+ 20.1Prepare a student guide on naming of the following 6 C atoms MAXIMUM :- Amine Amide Ester Nitrile Alcohol Aldehyde Ketone Carboxylic acid Halide MUST : Include functional group with 2 examples of each homologous series SHOULD : include branched examples COULD : include primary, secondary, tertiary
+ Nucleophilic Substitution Reactions Nucleophiles Why is hydroxide better than water ?
+ 20.2.2 Identity of the halogen Consider the R-X bond RCl<RBr< RI
+ 20.2.3 Nature of halogen effect on rate Review : Sn1 Sn2 reaction mechanisms, and rate data Rate : Tertiary > Secondary > Primary Sn1 route is faster as transition state ofSn2 has a higher activation energy.
+ Study Questions Why don’t primary halogenoalkanes use the Sn1 mechanism. Why don’t tertairyhalogenoalkanes use Sn2 ?
+ Starter Explain how you would synthesise 2-methyl-amino-propane from the appropriate haloalkane.
+ Draw structural formulae of ….. 1,2-dichlorobenzene Describe the bonding in this structure 3-chlorohexanamide Describe the intermolecular forces present in this molecule The first 4 organic acids Describe and explain the boiling point trend.
+ Recap – what affects the relative rate of Sn1 Sn2 reactions Nature of leaving group Primary >> Secondary >> Tertiary For Sn2 increasing nucleophile concentration increases rate – no effect on Sn1. Increasing nucleophile strength favours Sn2 – hydroxide >>> water
+ Completing the picture Substitution happens in competition with elimination reactions. Both reactions require REFLUX with NaOH ( or KOH ) What decides elimination or substitution ? Primary – mainly substitution Tertiary – mainly elimination Water solvent favours substitution Ethanol solvent favours elimination Higher concentrations ( at elevated temperature ) of alkali favour elimination
+ E2 mechanism ( E2 not on IB but..) Elimination from symmetrical haloalkanes. Elimination from unsymmetrical haloalkanes.
+ Stereoisomerism What ? compounds with the same structural formula but with different arrangements of atoms in space. So give me some examples …… Consider 1,2-dibromoethane and 1,2-dibromoethene.
+ Physico-chemical properties of geometric isomers Cis and trans 1,2-dichloroethene boiling points are 60C and 47C – why ? Cis and trans but-2-ene-1,4-dioic acid Cis forms the anydride BUT trans has no reaction……
Constitutional Isomers Have same molecular CH2 CH3 form but different H3 C CH2 structural formula butane B.P = -0.5C Both isomers to the right are C4H10 CH3 These have different CH chemical and physical H3C CH3 properties isobutane B.P = -11.7 C
Stereoisomers Appear to have same structure in 2D In 3D have different structures Are non-superimposable mirror images of each other With modeling kit build a model: carbon atom with four different colors attached (use same size bonds!) Look at your model in a mirror, build what you see in the mirror. Are the two superimposable?
Allmolecules have a mirror image – but for most molecules it is the same molecule. H H C C H H H H F F fluoromethane
For some molecules the mirror imageis a different molecule (the mirrorimage is non-superimposable). OH OH C C COOH HOOC H H3C H CH3 (-) lactic acid (+) lactic acid in sour milk in muscles
• Left and right hands are an example of non-superimposable mirror images.
Chirality Game on blog please
Thisusually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C). Such molecules are said to be chiral or optically active. a a C C d d b c c b
The optical isomers are called enantiomers. These are distinguished by +/-, D/L or more correctly R/S. A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.
