E–Z notation

Tiny bits join to make shapes.

Some bits stay on the same side. We use the letter Z for this. Other bits stay on opposite sides.

We use the letter E for this. Scientists give each bit a rank. They look at the two top bits. They see if they are together. Or they see if they are apart. This helps us name the shapes. It is a way to tell them apart.

Chemists study how tiny parts join together. Sometimes they form double bonds. These bonds hold parts in a fixed shape. We use E–Z notation to name these shapes.

First, scientists use CIP rules. These rules give each part a priority. A priority is a rank. Higher parts have more priority. Scientists look at the two high parts. They see where they sit on the bond.

If the high parts are on the same side, we use Z. This comes from a German word for "together." If the high parts are on opposite sides, we use E. This comes from a German word for "opposite."

We write these letters in italics and in parentheses. We also use a hyphen after them. For example, a name might look like (E)-But-2-ene. Some big molecules have many double bonds. We use numbers to show each shape. One molecule, alitretinoin, has four double bonds. We can name each one using E or Z.

Chemists use special names to describe the shapes of tiny molecules. One way they do this is called E–Z notation. This method describes the absolute stereochemistry of double bonds. Stereochemistry is just the study of how atoms are arranged in space. This system is the preferred method used by IUPAC. IUPAC is the group that sets rules for naming chemicals. This notation is an extension of an older system called cis–trans. While cis–trans only describes relative shapes, E–Z can describe more. It works for bonds with two, three, or even four parts attached.

To use this system, scientists follow the Cahn–Ingold–Prelog rules. These are often called CIP rules. First, you must look at the parts attached to the double bond. You assign a priority to each part based on these rules. You can only use this if neither atom has two identical parts. After that, you find the two highest priority parts. Then, you see where they sit on the bond. If they are on opposite sides, the bond is E. This comes from the German word entgegen, which means opposite.

If the high priority parts are on the same side, we use Z. This letter comes from the German word zusammen. Zusammen means together in English. Scientists write these letters in italics and inside parentheses. They also put a hyphen after the parentheses. For example, a name might look like (E)-But-2-ene. These letters are always written in full capitals. They do not count as the first letter for English capitalization rules.

Some molecules are very large and have many double bonds. We use numbers, called locants, to show the shape of each bond. A molecule called alitretinoin is a great example. Its full name is (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid. This name tells us the shape of four different bonds. The bonds at positions 2, 4, and 8 are all E. The bond at position 6 is Z.

Sometimes, scientists are not sure about the exact shape. They can use the E/Z prefix to show this uncertainty. For drawings, they use wavy single bonds to show unknown shapes. This is also used for a mixture of different isomers. In the past, some people used a crossed double bond. IUPAC no longer says this is an acceptable style. However, some computer software might still require it. This helps keep all chemical names clear and organized.

In organic chemistry, scientists must describe exactly how atoms are arranged in space. This study of spatial arrangement is called stereochemistry. One important way to describe these shapes is through E–Z notation. This system is the preferred method used by IUPAC. IUPAC is the international group that sets rules for naming chemical substances. E–Z notation is an extension of an older system called cis–trans notation. While cis–trans only describes relative shapes, E–Z can describe more complex arrangements. It can be used for double bonds that have two, three, or even four substituents attached to them.

To use this notation, a chemist must follow specific steps called the Cahn–Ingold–Prelog rules. These are often called CIP rules. First, the chemist looks at the substituents, which are the groups of atoms attached to the double bond. Each substituent is assigned a priority based on the CIP rules. There is one strict requirement for using this system. Neither atom in the double bond can have two identical atoms or groups attached to it. If the atoms are identical, the E–Z system cannot be used to describe the shape.

Once priorities are assigned, the chemist compares the positions of the two highest-priority groups. The notation depends on whether these groups are on the same side or opposite sides of the bond. If the two higher-priority groups are on opposite sides, the configuration is called E. This letter comes from the German word *entgegen*, which means "opposite." If the two higher-priority groups are on the same side, the configuration is called Z. This letter comes from the German word *zusammen*, which means "together."

There are specific rules for how to write these names correctly in a scientific paper. The letters E and Z are always printed as full capitals. They are also written in italic type and placed inside parentheses. A hyphen must separate these parentheses from the rest of the chemical name. For example, a molecule might be named (E)-But-2-ene. It is important to note that these letters do not count as the first letter of the name for English capitalization rules.

Sometimes, the priority of atoms might seem surprising to a student. The CIP rules determine priority based on specific atomic properties. For instance, in the molecule (E)-1-Bromo-1,2-dichloroethene, the rules assign a higher priority to bromine than to chlorine. Chlorine is then assigned a higher priority than hydrogen. This specific order determines whether the molecule is labeled E or Z. Following these precise rules ensures that every chemist in the world understands the exact shape of the molecule being discussed.

Large molecules can have many double bonds, which makes naming them more difficult. To solve this, chemists use locants, which are numbers that indicate the position of each bond. This allows them to specify the geometry of every single E or Z detail in a long chain. A complex example is the molecule alitretinoin. Its full name is (2E,4E,6Z,8E)-3,7-dimethyl-9-(2,6,6-trimethyl-1-cyclohexenyl)nona-2,4,6,8-tetraenoic acid. This long name tells us the geometry of four different bonds. The alkenes at positions 2, 4, and 8 are all E, while the one at position 6 is Z.

There are times when a scientist might not be certain about a molecule's exact shape. In these cases, they can use the prefix "E/Z-" to indicate uncertainty in the stereochemistry. This is helpful when dealing with a mixture of different isomers. For graphical representations in textbooks or papers, scientists use specific symbols. Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry. In the past, some people used a crossed double bond to show this. However, IUPAC no longer considers the crossed double bond an acceptable style for general use, though some computer software may still require it.