Alkanes and Cycloalkanes Help
Alkanes are hydrocarbons constituting the homologous series with the general formula CnH2n+2, where n is an integer, for open chain (acyclic) compounds. This formula gives the maximum number of hydrogens per carbon atom, so alkanes are said to be saturated. They have only single bonds.
A chain of singly bonded C's can be twisted around the C–C bonds into any zigzag shape (conformation). Two such arrangements, for four consecutive C's (butane), are shown below.
The two extreme conformations of ethane—called eclipsed and staggered — can be shown in "wedge-and dash" drawings, "sawhorse" drawings, and in Newman projections. With the Newman projection, we sight along the C—C bond, so that the back C is hidden by the front C. The circle aids in distinguishing the bonds on the front C (touching at the center of the circle) from those on the back C (drawn to the circumference of the circle). In the eclipsed conformation, the bonds on the back C are, for visibility, slightly offset from a truly eclipsed view. The angles between a given C—H bond on the front C and the closest C—H bond on the back C are 60° (in staggered) or 0° (in eclipsed).
In the Newman projection of butane, it is possible to see 2 different kinds of staggered conformations. Conformations that place the two CH3 groups 60° apart are called gauche conformations.
The gauche conformation is lower in energy than the eclipsed conformation, but higher in energy than staggered conformations that place the CH3 groups 180° apart, known as the anti conformation.
Nomenclature of Alkanes
Alkanes are named by following a set of steps, demonstrated on the following compound:
- Find the longest unbroken chain ("backbone") of carbon atoms. Find that chain length in the table and add "ane" at the end to denote an alkane.
- Identify branches. Number the backbone from the end closest to the first branch.
- Assemble the name. Put the branches in alphabetical order (ignoring di, tri, etc.). Our example here is 4-ethyl-2,3-dimethylhexane.
The letter n (for normal), as in n-butane, denotes an unbranched chain of C atoms. The prefixes sec- and tert- before the name of the group indicate that the H was removed from a secondary or tertiary C, respectively. A secondary C has bonds to two other C's, a tertiary to three other C's, and a primary to two H's and one C. The H's attached to these types of carbon atoms are also called primary, secondary and tertiary (1°, 2° and 3°), respectively. A quaternary C is bonded to four other C's.
Cyclic hydrocarbons are called cycloalkanes. Cycloalkanes have the general formula CnH2n and are isomeric with alkenes, but, unlike alkenes, they are saturated compounds. They are named by combining the prefix cyclo- with the parent name to indicate the number of carbon atoms in the ring. Two or more substituents are listed alphabetically and are assigned the lowest possible numbers when numbering carbons.
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