Diastereomers usually differ from each other in their physical and chemical properties.
Enantiomers, on the other hand, have identical physical and chemical properties – with two important exceptions.
1 Optical activity
As we know from their name, optical isomers show a difference in a specifi c
interaction with light. A beam of ordinary light consists of electromagnetic waves that oscillate in an infi nite number of planes at right angles to the direction of travel. If, however, this light is passed through a device called a polarizer, only the light waves oscillating in a single plane pass through, while light waves in all other planes are blocked out. This is known as plane-polarized light. A similar effect is achieved in polarized sunglasses or windshields to reduce glare.
In the early 1800s it was discovered that when a beam of plane-polarized light passes through a solution of optical isomers, they rotate the plane of polarization.
The amount and direction of rotation can be measured with an instrument called a polarimeter, as shown in Figure 10.20.
chiral compound in solution plane
polarized light polarizer
ordinary light
analyser emerging light beam u
The solution of isomers is placed in the sample tube through which plane-polarized light is passed. Rotation of the polarization plane occurs and the light then passes through a second polarizer called the analyser. This can be rotated until the light passes through it and so the extent and direction of rotation can be deduced. In order to compare different solutions, the concentrations of the solutions, the wavelength of light used, and the sample path length must all be kept the same.
When drawing enantiomers make sure that you use the wedge-dash type representation.
It is best to write in the plane of the mirror fi rst and then ensure that the same groups in the two molecules are an equal distance from this plane.
Figure 10.20 Schematic representation of a
polarimeter.
Note that when you are looking for a chiral carbon atom in a molecule you must look at the whole group bonded to the carbon, not just the immediately bonded atom. For example,
—CH3 is a different group from —C2H5.
522
Organic chemistry
10
2 Reactivity with other chiral molecules
When a racemic mixture is reacted with a single enantiomer of another chiral compound, the two components of the mixture, the (+) and (−) enantiomers, react to produce different products. These products have distinct chemical and physical properties and so can be separated from each other relatively easily. This is therefore a means by which the two enantiomers can be separated from a racemic mixture, a process known as resolution.
The different reactivity of a pair of enantiomers with another chiral molecule is of particular signifi cance in biological systems because these are chiral environments.
An infamous example of this occurred in the 1960s when the drug thalidomide was prescribed to pregnant women for morning sickness. One enantiomer is therapeutic but the other produces severe malformations in the fetus. This tragedy largely spearheaded research into processes for the manufacture of a single enantiomer using a chiral catalyst. The process, known as asymmetric synthesis, led to the Nobel Prize in Chemistry in 2001. It is discussed in Chapter 15.
NATURE OF SCIENCE
The pioneer of polarimetry was Jean Baptise Biot (1774–1862) who was a French physicist and older friend of the famous French bacteriologist Louis Pasteur (1822–1895). Biot showed that some crystals of quartz rotated the plane of polarized light while other crystals rotated it to the same extent in the opposite direction. Later, by showing the same effect in liquids such as turpentine, and in solutions of naturally occurring substance such as sugar he realized it must be a molecular property and coined the term ‘optical activity’. In 1848 Pasteur, working on crystalline salts derived from wine, discovered that while tartaric acid showed optical activity, racemic acid with the same chemical composition does not, and deduced that this is because it contains an equal mixture of two isomers. Pasteur saw the huge signifi cance of this. He reasoned that reactions outside the cell always produce an optically inactive mixture whereas biological activity is specifi c to one isomer. In his later work on the origin of life this became his guiding distinction between living and inanimate material.
