All About Alkenes

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Alkene Nomenclature

Alkenes represent one of the most common functional groups in organic chemistry. An alkene contains only carbon and hydrogen (a hydrocarbon) and contains at least one double bond (termed an unsaturated hydrocarbon). Alkenes have the general formula CnH2n, thus, an alkene with 10 carbons (n = 10) will have 2(10) = 20 hydrogens, or the molecular formula C10H20; each double bond therefore contributes one degree of unsaturation.

The root, or parent name for an unbranched alkene is taken directly from the number of carbons in the chain according to a scheme of nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC), as described previously for alkanes.

To name alkenes:

1. Find the longest chain containing the alkene

The IUPAC name for an alkene is constructed of two parts: 1) a prefix (meth… eth… prop…, etc.) which indicates the number of carbons in the main, or parent, chain of the molecule, and 2) the suffix …ene to indicate that the molecule is an alkane.

For branched-chain alkanes, the name of the parent hydrocarbon is taken from the longest continuous chain of carbon atoms containing the double bond.

2. Number the chain, giving the double bond the lowest possible number.

Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the double bond, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the chain, the substituent of lower alphabetical order is given the lowest number.

If the same substituent occurs more than once in a molecule, the number of each carbon of the parent chain where the substituent occurs is given and a multiplier is used to indicate the total number of identical substituents; i.e., dimethyl… trimethyl… tetraethyl…, etc. In constructing the name, substituents are arranged in alphabetical order, without regard for multipliers.

3. For cycloalkenes, begin numbering at the double bond and proceed through the double bond in the direction to generate the lowest number at the first point of difference.

One of the most common mistakes in naming cycloalkenes is to generate the lowest number sequence around the ring, disregarding this rule. Once again, the numbering must begin at the double bond and proceed through the bond in the direction to generate the lowest number sequence.

4. Assign stereochemistry using the E-Z designation

Historically, alkenes have been named using cis- and trans- to represent stereochemistry around the double bond; cis- for compounds where the “main substituents” are on the same side of the double bond, and trans- when they are on opposite sides. This system clearly breaks down, however, in more complex molecules where decisions concerning the “main substituents” are not easily made, and the E-Z system provides a set of rules to aid in these decisions.

February 1, 2010 Posted by | Uncategorized | Leave a comment