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here i m giving u the information for my friends for the pathology subject

PARKINSONISM
Parkinsonism
Definition:
It’s a syndrome of chronic, progressive disorder of motor function & is clinically characterized by tremors inflexible rigidity .
Causes/etiology:
Its cased by several diseases (mainly Parkinson’s disease )
other causes trauma, toxic agents and drugs such as dopamine antagonists, idiopathic(self-generated) as well as cerebral arteriosclerosis.
Pathogenesis:
for the normal control of voluntary movements a balance is required between dopaminergic and cholinergic components of basal ganglia.
Any imbalance between these 2 components will leads to loss of normal control and initiation of involuntary movement.
Pathological changes:
Gross:
The brain is atrophic(decrease in size) or may be normal externally.
Microscopically :
The hall mark is depigmentation of substantia nigra and locus ceruleus due to loss of neuromelanin pigment in neurons and accumulation of neuromelanin pigment in the giant cell.
So one of the residual neurons in these areas contains intracytoplasmic esinophills elongated inclusions called lewy bodies.
As the nigro straital neurons are well dopaminergic, degeneration of some of them deprives the adequate dopamine input.
This allows cholinergic transmission to predominate in basal ganglia which is responsible for involuntary movement, tremor and rigidity.
Clinical Features :
these are explained by the combination of dopamine deficiency and cholinergic predopanderance.
As the disease progress more and more nigro straital neurons fall out.
Treatment:
These is progressive and incurable disease and treatment is symptomatic, imperical and life long.

Schizophrenia Stroke






New Page 1





Schizophrenia
stroke


DEFINITION


ETIOLOGY




TYPES


PHTHOPHYSIOLOGY


Definition : its defined as group of emotional
disorder, usually of psychotic proportion, characterized by withdrawal from
reality delusion, hallucination, regressive behavior and ambivalence.


Etiology : it includes…



  1. early
    life adverse experience


  2. persistent
    worries


  3. unhappy
    without any justified cause


  4. regular
    insomnia


  5. afraid
    without any cause


  6. loss
    of tamper often and easily


  7. wild
    fluctuation in mood



 


Typers :



  1. disorganized:
    incoherence (an productive) unusual effect.


  2. catatonic:
    stupor, muteness, rigidity, bizarre, posturing.


  3. paranoid:
    systematized fearful or hastile delusions



Pathophysiology:




  •  it
    causes due to neurotransmitter dysfunction, dopamine, deficiency and even
    over activity of  dopamine in
    lymphatic system.



  •      
    Pathology of schizophrenia is not clearly understood.



  •      
    Several theories are postulated



  •      
    The illness is thought of as a phenomenon of regression i.e.
    reversal to infantile and childhood patterns of psychological living, a
    state of organization where reality doesn’t exist.



  •      
    Thus the patient attempts to resolve his psychological
    conflicts by denying the harsh and painful real world and living in a state
    of fantasy and pleasure.







HYPERTENSION


Hypertension


Definition: It’s a state in which
systolic pressure is 150 mm-Hg or  more
and diastolic pressure is 95 mm-Hg or more.


ü     
Hypertension can be classified into two types.


1.     
primary or essential hypertension


2.     
secondary hypertension


 


Primary
or essential hypertension:-


Here,
the cause of increase in blood pressure is unknown.


ü    
it comprises 90-95%
patient.


 


Etiology:- Following factors are involved…


1.     
genetic factor


2.     
Racial factor and environmental factors.


3.     
Risk factors modifying the cause of essential hypertension.


Pathophysiology:-


1.     
high plasma level of catecholamines


2.     
increase in blood volume


3.     
increase in cardiac output


4.     
low renin essential hypertension


5.     
high renin essential hypertension


Secondary
Hypertension:


ü    
here, the increase in blood pressure is caused by disease of the
kidney, endocrine or some other organs.


ü    
It emprises 5-10% patients.