Different notations are used to distinguish between the two enantiomers of a pair. (+) and (−) refer to the direction in which the plane polarized light is rotated, (+) for a clockwise direction and (−) for anticlockwise rotation. The lower case d- (dextrorotatory) and l- (laevorotatory) respectively are used as alternates for this but are becoming obsolete. Confusingly, small upper case d- and l- are a different, unrelated notation based on spatial confi gurations in comparison with the reference molecule glyceraldehyde. This latter system is widely used in naming many biological molecules such as amino acids and sugars. Other molecules are described by their absolute confi guration, using R (rectus) for right or clockwise and S (sinister) for left or counterclockwise. The rules for determining the absolute confi guration are based on atomic number and mass. Happily, we will not adopt any particular system here and you will not be expected to identify the specifi c enantiomer in any of these examples.
The two enantiomers of a chiral compound rotate plane-polarized light in equal and opposite directions.
Separate solutions of enantiomers, at the same concentration, rotate plane-polarized light in equal amounts but opposite directions. This is what is meant by being optically active. A racemic mixture does not rotate the light and so is said to be optically inactive. Naturally occurring chiral molecules are optically active, in other words they exist as only one enantiomer. For example, morphine rotates plane-polarized light to the left so is said to be (−), whereas sucrose rotates it to the right and is said to be (+).
The human senses of smell and taste are responsive to chiral infl uences. For example, d-amino acids all taste sweet, whereas l-amino acids are often tasteless or bitter. We can distinguish between the smells of oranges and lemons due to the presence of different enantiomers of the compound limonene.
This is because taste buds on the tongue and sense receptors in the nose contain chiral molecules and so interact differently with the different enantiomers. As a result, stereochemistry is a major aspect of the food and perfume industries, as well as being a key factor in the pharmaceutical industry.
523
Practice questions
1 Which reaction type is typical for halogenoalkanes?
A electrophilic substitution C nucleophilic substitution B electrophilic addition D nucleophilic addition
2 Which statement about the reactions of halogenoalkanes with potassium hydroxide is correct?
A Primary halogenoalkanes react mainly by an SN1 mechanism.
B Bromoalkanes react faster than iodoalkanes.
C Tertiary halogenoalkanes react faster than primary halogenoalkanes.
D The primary product of the reaction is an aldehyde.
3 Which molecule exhibits optical isomerism?
A 3-iodopentane C 1,3-diiodopropane
B 2-iodo-2-methylpropane D 2-iodobutane 4 From which monomer is this polymer made?
C H
H C H
Cl C H
H C Cl
H C H
H
C C
H
H C Cl
H H
Cl
)
(
nA C
H Cl H
H
C C C
Cl C H
H H
H H
B C
Cl H Cl
H
C D C
Cl Cl H
H C
5 What is the correct order of reaction types in the following sequence?
C3H7Br ⎯→ C3H7OH ⎯→ C2H5COOH ⎯→ C2H5COOC2H5
I II III
A substitution oxidation condensation B addition substitution condensation C oxidation substitution condensation D substitution oxidation substitution
I II III
Exercises
27 Which compound can exist as optical isomers?
A CH3CHBrCH3 C CH3CHBrCOOH
B CH2ClCH(OH)CH2Cl D CH3CCl2CH2OH 28 Write the structure of the first alkane to show optical isomerism.
29 Draw and name the isomers of the following using the E/Z convention.
(a) pent-2-ene
(b) 2,3-dichlorobut-2-ene
524
Organic chemistry
10
Practice questions
1 Which reaction type is typical for halogenoalkanes?
A electrophilic substitution C nucleophilic substitution B electrophilic addition D nucleophilic addition
2 Which statement about the reactions of halogenoalkanes with potassium hydroxide is correct?
A Primary halogenoalkanes react mainly by an SN1 mechanism.
B Bromoalkanes react faster than iodoalkanes.
C Tertiary halogenoalkanes react faster than primary halogenoalkanes.
D The primary product of the reaction is an aldehyde.
3 Which molecule exhibits optical isomerism?
A 3-iodopentane C 1,3-diiodopropane
B 2-iodo-2-methylpropane D 2-iodobutane 4 From which monomer is this polymer made?
