5.2: Geometric Isomers and E/Z Naming System (2024)

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    Geometric Isomers of Alkenes

    In the discussions about 1,2-dimethylcyclohexane in Chapter 4, we have learned that there are two geometric isomers possible for that compound, that are cis and trans. The restricted C-C bond rotation of cyclic structure result in the cis or trans isomer of 1,2-dimethylcyclohexane. Restricted rotation also can be caused by a double bond, so geometric isomers apply to some alkenes as well.

    5.2: Geometric Isomers and E/Z Naming System (2)

    For the example of 2-butene, the condensed structural formula CH3-CH=CH-CH3 does not really represent the trigonal planar shape of the sp2 carbons with double bonds. To show the shape explicitly, we need to draw the Kekulé structure that show all the bond angles. Then it will be noticed that there are two different shapes of 2-butene, with the CH3 groups on either the same side or opposite side of the double bond.

    5.2: Geometric Isomers and E/Z Naming System (3)

    They are geometric isomers and can be labelled as cis or trans in a similar way as disubstituted cycloalkane. Cis/trans is the common designation for geometric isomers and might be ambiguous for some structures, here we will learn the IUPAC naming system for geometric isomers of alkene, that is the E/Z naming system.

    E/Z Naming System

    To do the E/Z designation, at first, the groups connected on each sp2 double bond carbon will be assigned the priority based on the atomic number (see following guidelines for details), then the isomer with same priority group on the same side of double bond is assigned as “Z”, and the isomer with the same priority group on the opposite side of double bond is called “E”. Both E and Z come from German, “Zusammen” means same side and “Entgegen” means opposite.

    5.2: Geometric Isomers and E/Z Naming System (4)

    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.

    5.2: Geometric Isomers and E/Z Naming System (5)

    For the above structure of 2-penetene: on the left side sp2 carbon, methyl group CH3 is higher than hydrogen atom because C > H; on the right side sp2 carbon, ethyl group CH2CH3, is also higher than hydrogen. With higher priority group on both side of the double bond, this is the Z isomer, the complete name of the compound is (Z)-2-pentene.

    The group withhigher priority is labelled as #1, and the group with lower priority is labelled as #2 in this book.

    2. If the two groups bonded directly on an sp2 carbon start with the same atom, means there is a tie from step 1, then we move on to the atoms that connected to the “tied” atom, priority increases as the atomic number of the next attached atom increases.

    5.2: Geometric Isomers and E/Z Naming System (6)

    For the above structure, it is obvious that Cl is higher than C (C of CH2CH3 group) on the right side sp2 carbon.

    On the left side sp2 carbon, we need to compare between methyl CH3 group and ethyl CH2CH3 group. Both groups has carbon atom attached directly on the sp2 carbon, that is a tie. In CH3 group, the carbon atom is bonded to H, H, H; while in CH2CH3 group, the carbon atom is bonded with H, H, C. So ethyl CH2CH3 is higher than methyl CH3 (see Note below). With higher priority group on opposite side of the double bond, this is the E isomer, the complete name of the compound is: (E)-3-chloro-4-methyl-3-hexene.

    Note #1: For this round of comparison between H, H, H and H, H, C, compare the single atom with the greatest number in one group verse the single atom with the greatest number in the other group. So H in one group verse C in the other group, since C > H, therefore CH2CH3 is higher than CH3. Remember do not add the atomic numbers. For example, if one group has C, C, C, and the other group has C, O, H, then the C, O, H side is higher because O is higher than C.

    Note #2: The above compound is cis-isomer if using the cis/trans naming system (both ethyl group are on the same side of double bond), but is E-isomer for E/Z system. So the cis/trans and E/Z are two different naming systems, don’t always match.

    3. Repeat step 2 if necessary, until the priority is assigned.

    Examples: What is the correct structural formula of (E)-2-bromo-3-chloro-2-butene?

    5.2: Geometric Isomers and E/Z Naming System (7)

    The answer is B.

    Examples: Draw the structure of (E)-3-methyl-2-pentene

    Answer

    5.2: Geometric Isomers and E/Z Naming System (8)

    Examples: Order the following groups based on increasing priority.

    5.2: Geometric Isomers and E/Z Naming System (9)

    Approach:

    1st round: C, C, C, C (tie);

    2nd round:

    A: C bonded to C, C, C; (3rd)

    B: C bonded to H, Cl, Cl; (Cl is the 2nd high)

    C: C bonded to H, C, C; (4th)

    D: C bonded to H, H, Br (Br is the highest)

    Solution: C < A < B < D

    Exercises 5.1

    Order the following groups based on decreasing priority for E/Z naming purpose.

