Stereochemistry Of Cyclic Hydrocarbons Biology Essay


What is organic chemistry? Organic chemistry is defined as the chemistry of carbon compounds. Of tens of millions of known chemical compounds,over 95% are compounds of carbon.(1)

Hydrocarbons are the simplest organic compounds. As their name implies, they are composed entirely of carbon and hydrogen.(3)

Four basic types:Alkanes,Alkenes,Alkynes,Aromatic hydrocarbons.


Alkanes are hydrocarbons that contain only single bonds.Because all carbon-to-carbon bonds are single bonds,alkanes are often called saturated hydrocarbons.

The general formula of alkanes is CnH2n+2.




They contain at least one carbon-carbon double bond and are unsaturated.They have fewer than maximum number of hydrogens.



Alkynes: Contain at least one carbon-carbon triple bond;Carbons in triple bond sp-hybridized and have linear geometry.Also unsaturated.(8)


Cyclic hydrocarbons are ring compounds. The simplest cyclic hydrocarbon is cyclopropane. The names of cyclic

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hydrocarbons begin with the prefix cyclofollowed by the name of the alkane with the same number of carbon atoms.



Alkenes are hydrocarbons which contain a carbon-to-Carbon double bond.Their general formulas are CnH2n. Their names begin with a prefix denoting the number of carbon atoms followed by the suffix -ene.

Ethylene is the simplest alkene.


Alkynes are hydrocarbons which contain a carbon-to-carbon triple bond. Their general formulas are CnH2n-2.

Their names begin with a prefix denoting the number of carbon atoms followed by the suffix -yne.

Ethyne (acetylene) is the simplest alkyne.

Both alkenes and alkynes are unsaturated hydrocarbons. A saturated hydrocarbon has the maximum number of

hydrogen atoms attached to each carbon and no double or triple bonds. Unsaturated hydrocarbons can undergo

an addition reaction:



Benzene is a unique organic compound in that it is a very stable six-sided ring. Aromatic hydrocarbons contain a benzene ring or have properties similar to those of benzene.




When hydrogen atom or atoms of a hydrocarbon are substituted by chlorine, a chlorinated hydrocarbon is

formed. Chlorinated hydrocarbons have many useful properties.Dichloromethane is used as a solvent and paint

remover.Trichloromethane (chloroform) is also a solvent and at one time was used as an anesthetic. It is now

considered hazardous.



Atoms or groups of atoms attached to hydrocarbon skeletons give the compounds characteristic chemical and physical properties and are known as functional groups. Double and triple bonds as well as halogen substituents are examples of functional groups.



These molecules are found mainly in petroleum but living organisms, eukaryotic or prokaryotic, contain frequently hydrocarbons which are directly derived from fatty acids. They are known to be present in living matter since 1892 when ShallC identified undecane in ants, and Etard A identified eicosane in Bryonia dioica. They are distinct from the terpenoid hydrocarbons. They have usually a straight chain of up to about 36 carbon atoms but may also be branched, with one or more methyl groups attached at almost any point of the chain. Usually, the methyl group is near the end of the chain (iso or anteiso). They are either saturated or unsaturated (mono or diunsaturated). In contrast with the diversity of methyl-branched alkanes found in insect species, n-alkanes predominate in plants. Among the least polar components of plant surface lipids hydrocarbons with the odd number carbon chains (C15 up to C33) are predominant. Allenic hydrocarbons, such as 9,10-tricosadiene, 9,10-pentacosadiene, and 9,10-heptacosadiene were isolated from Australian insects. (16)

They are also abundant at the outer surface of insects and several marine organism. They are thought to serve as a barrier to water influx in the organism, to act as sex attractants (or anti-aphrodisiacs), to affect the absorption of chemicals and microorganisms. Wild populations of Drosophila melanogaster use several cuticular hydrocarbons (mainly 7,11-heptacosene) as sexual pheromone. (17)


Hydrocarbons may be classified into monocyclic and polycyclic species.

Monocyclic hydrocarbons : Several branched-alkylbenzenes have been described in Archaebacteria such as Thermoplasma and Sulfolobus. They have mostly two methyl groups branched on a saturated chain of 9 to 12 carbon atoms. One of them is shown below.

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After the first hypothesis of a communication system (chemotaxis) by Thuret MG for the fertilization of brown algae, 117 years were needed to know the structure of the first algal pheromone. This compound, ectocarpene, is an unsaturated heptacyclic hydrocarbon found in the brown algae Ectocarpus, Adenocystis and Sphacelaria.

