The E-Z system for naming alkenes (2024)

  1. Last updated
  2. Save as PDF
  • Page ID
    846
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)

    \( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)

    ( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\id}{\mathrm{id}}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\kernel}{\mathrm{null}\,}\)

    \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\)

    \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\)

    \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    \( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)

    \( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)

    \( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)

    \( \newcommand{\vectorC}[1]{\textbf{#1}}\)

    \( \newcommand{\vectorD}[1]{\overrightarrow{#1}}\)

    \( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}}\)

    \( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)

    \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)

    \(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)

    The traditional system for naming the geometric isomers of an alkene, in which the same groups are arranged differently, is to name them as cis or trans. However, it is easy to find examples where the cis-trans system is not easily applied. IUPAC has a more complete system for naming alkene isomers. The R-S system is based on a set of "priority rules", which allow you to rank any groups. The rigorous IUPAC system for naming alkene isomers, called the E-Z system, is based on the same priority rules.These priority rules are often called the Cahn-Ingold-Prelog (CIP) rules, after the chemists who developed the system

    The general strategy of the E-Z system is to analyze the two groups at each end of the double bond. At each end, rank the two groups, using the CIP priority rules, discussed in Ch 15. Then, see whether the higher priority group at one end of the double bond and the higher priority group at the other end of the double bond are on the same side (Z, from German zusammen = together) or on opposite sides (E, from German entgegen = opposite) of the double bond.

    Example 1: Butene

    The Figure below shows the two isomers of 2-butene. You should recognize them as cis and trans. Let's analyze them to see whether they are E or Z.

    The E-Z system for naming alkenes (1)

    Start with the left hand structure (the cis isomer). On C2 (the left end of the double bond), the two atoms attached to the double bond are C and H. By the CIP priority rules, C is higher priority than H (higher atomic number). Now look at C3 (the right end of the double bond). Similarly, the atoms are C and H, with C being higher priority. We see that the higher priority group is "down" at C2 and "down" at C3. Since the two priority groups are both on the same side of the double bond ("down", in this case), they are zusammen = together. Therefore, this is (Z)-2-butene.

    Now look at the right hand structure (the trans isomer). In this case, the priority group is "down" on the left end of the double bond and "up" on the right end of the double bond. Since the two priority groups are on opposite sides of the double bond, they are entgegen = opposite. Therefore, this is (E)-2-butene.

    E,Z will always work, even when cis,trans fails

    In simple cases, such as 2-butene, Z corresponds to cis and E to trans. However, that is not a rule. This section and the following one illustrate some idiosyncrasies that happen when you try to compare the two systems. The real advantage of the E-Z system is that it will always work. In contrast, the cis-trans system breaks down with many ambiguous cases.

    Example 2

    The following figure shows two isomers of an alkene with four different groups on the double bond, 1-bromo-2-chloro-2-fluoro-1-iodoethene.

    The E-Z system for naming alkenes (2)

    It should be apparent that the two structures shown are distinct chemicals. However, it is impossible to name them as cis or trans. On the other hand, the E-Z system works fine... Consider the left hand structure. On C1 (the left end of the double bond), the two atoms attached to the double bond are Br and I. By the CIP priority rules, I is higher priority than Br (higher atomic number). Now look at C2. The atoms are Cl and F, with Cl being higher priority. We see that the higher priority group is "down" at C1 and "down" at C2. Since the two priority groups are both on the same side of the double bond ("down", in this case), they are zusammen = together. Therefore, this is the (Z) isomer. Similarly, the right hand structure is (E).

    E,Z will work, but may not agree with cis,trans

    Consider the molecule shown at the left.

    The E-Z system for naming alkenes (3)

    This is 2-bromo-2-butene -- ignoring the geometric isomerism for now. Cis or trans? This molecule is clearly cis. The two methyl groups are on the same side. More rigorously, the "parent chain" is cis.

    E or Z? There is a methyl at each end of the double bond. On the left, the methyl is the high priority group -- because the other group is -H. On the right, the methyl is the low priority group -- because the other group is -Br. That is, the high priority groups are -CH3 (left) and -Br (right). Thus the two priority groups are on opposite sides = entgegen = E.

    Note

    This example should convince you that cis and Z are not synonyms. Cis/trans and E,Z are determined by distinct criteria. There may seem to be a simple correspondence, but it is not a rule. Be sure to determine cis,trans or E,Z separately, as needed.

