In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gave deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the salt-like character of the compounds formed.
He called the relationship between color and salt formation halochromy of which malachite green is a prime example.
Carbocations are reactive intermediates in many organic reactions. This idea, first proposed by Julius Stieglitz in 1899 was further developed by Hans Meerwein in his 1922 study of the Wagner-Meerwein rearrangement. Carbocations were also found to be involved in the S1 reaction, the E1 reaction, and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the "Journal of the American Chemical Society" refused articles that mentioned them.
The first NMR spectrum of a stable carbocation in solution was published by Doering et al. in 1958. It was the heptamethylbenzenonium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride. The stable 7-norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established that it was non-classically bridged (the first stable non-classical ion observed).
In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert-butyl fluoride in magic acid. The NMR of the norbornyl cation was first reported by Schleyer et al. and it was shown to undergo proton-scrambling over a barrier by Saunders et al.
In organic chemistry, a carbocation is often the target of nucleophilic attack by nucleophiles like hydroxyl (OH) ions or halogen ions.
Carbocations are classified as primary, secondary, or tertiary depending on the number of carbon atoms bonded to the ionized carbon. Primary carbocations have one or zero carbons attached to the ionized carbon, secondary carbocations have two carbons attached to the ionized carbon, and tertiary carbocations have three carbons attached to the ionized carbon.
Stability of the carbocation increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher-order carbons are stabilized by hyperconjugation, unsubstituted (primary) carbons are not. Therefore, reactions such as the S1 reaction and the E1 elimination reaction normally do not occur if a primary carbocation would be formed. An exception to this occurs when there is a carbon-carbon double bond next to the ionized carbon. Such cations as "allyl" cation CH=CH–CH and "benzyl" cation CH–CH are more stable than most other carbocations. Molecules which can form allyl or benzyl carbocations are especially reactive.
Carbocations undergo rearrangement reactions from less stable structures to equally stable or more stable ones with rate constants in excess of 10/sec. This fact complicates synthetic pathways to many compounds. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about 1/3 3-chloropentane and 2/3 2-chloropentane.
Some carbocations such as the norbornyl cation exhibit more or less symmetrical three centre bonding. Cations of this sort have been referred to as non-classical ions. The energy difference between "classical" carbocations and "non-classical" isomers is often very small, and there is generally little, if any activation energy involved in the transition between "classical" and "non-classical" structures. The "non-classical" form of the 2-butyl carbocation is essentially 2-butene with a proton directly above the centre of what would be the carbon-carbon double bond. "Non-classical" carbocations were once the subject of great controversy. One of George Olah's greatest contributions to chemistry was resolving this controversy.
Cyclopropylcarbinyl cations can be studied by NMR:
In the NMR spectrum of a dimethyl derivative, two nonequivalent signals are found for the two methyl groups indicating that the molecular conformation of this cation not perpendicular (as in A) but is bisected (as in B) with the empty p-orbital and the cyclopropyl ring system in the same plane:
In terms of bent bond theory, this preference is explained by assuming favorable orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital.
Isodesmic reaction
An isodesmic reaction is a chemical reaction where the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product. This type of reaction is often used as a hypothetical reaction in thermochemistry.
An example of an isodesmic reaction is
Equation 1 describes the deprotonation of a methyl halide by a methyl anion. The energy change associated with this exothermic reaction can be calculated using a computer. The energy change increases going from fluorine to chlorine to bromine and iodine. This shows that the CHI anion is the most stable and least basic of all the halides. Each of these reactions are isodesmic. The energy change in each example also depends on the difference in bond energy of the C-X bond in the base and conjugate acid. In other cases, the difference may be due to steric strain (the blocking of atoms by other atoms). The steric strain is small in fluorine (which is a small diameter atom) but large in iodine (in favor of the anion). So, the energy change grows between each example halide despite the fact that C-F bonds are stronger than C-I bonds.
The related term homodesmotic reaction also takes into account orbital hybridization. Homodesmotic reactions also have the same number of carbon to hydrogen (C-H) bonds. An example of a homodesmotic reaction is:
Spit
A spit is a depositional landform found off coasts. Spits connect to the beach, and extend into the sea. A spit forms because the sand on the beach is moved by the waves along the shore, and the sand collects at the end to form a spit. The process of the waves moving the sand along the shore is known as longshore drift.
Sawfish
The sawfish are a family of rays which have a long body, like a shark. One feature that separates a sawfish from other rays is its long, saw-like snout. On either side of this "rostrum" are little teeth like a saw.
Sawfish have a mouth, nostrils, and gill slits under their body, just like a ray. Also, their pectoral fins are enlarged, like those of a ray. Also like rays, when they settle on the bottom, they take in water through two spiracles just behing the eyes. This gets water to the gills without sand.
Several species can grow to about .
The sawfish's most distinctive feature is the saw-like rostrum. The rostrum is covered with motion- and electro-sensitive pores. These allow sawfish to detect movement and even heartbeats of prey hiding under the ocean floor. The rostrum serves as a digging tool to unearth buried crustaceans.
If a suitable prey swims by, the normally lethargic sawfish springs from the bottom and slashes at it with its saw. This generally stuns or injures the prey sufficiently for the sawfish to devour it. Sawfish also defend themselves with their rostrum, against predators such as sharks and intruding divers. The 'teeth' protruding from the rostrum are not real teeth, but modified tooth-like structures called "denticles".
Eagle ray
Eagle rays are cartilaginous fish in the ray family Myliobatidae. They are mostly large species living in the open ocean rather than on the sea bottom.