Premed · Premed · Organic Chemistry 1
Lecture 3: Alkanes: Structure and Nomenclature
Organic Chemistry I
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and bonding of alkanes
- Distinguish between constitutional isomers and apply the molecular formula CnH2n+2
- Name alkanes and cycloalkanes using IUPAC nomenclature rules
- Identify and name common alkyl substituents and branches
- Draw and interpret bond-line (skeletal) structures
- Understand the physical properties of alkanes and how they relate to structure
- Describe the sources and uses of alkanes
Lecture Content
I. Introduction to Alkanes
Alkanes are hydrocarbons that contain only carbon-carbon single bonds, earning them the designation of saturated hydrocarbons because they carry the maximum possible number of hydrogen atoms. Acyclic alkanes follow the general molecular formula CnH2n+2, while cycloalkanes conform to CnH2n, losing two hydrogens for each ring present.
Every carbon in an alkane is sp3 hybridized, which gives each carbon center a tetrahedral geometry with bond angles of approximately 109.5 degrees. The C-C bond length is 1.54 angstroms, and the C-H bond length is 1.09 angstroms. Alkanes are relatively unreactive molecules because their C-C and C-H bonds are strong (approximately 350 and 410 kJ/mol, respectively), they lack polar functional groups, and they have no empty low-energy orbitals available for reaction. The primary reactions that alkanes undergo are combustion and radical halogenation.
II. Constitutional (Structural) Isomers
Constitutional isomers are molecules that share the same molecular formula but differ in the connectivity of their atoms. The number of possible constitutional isomers grows dramatically with increasing carbon count. Methane, ethane, and propane each have only one possible structure, but butane has two isomers (butane and isobutane), pentane has three, hexane has five, the C10 alkanes have 75, and by C20 the number reaches 366,319 isomers. Because constitutional isomers have different bonding arrangements, they exhibit different physical and chemical properties.
The degree of unsaturation, also called the index of hydrogen deficiency (IHD), provides a useful way to assess how a molecular formula compares to a fully saturated compound. It is calculated as IHD = (2C + 2 + N - H - X) / 2, where each ring or double bond contributes one degree of unsaturation. For alkanes the IHD is zero, and for cycloalkanes it is one.
III. IUPAC Nomenclature: Naming Straight-Chain Alkanes
The IUPAC naming system provides a systematic, unambiguous name for every organic compound. For straight-chain alkanes, the name is built from a prefix that indicates the number of carbons and the suffix -ane. The prefixes for the first ten members are: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C), non- (9C), and dec- (10C). Combining these with the -ane suffix gives methane, ethane, propane, butane, pentane, hexane, and so on.
IV. IUPAC Nomenclature: Naming Branched Alkanes
Naming branched alkanes requires a systematic procedure. First, identify the longest continuous carbon chain, which serves as the parent chain. If two chains have equal length, choose the one bearing more substituents. Second, number the carbons starting from the end nearest the first branch point, using the first point of difference rule to break ties. Third, identify each substituent, which is named by taking the corresponding alkane name and changing the -ane suffix to -yl (methyl for CH3-, ethyl for C2H5-, propyl for C3H7-, and so forth). Fourth, assign a locant (number) to each substituent based on its position along the parent chain. Fifth, list substituents in alphabetical order before the parent chain name, using the prefixes di-, tri-, and tetra- for multiple identical substituents (these multiplying prefixes do not affect alphabetical ordering). Hyphens separate numbers from letters, and commas separate numbers from each other. Finally, the entire name is written as a single word, as in 2,3-dimethylpentane or 4-ethyl-2-methylheptane.
<image>A step-by-step naming example for a branched alkane. The molecule 4-ethyl-2,4-dimethylhexane is shown as a bond-line structure. Panel A: The longest chain is identified and highlighted (6 carbons = hexane). Panel B: The chain is numbered from the end giving lowest locants. Panel C: Substituents are identified with arrows pointing to 2-methyl, 4-methyl, and 4-ethyl groups. Panel D: The complete IUPAC name is assembled showing alphabetical ordering of substituents.</image>
V. Common and Complex Substituent Names
Simple alkyl groups include methyl (-CH3), ethyl (-CH2CH3), and propyl (-CH2CH2CH3). Several branched substituents appear frequently and have widely used common names. Isopropyl, also called 1-methylethyl, refers to -CH(CH3)2. The four-carbon substituents come in several varieties: n-butyl (-CH2CH2CH2CH3), isobutyl, sec-butyl (-CH(CH3)CH2CH3), and tert-butyl (-C(CH3)3), the last of which features three methyl groups attached to a single carbon. Neopentyl refers to -CH2C(CH3)3.