    Multiple double bonds

    If the compound contains more than one double bond, then each one is analyzed and declared to be E or Z.

    Example 3

    The E-Z system for naming alkenes (4)

    The configuration at the left hand double bond is E; at the right hand double bond it is Z. Thus this compound is (1E,4Z)-1,5-dichloro-1,4-hexadiene.

    The double-bond rule in determining priorities

    Example 4

    Consider the compound below:

    The E-Z system for naming alkenes (5)

    This is 1-chloro-2-ethyl-1,3-butadiene -- ignoring, for the moment, the geometric isomerism. There is no geometric isomerism at the second double bond, at 3-4, because it has 2 H at its far end.

    What about the first double bond, at 1-2? On the left hand end, there is H and Cl; Cl is higher priority (by atomic number). On the right hand end, there is -CH2-CH3 (an ethyl group) and -CH=CH2 (a vinyl or ethenyl group). Both of these groups have C as the first atom, so we have a tie so far and must look further. What is attached to this first C? For the ethyl group, the first C is attached to C, H, and H. For the ethenyl group, the first C is attached to a C twice, so we count it twice; therefore that C is attached to C, C, H. CCH is higher than CHH; therefore, the ethenyl group is higher priority. Since the priority groups, Cl and ethenyl, are on the same side of the double bond, this is the Z-isomer; the compound is (Z)-1-chloro-2-ethyl-1,3-butadiene.

    The "first point of difference" rule

    Which is higher priority, by the CIP rules: a C with an O and 2 H attached to it or a C with three C? The first C has one atom of high priority but also two atoms of low priority. How do these "balance out"? Answering this requires a clear understanding of how the ranking is done. The simple answer is that the first point of difference is what matters; the O wins.

    The E-Z system for naming alkenes (6)

    To illustrate this, consider the molecule at the left. Is the double bond here E or Z? At the left end of the double bond, Br > H. But the right end of the double bond requires a careful analysis.

    At the right hand end, the first atom attached to the double bond is a C at each position. A tie, so we look at what is attached to this first C. For the upper C, it is CCC (since the triple bond counts three times). For the lower C, it is OHH -- listed in order from high priority atom to low. OHH is higher priority than CCC, because of the first atom in the list. That is, the O of the lower group beats the C of the upper group. In other words, the O is the highest priority atom of any in this comparison; thus the O "wins".

    Therefore, the high priority groups are "up" on the left end (the -Br) and "down" on the right end (the -CH2-O-CH3). This means that the isomer shown is opposite = entgegen = E. And what is the name? The "name" feature of ChemSketch says it is (2E)-2-(1-bromoethylidene)pent-3-ynyl methyl ether.

    Contributors

    >Robert Bruner (http://bbruner.org)

    The E-Z system for naming alkenes (2024)

    FAQs

    The E-Z system for naming alkenes? ›

    The E/Z

    E/Z
    Cis–trans isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes.
    https://en.wikipedia.org › wiki
    system analyzes the two substituents attached to each carbon in the double bond and assigns each either a high or low priority. If the higher priority group on both carbons in the double bond the same side the alkene is said to have a Z isomer (from German zusammen = together).

    What is the e and Z system of nomenclature? ›

    If both substituents ranked 1 are on the same side of the pi bond, the bond is given the descriptor Z (short for German Zusammen, which means “together”). If both substituents ranked 1 are on the opposite side of the pi bond, the bond is given the descriptor E (short for German Entgegen, which means “opposite”).

    What do e and Z mean in chemistry? ›

    Then, see whether the higher priority group at one end of the double bond and the higher priority group at the other end of the double bond are on the same side (Z, from German zusammen = together) or on opposite sides (E, from German entgegen = opposite) of the double bond.

    How to identify E-Z? ›

    The rigorous IUPAC system for naming alkene isomers, called the E-Z system, is based on the same priority rules. The general strategy of the E-Z system is to analyze the two groups at each end of the double bond. At each end, rank the two groups, using the CIP priority rules, discussed in Ch 15.

    How do you assign the alkene stereochemistry as E or Z? ›

    If the higher-ranked groups on each carbon are on the same side of the double bond, the alkene is said to have a Z configuration, for the German zusammen, meaning “together.” If the higher-ranked groups are on opposite sides, the alkene has an E configuration, for the German entgegen, meaning “opposite.” (For a simple ...

