Halogenoalkanes PDF

Preview

Summary

HalogenoalkanesNaming- always alphabeticalPolarity Increase in electronegativity up the group  Bonds between carbon and halogen are polar  Bigger dipole with F compared to IStereoisomers Same molecular formula however different structural arrangement in space  Conformational isomers  Geometric...

Description

Halogenoalkanes Naming- always alphabetical Polarity   

Increase in electronegativity up the group Bonds between carbon and halogen are polar Bigger dipole with F compared to I

Stereoisomers   

Same molecular formula however different structural arrangement in space Conformational isomers Geometric isomers

Chiral Structures  

Mirror image of another structure however non-superimposable 4 different groups surrounding a central C atom

Chiral v Achiral 

Chiral must have 4 different groups connected to the C atom, achiral does not

Stereocenter 

A chiral carbon atom is termed as an asymmetric carbon

Enantiomers   

Pairs of chiral molecules Racemic mixture= an equimolar mixture of the 2 enantiomers that have no overall effect on the plain of polarised light If starting materials are achiral the products are chiral- the reaction forms a racemic mixture

Racemic Mixture   

Many drugs are chiral with only 1 enantiomers possessing the desired properties Opposite enantiomers may be biologically active, active in a different way to produce undesirable effect Thalidomide drug given to pregnant women

Enantiomers in Nature  

Many chiral molecules are present in nature as single enantiomers as enzymes are only specific to 1 of the enantiomers Termed “enantiomerically pure”

Describing an Enantiomer R+S System 1. 2. 3. 4.

Identify chiral centre Assign priority number to each group via atomic number If molecules are the same look at which atoms they are bonded to Arrange the lowest priority group is pointing away from you

5. Arrange 1,2,3 around the circle, anticlockwise (S) or clockwise (R) Detecting Enantiomers  

They have identical properties Differ in the way they which they rotate the plain of polarised light

Diasterioisomers  

Molecules with more than 1 chiral centre Stereoisomers that are not mirror images of one another are called diasterioisomers

Reactions of Halogenoalkanes 1. Nucleophilic substitution (OH- is the nucleophile) 2. Elimination (where the nucleophile acts as a base Nucleophilic Substitution   

Nucleophile= A chemical that donates an electron pair SN1 mechanism -formation of a carbocation intermediate then the formation of a substituted product SN 2 -leaving group leaves and

SN2 Mechanism 1. Only 1 transition state 2. Rate is dependent on [nucleophile] and [halogenoalkane] therefore is 2nd order reaction 3. Rate=k[nucleophile][halogenoalkane] SN1 Mechanism 1. Cl- leaves forming a carbocation (slow step) Rate is completely dependent on this step 2. Nucleophile attacks (fast step) this step can form enantiomer due to the nucleophile can attack from various angles 3. 2 transition states 4. Rate is first order 5. Rate=k[halogenoalkane] Hyperconjugation     

One bonding pair stabilising empty p-orbital in primary carbocation Two bonding pairs for secondary Three boding pairs for tertiary Therefore SN2 reaction occurs for primary carbon structures SN1 for tertiary carbon structures

Elimination

1. E1- Halogen leaves, nucleophile will the removed H Rate=k[alkyl halide] 2. E2- Both steps occur at the same time Rate=k[alkyl halide][nucleophile] E2 

Electronegative Oxygen will remove the H

E v Sub   

Dependant on the nucleophile Strong base attacks H Weak base will attack at carbon atoms and undergo substitution...