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Conformational Analysis, PhysicalOrganic chemistryAn important concept of.The simplest conformational analysis is based on ethane.Most importantConformational analysisIs based oncyclohexaneConformational analysis on.
Molecular conformational disorderThermodynamic function。For those composed of thousands of C-C single bondsPolymer chainEach single bond rotates internally to different degrees around its adjacent single bond, and the arrangement of atoms in the molecule constantly changes to take different conformations.If W is used to represent all the desirable conformations of the polymer chain in space, the relationship between the conformational entropy S and W of the polymer chain followsBoltzmann formula：S=KlnW。Here K isBoltzmann constant。When the polymer chain takes an extended form, there is only one conformationentropyEquals zero.If the polymer chain takes a curled form, the number of desirable conformations of the molecule will be large.The larger the number of conformations, the larger the corresponding conformational entropy, and the more severe the molecular chain curl.According to the principle of entropy increase, isolated polymer chains always spontaneously take the form of curl without external force, which makes the conformation entropy tend to maximum.This is it.Polymer chain flexibilityThe essence of.
Tectonic conformation
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Molecular structure refers to the way and order in which atoms in molecules connect with each other. It has been called molecular structure for a long time in the past. According to the proposal of the International Union of Pure and Applied Chemistry, it is changed to "structure".The term "structure" is used in a wide range, such as material structure, atomic electronic structure, etc.Molecular structure also has a wide range of meanings. In addition to molecular structure, it also includes molecular configuration and conformation.From molecular structure to molecular configuration and molecular conformation, chemists gradually realized the properties of chemical substances one by one, which promoted the development of chemical science.
Molecular structure is studied fromIsomerThe occurrence of began.
At the end of the 18th century and the beginning of the 19th century, due to the rapid development of analytical chemistrychemical compoundIt is constantly discovered.In the 1920s, chemists began to find that some substances had the same chemical composition but different properties.At that time, it was inconceivable.At that time, chemists believed that the nature of matter was only determined by its composition, and each substance had a fixed composition, which was established at the end of the 18th centuryLaw of definite composition。Later, the inverse law of this law was also confirmed, that is, substances with the same composition must be the same kind of substance.Now there are substances with the same composition but different properties.
These substances were first discovered in 1823.This year, German chemistsLiebig (Liebig, Justusvonl803~1873). Its composition is analyzed. Silver oxide accounts for 77.53%, and cyanide accounts for 22.47%. Its chemical formula is determined. Now it is known as AgOCN.Chemical group obtained from this analysis resultCheng ZhengheIt was made by his fellow chemist Wohler (Friedrich 1800-1882) a year agoSilver cyanateAgOCN is exactly the same, Waller analysisSilver cyanateThe result is that silver oxide accounts for 77.23%, and cyanide accounts for 22.77%.The composition of the two is almost the same, but the properties are completely different. Silver fulminate is a violent explosive, while silver cyanate is not.
When they jointly studied and analyzed the problems encountered, they found a more difficult situation to explain, that is, cyanic acid is not only the same as fulvic acid, but also the same as cyanuric acid (also known as cyanuric acidCyanuric acidH3O3C3N3)
In 1828, Waller used ammonium chloride andSilver cyanateRespond and expect to getAmmonium cyanateNH4OCN results in urea CO (NH2) 2.This not only destroyed the "vitality" theory of the generation of organic compounds at that time, uprooted the insurmountable gap between inorganic compounds and organic compounds at that time, but also provided another reliable example for this incredible phenomenon.
Urea composition
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About the composition of urea, British chemistProut(Prout, william1785-1850) has been analyzed and the results are as follows:
Element percentage Atomic number
Nitrogen 46.650 2
Carbon 19.975 1
Hydrogen 6.670 4
Oxygen 26.650 1
ninety-nine point eight seven five
Wuler analyzed the composition of ammonium cyanate and obtained the following results:
Element percentage Atomic number
Nitrogen 46.78 2
Carbon 20.19 1
Hydrogen 6.59 4
Oxygen 26.24 1
ninety-nine point eight zero
The similarity of the results of the two analyses attracted the attention of Bergelius (Jons Jakob 1779~1848), a leading figure in the European chemical field, a teacher and friend of Waller, and a Swedish chemist at that time.When Wuler and Libich found silver fulminate and silver cyanate, two substances with the same composition but different properties, he once thought that one of the two must have made an analytical error.In 1830, he also found that tartaric acid and grape acid were also two substances with the same composition but different properties.Tartaric acid is separated from tartar. It was discovered by Swedish chemist Scheele (Carl Wilhelm 1742~l786) as early as 1769. Grape acid was first separated by French chemical manufacturer Kestner (Charles) as a by-product of tartar and was sold as oxalic acid before 1819.In 1826, the French chemist Gay Lussac (Joseph Louis 1778 ～ 1850) studied it and determined that the neutralization ability of grape acid and tartaric acid was the same.However, grape acid is not easily soluble in water and does not form Rochelle salt, that isPotassium sodium tartrateKNaC4H4O6 · 4H2O, Rochelle is a seaport city in western France, from which a large number of wines are exported.Grape acid is the name given by Geluzac
difference
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After discovering that it has the same composition but different properties with tartaric acid, Bezieus called it iso tartaric acid, which is now calledRacemic tartaric acid。
Therefore, Bezilius published an article in the Annual Report of Progress in Physical Chemistry edited by him in 1832, pointing out that "they are composed of the same number of atoms of the same element, but their atoms are arranged in different ways, so they have different chemical properties." He also proposed that substances with the same composition but different properties are called isomers.This word comes from Greek, "iso" is "the same", "mer" is "part", and the literal translation is "the same part", which is now translated as“Isomer”。
The discovery of isomer phenomenon started the study of molecular structure.This phenomenon shows that the chemical and physical properties of material molecules depend not only on the type and number of element atoms that make up the molecule, but also on the arrangement of atoms.