 Several C11 alkylbenzenes similar to ectocarpene play a role of pheromone in marine brown algae, dityotene in Dictyota sp, desmarestene in Desmarestia sp and Cladostephus sp, lamoxirene in Laminaria sp, Alaria sp and many others. (18)



The nature of these compounds was discovered during the 19th century. In 1831, Wachen roder H. proposed the term "carotene" for the hydrocarbon pigment he had cristallized from carrot roots. Berzelius J. called the more polar yellow pigments extracted from autumn leaves "xanthophylls" and Tswett M., who separated many pigments by column chromatography, called the whole group "carotenoids".

Among this important group, the numerous compounds consist of C40 chains (tetraterpenes) with conjugated double bonds, they show strong light absorption and often are brightly colored (red, orange). They occur as pigments in bacteria, algae and higher plants. Carotenoids perform three major functions in plants : accessory pigments for light harvesting, prevention of photooxidative damage and pigmentation attracting insects.

The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls.

Carotenoids are important components of the light harvesting in plants, expanding the absorption spectra of photosynthesis. The major carotenoids in this context are lutein, violaxanthin and neoxanthin. Additionally, there is considerable evidence which indicates a photoprotective role of xanthophylls preventing damage by dissipating excess light. In mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. b-Carotene was thus shown to enhance cell-mediated immune responses. (20)

Carotenoids consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position relationship and the remaining non-terminal methyl groups are in a 1,5-position relationship. They are, by far the predominant class of tetraterpenes. They may be also classified in the terpenoids.

Carotenoids can be considered derivatives of lycopene, found in tomatoes, fruits and flowers. Its long straight chain is highly unsaturated and composed of two identical units joined by a double bond between carbon 15 and 15'. Each of these 20 carbon units may be considered to be derived from 4 isoprene units. Lycopene is a bioactive red colored pigment naturally occurring in plants. Interest in lycopene is increasing due to increasing evidence proving its antioxidant activities and its preventive properties toward numerous diseases. In vitro, in vivo and ex vivo studies have demonstrated that lycopene-rich foods are inversely associated to diseases such as cancers, cardiovascular diseases, diabetes, and others. A review of all these aspects may be consulted.(21)

Carotenoids may be acyclic (seco-carotenoids) or cyclic (mono- or bi-, alicyclic or aryl). Oxyfunctionalization of various carotenoids leads to a large number of xanthophylls in which the function may be a hydroxyl, methoxyl, carbonyl, oxo, formyl or epoxy group.

Only some of the most common carotenes and xanthophylls are given below:


Now let us learn about the CYCLIC HYDROCARBONS……….!!!!


Many important hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple cycloalkanes of formula (CH,), make up a particularly important homologous series in which

the chemical properties change in a much more dramatic way with increasing n than do those of the acyclic hydrocarbons CH,(CH,),,-,H. The cycloalkanes with small rings (n = 3-6) are of special interest in exhibiting chemical

properties intermediate between those of alkanes and alkenes. In this chapter we will show how this behavior can be explained in terms of angle strain and steric hindrance, concepts that have been introduced previously and will be used with increasing frequency as we proceed further.We also discuss the conformations of cycloalkanes, especially cyclohexane,in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cycloalkenes and cycloalkynes. (23)

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There is considerable similarity in the spectroscopic properties of alkanes and cycloalkanes. We mentioned previously the main features of their infrared spectra, and that their lack of ultraviolet absorption at wavelengths

down to 200 nm makes them useful solvents for the determination of ultraviolet spectra of other substances.

The proton nmr spectra of alkanes and cycloalkanes are characteristic but dificult to interpret because the chemical shifts between the various kinds of protons are usually small. Although proton spectra of simple cycloalkanes,(CH,),,, show one sharp line at room temperature, when alkyl substituents are present, small differences in chemical shifts between the ring hydrogens occur and, with spin-spin splitting, provide more closely spaced lines than normally can be resolved. The complexity so introduced can be seen by comparing the proton spectra of cyclooctane and methylcyclohexane.For methyl-substituted cycloalkanes the methyl resonances generally stand out as high-field signals centered on 0.9 ppm, and the area of these signals relative to the other C-H signals may be useful in indicating how many methyl groups there are. However, in cyclopropanes the ring protons have abnormally small chemical shifts (6 = 0.22for cyclopropane), which often overlap with the shifts of methyl groups(6 = 0.9 ppm).Although proton spectra are not very useful for identification purposes,13C nmr spectra are very useful. Chain-branching and ring-substitution normally cause quite large chemical-shift changes, and it is not uncommon to observe 13C shifts in cycloalkanes spanning 35 ppm. Some special features of

application of 13C nmr spectra to conformational analysis of cycloalkanes are described. (24)


Angle Strain - also called Ring Strain is the strain due to expansion or compression of bond angles.

