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Isomerism Isomerism of organic compound What are isomers? Isomers are molecules with the same molecular formula, but different arrangements of atoms. There are different types of isomers, shown by the diagram.

Isomerism - Warszawski Uniwersytet Medyczny · 2016. 10. 21. · Chain Isomerism Chain isomerism occurs when the way carbon atoms are linked together is different from compound to

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  • Isomerism

    Isomerism of organic compound

    What are isomers?

    Isomers are molecules with the same

    molecular formula, but different

    arrangements of atoms. There are different

    types of isomers, shown by the diagram.

  • Isomerism

    ../chain.htm../isomers.htm#structural../positional.htm../functional.htm../geometrical.htm../isomers.htm#stereo../optical.htm

  • Structural Isomerism

    Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of at course: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism (where there are two isomers are known as the keto and enol isomers).

    http://www.creative-chemistry.org.uk/molecules/chain.htmhttp://www.creative-chemistry.org.uk/molecules/positional.htmhttp://www.creative-chemistry.org.uk/molecules/functional.htmhttp://www.creative-chemistry.org.uk/molecules/functional.htm

  • Chain Isomerism

    Chain isomerism occurs when the way carbon atoms are linked together is different from compound to compound. It is an example of structural isomerism, and is also called nuclear isomerism.

    see the isomers of pentane pentane 2-methylbutane 2,2-dimethylpropane

    C5H12

    http://www.creative-chemistry.org.uk/molecules/hexane.htm

  • Positional Isomerism

    Positional isomerism, an example of

    structural isomerism, occurs when

    functional groups are in different positions

    on the same carbon chain. You usually

    meet positional isomers of alcohols and

    alkenes.

    butan-1-ol butan-2-ol

  • but-1-ene but-2-ene

    Note: this is cis-

    but-2-ene, which

    has a geometric

    isomer called

    trans-but-2-ene

    (select here to find

    out more)

    2-methylphenol 3-methylphenol 4-methylphenol

    http://www.creative-chemistry.org.uk/molecules/geometrical.htm

  • Functional Isomerism

    Functional isomerism, an example of

    structural isomerism, occurs substances

    have the same molecular formula but

    different functional groups. This means that

    functional isomers belong to different

    homologous series. There are two functional

    group isomers of which you need to be

    aware:

    alcohols and ethers

    aldehydes and ketones

  • Ethanol methoxymethane C2H6O Alcohols have the

    hydroxyl group,

    –OH.

    Ethers have the

    functional group

    R–O–R'.

  • Propanal propanone (acetone) C3H6O

    Aldehydes and ketones both have the carbonyl group C=O. In ketones this is attached to two carbon atoms; in aldehydes it is attached to 1 or 2 hydrogen atoms.

  • Stereo-isomerism

    Stereo-isomerism occurs when the atoms in a molecule can have different arrangements in space. There are two types of stereo-isomerism: geometrical isomerism and optical isomerism. Geometrical isomers can have very different physical properties, such as different melting points, but they tend to have the same chemical properties. Optical isomers have the same chemical and physical properties, except that one structure rotates the plane of polarised light to the right and the other rotates it to the left.

    http://www.creative-chemistry.org.uk/molecules/geometrical.htmhttp://www.creative-chemistry.org.uk/molecules/optical.htm

  • Geometrical Isomerism

    Geometrical isomerism is an example of stereo-isomerism. This occurs when substances have the same molecular formula, but a different arrangment of their atoms in space. There are three ways that this can happen:

    where there is a C=C bond in the molecule;

    where a molecule has rings; or

    where there is a >C=N bond.

  • cis-but-2-ene trans-but-2-ene

    Where like groups are on the same side of the double bond, we

    call it a cis isomer; where they are on opposite sides we call

    it a trans isomer. but-1-ene

    Although but-1-ene contains a C=C bond, it does not form

    geometrical isomers. Take care - look for different groups

    on the double-bonded carbon atoms!

    C C

    CH3

    H

    CH3

    H

    C C

    H

    H

    CH3

    CH3

    C C

    H

    H

    H

    CH2 CH3

  • Optical Isomerism

    Optical isomerism is an example of stereoisomerism. It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram).

    No matter how hard you try, the molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers

  • Optical isomers can occur when there is an asymmetric carbon atom.

    An asymmetric carbon atom is one which is bonded to four different

    groups. The groups can be something hideously complex, or

    something comfortably simple like a hydrogen or chlorine atom.

    Remember:

    * there must be four groups, and

    * they must be different.

  • Tro, Chemistry: A Molecular

    Approach

    15

    Nonsuperimposable Mirror

    Images

    mirror image cannot be rotated so all its atoms align

    with the same atoms of the original molecule

  • Tro, Chemistry: A Molecular

    Approach

    16

    Chirality

    any molecule with a nonsuperimposable mirror image is said to be chiral

    any carbon with 4 different substituents will be a chiral center

    a pair of nonsuperimposable mirror images are called a pair of enantiomers

  • Tro, Chemistry: A Molecular

    Approach

    17

    Optical Isomers of 3-methylhexane

  • Tro, Chemistry: A Molecular

    Approach

    18

    Plane Polarized Light

    light that has been filtered so that only

    those waves traveling in a single plane are

    allowed through

  • Tro, Chemistry: A Molecular

    Approach

    19

    Optical Activity

    a pair of enantiomers have all the same physical properties except one – the direction they rotate the plane of plane polarized light

    each will rotate the plane the same amount, but in opposite directions

    dextrorotatory = rotate to the right

    levorotatory = rotate to the left

    an equimolar mixture of the pair is called a racemic mixture

    rotations cancel, so no net rotation

  • 20

    Chemical Behavior of

    Enantiomers

    a pair of enantiomers will have the same chemical

    reactivity in a non-chiral environment

    but in a chiral environment they may exhibit different

    behaviors

    enzyme selection of one enantiomer of a pair

  • Optical isomers can rotate the plane of polarisation of plane-polarised light:

    one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;

    the other rotates the polarised light anticlockwise (to the left) and is called the (–) enantiomer.