Etiology:-


ü    
Renal
disease


A.    
Reno-vascular


B.    
Renal-parenchymal


ü    
Endocrine
disease


A.    
Adrenocortical hyper function


B.    
 Hyper parathyroidism


C.    
Oral contraceptives


ü    
Co-aractation
of aorta


ü    
Neurogenic


Pathology:-


1.     
Renal
hypertension


Ø     
Activation
of renin-angiotensin system


Ø     
Sodium
and water retention


Ø     
Release
of vasopressor material


 


2.     
Endocrine
hypertension


Ø     
Adrenal
gland E.g.: Cushing syndrome


Ø     
Parathyroid
gland E.g.: hyper parathyroidism


3.     
Oral
contraception


4.     
Neurogenic



Ø     
Vagus
(Both sympathetic and parasympathetic)


For
both primary and secondary hypertension it has been shown that hypertension have
tachycardia, increased cardiac output, but normal peripheral resistance.








Angina (angina pectoris)


Definition:- it’s a clinical syndrome of IHD
(Ischaemic heart disease) resulting from transient, myocardial Ischaemia.
It is characterized by proximal pain in the substernal and precardial region of
the chest which is caused due to increase in the demand of heart and relive by a
decrease in work of heart.


Ø     
Some times the pain transfer to left arm, new jaw or right arm.


Ø     
There are mainly three types of Angina, with some different in
their pathogenesis.


1.     
stable or typical angina


2.     
prinzmetal’s variant angina


3.     
unstable or crescendo angina



  1. Stable
    Or Typical Angina:-



Ø     
This is the most common pattern


Ø     
It is characterized by attack of pain following physical exertion
or emotional excitement and relived by rest.


Pathogenesis: includes coronary arteriosclerosis
depression of ST segment in ECG during the attack, no elevation of enzyme in the
blood as there is no irreversible myocardial injury.



  1. Prinzmetal’s
    variant angina :-



Ø     
Its characterized by pain at rest and has no relationship with
physical activity.


Ø     
Pathogenesis is not clear but it caused due to sudden vasospasm of
 coronary trunk induced by coronary
atheriosclerosis.


Ø     
ECG shows ST segment elevation due to Tran mural Ischaemia.


3.     
Unstable Or Crescendo Angina:-


Ø     
Also referred to as “preinfraction angina” or ‘acute
coronary insufficiency”. This is the most serious pattern of angina.


Ø     
Its characterized by more frequent onset of pain of prolonged
duration and occurring often at rest.


Pathogenesis:- includes…


Steno sing coronary atheriosclerosis, complicated coronary
plaques, platelet thrombi over atherosclerotic plaque and vasospasm of coronary
arteries. More often the lesions lie in a branch of major coronary trunk.


Congestive
cardiac failure (CCF) or Congestive heart failure (CHF)


Definition:- Its defined as pathological state
resulting from impaired cardiac function which is unable to maintain an output
sufficient for metabolic need s of tissue of body.


Etiology
:-


Ø    
Decreased myocardial contractility 
because of systolic dysfunction.


Ø    
Diminished outflow due to excessive pressure. Stoke volume
load is imposed on the heart because of diastolic dysfunction.


Classification:- They are of two types…


Ø    
Left sided heart failure:-


o      
It’s the failure of the left ventricle.


o      
Its initiated by the stress to the left heart.


Ø    
The major causes are…


o      
Ischaemic heart disease (IHD)


o      
Systemic hypertension


o      
Mitral or aortic valve diseases(sepsis)


o      
Myocardial disease like cardiomyopathic myocarditis




 







Ring Conformers




Ring Conformations



Although the customary line drawings of simple cycloalkanes are geometrical
polygons, the actual shape of these compounds in most cases is very different.



Cyclopropane is necessarily planar (flat), with the carbon atoms at
the corners of an equilateral triangle. The 60º bond angles are much smaller
than the optimum 109.5º angles of a normal tetrahedral carbon atom, and the
resulting angle strain dramatically influences the chemical behavior of
this cycloalkane. Cyclopropane also suffers substantial eclipsing
strain
, since all the carbon-carbon bonds are fully eclipsed. Cyclobutane
reduces some bond-eclipsing strain by folding (the out-of-plane dihedral angle
is about 25º), but the total eclipsing and angle strain remains high. Cyclopentane
has very little angle strain (the angles of a pentagon are 108º), but its
eclipsing strain would be large (about 10 kcal/mol) if it remained planar.
Consequently, the five-membered ring adopts non-planar puckered conformations
whenever possible. Rings larger than cyclopentane would have angle strain if
they were planar
. However, this strain, together with the eclipsing strain
inherent in a planar structure, can be relieved by puckering the ring. Cyclohexane
is a good example of a carbocyclic system that virtually eliminates eclipsing
and angle strain by adopting non-planar conformations, such as those shown
below. Cycloheptane and cyclooctane have greater strain than cyclohexane, in
large part due to transannular crowding (steric hindrance by groups on
opposite sides of the ring).