Torsional Strain - the strain due to eclipsing of bonds on neighboring atoms.

Steric Strain - the strain due to repulsive interactions when atoms approach each other too closely.


Cyclopropane has angles of 90o when we know that SP3 carbons like to bond at 109.5o. In cyclopropane there is angle strain. Example cyclopropane and cyclohexane..



Is the strain due to eclipsing of bonds on neighboring atoms.

When atoms are eclipsed then there is a natural tendency to move away from each other this is called torsional strain.



Steric strain focuses on the strain of functional groups bound due to size onto the cyclic ring. Because rings don't rotate freely in space like linear chains then energy is higher when they are on the same side. FLOWER ENERGY




The C-C=C angle in alkenes normally is about 122", which is 10" larger than the normal C-C-C angle in cycloalkanes. This means that we would expect about 20" more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear,but may be connected in part with the C-H bond strengths.Cyclopropene has rather exceptional properties compared to the other cycloalkenes. It is quite unstable and the liquid polymerizes spontaneously although slowly, even at -80". This substance, unlike other alkenes, reacts rapidly with iodine and behaves like an alkyne in that one of its double-bond hydrogens is replaced in silver-ammonia solution to yield an alkynide-like silver complex.One of the most interesting developments in 'the stereochemistry of organic compounds in recent years has been the demonstration that transcyclooctene

(but not the cis isomer) can be resolved into stable chiral isomers. In general, a trans-cycloalkene would not beexpected to be resolvable because of the possibility for formation of achiral conformations with a plane of symmetry. Any conformation with all of the carbons in a plane is such an achiral conformation (Figure 12-20a). However,when the chain connecting the ends of the double bond is short, as in transcyclooctene,steric hindrance and steric strain prevent easy formation of planar conformations, and both mirror-image forms (Figure 12-20b) are stable and thus resolvable. (29)

Now let us come to the main part of this presentation…..STEREOCHEMISTRY!!!!!


Why do think stereochemistry is important?

Stereochemistry is the study of the static and dynamic aspects of the three-dimensional shapes of molecules. It has long provided a foundation for understanding structure and reactivity.At the same time, stereochemistry constitutes an intrinsically interesting research field in itsownright.Manychemists find this area of study fascinating due simply to the aesthetic beauty associated with chemical structures, and the intriguing ability to combine thefields of geometry, topology, and chemistry in the study of three-dimensional shapes. In addition,there are extremely important practical ramifications of stereochemistry. Nature isinherently chiral because the building blocks of life (_-amino acids, nucleotides, and sugars)are chiral and appear in nature in enantiomerically pure forms. Hence, any substances created by humankind to interact with or modify nature are interacting with a chiral environment.This is an important issue for bioorganic chemists, and a practical issue for pharmaceutical

chemists. The Food and Drug Administration (FDA) now requires that drugs be produced in enantiomerically pure forms, or that rigorous tests be performed to ensure that both enantiomers are safe.In addition, stereochemistry is highly relevant to unnatural systems. As we will describe herein, the properties of synthetic polymers are extremely dependent upon the stereochemistry of the repeating units. Finally, the study of stereochemistry can be used to probe reaction mechanisms. (30)


Stereochemistry is a field that has often been especially challenging for students. No doubt one reason for this is the difficulty of visualizing three-dimensional objects, given twodimensional representations on paper. Physical models and 3-D computer models can be of great help here, and the student is encouraged to use them as much as possible when working through this chapter. However, only simple wedges and dashes are given in most of our

drawings. It is these kinds of simple representations that one must master, because attractive,computer generated pictures are not routinely available at the work bench. The most common convention is the familiar ''wedge-and-dash'' notation. Note that there is some variability in the symbolism used in the literature. Commonly, a dashed wedge that gets larger as it emanates from the point of attachment is used for a receding group. However,

considering the art of perspective drawing, it makes no sense that the wedge gets bigger as

it moves further away. Yet, this is the most common convention used, and it is the convention we adopt in this book. Many workers have turned to a simple dashed line instead (see above), or a dash that does get smaller. Similarly, both a bold wedge and a bold line are used to represent forward-projecting substituents. Another common convention is the bold ''dot'' on a carbon at a ring junction, representing a hydrogen that projects toward the viewer.

