ahl organic
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AHL Organic ChemistrySyllabus
+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 tertairy halogenoalkanes 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.
+Cyclics also exhibit geometric isomerism
Cyclopropane
Cyclobutane
+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 form but different structural formula
Both isomers to the right are C4H10
These have different chemical and physical properties
H3C
CH2
CH2
CH3
H3CCH
CH3
CH3
butane
isobutane
B.P = -0.5C
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?
All molecules have a mirror image – but for most molecules it is the same molecule.
fluoromethane
H
CH F
H
H
CHF
H
+
For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).
OH
CH CH3
COOH
OH
CHH3C
HOOC
(-) lactic acid (+) lactic acidin sour milk in muscles
• Left and right hands are an example of non-superimposable mirror images.
Chirality Game on blog please
This usually 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
Cb c
d
a
Cbc
d
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.
TASK Which of the following molecules are optically active?
1) propan-2-ol2) 2-chlorobutane3) 1-chlorobutane4) 3-methylhexane
5) butanone6) 2-methylbutanoic acid7) butan-2-ol8) 1-chloro-3-methylpentane
propan-2-ol
NOT OPTICALLY ACTIVE
CH3 CH CH3
OH
2-chlorobutane CH3 CH CH2 CH3
Cl
OPTICALLY ACTIVE
CH2CH3
CH Cl
CH3
CH2CH3
CHCl
H3C
1-chlorobutane
NOT OPTICALLY ACTIVE
CH2 CH2 CH2 CH3
Cl
3-methylhexane
OPTICALLY ACTIVE
CH2 CH CH2 CH2
CH3
CH3CH3
CH2CH2CH3
CH CH2CH3
CH3
CH2CH2CH3
CHCH3CH2
CH3
butanone
NOT OPTICALLY ACTIVE
C CH2 CH3CH3
O
2-methylbutanoic acid
OPTICALLY ACTIVE
CH3 CH2 CH
CH3
C
O
OH
CH2CH3
CCH3 COOH
H
CH2CH3
CCH3
HHOOC
butan-2-ol
OPTICALLY ACTIVE
CH3 CH2 CH
OH
CH3
CH2CH3
CCH3 OH
H
CH2CH3
CCH3
HHO
1-chloro-3-methylpentane
OPTICALLY ACTIVE
CH3 CH2 CH
CH3
CH2 CH2
Cl
CH2CH3
CCH3 CH2CH2Cl
H
CH2CH3
CCH3
HCH2ClCH2
How to draw 3D: Dash-Wedge Formula Draw two straight lines
about 110 from each other Bond angle is 109.5 These represent the bonds
in the plane of the page
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
OH
H
Cl
CH3
+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
C=O, never chiral (C only attached to 3 things)
CH3, CH2 groups, never chiral because they are symmetric:
OH
H
CH2CH3
H3C
OH
H
H3CH2C
CH3
+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 Length of sample tube Concentration of enantiomers
Both isomers 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.
normal light(waves vibrate in all directions)
plane-polarised light(vibrates in only one direction)
plane-polarised light after clockwise rotation
The wave vibrations are perpendicular to the direction of travel of the wave.
Optical isomers rotate the plane of plane polarised light.
(-)-enantiomer(anticlockwise rotation)
(±)-racemate(no overall effect)
(+)-enantiomer(clockwise rotation)
+
• 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.
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H2CCH2
C
NHO O
H
N
O
O
H2CCH2
C
NH OO
H
N
O
O
S thalidomide (effective drug)
The body racemises each enantiomer, so even pure S is dangerous as it converts to R
in the body.
R thalidomide (dangerous drug)
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).
S carvone (caraway seed) R carvone (spearmint)
O
CH3
H C CH2
H3C
O
CH3
HCH2C
CH3
Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed
overtones.
S limonene (lemons) R limonene (oranges)
CH3
HCCH2
CH3
CH3
H C CH2
H3C