Carbocation and carbanion are two terms that are frequently used in organic chemistry. These are organic chemical species bearing an electrical charge on a carbon atom.
Carbocations and carbanions are often found as intermediates of some reactions. The main difference between carbocation and carbanion is that carbocation contains a carbon atom bearing a positive charge whereas carbanion contains a carbon atom bearing a negative charge.
Trigonal Planar, Pyramidal.
The term carbocation can be defined as an ion containing a positively charged carbon atom. Carbocation refers to the whole molecule, not only the positively charged carbon atom.
A carbocation may have one or more positive charges. These carbocations are generally unstable because p orbitals of the carbon atom are free due to loss of electrons. Therefore, carbocations are very often reactive. This favors the reaction between a carbocation and a nucleophile. Carbocations are paramagnetic due to incomplete electron pairing.
Typically, carbocations show sp 2 hybridization. This is because a carbon atom with a positive charge can have only three bonds around it. The geometry around this carbon is trigonal planar. Generally, carbocations are divided into four groups according to the number of carbon atoms that the positively charged carbon atom is attached to.
These carbocations contain a positively charged carbon atom that is not attached to any other carbon atoms. Figure Methyl Carbocation.
Here, the positively charged carbon atom in the carbocation is connected to another carbon atom through a covalent bond. This type of carbocations are stable than methyl carbocations but are less stable than other carbocations. Figure Primary carbocation. Here a —HH3 group is attached to the positively charged carbon atom. The positively charged carbon atom is bonded to two other carbon atoms. These carbocations are stable than primary carbocations.
Figure A secondary Carbocation. Here, the positively charged Carbon atom is bonded to two other carbon atoms. These two carbon atoms are shown in red circles. The positively charged carbon atom is attached to three other carbon atoms. This form is very stable. Figure Tertiary Carbocation. If the organic molecule has a good leaving group, it can leave the molecule through ionization. This ionization gives the bonding electron pair to the leaving group, resulting in a positive charge on the carbon atom.
An electrophile can attack a pi bond and make a covalent bond with one of the vinyl carbon atoms. This causes the other vinyl carbon atom to get a positive charge due to the lack of electrons.Carbanionany member of a class of organic compounds in which a negative electrical charge is located predominantly on a carbon atom. Carbanions are formally derived from neutral organic molecules by removal of positively charged atoms or groups of atoms, and they are important chiefly as chemical intermediates —that is, as substances used in the preparation of other substances.
Important industrial products, including useful plastics, are made using carbanions. In discussing the structures of carbanions, one must distinguish between localized and delocalized ions. In the former, the negative charge is confined largely to one carbon atom, whereas, in the latter, it is distributed over several atoms.
Difference Between Carbocation and Carbanion
The simplest localized carbanion is the methide ion CH - 3. It is isoelectronic it has identical electron configuration with the neutral molecule ammonia formula NH 3N being the chemical symbol for the nitrogen atom. The geometry of the methide ion is best represented by a pyramid with the carbon atom at the apex, a structure similar to that of the ammonia molecule.
Both structures are shown below:. The allyl carbanion formula, C 3 H - 5a somewhat more elaborate unit than the methide ion, serves as the prototype for the structures of delocalized carbanions. A substance like the allyl carbanionwhose structural formula is expressed in terms of separate resonance forms, is considered to have a hybrid structure similar to all the resonance forms but truly expressed by none of them alone. An additional example of this kind of carbanion is the benzylic anion shown belowin which the negative charge can be distributed over a much more extended pi-bond system, which includes an aromatic ring a circle of carbon atoms joined by sigma and pi bonds.
A resonance formulation of this anion is given below:. Closely related to the allyl carbanion are the enolate anions, in which one of the carbon atoms is replaced by an oxygen atom. Enolate ions are derivatives of ketones and aldehydes compounds containing a double bond between carbon and oxygen atomsfrom which they can be generated by abstraction of a proton from the carbon atom that is located next to the carbon of the carbonyl group. The resonance forms of an enolate ion are as depicted below:.Picasa 3 for mac
Because of the greater attraction for electrons electronegativity of oxygen as compared to carbon, the resonance structure with negative charge on oxygen contributes more than half to the true representation of the compound.