C
5 What is the correct order of reaction types in the following sequence?
C3H7Br ⎯→ C3H7OH ⎯→ C2H5COOH ⎯→ C2H5COOC2H5
I II III
A substitution oxidation condensation B addition substitution condensation C oxidation substitution condensation D substitution oxidation substitution
I II III
6 Which reactants could be used to form the compound below?
H
A butanoic acid and ethanol C ethanoic acid and propan-1-ol B propanoic acid and ethanol D ethanoic acid and butan-1-ol
7 Halogenoalkanes can undergo SN1 and SN2 reactions with aqueous sodium hydroxide. Which halogenoalkane will react fastest with a 0.1 mol dm–3 solution of aqueous sodium hydroxide?
A 2-chloro-2-methylpropane C 1-chlorobutane B 2-iodo-2-methylpropane D 1-iodobutane
8 How many isomers can exist for a compound with the molecular formula C2H2Cl2?
A 1 B 2 C 3 D 4
9 Which substances are possible products of the incomplete combustion of octane?
A carbon dioxide and hydrogen gas C carbon monoxide and hydrogen gas B carbon monoxide and water vapour D methane and hydrogen gas 10 How many chiral carbon atoms are present in a molecule of 2,3-dibromobutane?
A 1 B 2 C 3 D 4
11 Which reaction occurs via a free-radical mechanism?
A C2H6 + Br2 → C2H5Br + HBr C C4H9I + OH− → C4H9OH + I− B C2H4 + Br2 → C2H4Br2 D (CH3)3CI + H2O → (CH3)3COH + HI 12 Which compound could rotate the plane of polarization of polarized light?
A (CH3)2CHCH2Cl B CH3CH2CH2CH2Cl C CH3CH2CHClCH3 D (CH3)3CCl 13 Which conditions are required to obtain a good yield of a carboxylic acid when ethanol is
oxidized using potassium dichromate(VI), K2Cr2O7(aq)?
I add sulfuric acid
II heat the reaction mixture under refl ux
III distil the product as the oxidizing agent is added
A I and II only B I and III only C II and III only D I, II, and III 14 Which statement is correct about the enantiomers of a chiral compound?
A Their physical properties are different.
B All their chemical reactions are identical.
C A racemic mixture will rotate the plane of polarized light.
D They will rotate the plane of polarized light in opposite directions.
525
15 (a) Below are four structural isomers with molecular formula C4H9Br. State the name of each of the isomers A, B, C, and D.
Carbon Hydrogen Bromine Key:
A B
C D (4)
(b) (i) Identify the isomer(s) which will react with aqueous sodium hydroxide almost exclusively by an SN1 mechanism. State the meaning of the symbols in the term SN1 mechanism. (2) (ii) Using the formula RBr to represent a bromoalkane, state an equation for the
rate-determining step of this SN1 reaction. (1)
(iii) Identify one isomer that will react with aqueous sodium hydroxide almost exclusively by an SN2 mechanism. Draw the mechanism for this reaction using curly arrows to represent the movement of electron pairs. Include the structural formulas of the
transition state and the organic product. (4)
(c) State and explain how the rates of the reactions in parts (b) (i) and (b) (iii) are affected when the concentration of the sodium hydroxide is doubled. (2) (d) State and explain how the rate of reaction of 1-bromobutane with sodium hydroxide
compares with that of 1-chlorobutane with sodium hydroxide. (2) (e) Identify the isomer of C4H9Br that can exist as stereoisomers. Outline how a polarimeter will
distinguish between the isomers, and how their physical and chemical properties
compare. (5) (Total 20 marks) 16 Alkenes are an economically and chemically important family of organic compounds.
(a) The reaction of alkenes with bromine water provides a test for unsaturation in the laboratory.
Describe the colour change when bromine water is added to chloroethene. (1) (b) Deduce the Lewis structure of chloroethene and identify the formula of the repeating unit of
the polymer poly(chloroethene). (2)
(c) Besides polymerization, state two commercial uses of the reactions of alkenes. (2) (Total 5 marks)