Some Conformations of Cyclohexane Rings




A planar structure for cyclohexane is clearly improbable. The bond angles
would necessarily be 120º, 10.5º larger than the ideal tetrahedral angle.
Also, every carbon-carbon bond in such a structure would be eclipsed. The
resulting angle and eclipsing strains would severely destabilize this
structure. If two carbon atoms on opposite sides of the six-membered ring are
lifted out of the plane of the ring, much of the angle strain can be
eliminated. This boat structure still has two eclipsed bonds (colored
magenta in the drawing) and severe steric crowding of two hydrogen atoms on
the "bow" and "stern" of the boat. This steric crowding
is often called steric hindrance. By twisting the boat conformation,
the steric hindrance can be partially relieved, but the twist-boat
conformer still retains some of the strains that characterize the boat
conformer. Finally, by lifting one carbon above the ring plane and the other
below the plane, a relatively strain-free chair conformer is formed.
This is the predominant structure adopted by molecules of cyclohexane.


These conformations may be examined as Chime models by
.



On careful examination of a chair conformation of cyclohexane, we find that
the twelve hydrogens are not structurally equivalent. Six of them are located
about the periphery of the carbon ring, and are termed equatorial. The
other six are oriented above and below the approximate plane of the ring
(three in each location), and are termed axial because they are aligned
parallel to the symmetry axis of the ring. In the stick model shown on the
left below, the equatorial hydrogens are colored blue, and the axial hydrogens
are red. Since there are two equivalent chair conformations of cyclohexane in
rapid equilibrium, all twelve hydrogens have 50% equatorial and 50% axial
character.



Because axial bonds are parallel to each other, substituents larger than
hydrogen generally suffer greater steric crowding when they are oriented axial
rather than equatorial. Consequently, substituted cyclohexanes will
preferentially adopt conformations in which large substituents assume
equatorial orientation. In the two methylcyclohexane conformers shown above,
the methyl carbon is colored blue. When the methyl group occupies an axial
position it suffers steric crowding by the two axial hydrogens located on the
same side of the ring. This crowding or steric hindrance is associated with
the red-colored hydrogens in the structure. A careful examination of the axial
conformer shows that this steric hindrance is due to two gauche-like
orientations
of the methyl group with ring carbons #3 and #5. The use of
models, including Chime, is particularly helpful in recognizing and evaluating
these relationships.

These conformations may be examined as Chime models by


To view an animation of the interconversion of
cyclohexane chair conformers


The relative steric hindrance experienced by different substituent groups
oriented in an axial versus equatorial location on cyclohexane may be
determined by the conformational equilibrium of the compound. The
corresponding equilibrium constant is related to the energy
difference
between the conformers, and collecting such data allows us to
evaluate the relative tendency of substituents to exist in an equatorial or
axial location. A table of these free energy values (sometimes referred to as
A values) may be examined by .
Clearly the apparent "size" of a substituent is influenced by its
width and bond length to cyclohexane, as evidenced by the fact that an axial
vinyl group is less hindered than ethyl, and iodine slightly less than
chlorine.