The challenge of seeing, thinking, and drawing in three dimensions is not the only cause for confusion in the study of stereochemistry. Another major cause is the terminology used. Hence, we start this chapter off with a review of basic terminology, the problems associated with this terminology, and then an extension into more modern terminology. (31)


All introductory organic chemistry texts provide a detailed presentation of the various rules for assigning descriptors to stereocenters. Herewe provide a brief review of the terminology to remind the student of the basics.Many of the descriptors for stereogenic units begin with assigning priorities to the attached ligands. Higher atomic number gets higher priority. If two atoms under comparison are isotopes, the one with higher mass is assigned the higher priority. Ties are settled by moving out from the stereocenter until a distinction is made. In other words, when two attached atoms are the same, one examines the next atoms in the group, only looking for a

winner by examining individual atomic numbers (do not add atomic numbers of several atoms).

Multiple bonds are treated as multiple ligands; that is, C_O is treated as a C that is singly bonded to two oxygens with one oxygen bound to aC. For example, the priorities shown below for the substituted alkene are obtained, giving an E-stereochemistry.


R,S System

For tetracoordinate carbon and related structures we use the Cahn-Ingold-Prelog system.The highest priority group is given number 1, whereas the lowest priority group is given number 4. Sight down the bond from the stereocenter to the ligand of lowest priority behind. If moving fromthe highest (#1), to the second (#2), to the third (#3) priority ligand involves a clockwise direction, the center is termed R. A counterclockwise direction implies S.

E,Z System

For olefins and related structures we use the same priority rules, but we divide the double bond in half and compare the two sides. For each carbon of an olefin, assign one ligand high priority and one low priority according to the rules above. If the two high priority ligands lie on the same side of the double bond, the system is Z (zusammen); if they are on opposite sides, the system is E (entgegen). If an H atom is on each carbon of the double bond,however,we can also use the traditional ''cis'' and ''trans'' descriptors.

d andl

The descriptors d and l represent an older system for distinguishing enantiomers,relating the sense of chirality of any molecule to that of d- and l-glyceraldehyde. d- and l-glyceraldehyde are shown below in Fischer projection form. In a Fischer projection, the horizontal lines represent bonds coming out of the plane of the paper, while the vertical lines represent bonds projecting behind the plane of the paper. You may want to review an introductory

text if you are unfamiliar with Fischer projections. The isomer of glyceraldehyde that rotates plane polarized light to the right (d) was labelled d, while the isomer that rotates plane polarized light to the left (l) was labelled l.

To name more complex carbohydrates or amino acids, one draws a similar Fischer projection where theCH2OHorRis on the bottom and the carbonyl group (aldehyde, ketone, or carboxylic acid) is on the top. The d descriptor is used when the OH or NH2 on the penultimate (second from the bottom) carbon points to the right, as in d-glyceraldehyde, and l is used when theOHorNH2 points to the left. See the following examples.

Erythro and Threo

Another set of terms that derive fromthe stereochemistry of saccharides are erythro and threo. The sugars shown below are d-erythrose and d-threose, which are the basis of a nomenclature system for compounds with two stereogenic centers. If the two stereogenic centers have two groups in common, we can assign the terms erythro and threo. To determine the use of the erythro and threo descriptors, draw the compound in a Fischer projection with the distinguishing groups on the top and bottom. If the groups that are the same are both on the right or left side, the compound is called erythro; if they are on opposite sides, the compound is called threo. See the examples given below. Note that these structures have enantiomers,and hence require R and S descriptors to distinguish the specific enantiomer. The erythro/threo system distinguishes diastereomers. (33)

Optical Activity and Chirality

Historically, the most common technique used to detect chirality and to distinguish enantiomers has been to determine whether a sample rotates plane polarized light. Optical activity and other chiroptical properties that can be measured usingORDandCD(see below)have long been essential for characterizing enantiomers. Their importance has lessened somewhat with the development of powerful NMR methods and chiral chromatographic

methods, but their historical importance justifies a brief discussion of the methodology.All introductory organic chemistry textbooks cover the notion of optical activity-the ability of a sample to rotate a plane of polarized light.We check to see if the plane in which the polarized light is oscillating has changed by some angle relative to the original plane of oscillation on passing through the sample.Asolution consisting of a mixture of enantiomers

at a ratio other than 50:50 can rotate plane polarized light to either the right (clockwise) or the left (counterclockwise).Arotation to the right is designated (_); a rotation to the left is designated (-). Earlier nomenclature used dextrorotatory (designated as d) or levorotatory (designated as l) instead of (_) or (-), respectively. Typically, light of one particular wavelength,the Na ''D-line'' emission, is used in such studies. However, we can in principle use any wavelength, and a plot of optical rotation vs. wavelength is called an optical rotatory dispersion (ORD) curve. Note that as we scan over a range of wavelengths, any sample will have some wavelength regions with_rotation and others with - rotation. Since the rotation must pass through zero rotation as it changes from_to -, any chiral sample will be optically inactive at some wavelengths. If one of those unique wavelengths happens to be at (or near) the Na D line, we could be seriously misled by simple optical activity measurements. Furthermore,at the Na Dline, rotation is often small for conventional organic molecules. In addition,we previously discussed instances in which a chiral sample might be expected to fail to rotate plane polarized light. Thus, optical activity establishes that a sample is chiral, but a lack of optical activity does not prove a lack of chirality.

Why is Plane Polarized Light Rotated by a Chiral Medium?

We have said that we need a chiral environment to distinguish enantiomers, and so it may seem odd that plane polarized light can do so. To understand this, we must recall that electromagnetic radiation consists of electric and magnetic fields that oscillate at right angles to each other and to the direction of propagation (see Figure 6.5 A). In normal light (such as that coming from a light bulb or the sun), the electric fields are oscillating at all possible angles when viewing the radiation propagating toward you (Figure 6.5B). Plane polarized light has all the electric fields oscillating in the same plane (Figure 6.5 B and C), and can be viewed as the single oscillation shown in Figure 6.5A. The representation in Figure 6.5A


Stereochemical Issues in Polymer Chemistry

Many unnatural polymers of considerable commercial importance have one stereocenter per monomer, such as in polypropylene and polystyrene (Figure 6.16). Unlike the ''polymerization'' involved in forming a protein or nucleic acid (see the next section), these unnatural systems typically start with a simple, achiral monomer (propene or styrene), and the polymerization generates the stereogenic centers. Control over the sense of chirality for each polymerization step is often absent. As a result, considerable stereochemical complexity can be expected for synthetic polymers. For example, molecular weight 100,000 polypropylene has approximately 2400 monomers, and so 2400 stereogenic centers (look at the next Going Deeper highlight for an interesting ramification of this). There are thus 22400 or approximately 10720 stereoisomers! The R,S system is not very useful here. Hence, polymer stereochemistry is denoted by a different criterion called tacticity. Tacticity describes only local, relative configurations of stereocenters. The terms are best defined pictorially, as in Figure 6.16. Thus, isotactic polypropylene has the same configuration at all stereocenters. Recall the two faces of propylene are enantiotopic,and the isotactic polymer forms when all new bonds are formed on the same face of the olefin. If, instead there is an alternation of reactive faces, the polymer stereocenters alternate, and a syndiotactic polymer is produced. Finally, a random mixture of stereocenters produces atactic polymer.Control of polymer stereochemistry is a major research area in academic and industrial laboratories. This is because polymers with different stereochemistries often have very different properties. For example, atactic polypropylene is a gummy, sticky paste sometimes used as a binder, while isotactic polypropylene is a rugged plastic used for bottle caps. Recent advances (see the Going Deeper highlight on the next page and Chapter 13) have greatly improved the ability to control polymer stereochemistry, leading to commercial production of new families of polymers with unprecedented properties.Another stereochemical issue is helicity, as some simple polymers can adopt a helical shape.We defer discussion of this to Section 6.8.2, in which we discuss helicity in general.