In a typical enolate ion, in other words, the oxygen atom bears more of the negative charge than the carbon atom.
In a solution containing carbanions there must exist a corresponding cation positive ion for each carbanion. If the two ions of opposite charge are in close contact with each other, a covalent nonionic bond may form. This reaction is represented by the equilibrium that follows:. Because for a given carbanion the reaction of ionization is favoured by a low electron affinity of the cation, the largest carbanion character of such a compound is exhibited when the atom M is an alkali metal—lithium, sodium, potassium, cesium, or rubidium.
Even in cases in which the tendency to form covalent bonds is negligible, however, the properties of free carbanions cannot always be observed. This situation arises from the fact that there is a strong attraction between the cation and the anion, leading to the pairing of these ions of opposite charge.This module covers the factors that influence the stability of carbocations. The purpose of this module is to provide a review of the fundamental nature of carbocations, and to describe different means by which carbocations are stabilized by nearby atoms or groups of atoms.
This is important because carbocations are formed as intermediates in many organic reactions; the specific involvement of carbocations in various reactions is covered in related modules. They have sp 2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents see diagrams shown to the right.
Carbocations are "hypovalent" species which have only three shared pairs of electrons around the carbon, instead of the usual four. This incomplete octet around carbon makes carbocations very unstable and very reactive. Nevertheless, carbocations are known to be formed as intermediates in many types of organic reactions. The relatively high energy of a carbocation, however, means that it will usually be formed in the rate-determining step for a reaction, and so it is important to understand how the substituents -- shown as R 1R 2and R 3 in the diagrams -- can act to stabilize a carbocation, thus making it easier to form from a neutral molecule in the first place.
This turns out to be a trivial point, however, because the methyl carbocation is so unstable that it is never invoked as an intermediate in an organic chemical reaction. You should view with great skepticism any mechanism that purports to involve the methyl carbocation as an intermediate.
Identify each of the following six carbocations as being either primary, secondary, or tertiary by clicking on the correct answer.
It was long believed that the reason for the order of alkyl carbocation stability was simply inductive stabilization of the positively charged carbon by its attached electron-releasing alkyl substituents. More recently, however, this notion has been more or less discarded in favor of " hyperconjugation " as the means by which alkyl substituents can act to stabilize a positive center to which they are attached.
In molecular orbital terms, hyperconjugation is the overlap of the filled sigma orbitals of the C-H bonds adjacent to the carbocation with the empty "p" orbital on the positively charged carbon atom see diagram below. This electronic "spillover" helps delocalize the positive charge onto more than one atom. The more alkyl substituents, the more sigma bonds for hyperconjugation. Mouse over the diagram to see the image change.
Note that it is not the sigma bonds that are directly attached to the carbocation that are involved in hyperconjugation; these orbitals are perpendicular to the empty "p" orbitals and cannot overlap with it. Rather, it is the sigma bonds one atom removed from the positively charged carbon atom that help to stabilize it. These bonds can rotate into an "eclipsed" conformation with the empty "p" orbital thus interacting with it.
Another way of viewing the effect of hyperconjugation is via Lewis formulas, using "no-bond resonance " see diagrams below.
The image above shows the expanded version of the static image on the right. Mouse over the image on the right to see the animation. The animation shows a depiction of the resonance effect. Note: The positive charge does not really "move"; it is simply "delocalized" smeared out over these four atoms.
Can you think of a reason why the hydrogen atoms on carbons adjacent to a carbocation center are much more acidic than ordinary alkane hydrogen atoms? Hint: Think of the second step in an E1 elimination reaction mechanism. A hydrogen atom on a carbon adjacent to a carbocation center already has a substantial amount of positive charge built up on it.
This occurs via the hyperconjugative delocalization of the C-H bonding electrons into the vacant p orbitals of the positively charged carbon atom. No similar charge is build-up is found on hydrogens in ordinary alkanes. It is relevant at this point to note that carbocations are positively charged species.