 







Substituted Cyclohexanes




Substituted Cyclohexane Compounds



Because it is so common among natural and synthetic compounds, and because
its conformational features are rather well understood, we shall focus on the
six-membered cyclohexane ring in this discussion. In a sample of cyclohexane,
the two identical chair conformers are present in equal concentration, and the
hydrogens are all equivalent (50% equatorial & 50% axial) due to rapid
interconversion of the conformers. When the cyclohexane ring bears a
substituent, the two chair conformers are not the same. In one conformer the
substituent is axial, in the other it is equatorial. Due to steric hindrance
in the axial location, substituent groups prefer to be equatorial and that
chair conformer predominates in the equilibrium.
We noted earlier that cycloalkanes having two or more substituents on
different ring carbon atoms exist as a pair (sometimes more) of
configurational stereoisomers. Now we must examine the way in which favorable
ring conformations influence the properties of the configurational isomers.
Remember, configurational stereoisomers are stable and do not easily
interconvert, whereas, conformational isomers normally interconvert rapidly.
In examining possible structures for substituted cyclohexanes, it is useful to
follow two principles.



(i)   Chair conformations are generally more stable than
other possibilities.
(ii)   Substituents on chair conformers prefer to occupy
equatorial positions due to the increased steric hindrance of axial
locations.



The following equations and formulas illustrate how the presence of two or
more substituents on a cyclohexane ring perturbs the interconversion of the
two chair conformers in ways that can be predicted.



Conformational Structures of Disubstituted Cyclohexanes


 































1,1-dimethylcyclohexane  
1-t-butyl-1-methylcyclohexane  
cis-1,2-dimethylcyclohexane  
trans-1,2-dimethylcyclohexane  
cis-1,3-dimethylcyclohexane  
trans-1,3-dimethylcyclohexane  
cis-1,4-dimethylcyclohexane  
trans-1,4-dimethylcyclohexane  


In the case of 1,1-disubstituted cyclohexanes, one of the substituents must
necessarily be axial and the other equatorial, regardless of which chair
conformer is considered. Since the substituents are the same in
1,1-dimethylcyclohexane, the two conformers are identical and present in equal
concentration. In 1-t-butyl-1-methylcyclohexane the t-butyl group is much
larger than the methyl, and that chair conformer in which the larger group is
equatorial will be favored in the equilibrium( > 99%). Consequently, the
methyl group in this compound is almost exclusively axial in its orientation.
In the cases of 1,2-, 1,3- and 1,4-disubstituted compounds the analysis is a
bit more complex. It is always possible to have both groups equatorial, but
whether this requires a cis-relationship or a trans-relationship depends on
the relative location of the substituents. As we count around the ring from
carbon #1 to #6, the uppermost bond on each carbon changes its orientation
from equatorial (or axial) to axial (or equatorial) and back. It is important
to remember that the bonds on a given side of a chair ring-conformation
always alternate in this fashion
. Therefore, it should be clear that for
cis-1,2-disubstitution, one of the substituents must be equatorial and the
other axial; in the trans-isomer both may be equatorial. Because of the
alternating nature of equatorial and axial bonds, the opposite relationship is
true for 1,3-disubstitution (cis is all equatorial, trans is
equatorial/axial). Finally, 1,4-disubstitution reverts to the 1,2-pattern.


The conformations of some substituted cyclohexanes may
be examined as Chime models by
.



 







Practice Problems



These four problems concern the recognition of different conformations of a
given constitutional structure. Axial and equatorial relationships of
cyclohexane substituents are also examined.



















Return
to Table of Contents


 





 







Stereoisomers Part II




As chemists studied organic compounds isolated from plants and animals, a new
and subtle type of configurational stereoisomerism was discovered. For
example, lactic acid ( a C3H6O3 carboxylic
acid) was found in sour milk as well as in the blood and muscle fluids of
animals. The physical properties of this simple compound were identical,
regardless of the source (m.p, 53 ºC & pKa 3.80), but there
was evidence that the physiological behavior of the compound from the two
sources was not the same. Another natural product, the fragrant C10H14O
ketone carvone, was isolated from both spearmint and caraway. Again, all the
physical properties of carvone from these two sources seemed to be identical
(b.p. 230 ºC), but the odors of the two carvones were different and reflected
their source. Other examples of this kind were encountered, and suspicions of
a subtle kind of stereoisomerism were confirmed by the different interaction
these compounds displayed with plane polarized light. We now know that this
configurational stereoisomerism is due to different right and left-handed
forms that certain structures may adopt, in much the same way that a screw may
have right or left-handed threads but the same overall size and shape.
Isomeric pairs of this kind are termed enantiomers (from the Greek enantion
meaning opposite).



