Organic Chemistry

Classes of Hydrocarbons

Alkanes

Alkanes are saturated hydrocarbons that contain only single carbon-carbon (CC{\rm{C}{-}{C}}) and carbon-hydrogen (CH{\rm{C}{-}{H}}) bonds.

Alkanes are the most basic organic molecules. An alkane is a hydrocarbon containing only CC{\rm{C}{-}{C}} 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 (CH{\rm{C}{-}{H}}) 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 CC{\rm{C}{-}{C}} 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
1 Methane CH4
2 Ethane C2H6
3 Propane C3H8
4 Butane C4H10
5 Pentane C5H12
6 Hexane C6H14
7 Heptane C7H16
8 Octane C8H18
9 Nonane C9H20
10 Decane C10H22

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 CH{\rm{C}{-}{H}} bond in an organic compound use the prefixes di-, tri-, tetra-, etc., and repeat the carbon number if necessary. For straight chains (alkanes without CC{\rm{C}{-}{C}} 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.
IUPAC rules are used to name organic compounds. The end carbon atom closest to the substituent is numbered 1, and the remaining carbon atoms are numbered sequentially after that.
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.

Structural Isomers

The five structural isomers of C6H14 have the same chemical formula but different molecular structures. The straight chain is a hexane, but when the carbons branch off, the parent chain becomes the shorter pentane or butane, depending on the location of the methyl substituents.
An important note is that there are eight common names for small molecular weight alkyl groups that are acceptable by IUPAC. Common names are names that clearly define a chemical, but do not follow the IUPAC naming rules. In addition, the prefixes n-, s-, and t- stand for primary, secondary, and tertiary, respectively. A carbon bound to another carbon is a primary carbon (normal or n-). A carbon bound to two other carbons is called a secondary carbon (sec- or s-). A carbon bound to three other carbons is a tertiary carbon (tert- or t-). The prefix iso- is used for propyl and butyl groups that are bound to the main chain by a secondary carbon and leave a Y-shaped tail. The prefix iso- means "same," and the isopropyl and isobutyl groups have two methyl groups attached to a central methine (CH) center, so they are the same on both sides of the CH. For example, isopropanol is the common name for the IUPAC name 2-propanol.

Common Alkyl Groups

Alkyl Group Common Name (Alternative Name) Molecular Structure
Methyl (methyl-)
Ethyl (ethyl-)
Propyl (n-propyl-)
Isopropyl (iso-propyl; 2-propyl-)
Butyl (n-butyl-)
Isobutyl (iso-butyl; 2-methylethyl-)
Sec-butyl (s-butyl, 2-butyl-)
Tert-butyl (t-butyl; 2-methylpropyl-)
Cyclopropyl
Cyclobutyl
Cyclopentyl
Cyclohexyl

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 CC{\rm{C}{-}{C}} 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.

Cyclohexane

The molecular structure of cyclohexane in the chair conformation is shown on the left. The hydrogen atoms shown in blue are axial, and the red hydrogen atoms are equatorial. A ball-and-stick model of the same conformation is on the right. Note how the axial hydrogen atoms have less space around them than the equatorial hydrogen atoms do.
Hydrocarbons are essentially nonpolar, as the electronegativities of the carbon and hydrogen atoms are nearly identical. This makes them hydrophobic (insoluble in water). The physical properties of alkanes depend on their particular structures and the arrangements of aliphatic groups that are present. There is little difference in the intermolecular forces between similarly shaped molecules such as n-hexane and n-pentane, so their physical properties such as boiling points, melting points, and solubilities are very similar. However, an elongated molecule, such as n-hexane, has similar intramolecular forces as a more spherical molecule such as 2,2-dimethylbutane. Since hexane is linear, it can intertwine with other molecules and stabilize a greater instantaneous dipole moment than smaller molecules (by volume), which generally have weaker intramolecular forces. Their properties, therefore, tend to be different even though their chemical formulas are the same. For example, hexane has a higher boiling point (69°C) than 2,2-dimethylbutane (49.73°C) because of its stronger intramolecular forces. Alkanes are not very reactive. Halogenation, which is the replacement of CH{\rm{C}{-}{H}} bonds with C-halogen bonds, can occur under low or high temperatures, depending of the halogen and alkane present. This type of reaction, when one atom is substituted for another, is called a substitution reaction. Alkanes are also flammable; they oxidize in a combustion reaction.
Halogenation: CH4(g)+Cl2(g)hνCH3Cl(g)+HCl(g)Combustion: 2C2H6(g)+7O2(g)hν4CO2(g)+6H2O(g)\begin{aligned}\text{Halogenation: }&{\rm{CH}_{4}}(g)+{\rm{Cl}_{2}}(g)\xrightarrow{{h}\nu}{\rm{CH}_{3}}{\rm{Cl}}(g)+{\rm{HCl}}(g)\\\\\text{Combustion: }&2{\rm {C}_{2}}{\rm {H}_{6}}(g)+7{\rm {O}_{2}}(g)\xrightarrow{{h}\nu}4{\rm{CO}_{2}}(g)+6{\rm {H}_{2}}{\rm {O}}(g)\end{aligned}
The symbol hν{h}\nu in the equations indicates that light provides energy for the reactions.