    What is the EZ system of naming alkenes? ›

    The E/Z system analyzes the two substituents attached to each carbon in the double bond and assigns each either a high or low priority. If the higher priority group on both carbons in the double bond the same side the alkene is said to have a Z isomer (from German zusammen = together).

    How to determine priority for EZ? ›

    To do so, each substituent is assigned a priority using the Cahn–Ingold–Prelog rules. If the two higher priority substituents are on same side of the double bond, the configuration of the bond is Z. If the two higher priority substituents are on opposite sides of the double bond, the configuration of the bond is E.

    What is Z alkene? ›

    Determine the relative position of the two higher priority groups. If they are on the same side then it is a (Z)-alkene (German; zusammen = together) If they are on opposite sides then it is an (E)-alkene (German; entgegen = opposite)

    What does Z mean in chemistry nomenclature? ›

    Z: Describes the configuration of a double bond in which the two groups of highest Cahn-Ingold-Prelog priority are cis. From the German zusammen, meaning together. (2E,4Z)-2,4-hexadiene. (1E,3Z)-2-bromo-1-chloro-4-iodo-1,3-pentadiene.

    Which alkene shows EZ isomerism? ›

    For example, an alkene such as but-2-ene has two different groups on each alkene carbon. It can exist as E−Z isomers that differ in the positions of the substituents on the double-bonded atoms.

    Why do some alkenes exist as E-Z stereoisomers? ›

    E/Z isomerism occurs in alkenes. Alkanes are unsaturated hydrocarbons with C=C bonds and general formula CnH2n. E/Z isomerism occurs due to restricted rotation about C=C double.

    What is the E-Z notation in stereochemistry? ›

    E and Z notation are only used when a compound doesn't have two identical substituents. Following the Cahn–Ingold–Prelog priority rules (CIP rules), each substituent on a double bond is assigned a priority, then positions of the higher of the two substituents on each carbon are compared to each other.

    Is E-Z based on atomic number? ›

    The guidelines for assigning group priority in E/Z naming system. 1. Priority is assigned based on the atomic number of the atoms bonded directly to the sp2 double bond carbon, the larger the atomic number, the higher the priority (isotopes with higher mass number has higher priority). For example: S > O > N > C > H.

    Is there stereochemistry of alkenes? ›

    Shown below is an example of an E-alkene: notice that, although the two methyl groups are on the same side relative to one another, the alkene has E stereochemistry according to the rules of the E/Z system because one of the methyl groups takes a higher priority (relative to a hydrogen) and the other takes lower ...

    Are E and Z double bonds enantiomers? ›

    E/Z isomers of alkenes are "stereoisomers that are not enantiomers" too! So they are diastereomers. The double bond is the non-chiral source of the stereochemistry.

    What is the relationship between E and Z isomers? ›

    Then, the isomer with the same priority group on the same side of the double bond is assigned “Z”, and the isomer with the same priority group on the opposite side of the double bond is called “E”. Both E and Z come from German: “Zusammen” means “same side” and “Entgegen” means “opposite”.

    How do you know if a structure is E or Z? ›

    Determine whether the two highest priority groups on each carbon atom are on the same side (Z) or opposite sides (E) of the double bond. If the two highest priority groups on each carbon atom are on the same side of the double bond, it is a Z isomer. If they are on opposite sides, it is an E isomer.

    What is the difference between Z and E isomers? ›

    Then, the isomer with the same priority group on the same side of the double bond is assigned “Z”, and the isomer with the same priority group on the opposite side of the double bond is called “E”. Both E and Z come from German: “Zusammen” means “same side” and “Entgegen” means “opposite”.

    What does the E stand for in chemistry? ›

    A common abbreviation for electron is e or e-.

    Top Articles
    Latest Posts
    Recommended Articles
    Article information

    Author: Pres. Lawanda Wiegand

    Last Updated:

    Views: 6287

    Rating: 4 / 5 (51 voted)

    Reviews: 82% of readers found this page helpful

    Author information

    Name: Pres. Lawanda Wiegand

    Birthday: 1993-01-10

    Address: Suite 391 6963 Ullrich Shore, Bellefort, WI 01350-7893

    Phone: +6806610432415

    Job: Dynamic Manufacturing Assistant

    Hobby: amateur radio, Taekwondo, Wood carving, Parkour, Skateboarding, Running, Rafting

    Introduction: My name is Pres. Lawanda Wiegand, I am a inquisitive, helpful, glamorous, cheerful, open, clever, innocent person who loves writing and wants to share my knowledge and understanding with you.