    5.2: Geometric Isomers and E/Z Naming System (10)

    Answers to Practice Questions Chapter 5

    4. When multiple bond is part of the group, the multiple bond is treated as if it was singly bonded to multiple of those atoms. Specifically:

    5.2: Geometric Isomers and E/Z Naming System (11)

    For these three groups involve multiple bonds, they all start with the carbon atom (the carbon atom highlighted in blue color), and we should compare the group of atoms that connected on the blue carbon by converting the multiple bond to “multiple single bonds”, as shown above. So, if we compare the order of these three groups, it is:

    5.2: Geometric Isomers and E/Z Naming System (12)

    Examples: Assign E/Z of the circled double bond.

    5.2: Geometric Isomers and E/Z Naming System (13)

    Thinking:

    5.2: Geometric Isomers and E/Z Naming System (14)

    The answer is: Z-isomer.

    5.2: Geometric Isomers and E/Z Naming System (2024)

    FAQs

    5.2: Geometric Isomers and E/Z Naming System? ›

    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 name Z and e isomers? ›

    To do the E/Z designation, at first, the groups connected on each sp2 double bond carbon will be assigned the priority based on the atomic number (see following guidelines for details), then the isomer with same priority group on the same side of double bond is assigned as “Z”, and the isomer with the same priority ...

    How do you name geometric isomers? ›

    Cis-trans (geometric) isomerism exists when there is restricted rotation in a molecule and there are two nonidentical groups on each doubly bonded carbon atom. The IUPAC naming is the same as alkene except for the addition of the cis or trans prefix.

    What does E and Z mean in Iupac naming? ›

    priority groups on same side. (Z) configuration. The Z isomer is designated as the isomer in which the top priority groups are on the same side (Z is taken from the German word zusammen- together). The E isomer has these groups on opposite sides (E, German for entgegen across).

    What are the various naming systems of geometrical isomers? ›

    Based on the position of the functional group the isomers are named cis and trans. When a similar group is placed at an adjacent position the compound is named with the prefix cis. When a similar group is placed at the opposite position the compound is named with the prefix trans.

    What is the E and Z nomenclature of geometrical 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 are the rules for E and Z isomers? ›

    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 are the rules for geometric isomers? ›

    To get geometric isomers you must have: restricted rotation (often involving a carbon-carbon double bond for introductory purposes); two different groups on the left-hand end of the bond and two different groups on the right-hand end.

    How to separate e and Z isomers? ›

    E and Z isomers of alkene alcohols and/or alkene alcohol derivatives are separated by substantially continuously contacting an ion exchange medium which is ion exchanged with silver and/or copper ions with the feed stream comprising the E and Z isomers of at least one alkene alcohol and/or at least one alkene alcohol ...

    What are examples of geometric isomers? ›

    Geometric Isomers Examples

    In cis-pentene, the carbons adjacent to the double bond are on the same side of the double bond. In trans-pentene, the carbons adjacent to the double bond are on different sides of the double bond. Difluoroethene is a substituted alkene.

    Are E and Z isomers 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 difference between E and Z in chemistry? ›

    The E stands for “entgegen,” meaning “opposite” in German. It indicates that the higher-priority substituents are on opposite sides of the double bond. The Z stands for “zusammen,” meaning “together” in German. It indicates that the higher-priority substituents are on the same side of the double bond.

    How do you distinguish e and Z isomers by NMR? ›

    It can be seen that the E -isomer forms aggregates at a lower f w of 70% than the Z -isomer, which indicates that the Z -isomer is more polar than the E -isomer.

    How to determine Z and e configuration? ›

    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.

    How to find geometrical isomers? ›

    Geometric isomers can occur where there is restricted rotation about a bond. To know whether a molecule exhibits geometrical isomerism or not, the molecule must: Restrict Rotation involving a carbon-carbon double bond. There should be two different compounds on the left hand side and right-hand side of the double bond.

    What is the difference between geometric isomers and isomers? ›

    Isomers: Compounds with the same molecular formula but different arrangements. Structural Isomers: Compounds with the same molecular formula but different arrangements of atoms in bonding. Geometric Isomers: Compounds with the same molecular formula but different arrangement of atoms in space and restricted rotation.

    How do you specify E or Z in stereochemistry? ›

    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”).

    How do you name different isomers? ›

    There are three parts in the naming of isomers:
    1. Prefix indicated by the branch group, which is alkyl group, attached to the longest carbon chain.
    2. The root name showing the number of carbon atoms in the longest carbon chain.
    3. The suffix showing the homologous series.

    How to separate e and z isomers? ›

    E and Z isomers of alkene alcohols and/or alkene alcohol derivatives are separated by substantially continuously contacting an ion exchange medium which is ion exchanged with silver and/or copper ions with the feed stream comprising the E and Z isomers of at least one alkene alcohol and/or at least one alkene alcohol ...

    Are E and Z 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.

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