Molecular optical activity was first discovered by Pasteur in the 19th century.He found that tartaric acid crystal has two relative crystal forms, which will make light rotate in the opposite direction when forming solution, so he determined that the molecule has different structures of left and right rotation.When ordinary light passes through apolarizationOflensorNicol prismPart of the light is blocked when the vibration direction is parallel to the crystal axis of the prism.This kind of light that vibrates on only one plane is calledPlane polarized light, referred to as polarized light.PolarizedVibrating surfaceChemically, it is customarily calledPlane of polarization。When plane polarized light passes throughChiral compoundAfter the solution, the direction of the polarization plane is rotated by an angle.This property that can rotate the polarization plane is called optical rotation.
As the French physicist Marius first discovered in 1808, the reflected light is often partially plane polarized light (he usedNewtonThe argument about the poles of light particles - which is very difficult to explain the fluctuation, but the concept of photons shows that this argument is correct - creates the term polarization.Therefore, wearPolarizerSunglasses can soften the strong sunlight reflected from buildings and car windows, even from the road surface to the eyes.
Plane polarization
optical rotation
Light waves normally vibrate in all planes.Nicol prismOnly light that vibrates in one plane is allowed to be reflected through the rest of the light.Therefore,Transmitted lightbyPlane polarized light。In 1815, French physicist Biot found that when plane polarized light passes throughquartz crystalThe polarization plane will rotate.In other words, light enters one plane in the form of waves and shoots out from another plane in the form of waves.The substance with this effect is called optically active substance.Some quartz crystals can make the vibration planeClockwiseRotate (right rotation), while some quartz crystals can make it rotate counterclockwise (left rotation).Biot also found that someOrganic compound, such as camphor and tartaric acid, also have the same effect.He believes that the reason for the rotation of the light beam is probably due to some asymmetry of the arrangement of atoms in the molecule.However, in the following decades, this view is still a pure theoretical speculation.
Direction of rotation
In 1844,Pasteur(He was only 22 at the time) was fascinated by this interesting question.He studied two substances: tartaric acid and exogenousRacemizationAcid（2,3-dihydroxysuccinic acid)。Although they have the same chemical composition, tartaric acid canpolarized lightThe vibration plane of the racemic acid could not rotate.Pasteur conjectured that it might be possible to prove that the crystal of tartrate is asymmetric, while the crystal of racemate is symmetric.To his surprise, by observing the crystals of these two groups of salts under a microscope, he found that both were asymmetric.However, racemate crystals have two forms of asymmetry: one half is the same shape as the tartrate crystal, and the other half is mirror image.In other words, half of the racemic acid salt crystals are left-handed and half are right-handed.
Rotation confirmation
Pasteur painstakingly separated the left-handed and right-handed racemic acid salt crystals, then made solutions respectively, and let the beam pass through each solution.Sure enough, crystals with the same asymmetry as tartaric acid crystals, whose solution, like tartratepolarized lightThe vibration surface of is rotated, and the rotation angle is the same.These crystals are tartrate.The solution of another group of crystals makes the vibration of polarized light rotate in the opposite direction with the same rotation angle.It can be seen that the reason why the original racemate does not show optical activity is that these two opposing tendencies offset each other.
Then Pasteur added hydrogen ion to these two solutions to make these two racemic acid salts become racemic acid again.(By the way, a salt is a compound formed when one or more hydrogen ions in an acid molecule are replaced by positive ions such as potassium or sodium).He found that both racemic acids are optically active, one of which makes polarized light rotate in the same direction as tartaric acid (because it is tartaric acid), while the other makes polarized light rotate in the opposite direction.
Enantiomer
Later, many mirror compounds were found, namely enantiomers (from Greek, meaning "opposite shape").In 1863, German chemist Wesley Zenus found that lactic acid (the acid in sour milk) can form such compounds.He further proved that the other properties of these two kinds of lactic acid are exactly the same except for their different effects on polarized light.It was later confirmed that this was generally true for various mirror compounds.