How the atoms in the molecule are arranged.The appearance of elemental valence provides some evidence.1852 Frank Nande, Professor of Chemistry, University of Manchester, England（Framk-Land, Edwardl 825 ～ 1899) published an article "On a New Series of Organic Compounds Containing Metals", listing the chemical formulas of some metal compounds and writing: "When studying the chemical formulas of inorganic compounds, even a superficial observer will also know about them.
impression
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The structure is generally symmetrical, especially the compounds of nitrogen, phosphorus, antimony and arsenic, showing the tendency of these elements to form compounds containing 3 or 5 equivalent other elements. It is precisely at this ratio that their affinity is maximized, so there are NO3, NH3, NI3, NS3, PO5, PH3, PCl3, SbO3, SbH3, SbCl3, AsO3, AsH3AsCl3, etc. There are NO5, NH4O, NH4I, PO5, PH4I, etc. in the pentaatomic group.According to the examples given above, it is enough to show that there is no need to provide any hypothesis about the cause of this symmetric combination of atoms. There is a general trend or law that, regardless of the nature of the combined atoms, the combining power of the connected elements is always satisfied by the same number of these atoms if I am allowed to use this term. "
Because the atomic weight measurement was not correct at that time, some of the chemical formulas listed by Frank Nande were wrong, but he summarized the concept of binding force from these chemical formulas, which led to the concept of valence in modern chemistry.
noun
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Then some other chemists put forward the atomic cardinal numberAtomic number, affinity number, chemical unit, substitution value and other terms with the same meaning are all today's concept of valence, and each of them proposes hydrogenhalogenIt is 1-valent, oxygen is 2-valent, carbon is 4-valent, etc., which correctly expresses the chemical valence of elements.
In the 1850s, while establishing the concept of valence, European chemists established the theory of atomic bonding in molecules, or the law of atomic bonding.They proposed that atoms in molecules are indirectly bound with a certain valence, and various graphical expressions are used to mark the valence of various elements, indicating the way and order of bonding among atoms in molecules, and the molecular structural formula appears.For example, the British chemist Couper,
formula
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Archibald Scott (1831~l892) gave the molecular structural formula of methanol and ethanol as follows:
The dotted line is used to indicate the combination price,carbonIt is 4, oxygen is 2, and hydrogen is 1, which is correct, but the chemical composition of methanol and ethanol is still wrong.The German chemist Kekul e, Friedrich August 1829-1896, published his modified hexagonal molecular formula of benzene in 1867, laying the foundation for the molecular formula of benzene.
However, the molecular structural formula listed by chemists at that time was just to express the chemical valence of elements, and in essence, it did not take into account the relationship between the way and order of atomic interconnection in molecules and the properties of matter.Although chemists at that time had put forward the concept of molecular structure, there was no method to determine the molecular structure according to the properties of substances at that time, so they fell into the ignorance of molecular structure and thought that molecular structure was unrecognizable.Kekule insists that the valence of an element is constant. When he and his students express the interconnection between atoms in oxyacid molecules, they write the following:
HClOH-O-Cl
HClO3H-O-O-O-Cl
H2SO3H-O-S-O-O-H
H2SO4H-O-S-O-O-O-H
H2S2O3H-S-S-O-O-O-H
In order to keep the valence of one element fixed, the following "structural formulas" of different "designs" were obtained at that time:
Fe=3
Sn=4
This is purely to indicate the valence of elements, rather than the true molecular structure.
In 1861, Russ chemist Boutroff attended the 36th Congress of Natural Scientists and Physicians held in Speyer, Germany, and delivered a speech titled "On the Structure of Matter",It is clearly pointed out that "the nature of a molecule depends on the nature, quantity and arrangement of its basic components, which is a known rule. This rule can now be revised as follows: the chemical properties of a molecule depend on the nature, quantity and chemical structure of its basic components."
Butelierov made it clear that the molecular structure is knowable, and proposed the way to determine the molecular structure.He said: "The best way to study the chemical structure of different compounds is probably to synthesize them... On the other hand, the chemical structure can also be determined from the decomposition process to some extent." Butelierov's "chemical structure" is exactly "molecular structure".
Butelierov also pointed out that the role of direct coupling between atoms in molecules is different from that of indirect coupling.He called the former the first interaction and the latter the second interaction.He believes that the first interaction is the main one, which determines the typical reaction performance of the molecular atomic group, and the second interaction is the secondary one, which determines the particularity of each reaction belonging to the same typical one.For example, when acetic acid CH3COOH andMonochloroacetic acidIn CH2ClCOOH, all H atoms connected with O atoms have acidity, which is determined by the first interaction, but CH2ClCOOH is more acidic than CH3COOH, which is due to the existence of Cl atoms not directly connected with H atoms.
Inspired by Butlerov, chemists have successively developed various physical and chemical methods for determining molecular structure.For example, cyclohexane and hexene have the same composition C6H12, and their different molecular structures are determined according to the different heat released during combustion.The molecular combustion heat of cyclohexane is smaller than that of hexene. This is because the energy of single bond and double bond is different, so we can get their different molecular structures: CH3CH2CH2CH2CH=CH3 hexene
Another example is that ethanol and ether have the sameC2H6OHowever, ethanol can react with metal sodium to release hydrogen. No matter how much excess sodium is used, only one hydrogen atom is replaced in the alcohol molecule, indicating that one hydrogen atom in the alcohol molecule is different from all the other hydrogen atoms, while ether has no such chemical property, so their different molecular structures are obtained:
