Chemistry 114
Alkanes and Cycloalkanes

Structure and Properties


General formula: CnH2n+2
All carbon-carbon and carbon-hydrogen single bonds (saturated)
Tetrahedral arrangement around each carbon
sp3 hybridized
Free rotation around single bonds

Boiling points of alkanes

Increase with increasing carbon number (increased molecular weight) [See Figures 15.2 and 15.6]
At room temperature (RT) and 1 atm pressure
C1 to C4 are gases
C5 to C17 are liquids
C18 and higher are solids
Increased boiling points (and melting points) are due to stronger intermolecular forces.

In alkanes the only significant intermolecule force is the VanderWaals interaction (aka London dispersion forces or induced dipoles). This attractive force results from charge-charge interactions between molecules due to transient uneven distributions of electrons. Extra electron density in one part of a molecule (resulting in slight negative charge) repels the electrons density in a neighboring molecule (resulting in a slight positve charge). The positive and negative charges attract. The more carbons there are in a hydrocarbon (i.e. the higher the molecular weight) results in more electrons that produce a more significant intermolecular interaction.

Branched alkanes have a lower boiling point than their corresponding unbranched isomer. Branching produces less efficient packing and thus weakens the intermolecular interactions.

Solubilities of alkanes


Very low solubility in aqueous solvents (immiscible). Alkanes (and other organic solvents) tend to separate into a separate layer when mixed with water.

Since C-H groups are not hydrogen bond donors, alkanes disrupt the hydrogen bonding network of water. Also, when a non-polar molecule is incorporated into an aqueous solution, the waters form a rigid molecular cage around the non-polar molecule. This is comparable to freezing and is an unfavorably process due to the reduced entropy (disorder).

Like dissolves like. Alkanes are very soluble in other organic solvents.

Nomenclature of alkanes


Straight chained hydrocarbon (and parent carbon chains): prefix for the number of carbons + -ane
For a subsituent group based on an alkane (an alkyl group): prefix for the number of carbons + -yl

Know all of the prefixes listed in this table. 
Number of Carbons  Prefix    alkane      alkyl group

        1            meth-   methane        methyl
        2             eth-    ethane         ethyl
        3            prop-   propane        propyl
        4             but-    butane         butyl
        5            pent-   pentane        pentyl
        6             hex-    hexane         hexyl
        7            hept-   heptane        heptyl
        8             oct-    octane         octyl
        9             non-    nonane         nonyl
       10             dec-    decane         decyl

Special alkyl grops that you must know (see Tables 15-2 and 15-3):
isopropyl
sec-butyl
isobutyl
tert-butyl

General Rules of Nomenclature for Alkanes (IUPAC)


1. The longest continuous carbon is the parent compound. The base name for the molecule is the prefix for the number of carbons of the parent compound + the suffix -ane. Number the carbons in the parent chain so that substituents have the smallest possible numbers.
2. Find each alkyl substituent and assign it a number according to which carbon it is attached.
3. List the substituents (with their carbon number) in alphabetical order followed by the name of the parent alkane. If there are more than one of a given substituent use the prefixes di-, tri-, tetra-. Prefixes are not used to determine alphabetical order.
4. Use commas between numbers and dashes between numbers and words. Do not leave spaces in the name.

Examples:
2,2-dimethylpropane
4-ethyl-2,3-dimethyloctane

Nomenclature for Organohalogens


1. Organohalogens are named just like their parent compound alkane with the halogen treated exactly as alkyl substituents.
2. -F, -Cl, -Br and -I substituents are labeled fluoro-, chloro-, bromo- and iodo- , respectively.

Nomenclature for Cycloalkanes


1. Cycloalkanes are alkanes in which a bond is formed between the two terminal carbons in the chain to for a cyclical or ring structure. The prefix cyclo- is written before the name designating the carbon number in the ring (e.g. cyclohexane).
2. Substituents are named similarly to straight-chained alkanes. The carbons in the ring are numbered so that the substituents have the smallest numbers.
3. Because the ring structure prevents free rotation around the C-C bonds, there is a spatial difference between above the plane of the ring and below the plane of the ring. If two substituents are attached to two different carbons on opposite sides of the plane of the ring, they are said to be trans; if they are on the same side of the plane of the ring, they are said to be cis. If the plane (or approximate plane) of the ring is drawn in the plane of the drawing surface, then a substituent coming up out of the plane is indicated by a thick wedge-shaped bond; a substitutent going into the plane is indicated by a dotted line. The cis or trans label is written at the beginning of the compounds name. The figure below illustrates several of these concepts.

Molecule A is trans-1,3-dimethylcyclohexane. Molecule B is cis-1,3-demethylcyclohexane.

The carbon chain ring of most cycloalkanes is not planar (cyclopropane must be and cyclobutane is very close). Because of free rotation around the single bonds, the molecules adopt a shape so that the bond angles are as close as possible to 109.5. See the discussion of conformation below.

Conformation of alkanes and cycloalkanes


Because of free rotation around C-C single bonds atoms or groups attached to the bond carbons can be in various spatial relationships to one another. These different spatial arrangements are known as different conformations. This is a very important topic in chemistry and biochemistry. In proteins, for example, the difference between a folded, biologically functional protein and an unfolded, denatured, inactive protein is simply a change in conformation, i.e. rotations around single bonds (although very many of them in a molecule as large as a protein.)

Ethane

The easiest way to show differences in conformation is by looking down the C-C bond being rotated and determine the relative positions of attached atoms or groups. This is visualized with the Newman projection illustrated in the figure below for ethane.

In each figure the structure on the left represents a 3D model of ethane whereas the structure on the right is the Newman projection. Figure A is known as the eclipsed conformation because the viewed down the C-C bond the hydrogens are aligned. Figure B is known as the staggered conformation. The staggered conformation is the lower energy (more favorable conformation) because the H atoms and the C-H bonds one one carbon are less crowded with the H atoms and the C-H bonds on the other carbon. Figure 15.13 in the textbook shows the relative energies of the conformations as the molecule is rotated around the C-C bond.

Butane

Rotation around the central C-C bond of butane is a bit more complicated. It can be treated like ethane but with a methyl group (-CH3) replacing an H on both the front and the back carbon. The main Newman projections for butane are shown in the figure below.




Notice that the front methyl group is unchanged in all of these structures. Conformations A, C and E are eclipsed, whereas conformations B, D and F are staggered. A is the highest energy (least favorable) conformation because of crowding between the two methyl groups when they are eclipsed. It is known as the syn conformation. D is the lowest energy (most favorable) conformation because the two methyl groups are farthest apart. This is know as the anti conformation. B and F are low energy conformations but not as low energy as the anti. They are known as gauche+ and gauche-. C and E are high energy conformations but not as high as the syn conformation.

Cyclohexane

Rotation ar