 







Chirality




Chirality and Symmetry



All objects may be classified with respect to a property we call chirality
(from the Greek cheir meaning hand). A chiral object is not
identical in all respects (i.e. superimposable) with its mirror image. An achiral
object
is identical with (superimposable on) its mirror image. Chiral
objects have a "handedness", for example, golf clubs, scissors,
shoes and a corkscrew. Thus, one can buy right or left-handed golf clubs and
scissors. Likewise, gloves and shoes come in pairs, a right and a left.
Achiral objects do not have a handedness, for example, a baseball bat (no
writing or logos on it), a plain round ball, a pencil, a T-shirt and a nail.
The chirality of an object is related to its symmetry, and to this end it is
useful to recognize certain symmetry elements that may be associated
with a given object. A symmetry element is a plane, a line or a point in or
through an object, about which a rotation or reflection leaves the object in
an orientation indistinguishable from the original. Some examples of symmetry
elements are shown below.




The face playing card provides an example of a center or point of symmetry.
Starting from such a point, a line drawn in any direction encounters the same
structural features as the opposite (180º) line. Four random lines of this
kind are shown in green. An example of a molecular configuration having a
point of symmetry is (E)-1,2-dichloroethene. Another way of describing a point
of symmetry is to note that any point in the object is reproduced by
reflection through the center onto the other side. In these two cases the
point of symmetry is colored magenta.
The boat conformation of cyclohexane shows an axis of symmetry (labeled C2
here) and two intersecting planes of symmetry (labeled σ). The notation
for a symmetry axis is Cn, where n is an integer chosen so that
rotation about the axis by 360/nº returns the object to a position
indistinguishable from where it started. In this case the rotation is by 180º,
so n=2. A plane of symmetry divides the object in such a way that the points
on one side of the plane are equivalent to the points on the other side by
reflection through the plane. In addition to the point of symmetry noted
earlier, (E)-1,2-dichloroethene also has a plane of symmetry (the plane
defined by the six atoms), and a C2 axis, passing through the
center perpendicular to the plane. The existence of a
reflective symmetry element (a point or plane of symmetry) is sufficient to
assure that the object having that element is achiral
. Chiral
objects, therefore, do not have any reflective symmetry elements, but may have
rotational symmetry axes, since these elements do not require reflection to
operate. In addition to the chiral vs achiral distinction, there are two other
terms often used to refer to the symmetry of an object. These are:


(i)   Dissymmetry: The absence of reflective symmetry
elements. All dissymmetric objects are chiral.
(ii)  Asymmetry: The absence of all symmetry elements. All
asymmetric objects are chiral.



George Hart has produced a nice treatment of symmetry
in polyhedra that makes use of VRML. To view this site
Click
Here









The symmetry elements of a structure also provide insight
concerning the structural equivalence or nonequivalence of similar
component atoms or groups
Examples of this symmetry analysis may be viewed by Clicking
Here
.





 







Enantiomorphism




Enantiomorphism



A consideration of the chirality of molecular configurations explains the
curious stereoisomerism observed for lactic acid, carvone and a multitude of
other organic compounds. Tetravalent carbons have a tetrahedral configuration.
If all four substituent groups are the same, as in methane or
tetrachloromethane, the configuration is that of a highly symmetric
"regular tetrahedron". A regular tetrahedron has six planes of
symmetry and seven symmetry axes (four C3 & three C2)
and is, of course, achiral. Examples of these Axes
and Planes
are found on George Hart's VRML site.

If one of the carbon substituents is different from the other three, the
degree of symmetry is lowered to a C3 axis and three planes of
symmetry, but the configuration remains achiral. The tetrahedral configuration
in such compounds is no longer regular, since bond lengths and bond angles
change as the bonded atoms or groups change. Further substitution may reduce
the symmetry even more, but as long as two of the four substituents are the
same there is always a plane of symmetry that bisects the angle linking those
substituents, so these configurations are also achiral.



