In type I rearrangements (Y-A-B-X conversion to X-A-B-Y), the two migrating groups are oriented trans to each other. The reaction leaves both groups on the opposite sides. The first example of a dyotropic rearrangement involving a carbon-carbon bond was reported by Cyril A. Grob and Saul Winstein. They saw the exchange of two bromine atoms in a certain steroid.
In a simple example, the two bromine atoms in 3-tert-butyl-trans-1,2-dibromohexane mutarotate by heating. In the transition state both bromine atoms connect symmetrically two both carbon atoms on opposite sides and the reaction is concerted. Chemists have also investigated stepwise mechanisms in dyotropic reactions.
In organic synthesis, an important dyotropic reaction is the conversion of 4-substituted-gamma-lactones to butyrolactones. Type I dyotropic rearrangements also occur around carbon-oxygen bonds. One example is using heat to change RRSiRC-O-SiR to RRSiRC-O-SiR. (This reaction can go in both directions depending on temperature.) Another example is the 1,2-Wittig rearrangement. Dyotropic reactions can also happen with N-O bonds or N-N bonds.
Type II rearrangements often involve two hydrogen atoms moving along a carbon skeleton. This reaction type can be found in certain transfer hydrogenations. An example is hydrogen transfer in syn-sesquinorbornene
disulfones.
Diels–Alder reaction
The Diels–Alder reaction (DA) is a chemical reaction between organic compounds. The reaction causes the compounds to form a new six-sided compound. This is called a cyclohexene. A conjugated diene joins with an alkene to make the cyclohexene. This compound looks like a ring.
This reaction was discovered by Otto Diels and Kurt Alder in 1928. In 1950, they were given the Nobel Prize in Chemistry for their work on this reaction. The DA reaction is very useful as it means cyclohexenes can be made with very little energy. Cyclohexenes are used to make complex organic molecules. One of the first uses of the DA reaction was to make insecticides.
The DA reaction makes a new hexagonal ring-shaped compound. A compound with two double bonds which are one carbon atom apart from each other is joined to another compound with at least one double bond. A DA reaction can also happen if some of the atoms in the newly formed ring are not carbon. Some of the DA reactions are reversible. The reaction caused during this breaking up of the cyclic system is called the retro-Diels–Alder. These retro-Diels–Alder compounds are often seen when they are analyzed by mass spectrometry.
Some chemists call the Diels–Alder reaction the 'Mona Lisa' of organic reactions. Like the simple smile on the painting, the reaction may be far more complex than chemists have so far discovered.
Lewis acids (AlCl, ZnCl, and others) act as catalysts to the reaction.
Dagestan
Dagestan is a federal subject of the Russian Federation in the Caucasus region. The name comes from two Persian words, "dag" which means mountain, and "stan" which means land. Dagestan has a population of nearly 3 million, and its people are known as "Dagestani". Its capital city is Makhachkala.
Dagestan has the status of "republic" in Russia and has its own constitution and official languages. Republics in Russia are the home of its ethnic minorities (those who are not Russians), and usually one or two nationalities have a majority of the population in each republic. But in Dagestan, there are over 30 different nationalities (the largest of which are the Avar, Lak, Dargwa, Lezgian, and Tabasaran), and none of these form a majority. This is because the mountains in the region make it hard for people to move around. These people speak 30 different languages, but use Russian as their lingua franca to communicate between each other.
Most people of Dagestan are Muslims and belong to the Shafi`i school of Sunni Islam.
Theoretical chemistry
Theoretical chemistry tries to explain data from chemistry experiments. It uses mathematics and computers. Theoretical chemistry predicts what happens when atoms combine to form molecules. It also predicts chemical properties (characteristics) of molecules. An important part of theoretical chemistry is quantum chemistry. This is using quantum mechanics to understand valency (the number of bonds formed by an atom of an element). Other important parts include molecular dynamics, statistical thermodynamics and theories of electrolyte solutions, reaction networks, polymerization and catalysis.
Theoretical chemists use a wide range of tools. These tools include analytical models (for example, LCAO-MOs to approximate the behaviors of electrons in molecules) and computational and numerical simulations.
Theorists in chemistry create theoretical models. Then, they find things that experimental chemists can measure from those models. This helps chemists to look for data that can prove a model not true. The data helps to chose between several different or opposite models.
Theorists also try to generate or modify models to fit any new data, If the data can not fit the model, chemists try to make the smallest change to the model to fit the data. In some cases, chemists throw out a model if a lot of data will not fit, over time.
Theoretical chemistry uses physics to explain or predict chemical observations. In recent years, it has been mainly about quantum chemistry (the application of quantum mechanics to problems in chemistry). The main parts of theoretical chemistry are electronic structure, dynamics, and statistical mechanics.
All of these areas are used in the process of predicting chemical reactivities. Other less central research areas include the mathematical description of bulk chemistry in various phases. Theoretical chemists want to explain chemical kinetics (the pathway that molecules combine).
Scientists call a lot of this work "computational chemistry". Computational chemistry usually uses theoretical chemistry to work on industrial and practical problems. Examples of computational chemistry are projects to approximate chemical measurements such as certain types of post Hartree-Fock, Density Functional Theory, semiempirical methods (such as PM3) or force field methods. Some chemical theorists use statistical mechanics to create a link between the microscopic phenomena of the quantum world and the macroscopic bulk properties of systems.
Historically, researchers use theoretical chemistry to study:
Pebble
A pebble is a piece of rock with a particle size of 4 to 64 millimetres. They are often made of flint.
Pebbles are larger than granules (2 to 4 millimetres diameter) and smaller than cobbles (64 to 256 millimetres diameter). A rock made predominantly of pebbles is termed a conglomerate. Pebble tools are among the earliest known man-made artifacts, dating from the Palaeolithic period of human history.
A beach composed chiefly of surface pebbles is commonly termed a shingle beach. This type of beach resists wave erosion, and has ecological niches which can provide habitat for animals and plants.
MESSENGER
MESSENGER, MErcury Surface, Space ENvironment, GEochemistry and Ranging, was an unmanned NASA and APL spacecraft. It was orbiting and studying the planet Mercury. Its mission lasted 10 years, 8 months and 28 days.
It was launched on August 3, 2004 at Cape Canaveral Air Force Station. It was aboard a Boeing Delta II rocket. After launch, the probe did several fly-bys and deep space manoeuvres to gain the right trajectory and speed.
It completed 30% mapping of Mercury on January 14, 2008. MESSENGER made one more pass by Mercury in 2009, and on March 18, 2011 began to orbit Mercury. 100% mapping was completed in March 2013 and the probe continued its studies. On April 30, 2015, it crashed into Mercury. It crashed near the crater Janáček.
Compensation (psychology)