They are much more likely to be formed in acidic that in basic media; in fact, carbocations are never seen under conditions that are strongly basic. Which of the compounds shown below would ionize to form the most stable carbocation?A carbanion is an anion in which carbon is trivalent forms three bonds and bears a formal negative charge in at least one significant resonance form.
Formally, a carbanion is the conjugate base of a carbon acid :. The carbanions formed from deprotonation of alkanes at an sp 3 carbonalkenes at an sp 2 carbonarenes at an sp 2 carbonand alkynes at an sp carbon are known as alkylalkenyl vinylaryland alkynyl acetylide anionsrespectively. A carbanion is one of several reactive intermediates in organic chemistry. In organic synthesis, organolithium reagents and Grignard reagents are commonly treated and referred to as "carbanions.
By Bent's ruleplacement of the carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively.
This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations. However, delocalized carbanions may deviate from these geometries. Instead of residing in a hybrid orbital, the carbanionic lone pair may instead occupy a p orbital or an orbital of high p character.
As a consequence, alkyl carbanions with neighboring conjugating groups e. Likewise, delocalized alkenyl carbanions sometimes favor a linear instead of bent geometry. More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only slightly less stable, resulting in facile equilibration between the E and Z isomers of the bent anion through a linear transition state.
Carbanions are typically nucleophilic and basic. The basicity and nucleophilicity of carbanions are determined by the substituents on carbon. These include.Past tense spanish conjugation practice
Geometry also affects the orbital hybridization of the charge-bearing carbanion. The greater the s-character of the charge-bearing atom, the more stable the anion. Organometallic reagents like butyllithium hexameric cluster, [BuLi] 6 or methylmagnesium bromide ether complex, MeMgBr OEt 2 are often referred to as "carbanions," at least in a retrosynthetic sense.
However, they are really clusters or complexes containing a polar covalent bond, though with electron density heavily polarized toward the carbon atom. In fact, true carbanions without stabilizing substituents are not available in the condensed phase, and these species must be studied in the gas phase. For some time, it was not known whether simple alkyl anions could exist as free species; many theoretical studies predicted that even methanide anion CH 3 — should be an unbound species i.
Such a species would decompose immediately by spontaneous ejection of an electron and would therefore be too fleeting to observe directly by mass spectrometry. Simple primary, secondary and tertiary sp 3 carbanions e.Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up. This doesn't really make sense. Positive charge arises from the lack of electrons.
How does holding an empty orbital regardless of whether that makes any physical sense closer affect your stability if the empty orbital is So, what is the exact reason sp2 aren't most carbocations sp2 though? The only reason I can think of is that it's energetically expensive to make an sp carbocation Here are links to 2 earlier answers where this question has been addressed for vinylic and aryl carbocations.
The more s character in a bond, the lower the energy of the electrons in that bond because s orbitals are lower in energy than p orbitals. Once rehybridized the carbocations overall energy has been lowered and the carbocation is more stable. Examples would be the phenyl carbocation and vinylic carbocations in small rings, like the cyclohexenyl carbocation see figure below.
They exist, but again, realize that since the empty orbital is not p hybridized they will be higher in energy and harder to generate then carbocations in which the empty orbital is p hybridized.
Let's just tackle the obvious part first: There is no sp3 carbocation. One reason for this is the same, as for why higher substituted carbocations are more stable than lower substituted.
Electrons are more stable in orbitals with high s-character.Freshly vs kettlebell kitchen
All s character is used in occupied orbitals. My first draft of answering did also assume this. There will be interactions with some carbon hydrogen bonds. Hyperconjugation, see below. This effect is most likely much less than in the prior case, causing it to be less stable. In addition to this, the molecule is much more constrained in its geometry.
This linear arrangement is probably also only true for this very simple arrangement. Dependent on the moieties attached to it, a bent structure might be more stable and a different bonding angle will be observed.Except for a tiny misstep with my second order not automatically showing on the Aftership dashboard, everyt.
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