A carbon atom that is bonded to four different atoms or groups loses all
symmetry, and is often referred to as an asymmetric carbon. The
configuration of such a molecular unit is chiral, and the structure may exist
in either a right-handed configuration or a left-handed configuration. This
type of configurational stereoisomerism is termed enantiomorphism, and
the non-identical, mirror-image pair of stereoisomers that result are called enantiomers.
The structural formulas of lactic acid and carvone are drawn on the right with
the asymmetric carbon colored red. Consequently, we should expect these
compounds to exist as pairs of enantiomers. The presence of a single
asymmetrically substituted carbon atom in a molecule is sufficient to render
the whole configuration chiral, and modern terminology refers to such
asymmetric (or dissymmetric) groupings as chiral centers. Most of the
chiral centers we shall discuss are asymmetric carbon atoms, but it should be
recognized that other tetrahedral or pyramidal atoms may become chiral centers
if appropriately substituted. Also, linear and planar regions of chirality may
exist if appropriate rigid configurations are present, as in the case of allenes.
When more than one chiral center is present in a molecular structure, care
must be taken to analyze their relationship before concluding that a specific
molecular configuration is chiral or achiral. This aspect of stereoisomerism
will be treated later.


The identity or non-identity of mirror-image
configurations of some substituted carbons may be examined as Chime models by
.



A useful first step in examining structural formulas to determine whether
stereoisomers may exist is to identify all stereogenic centers. A stereogenic
center
is a focus of stereoisomerism, such that an interchange of two
groups attached to the atom constituting the center leads to a stereoisomer. A
chiral center, such as an asymmetric carbon, is a stereogenic center, since
interchanging any two substituent groups converts one enantiomer to the other.
Stereogenic centers need not be chiral. The double bond carbon atoms of an
alkene bearing two different groups on each carbon (e.g. abC=Cab) are achiral
stereogenic centers (interchanging substituents at one of the carbons changes
the cis/trans configuration of the double bond. Other stereogenic elements,
such as axes or planes, may be present in a molecular configuration, but these
are less common as stereogenic centers and will not be discussed here.


Structural formulas for eight organic compounds are displayed in the frame
below. Some of these structures are chiral and some are achiral. First, try to
identify all chiral stereogenic centers. Formulas having no chiral centers are
necessarily achiral. Formulas having one chiral center are always chiral; and
if two or more chiral centers are present in a given structure it is likely to
be chiral, but in special cases, to be discussed later, may be achiral. Once
you have made your selections of chiral stereogenic centers, check them by
pressing the "Show Chiral Centers" button. The chiral centers will
be identified by red dots.






Structures F and G are achiral. The former has a plane of
symmetry passing through the chlorine atom and bisecting the opposite
carbon-carbon bond. The similar structure of compound E does not have
such a symmetry plane, and the carbon bonded to the chlorine is a chiral
stereogenic center (the two ring segments connecting this carbon are not
identical). Structure G is essentially flat. All the carbons except
that of the methyl group are sp2 hybridized, and therefore
trigonal-planar in configuration. Compounds C, D & H have
more than one chiral center, and are also chiral. Remember, all chiral
structures may exist as a pair of enantiomers
. Other configurational
stereoisomers are possible if more than one stereogenic center is present in a
structure.



 







Polarimetry




Optical Activity



Identifying and distinguishing enantiomers is inherently difficult, since
their physical and chemical properties are largely identical. Fortunately, a
nearly two hundred year old discovery by the French physicist Jean-Baptiste
Biot has made this task much easier. This discovery disclosed that the right-
and left-handed enantiomers of a chiral compound perturb plane-polarized light
in opposite ways. This perturbation is unique to chiral molecules, and has
been termed optical activity.


Plane-polarized light is created by passing ordinary light through a
polarizing device, which may be as simple as a lens taken from polarizing
sun-glasses. Such devices transmit selectively only that component of a light
beam having electrical and magnetic field vectors oscillating in a single
plane. The plane of polarization can be determined by an instrument called a polarimeter,
shown in the diagram below.



