## Chapter 4

## Q. 4.5

What is the configuration of the following stereoisomer of 3-methyl-2-pentene? (The numbers are for reference in the solution.)

\\\quad H_3C \enspace 2 \quad 3\quad CH_3 \\\qquad \diagdown \downarrow \quad \downarrow\diagup \\\qquad\enspace C=C \\\qquad \diagup \qquad \diagdown \\\quad H \qquad CH_2CH_3## Step-by-Step

## Verified Solution

Assign the relative priorities of the two groups attached to each carbon. The two atoms directly attached to carbon-2 are C and H. Because C has a higher atomic number (6) than H (1), the CH_3 group is assigned the higher priority, as shown in the example for step 1. Now consider the groups attached to carbon-3. The example accompanying step 2 and rule 2 shows that ethyl has higher priority than methyl. The priority pattern is therefore as follows:

Because groups of like priority are on opposite sides of the double bond, this alkene is the *E* isomer; its complete name is (*E*)-3-methyl-2-pentene.