-butyl (s-butyl, 2-butyl-)|
|Tert-butyl (t-butyl; 2-methylpropyl-)|
Alkanes can also form ring structures, which are called alicyclic molecules or cycloalkanes. An alicyclic molecule is a cyclic hydrocarbon that contains nonaromatic rings. The rings impose spatial restrictions on the atoms so that the rotation around the bonds is severely restricted. The bond angles in the 3- and 4-membered rings induce greater bond strain than that present in the 5- and 6-membered cycloalkane analogues because the sp3 orbitals are forced away from their ideal 109.5 degree geometry. This means that cyclobutanes are more reactive than cyclopentane or cyclohexanes because of the ring strain around the carbon atoms. Of cyclic alkanes, cyclopropane is the most reactive. Much like alkanes, functional groups present in cycloalkanes are also numbered according to their lowest numbers in alphabetical order. When naming cycloalkanes, the carbons are numbered so that the functional groups are on the lowest-numbered carbons possible.
Alicyclic structures also introduce another form of isomerism, cis- and trans-isomerism, which is a form of geometric or configurational isomerism. Molecular strain because of the tetrahedral orientation of covalent bonds at each carbon atom causes cycloalkanes to buckle so that they do not lie in a flat plane. In other words, the hydrogen atoms and functional group are forces to orient themselves up, down, or away from the average plane containing the cycloalkane ring. The atoms that point away from the ring, but are approximately in the plane of the ring, are designated equatorial, and the other atoms, up or down, are called axial.
Axial groups that are attached to adjacent carbon atoms, groups with a 1,2 relationship to each other, will point in opposite directions in a trans-configuration. One axial group will point up from the plane of the ring, and the axial group on the neighboring carbon will point down from the plane of the ring. For example, two axial hydrogen atoms attached to the 1 and 2 carbon atoms in a ring will have a trans-configuration. An axial group next to a carbon with an equatorial group will have cis-configuration. Like groups (axial/axial or equatorial/equatorial) attached to carbon atoms that are separated by a middle carbon atom have a 1,3 relationship and have a cis-configuration. Unlike groups, an axial group and an equatorial group, with a 1,3 relationship have a trans-configuration to each other.Cycloalkanes are able to bend and shift conformations, thereby switching which hydrogen atoms are axial and which are equatorial. But equatorial groups have more space around them than axial groups do, so cycloalkanes prefer conformations in which larger functional groups remain in the equatorial sites. The most stable conformation of cyclohexane is called the chair conformation in which one side of the ring points up and the other side points down.
Alkenes and Alkynes
An alkene is a hydrocarbon containing at least one carbon-carbon double bond () with CnH2n stoichiometry. Alkenes with only one double bond have the generic formula CnH2n . They lose two hydrogen atoms for every double bond. An alkyne is a hydrocarbon that contains at least one carbon-carbon triple bond () with CnH2n–2 stoichiometry. A hydrocarbon loses four hydrogen atoms for every triple bond. A hydrocarbon that has at least one multiple bond is unsaturated. An unsaturated hydrocarbon is an organic compound that contains bonds in addition to one or more carbon-carbon or bonds.Alkenes and alkynes are named just like alkanes, but have -ene and -yne endings, respectively. The locations of the multiple bonds are indicated by the lowest number possible of the carbon on the base chain, much like the location of the functional groups.
Representative Alkenes and Alkynes
Reactions with Propene
An aromatic compound is a planar hydrocarbon with CnHn stoichiometry that consists of alternating and bonds. Benzene (C6H6) is aromatic. It has one hydrogen atom attached to each carbon atom, making a ring with three carbon-carbon double bonds. The double bonds form resonance hybrid orbitals, which means the electrons are spread out over the whole molecule. Benzene molecules are not buckled like cyclohexane because of the resonance. Instead, they lie in a flat plane. Benzene is represented in a skeletal structure as a hexagon with a circle inside. The circle represents the hybrid resonance structure.There are a few additions to the IUPAC rules when naming aromatic compounds. First, benzene with one methyl group is called toluene (C6H5CH3). A benzene ring with two methyl groups is called xylene (C6H4(CH3)2). Second, if any of these compounds contain additional substituents, the carbon numbering starts at the functional group, for example, the carbon atom bound to the methyl group (CH3) in toluene. The carbon atom attached to the methyl group in toluene is a "1." All others in o- (1,2-) and m-xylene (1,3-) are numbered in relation to this. If the benzene ring is bound to a functional group that does not result in a molecule with a common name, the numbering starts at the functional group. Finally, in addition to numbered positions, benzene retains the common-name prefixes ortho- (o-), para- (p-), and meta- (m-) for di-substituted benzene rings. These prefixes designate ortho- as 1,2-substituted, meta- as 1,3 substituted, and para- as 1,4-substituted on a C6H4XY scaffold. If a benzene ring is a functional group, it is called a phenyl group.
Even though aromatic compounds have double bonds, the bonds are not localized. The compounds, therefore, do not easily undergo addition reactions but instead undergo substitution reactions more readily.