Law of character
It was not until 1874, the 12th year after Biot's death, that the answer was finally found.Two young chemists -- one namedVantovA 22 year old Dutch and a 27 year old French named Lebel independently put forward new theories on carbon valence bonds, thus solving the problem of the composition of mirror molecules.(Since then, Vantov has devoted his life to the study of the properties of substances in solution and proved that the law governing the properties of liquids is similar to the law governing the properties of gases. Because of this achievement, he became the first person to obtainNobel Prize in ChemistryPeople.)
Kekule Drawing all four valence bonds of carbon atoms in the same plane is not necessarily because the carbon bonds are indeed arranged like this, but simply because it is easier to draw them on a flat sheet of paper.Van Toff and Lebel proposed a three-dimensional model.In this model, they assigned four valence bonds to two mutually perpendicular planes, each with two valence bonds.The best way to describe this model is to imagine that any three of the four valence bonds act as legs to support the carbon atoms, while the fourth valence bond points directly above.If we assume that the carbon atom is located atRegular tetrahedron(4 faces are equilateral triangularGeometry）The four valence bonds point to the four vertices of the tetrahedron.Therefore, this model is called the tetrahedron model of carbon atoms.
Asymmetry
In this way, Vantov and Lebel uncovered the secret of the asymmetry of optically active matter.Originally causing light to rotate in the opposite directionMirror materialOfcarbon atomIts valence bond is connected with four different atoms or atomic groups.There are two possible arrangements of these four atoms or atomic clusters, one is to makepolarized lightThe other makes polarized light rotate to the left.More and more evidences strongly support the regular tetrahedron model of carbon atom proposed by Vantov and Lebel.By 1885, their theory had been widely recognized (thanks in part to the enthusiastic support of Veslitzenus).
The concept of three-dimensional structure has also been applied to atoms other than carbon atoms.German chemist Meier successfully applied this concept to nitrogen atoms, while British chemist Popper applied it to sulfur, selenium and tin atoms.Swiss chemist of German originWerner Apply it to more elements.He also started to create a coordinate theory in the 1890s, that is, by carefully studying the distribution of atoms and atomic clusters around a central atom, to explain the structure of complex inorganic substances.For this achievement, Werner won the Nobel Prize in Chemistry in 1913.Pasteur named the two separated racemic acids d tartaric acid (dextral) and l tartaric acid (left-handed) respectively, and wrote the mirror structure formula for them.However, there is no way to distinguish which is a true dextral compound and which is a left-handed compound.
In order to provide chemists with reference materials or comparison standards to distinguish dextral and left-handed substances, the German chemist E. Fischer chose a close relative of sugar, a simple compound called glyceraldehyde.It was one of the most thoroughly studied optical compounds at that time.He arbitrarily defined one of its forms as left-handed, called it LGlyceraldehydeIts mirror compound is defined as dextral, which is called D glyceraldehyde.
Any compound, as long as it can be proved by appropriate chemical methods (this is a rather meticulous work) that it has a structure similar to L glyceraldehyde, no matter whether its effect on polarized light is left-handed or right-handed, is considered to belong to the L series, and its name is preceded by L.Later, it was found that tartaric acid, which was previously thought to be in the left-handed form, originally belonged to the D series rather than the L series.For compounds that belong to the D series in structure and cause light to rotate to the left, we prefix their names with D (-);Similarly, some compounds should be preceded by D (+), L (+) and L (-).
Direction difference
The direction in which the polarization plane is rotated isDextral rotation(clockwise) and left-hand (counterclockwise).The sign (+) indicates right rotation, and (-) indicates left rotation.For example: (+) - 2-butanol represents dextral;(-) - 2-butanol indicates levorotatory.AllOptically active compoundEither right or left.