Monochromatic (single wavelength) light, is polarized by a fixed polarizer
next to the light source. A sample cell holder is located in line with the
light beam, followed by a movable polarizer (the analyzer) and an eyepiece
through which the light intensity can be observed. In modern instruments an
electronic light detector takes the place of the human eye. In the absence of
a sample, the light intensity at the detector is at a maximum when the second
(movable) polarizer is set parallel to the first polarizer (α = 0º). If
the analyzer is turned 90º to the plane of initial polarization, all the
light will be blocked from reaching the detector.


Chemists use polarimeters to investigate the influence of compounds (in the
sample cell) on plane polarized light. Samples composed only of achiral
molecules (e.g. water or hexane), have no effect on the polarized light beam.
However, if a single enantiomer is examined (all sample molecules being
right-handed, or all being left-handed), the plane of polarization is rotated
in either a clockwise (positive) or counter-clockwise (negative) direction,
and the analyzer must be turned an appropriate matching angle, α,
if full light intensity is to reach the detector. In the above illustration,
the sample has rotated the polarization plane clockwise by +90º, and the
analyzer has been turned this amount to permit maximum light transmission.
The observed rotations (α) of enantiomers are opposite in direction. One
enantiomer will rotate polarized light in a clockwise direction, termed dextrorotatory
or (+), and its mirror-image partner in a counter-clockwise manner, termed levorotatory
or (–). The prefixes dextro and levo come from the Latin dexter,
meaning right, and laevus, for left, and are abbreviated d and l
respectively. If equal quantities of each enantiomer are examined , using the
same sample cell, then the magnitude of the rotations will be the same, with
one being positive and the other negative. To be absolutely certain whether an
observed rotation is positive or negative it is often necessary to make a
second measurement using a different amount or concentration of the sample. In
the above illustration, for example, α might be –90º or +270º rather
than +90º. If the sample concentration is reduced by 10%, then the positive
rotation would change to +81º (or +243º) while the negative rotation would
change to –81º, and the correct α would be identified unambiguously.
Since it is not always possible to obtain or use samples of exactly the same
size, the observed rotation is usually corrected to compensate for variations
in sample quantity and cell length. Thus it is common practice to convert the
observed rotation, α, to a specific rotation, [α], by the
following formula:
















Specific Rotation =     where l = cell length in dm, c = concentration in g/ml
    D is the 589 nm light from a sodium lamp


 



Compounds that rotate the plane of polarized light are termed optically
active
. Each enantiomer of a stereoisomeric pair is optically active and
has an equal but opposite-in-sign specific rotation. Specific rotations are
useful in that they are experimentally determined constants that characterize
and identify pure enantiomers. For example, the lactic acid and carvone
enantiomers discussed earlier have the following specific rotations.


























Carvone from caraway:   [α]D = +62.5º this isomer may be referred to as (+)-carvone or d-carvone
Carvone from spearmint:   [α]D = –62.5º this isomer may be referred to as (–)-carvone or l-carvone
Lactic acid from muscle tissue:   [α]D = +2.5º this isomer may be referred to as (+)-lactic acid or d-lactic
acid
Lactic acid from sour milk:   [α]D = –2.5º this isomer may be referred to as (–)-lactic acid or l-lactic
acid


 


A 50:50 mixture of enantiomers has no observable optical activity. Such
mixtures are called racemates or racemic mixtures, and are designated (±).
When chiral compounds are created from achiral compounds, the products are
racemic unless a single enantiomer of a chiral co-reactant or catalyst is
involved in the reaction. The addition of HBr to either cis- or trans-2-butene
is an example of racemic product formation (the stereogenic center is colored
red in the following equation).











CH3CH=CHCH3 + HBr (±) CH3CH2CHBrCH3


 


Chiral organic compounds isolated from living organisms are usually
optically active, indicating that one of the enantiomers predominates (often
it is the only isomer present). This is a result of the action of chiral
catalysts we call enzymes, and reflects the inherently chiral nature of life
itself. Chiral synthetic compounds, on the other hand, are commonly racemates,
unless they have been prepared from enantiomerically pure starting materials.


There are two ways in which the condition of a chiral substance may be
changed:
        1.  A racemate may be
separated into its component enantiomers. This process is called resolution.
        2.  A pure enantiomer
may be transformed into its racemate. This process is called racemization.










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