Configurational isomers PDF

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Configurational isomers (stereoisomers): -

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Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but that differ only in the three-dimensional orientations of their atoms in space. Configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter (e.g., R- vs. S- or E- vs. Z-). This means that configurational isomers can be interconverted only by breaking covalent bonds to the stereocenter, for example, by inverting the configurations of some or all of the stereocenters in a compound. Interconversion of these stereoisomers requires the “breaking” of bonds. This does not occur at room temperature so the two forms may be “separated” from one another.

Conformational isomers (stereoisomers): -

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In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds (refer to figure on single bond rotation). Alkanes are free to rotate about the carbon-carbon single bond. o Giving rise to different spatial orientations of the hydrogen’s of the one carbon relative to those of the other These different structures are called conformational isomers or conformers. Conformational isomers are part of a class of isomers called stereoisomers. Stereoisomers possess the same constitution but differ in the arrangement of atoms in space. Different conformers cannot usually be isolated from one another as they rapidly interconnect under normal conditions due to low energy barriers.

8.3 Draw the representation cycloalkane

chair of 6-membered rings

The two ends of the alkane can 'meet up' to form a ring. These molecules are called cycloalkanes. When substituents are added on different positions on the ring, stereoisomers are possible.

Cycloaklanes are non-polar. Most of its bond angles are ~109.5o and all hyrdogens on adjacent carbons are perfectly staggered. The hydrogens also occupy two positions - axial and equatorial which are interconverted by ring "flipping".

Substituents n the ring prefers to occupy equatorial positions, minimising steric repulsions. This is because bulky groups prefer to be away from the H. Anything that is bulky prefers to be in equatorial positions to avoid clashing. The same is said for two substituents, in which they both prefer to be in the equatorial position.

Alkenes: Alkenes are “unsaturated” hydrocarbons that contain the carbon-carbon double bond. These molecules have the general formula CnH2n , and usually contain one or more carbon - carbon double bonds. The simplest alkane is ethene:

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All bond angles are 120˚ All bond lengths are 108 pm (picometre) N.B: 1 pm = 1 x 10-12m All C-H bond lengths are the same (108 pm) All H-C-H and H-C-C bond angles (120˚C) are the same C=C bond is composed of one σ bond and a weaker π bond (less efficient overlap of orbitals)

9.1 Identify alkene diastereomers as E or Z. Diastereomers: Configurational isomers which are not “mirror images” of each other – also known as diastereomers

The cis- trans- naming system only works with DISUBSTITUTED alkenes! Higher substituted alkenes must be described using (E)- and (Z)-, these are considered as configurational isomers. The E/Z system must be used for tri- and tetrasubstitued alkenes. If the substituents are all on one carbon atom or there is only one, then the (E)- and (Z)- is not needed.

The priority rules: 1. Look at the two atoms “directly attached” and assign priority to them based on their “atomic number”. a. The higher the atomic number of the atom attached, the higher the priority: Br > Cl >O>N>C>H 2. If identical atoms are attached, look at the second, third or fourth atom away until the first “difference” is found then go by atomic number. 3. If groups of higher priority are on the same side of the double bond, the configuration of the alkene is Z. (Ze Zame Zide) 4. If group of higher priority are on opposite sides of the double bond, the configuration is E.

Example

Name each of the following alkenes and specify its configuration using the E/Z system: Compound

Name

9.2 Identify the electrophiles and nucleophiles in a reaction. Electrophiles (positive, seeks negative): A species that seeks an electron pair. Nucleophiles (negative, seeks positive): A species that supplies an electron pair.

Consider hydrohalogenation: (addition of an unsymmetrical hydrogen halide reagent)

nucleophile

electrophile

CLASSIFICATION OF REACTION: ELECTROPHILIC ADDITION (we are adding an electrophile)

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The addition of HX to a symmetrical alkene can give only one product:

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the addition of HX to an unsymmetrical alkene can give two different products (constitutional isomers):

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The major product can always be predicted with Markovnikov’s rule, which states that the hydrogen of an unsymmetrical reagent adds to the end of the double bond that has the greater number of hydrogen atoms already attached. To rationalise this, we must consider the MECHANISM of the reaction, which leads to the introduction of the Curly Arrow Notation.

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Curly Arrow notation is used in organic chemistry as a way of visualising how bonds form and break during the course of a reaction. A curly arrow starts at the electron pair that moves which is either: o a lone pair on an atom (a new bond is formed) o a pair of electrons involved in a bond (an existing bond is broken) A curly arrow ends where the electron pair is in the products which is either: o Half a bond away from the atom to which the electron pair has moved (a new bond is formed) o Or on the atom to which electron pair has moved (an existing bond is broken)

9.3 Follow a reaction mechanism using curly arrows. 9.4 Predict the major products obtained from the reaction of alkenes with electrophiles (using markovnikov's rule)

Hydrohalogenation of Alkenes: Electrophilic addition  Markovnikov’sruleapplies:

Dilute H2SO4 - sulfuric acid is a strong acid and it dissociates in water to give H3O+ and HSO4-. These conditions are a source of H3O+.

2 main steps ...... also a 3 rd mini step!

Step 1: x

H+ adds to give a carbocation – secondary carbocation is more stable

Step 2: x

H2O intercepts the carbocation (using spare electrons on O)

Step 3: x

H+ is removed by HSO4- (the conjugate base of H2SO4) or by another water molecule.

Which carbocation is more stable depends on how many hydrogen atoms are on the carbon already, the carbocation with the addition of hydrogen to where there is already hydrogen is more stable.

The tertiary carbocation intermediate is formed more easily than the primary carbocation intermediate.

Hydration of Alkenes: Electrophilic Addition These reactions occur by 2 discrete steps:

Hydrogenation - electrophilic addition x

Catalyst required (e.g. palladium on charcoal) to break strong H-H bond

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Both H's added to same face of C=C

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Mechanism more complex (H2 adsorbed onto Pd surface)

Halogenation - electrophilic addition

 Same sort of reaction mechanism – Markovnikov’srule“applies”:

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As the halogen approaches, it is polarised by the alkene.

Step 1: x

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"Cl " adds to give a carbocation (the alkene reacts with one end of the polarised halogen)

Thereisno“H+”toadd,so Markovnikov’srulestrictlydoes not apply. Cl+ adds just like H+ did with hydrohalogenation

Step 2: x

X2 must be added using a non-polar organic solvent (such as CCl4). No water can be present because it will interfere with the reaction.

Cl- adds (the carbocation is intercepted by the chloride anion)...