optical rotation
In the second half of the 19th century, chemists discovered a particularly wonderfulIsomerismIt was later proved that this phenomenon was extremely important in life chemistry.This discovery is that someOrganic compoundThere is a strange asymmetric effect on the light beams passing through them.It can be seen from a cross section of an ordinary beam that the countless waves constituting the beam vibrate up and down, left and right, and obliquely in all planes.This kind of light is called nonpolarized light。However, when the light beam passes through the crystal of transparent material (such asIceland stone）When, refraction will occur, making the outgoing light become polarized light.This seems to be theAtomic latticeOnly certain undulating surfaces are allowed to pass through (just like the fence only allows pedestrians to squeeze through sideways, but cannot let people swagger through it).Some devices, such as those invented by Scottish physicist Nichol in 1829Nicol prism, only light is allowed to pass through one plane.This prism has been made of other materials, such asPolarizer(A set of inlaidNitrocelluloseParallel to crystal axisQuinine sulfateWith iodine).The first polarizer isRand It was made in 1932.[1-2]
Overview of basic concepts
Announce
edit
polarized light
Light of various wavelengths in ordinary light vibrates in various planes perpendicular to the direction of travel. The plane formed by the vibration plane and the direction of travel of light waves is called the vibration plane.The vibration surface of light is limited to a fixed direction, which is called plane polarized light, or polarized light for short.
Optical rotation
When the plane polarized light passes through the rotator tube containing the optically active compound, the polarization plane will be rotated (to the right or left) for an angle, and then the polarized light cannot pass unimpeded through the polarizer parallel to the prism axisPolarizer。Only the polarizer rotates (right or left) at the same angle
Only rotated plane polarized light can pass through completely.Observe the rotation angle of the dial carried on the polarizer
, that is, theOptical rotation。polarizationThe angle at which the surface is rotated by the optically active material is called the optical rotation.Expressed with.
Specific rotation
The length of the sample tube, the type of solvent, the concentration of the solution, the temperature and the wavelength of the light used have an impact on the value of the optical rotation of a specific substance.In order to make its optical rotation a characteristic physical constant, a 1dm long optical rotation tube is usually used. The concentration of the substance to be measured is 1g/mL. The optical rotation measured under the condition of sodium light (D line) with a wavelength of 589nm is called specific optical rotation.
In the above formula,
t: Temperature during measurement (℃)
D: Sodium light D-line wavelength 589nm
α: Rotational value observed in experiment (°)
l: Length of optical rotator tube (dm)
C: Solution concentration (g/ml),
(Density for pure liquid g/cm3)
Enantiomer
A pair of enantiomers is a pair of mirror isomers that cannot overlap in space, that is, chiral molecules.
Racemate
The equivalent mixture of a pair of enantiomers is called racemic mixture or racemate.Usually expressed as (±) or dl.Racemes are mixtures.
A pair of enantiomers have the same melting point, boiling point, density, pKa, and their specific rotations are equal in size and opposite in direction.The physical properties of racemate are somewhat different from that of single enantiomer. It has no optical activity, and its melting point, density and solubility are often different.But the boiling point, pKa and pure enantiomer are the same.
Diastereomer
Stereoisomers that do not mirror each otherDiastereomer。Diastereomers have different physical properties.For example, boiling point, solubility and optical activity are different.
Two chiral compounds containing multiple chiral carbon atoms, if they have the same structure except for one chiral carbon atom, they areEpimer。Epimer is a kind of diastereomer.
Racemic compound
Although some organic molecules have chiral centers, they are nonchiral as a whole due to the different number of chiral centers and connection modes. Such substances areRacemic compound。
The racemic compound is pure and has no optical activity.[1]
significance
Announce
edit
It is of great significance to study the details of optical activity with great concentration, and it is not a useless work driven by curiosity.Coincidentally, almost all compounds in living organisms contain asymmetric carbon atoms.Moreover, living organisms always use only one of the two mirror forms of compounds.In addition, similar compounds generally belong to the same series.For example, all themonosaccharideIn fact, they belong to the D series, while all amino acids (except glycine, the basic unit of protein) belong to the L series.
In 1955, Dutch chemist Bijwart finally determined what kind of structure would make polarized light rotate leftward and what kind of structure would make polarized light rotate rightward.It was only then that people knew that E. Fischer had just guessed right by accident when naming left-handed and right-handed forms.[3-4]