Carbons in organic molecules are classified by how many other carbon atoms they are directly bonded to. A primary carbon is bonded to one other carbon, a secondary carbon to two, a tertiary carbon to three, and a quaternary carbon to four. Hydrogens are classified according to the carbon to which they are attached.
VI. Cycloalkane Nomenclature
Cycloalkanes are named by adding the prefix "cyclo-" to the name of the corresponding straight-chain alkane, giving names such as cyclopropane, cyclobutane, cyclopentane, and cyclohexane. For substituted cycloalkanes, if the ring contains more carbons than any substituent chain, the ring serves as the parent. If a chain is longer, the ring becomes a substituent (for example, cyclopropyl-). The ring is numbered to give the lowest set of locants to the substituents, and when two substituents are present, alphabetical order determines which receives position 1.
Bicyclic and polycyclic systems are named using the bicyclo[X.Y.Z]alkane convention, where X, Y, and Z indicate the number of carbons in each bridge connecting the bridgehead carbons. Bicyclo[2.2.1]heptane, commonly known as norbornane, is a classic example.
VII. Drawing Organic Structures
Organic structures can be represented at three levels of detail. Structural formulas show every atom and every bond explicitly. Condensed structural formulas group atoms together for compactness, as in CH3CH2CH3 for propane or (CH3)3CH for isobutane. Bond-line (skeletal) structures are the standard representation in organic chemistry. In this notation, each vertex and line endpoint represents a carbon atom, hydrogens on carbon are implied in sufficient number to give each carbon four bonds, and heteroatoms such as oxygen, nitrogen, and sulfur are shown explicitly along with their attached hydrogens. Single bonds appear as single lines, double bonds as two lines, and triple bonds as three lines. To read a bond-line structure, count the number of lines meeting at each vertex to determine the number of explicit bonds, then add enough hydrogens to bring each carbon to a total of four bonds.
<image>Panel A: The same molecule (2-methylbutane) drawn in three different representations: full structural formula showing all C-H bonds, condensed formula CH3CH(CH3)CH2CH3, and bond-line/skeletal structure showing only the carbon backbone as a zigzag line with the methyl branch. Panel B: A more complex molecule (4-ethyl-2-methylhexane) shown as a bond-line structure with annotations pointing to each vertex identifying the number of hydrogens implied at each carbon position.</image>
VIII. Physical Properties of Alkanes
The boiling points of alkanes increase with molecular weight because larger molecules experience stronger London dispersion forces. Branching decreases the boiling point because a more spherical molecular shape reduces surface area and weakens intermolecular contact. Representative boiling points illustrate the trend: methane boils at -161 degrees C, ethane at -89 degrees C, pentane at 36 degrees C, and decane at 174 degrees C. Melting points follow a similar general trend with molecular weight, and more symmetric molecules tend to pack more efficiently into crystal lattices, raising their melting points.
Alkanes are nonpolar molecules, and the principle "like dissolves like" governs their solubility behavior. They are insoluble in water but dissolve readily in nonpolar solvents such as hexane, toluene, and diethyl ether. Alkanes are less dense than water and therefore float on its surface. The only intermolecular forces available to alkanes are London dispersion forces (induced dipole-induced dipole interactions), since they lack the capability for hydrogen bonding or permanent dipole interactions. Larger surface area translates directly into stronger London dispersion forces.
IX. Sources and Reactions of Alkanes
Petroleum, or crude oil, is the primary source of alkanes. Fractional distillation of crude oil separates its components by boiling point into fractions that include gasoline, kerosene, diesel fuel, and lubricating oils. Natural gas consists primarily of methane with smaller amounts of ethane and propane.
Combustion is the most important reaction of alkanes. The balanced equation for complete combustion of an alkane is CnH2n+2 + excess O2 yielding nCO2 + (n+1)H2O plus energy. This reaction provides the energy that powers transportation and heating. Halogenation, which is discussed in detail in the radical reactions lecture, is the other significant reaction of alkanes. It requires heat or ultraviolet light for initiation, as exemplified by the reaction of methane with chlorine gas under UV light to produce chloromethane and HCl. Beyond these two reaction types, alkanes are remarkably unreactive at room temperature.
<image>A diagram showing the fractional distillation of crude oil in a distillation column. The column is shown with temperature decreasing from bottom to top. Different fractions are labeled at their collection points: refinery gas (C1-C4, collected at top), gasoline (C5-C10), kerosene (C10-C16), diesel (C14-C20), lubricating oil (C20-C50), and bitumen/asphalt (C50+, collected at bottom). Boiling point ranges are indicated for each fraction.</image>
