2-ethyl-l-pentene 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 substi tuent 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 substi tuents: i. e dimethyl.. trimethyl.. tetraethy l. the substituents are arranged in alphabetical order, without regard for multipliers 6, 6-dimethyl-3-heptene 3. For cycloalkenes, begin numbering at the double bo nd 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. 1 4-cyclohexadiene 3,3, 6-trimethylcyclohexene 4, 6, 6-trimethylcyclooctene 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 decisions2. 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