Alkenes and Alkynes

Alkenes are hydrocarbons with at least one carbon-carbon double bond (C=C{\rm{C}{=}{C}}). Alkynes are hydrocarbons with at least one carbon-carbon triple (CC{\rm{C}{\equiv}{C}}) bond.

An alkene is a hydrocarbon containing at least one carbon-carbon double bond (C=C{\rm{C}{=}{C}}) 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 (CC{\rm{C}{\equiv}{C}}) 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 CH{\rm{C}{-}{H}} bonds in addition to one or more carbon-carbon C=C{\rm{C}{=}{C}} or CC{\rm{C}{\equiv}{C}} 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

Alkenes are hydrocarbons containing at least one carbon-carbon double bond, and are named with -ene endings. Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond, and are named with -yne endings.
Because atoms are not able to spin freely around double bonds (high bond strength), alkenes exhibit geometric isomerism. A geometric isomer is one of two or more molecules that have different spatial arrangements of functional groups around a double bond, ring, or other rigid structures. If the substituents are on the same side of the double bond, they are designated cis-, and if they are on different sides, they are designated trans-. (Note that cycloalkanes can also exist as geometric isomers depending on whether functional groups are both pointed up or down in the cis-configuration or if one is up and one is down, in the trans-configuration.)

Geometric Isomerism

Structural formulas and ball-and-stick models show the relationship between the cis- and trans-isomers of butenedioic acid.
Alkenes and alkynes are prepared by removing the hydrogens from adjacent carbons, a type of reaction called a dehydrogenation or elimination reaction. Like alkanes, alkenes and alkynes can undergo substitution reactions, but because they are unsaturated they can also participate in addition reactions where a new functional group is added to each carbon atom involved in a double (C=C{\rm{C}{=}{C}}) or triple (CC{\rm{C}{\equiv}{C}}) carbon-carbon bond. In these reactions, a CC{\rm{C}{-}{C}} double bond is converted to a single one while the same reaction can occur once or twice for alkynes. Many addition reactions add water (HOH), dihydrogen (H2), halogens (e.g. Br2), and hydrogen halides (e.g. HCl), and can all be added to the double or triple bonds. These types of reactions are called hydration, hydrogenation, halogenation, and hydrohalogenation, respectively. Like alkanes, alkenes and alkynes can also undergo combustion reactions.

Reactions with Propene

In the presence of H2SO4 and heat, isopropyl alcohol undergoes an elimination reaction to form propene and water. Propene can undergo four different addition reactions: hydrogenation to form propane, hydration to form isopropyl alcohol, halogenation to form 1,2-dibromopropane, and hydrohalogenation to form 2-bromopropane.

Aromatics

Benzene is a hydrocarbon that is a 6-membered ring and is aromatic, giving it and its derivatives special properties.

An aromatic compound is a planar hydrocarbon with CnHn stoichiometry that consists of alternating CC{\rm{C}{-}{C}} and C=C{\rm{C}{=}{C}} 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.

Aromatic Compounds

Aromatic compounds are planar hydrocarbons with CnHn stoichiometry and alternating carbon-carbon single and double bonds. There are special IUPAC naming rules for certain aromatic compounds.
Benzene and benzene derivatives such as toluene are much more stable than cyclohexanes because of the hybrid resonance structure that leads to higher CC{\rm{C}{-}{C}} and CH{\rm{C}{-}{H}} bond strengths. The flat shape of the rings and spreading out of electrons in the p orbital above and below the molecular plane increases the attractive force (London dispersion force) between the benzene rings, so aromatic compounds boil at a higher temperature than analogous cyclohexanes. The flat rings also allow benzene molecules to pack closely together, so benzene and its derivatives melt at higher temperatures than straight-chain alkanes with the same number of carbon atoms and have higher boiling points. Benzene and benzene derivatives (many aromatics are polar and exist as ions) that do not have polar functional groups are hydrophobic and are not soluble in or miscible with water. Aromatics are so-named because many of the first aromatic molecules to be isolated were fragrant, such as benzaldehyde, which is present in cherries, peaches, and almonds.

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.