2. +
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
3. +
Nucleophilic Substitution Reactions
Nucleophiles
Why is hydroxide better than water ?
4. +
20.2.2 Identity of the halogen
Consider the R-X bond
RCl<RBr< RI
5. +
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.
6. +
Study Questions
Why don’t primary halogenoalkanes use the Sn1 mechanism.
Why don’t tertairyhalogenoalkanes use Sn2 ?
7. +
Starter
Explain how you would synthesise 2-methyl-amino-propane
from the appropriate haloalkane.
8. +
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.
9. +
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
10. +
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
11. +
E2 mechanism ( E2 not on IB but..)
Elimination from symmetrical haloalkanes.
Elimination from unsymmetrical haloalkanes.
12. +
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.
14. +
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……
15. 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
16. 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?
17. 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
19. For some molecules the mirror image
is a different molecule (the mirror
image is non-superimposable).
OH OH
C C
COOH HOOC
H H3C H
CH3
(-) lactic acid (+) lactic acid
in sour milk in muscles
20. • Left and right hands are an example
of non-superimposable mirror images.
22. 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
23. 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.
24. TASKWhich of the following molecules are optically
active?
1) propan-2-ol 5) butanone
2) 2-chlorobutane 6) 2-methylbutanoic acid
3) 1-chlorobutane 7) butan-2-ol
4) 3-methylhexane 8) 1-chloro-3-methylpentane
25. propan-2-ol
CH3 CH CH3
OH
NOT OPTICALLY ACTIVE
26. 2-chlorobutane
CH3 CH CH2 CH3
Cl
CH2CH3 CH2CH3
C C
CH3 H3C
H H
Cl Cl
OPTICALLY ACTIVE
30. 2-methylbutanoic acid CH3 O
CH3 CH2 CH C OH
CH2CH3 CH2CH3
C C
H H
CH3 CH3
COOH HOOC
OPTICALLY ACTIVE
31. butan-2-ol OH
CH3 CH2 CH CH3
CH2CH3 CH2CH3
C C
H H
CH3 CH3
OH HO
OPTICALLY ACTIVE
32. 1-chloro-3-methylpentane CH3 Cl
CH3 CH2 CH CH2 CH2
CH2CH3 CH2CH3
C C
H H
CH3 CH3
CH2CH2Cl CH2ClCH2
OPTICALLY ACTIVE
33. 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
34. +
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.
35. 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:
36. +
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
37. +
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
38. +
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
39. Optical Activity
The amount of rotation depends on
Lengthof sample tube
Concentration of enantiomers
Bothisomers present, rotation cancels out
called: racemic mixture
40. 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
41. Light is a form of electromagnetic
radiation.
42. 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
43. 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)
45. • 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.
46. • 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).
47. 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.
48. 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.
49. +
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.
50. 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).
51. CH3 CH3
O O
H C CH2 H2C C H
H3C CH3
S carvone (caraway seed) R carvone (spearmint)
Caraway Seed has a
warm, pungent, slightly bitter flavour with
aniseed overtones.
52. CH3 CH3
CH2 C H H C CH2
CH3 H3C
S limonene (lemons) R limonene (oranges)