TASKWhich of the following molecules are optically active?1) propan-2-ol 5) butanone2) 2-chlorobutane 6) 2-methylbutanoic acid3) 1-chlorobutane 7) butan-2-ol4) 3-methylhexane 8) 1-chloro-3-methylpentane
propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE
2-chlorobutane CH3 CH CH2 CH3 Cl CH2CH3 CH2CH3 C C CH3 H3C H H Cl Cl OPTICALLY ACTIVE
1-chlorobutane CH2 CH2 CH2 CH3 Cl NOT OPTICALLY ACTIVE
3-methylhexane CH3 CH2 CH CH2 CH2 CH3 CH3 CH2CH2CH3 CH2CH2CH3 C C CH3 CH3 H H CH2CH3 CH3CH2 OPTICALLY ACTIVE
butanone O CH3 C CH2 CH3 NOT OPTICALLY ACTIVE
2-methylbutanoic acid CH3 O CH3 CH2 CH C OH CH2CH3 CH2CH3 C C H HCH3 CH3 COOH HOOC OPTICALLY ACTIVE
butan-2-ol OH CH3 CH2 CH CH3 CH2CH3 CH2CH3 C C H H CH3 CH3 OH HO OPTICALLY ACTIVE
1-chloro-3-methylpentane CH3 Cl CH3 CH2 CH CH2 CH2 CH2CH3 CH2CH3 C C H HCH3 CH3 CH2CH2Cl CH2ClCH2 OPTICALLY ACTIVE
How to draw 3D:Dash-Wedge Formula Draw two straight lines H about 110 from each other Bond angle is 109.5 Cl OH These represent the bonds in the plane of the page CH 3 Draw dashed line for the bond that extends behind the plane of the page Draw wedge for bond that extends in front of the plane of the page
+ Drawing Enantiomers Draw the molecule you built, using the dash-wedge formula and colored pens Use a mirror to see the enantiomer Sketch the mirror image, using the dash-wedge formula Draw the two stereoisomers of 1-chloro-1-bromo-ethane. There is no convention for which atom is attached to a wedge.
Chiral centers Carbon attached to 4 different substituent groups Even if two substituent groups start with C, they could still be different H H H3CH2C OH CH2CH3 HO CH3 H3C C=O, never chiral (C only attached to 3 things) CH3, CH2 groups, never chiral because they are symmetric:
+ Vocabulary Stereoisomers have chiral centers. Carbon atoms can be chiral or achiral Asymmetric or symmetric If two molecules are stereoisomers, they are also called enantiomers Chiral molecules are optically active Enantiomers are optical isomers
+ Properties of Optical Isomers same physical and chemical properties rotate plane polarized light Each isomer rotates it in a different direction What is plane polarized light? Try polarizers
+ Optical Activity Stereoisomers are said to be optically active if the rotate plane polarized light Each type of enantiomer rotates light the same amount, but in different directions. Amount and direction of rotation must be experimentally determined using a polarimeter
Optical Activity The amount of rotation depends on Lengthof sample tube Concentration of enantiomers Bothisomers present, rotation cancels out called: racemic mixture
Molecules that are optical isomers are called enantiomers.• Enantiomers have identical chemical and physical properties, except: • Their effect on plane polarised light; • Their reaction with other chiral molecules
Light is a form of electromagnetic radiation.
The wave vibrations are perpendicular to the direction of travel of the wave. normal light plane-polaris ed light plane-polaris ed light after(w av es v ibrate in all direc tions ) (v ibrates in only one direc tion) c loc k w is e rotation
Optical isomers rotate the plane of plane polarised light. (-)-enantiomer (+)-enantiomer (±)-rac emate(antic loc k w is e rotation) (c loc k w is e rotation) (no ov erall effec t)
• Chiral molecules often react differently with other chiral molecules.• This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with.• Many natural molecules are chiral and most natural reactions are affected by optical isomerism.
• For example, most amino acids (and so proteins) are chiral, along with many other molecules.• In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left).
Many drugs are optically active, with one enantiomer only having the beneficial effect.In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide.
In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness.This led to many disabilities in babies and early deaths in many cases.
+ O NH O O NH O O O H2C C C H2C CH2 N N CH2 H H O O S thalidomide (effective drug) R thalidomide (dangerous drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body.
Thalidomide was banned worldwide when the effects were discovered.However, it is starting to be used again to treat leprosy and HIV.Its use is restricted though and patients have to have a pregnancy test first (women!) and use two forms of contraception (if sexually active).
CH3 CH3 O O H C CH2 H2C C H H3C CH3 S carvone (caraway seed) R carvone (spearmint) Caraway Seed has awarm, pungent, slightly bitter flavour with aniseed overtones.
CH3 CH3CH2 C H H C CH2 CH3 H3C S limonene (lemons) R limonene (oranges)