The Origin of Chirality in Nature

The molecules of life are for the most part chiral, and in living systems they are almost always enantiomerically pure. In addition, groups of biomolecules are generally homochiral -all amino acids have the same sense of chirality and all sugars have the same sense of chirality.As already discussed, the chirality of the amino acids leads to chiral enzymes, whichin turn produce chiral natural products. All the chiral compounds found in nature that are

readily accessible to synthetic chemists for the construction of more complex molecules are referred to as the chiral pool.Whatis the origin of the chirality of the molecules of life, and the reason for the homochirality?

We cannot distinguish enantiomers unless we have a chiral environment. Further, in a reaction that forms a stereocenter, we cannot create an excess of one enantiomer over another without some chirality to start with. In the laboratory today, all enantiomeric excesses that we exploit ultimately derive from natural materials. Whether it is the interaction with an enantiomerically pure amino acid froma natural source, or an individual manually separating enantiomorphous crystals (first achieved by Pasteur), the source of enantiomeric excess in modern chemistry is always a living system. But how was this achieved in the absence of life? This is a fascinating, complex, and controversial topic thatwecan touch on only briefly here. This question is often phrased as the quest for the origin of chirality in nature,but more correctly it is the origin of enantiomeric excess and homochirality we seek.

Models for the origin of life generally begin with simple chemical systems that, in time,evolve to more complex, self-organizing, and self-replicating systems. It is easy to imagine prebiotic conditions in which simple condensation reactions produce amino acids or molecules that closely resemble them, and indeed experiments intended to model conditions on the primitive earth verify such a possibility. However, it is difficult to imagine such conditions

producing anything other than a racemic mixture.Essentially, there are two limiting models for the emergence of enantiomeric excess in biological systems. They differ by whether enantiomeric excess arose naturally out of the

evolutionary process or whether an abiotic, external influence created a (presumably slight)initial enantiomeric excess that was then amplified by evolutionary pressure (maybe a type of sergeant-soldier effect). The first scheme is a kind of selection model. The building blocks (let's consider only amino acids here) are initially racemic. However, there is considerable advantage for an early self-replicating chemical system to use only one enantiomer. For example,consider a simple polymer of a single amino acid. If both enantiomers are used, the likely result is an atactic polymer, which may well have variable and ill-defined properties.However, if only a single enantiomer is used, only the isotactic polymer results. This kind of specificity could be self-reinforcing, such that eventually, only the single amino acid is used.The homochirality of nature could result because addition of a second amino acid to the mix might be less disruptive if the new one has the same handedness as the original. The details of how all this could happen are unknown, but the basic concept seems plausible. Certainly,the remarkable cooperativity seen in polyisocyanates provides an interesting precedent.

While we begin with racemic materials, there will never be exactly identical numbers of right- and left-handed molecules in a sample of significant size. This is a simple statistical argument.For example, earlier we considered the reduction of 2-butanone with lithium aluminum hydride under strictly achiral conditions (Figure 6.7), and stated that we expect a racemic mixture without a significant enantiomeric excess. However, if we start with 1023molecules of ketone, the probability that we will produce exactly 0.5_1023 molecules of (R)-and 0.5_1023 molecules of (S)-alcohol is essentially nil. There will always be statistical fluctuations.For example, for a relatively small sample of 107 molecules there is an even chance that one will obtain a _ 0.021% excess of one enantiomer over the other (we cannot anticipate which enantiomer will dominate in any given reaction). Perhaps such a small excess from a prebiotic reaction, or a significantly larger excess from a statistical fluke, got amplified through selective pressure, and ultimately led to the chirality of the natural world.The alternative type of model emphasizes the possible role of an inherently chiral bias of external origin. One possibility for this bias is the inherent asymmetry of our universe reflected in the charge-parity (CP) violation of the weak nuclear force. In particular, _ decay of 60Co nuclei produces polarized electrons with a slight excess of the left- over the righthanded form. From this point, several mechanisms that translate the chirality of the emission to a molecular enantiomeric excess can be envisioned. Unfortunately, all attempts to measure such enantiomeric enrichment in the laboratory have produced at best extremely small enrichments that have proven difficult to reproduce. An alternative proposal for an external chiral influence is an enantioselective photochemical process involving circularly polarized light, which is well established in the laboratory to give significant enantiomeric excesses. At present, however, no clear mechanism for creating circularly polarized light with an excess of one handedness in the prebiotic world has been convincingly demonstrated,although models have been proposed. Only further experimentation in the lab, or perhaps examination of the chirality of extraterrestrial life forms, will resolve this issue.