    A mixture containing equal concentrations of the (+) and (–) enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate.

  • (+)-lactic acid (–)-lactic acid

    Lactic acid (2-hydroxypropanoic acid) is a fairly

    common and simple example of optical isomerism. The

    (+) enantiomer of lactic acid is found in muscle. Sour

    milk contains a racemic mixture of the two enantiomers.

  • D-glyceraldehyde L-glyceraldehyde

    2,3-dihydroxypropanal (glyceraldehyde) is a fairly common and simple example of optical isomerism, too. This is because glyceraldehyde is used as a standard by which other chiral molecules are compared. There are two enantiomers of glyceraldehyde, depending on the position of the –OH (hydroxyl) group on the molecule. These are known as D-glyceraldehyde and L-glyceraldehyde.

    The little capital letters D and L are deliberate. The positions of the hydroxyl groups on other chiral molecules can be compared with glyceraldehyde to see if they are the D-enantiomer or the L-enantiomer. This is very common in Biology (natural sugars are D-enantiomers and amino acids are L-enantiomers). However, knowing whether a molecule is the D or L-enantiomer does not tell us whether it is the (+) or (–) enantiomer – so be careful!

  • MOLECULES WITH TWO OR MORE

    STEREOCENTERS

    To generalize, for a molecule with n stereocenters, the

    maximum number of stereoisomers possible is 2n2.

    For a molecule with one srereocenter, 21 = 2

    stereoisomers are possible, for a molecule with two

    srereocenters, 22 = 4 stereoisomers are possible.

  • Enantiomers and Diastereoisomers

    C

    C

    CHO

    OHH

    H OH

    CH2OH

    C

    C

    CHO

    HHO

    HO H

    CH2OH

    C

    C

    COH

    HHO

    H OH

    CH2OH

    C

    C

    CHO

    OHH

    HO H

    CH2OH

    (a) (b)

    One pair of enantiomers(Erythrose)

    A second pair of enantiomers(Erythrose)

    (d)(c)

    Stereoisomers (a) and (b) are nonsuperposable mirror images and are,

    therefore, a pair of enantiomers. Stereoisomers (c) and (d) are also

    nonsuperposable mirror images and are a second pair of enantiomers.

    Diastereoisomers are stereoisomers that are not mirror images.

    Compound (a) and (c), (a) and (d), (b) and (c), (b) and (d) are

    diastereisomers.

  • 26

    Figure 5.8 Summary: The four

    stereoisomers of 2,3-

    dibromopentane

  • Meso Compounds

    C

    C

    COOH

    OHH

    H OH

    COOH

    C

    C

    COOH

    HHO

    HO H

    COOH

    C

    C

    COOH

    HHO

    H OH

    COOH

    C

    C

    COOH

    OHH

    HO H

    COOH

    (a) (b)

    A pair of enantiomers A meso compound

    (d)(c)

    A plane of symmetry

    In tartaric acid, carbon 2 and 3 are stereocenters, and, using the 2n rule, the maximum number of stereoisomers possible is

    22 = 4. Structures (a) and (b) are nonsuperposable mirror images and are, therefore, a pair of enantiomers. Structures (c)

    and (d) are also mirror images, but they are superposable. Therefore, (c) and (d) are not different molecules; they are the

    same molecule, just oriented differently. Because (c) and its mirror image are superposable, (c) is achiral.

    Another way to determine that (c) is achiral is to see that it has a plane of symmetry that bisects the molecule in such a way

    that the top half is the reflection of the bottom half. Thus, even though (c) has two stereoisomers, it is achiral . The

    stereoisomer of tartaric acid represented by (c) or (d) is called a meso compound. A meso compound is an achiral

    compound that contains two or more stereocenters.

  • 28

    Figure 5.9 Summary: The three

    stereoisomers 2,3-

    dibromobutane

  • • Larger organic molecules can have two, three or

    even hundreds of stereogenic centers.

  • 30

    How many stereogenic centers does each molecule have?

    a)

    Br

    Br

    b)

  • 31

    c)

    H2N

    HN

    NH

    OH

    CO2H

    O

    SH

    O

    O

    Only carbons attached to four different groups.

  • • Stereogenic centers may also occur at carbon

    atoms that are part of a ring.

    • To find stereogenic centers on ring carbons, always

    draw the rings as flat polygons, and look for

    tetrahedral carbons that are bonded to four different

    groups.

  • 33

    Locate the stereogenic center in the following:a)

    and b)

    No stereogenic

    centers.

    b)

    O

    a)